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Dynaco ST-150: can't adjust bias

After running about an hour, I get a random loud pop every few minutes, and soft "rustling" noises (both channels). Adjusting bias, one channel will only drop to about 400ma (should be around 75) and the DC null adjustment at the speaker output will only get down to about 400mv (should be near zero, less than 25mv). The other channel is OK. With the noise in both channels, maybe the bias is unrelated? The affected channel runs hotter.

Maybe in a 40 year old amp, I should just replace all the electrolytic caps for starters, then the transistors?

John



Re: Dynaco ST-150: can't adjust bias

Sheesh - yeah saw the surge after the fact. Happened soooooo quick and I barely moved the variac dial

https://youtu.be/QYBnujJqumE

Gonna talk to someone local. The weird thing is I am visually unable to find the component that went poof.

Re: Dynaco ST-150: can't adjust bias

Andy
Ref your: "First, lets identify the fuse # that you were measuring continuity across with the fuse removed. If it was the right channel B+ fuse, it would be #F204. What was the resistance measurement you recorded when you say the continuity was "strong"? Was it close to a dead short 0 ohms? I would clip your black meter lead on the common ground connection on top of the 2 main filter caps, and then probe both fuse clips with the other meter lead. You should see a fairly high reading that will be constantly changing on one of the fuse clips; it may increase or decrease depending on the polarity of the meter leads since you are measuring across those large filter caps. The other fuse clip should show a very high or open connection.
I'm not familiar with the (2) 2200mfd caps you referred to. Where were these installed?"

Andy
yes F204 - was just going by the convention in the assembly manual discussions and here:https://www.updatemydynaco.com/Stereo150.html
My bad
By strong continuity I guess I mean that the buzzer on continuity/Diode checker was a strong steady tone indicating close to 0 Ohms. I had since gone back and cleaned both sides of the PC-37 with 99% IPA and long stick cotton swabs to get rid of any film and Rosin remnants thinking maybe it was a filming scum that was conducting between the top clip and bottom clip of the F204 fuse holder. Since then the short has not been repeatable. Before the cleaning it was. Missed a zero on the filter caps as I could only read 2200 from my vantage point when hastily writing my note. Will try the measurement technique above - hoping the cleaning may have sorted out the issue as well.
Also was getting continuity across the top clips from B+ F204 to both B- tops (don't have the fuse numbers handy) - when I flipped the leads I got no continuity. Then I looked at the schematic and saw the Diode bridge arrangement on PC-37 and now see why it makes sense...
So many circuit dynamics going on, as a newb it is easy to miss things that are more common sense to pros like you guys - It is why I really appreciate taking the time to coddle me along ;-)
Cheers
Tim

Edit addition: Bit confused about the common ground on the filter caps. There is the aluminum bar that connects both 22000 uF filter caps together only the one on the left channel has the pos terminal hooked to that bar and the one on the right channel side has the neg filter cap attached to that common bar. Is that what you mean because I get high Ohm readings from there to both the upper and lower clips....

Re: Dynaco ST-150: can't adjust bias

REF: Edit addition: Bit confused about the common ground on the filter caps. There is the aluminum bar that connects both 22000 uF filter caps together only the one on the left channel has the pos terminal hooked to that bar and the one on the right channel side has the neg filter cap attached to that common bar. Is that what you mean because I get high Ohm readings from there to both the upper and lower clips.

That is a classic hookup for a split plus/minus power supply. The 2 caps connected together with the metal bar serves as the main amplifier ground point. All DC voltage measurements are referenced to that point. Above, you made reference to the left channel cap. There are no left or right channel caps. C301 supplies plus 50volts to BOTH LEFT AND RIGHT CHANNELS. Thats why there are 2 B+ and 2 B- minus fuses; 1 for protection on each channel. Then, C302 supplies BOTH channels with minus 50volts. Don't think of the power caps as left channel or right channel caps. If you connect your black (negative) meter lead to the metal bar, and measure the voltage on the remaining terminal of each 22000mfd cap, you should read plus 50volts on one and minus 50volts on the other. If you get those reading, the power supply/diode bridge are all OK and you can concentrate your troubleshooting else ware.

RE: I get high Ohm readings from there to both the upper and lower clips

That is what you should be getting. The fact that you can't see the caps "charging up" simply means your meter can't supply enough voltage or current. If it did, you would see the meter reading varying (cap charging) when you where on one of the fuse clips. I don't want you to take this the wrong way, but your equipment is not ideal for the type of work you are undertaking.

There have been so many "unrelated issues" that have popped up during this quest, that it's hard to say where you need to go at this point. The information I suggest to you is based on a "normal" type repair of the ST-150. You have to admit that what has happened with this 150 has been anything but normal. Just an FYI; I can repair BOTH blown out channels in these amps and have it buttoned up and ready to go in about 3 hours tops. But, I have to give you credit for not throwing in the towel yet.

As to what smoked, most likely a resistor or maybe a transistor again. Time to go back to checking if there are any shorted devices; I suspect you will find some again.

Re: Dynaco ST-150: can't adjust bias

Many thanks Andy. You have been extremely patient and super helpful to this point. I have come to the realization that I have dug myself a deep hole on this one that I am unable to extricate myself from. Last night I chatted with a really good tech I know ( I would say the best best in town here) who, like you, has oodles of experience with these amps and many others. He is the go to guy for many audio gear-heads around here and has all the necessary test gear for such work. He agreed to look at it for me which is a huge relief. Sometimes a fella has to bite the bullet and admit defeat. Kicking myself for having got the amp up and working and actually sounding good but could not leave it at that because i had not properly set the bias and DC Offset correctly and then did a bone-head move at that point which got me into this mess. Taking it today to get looked at by him with hat in hand. Thank you immensely for all your help. Will follow up with outcome of course.
Cheers
Tim

Re: Dynaco ST-150: can't adjust bias

Tim,

I was hoping that one day I would see a new message from you that read the amp was finally finished and sounding great, but things don't always go the way you hope :(. Your new tech should be able spot the problem and finally fix it for good. Sometimes a fresh set of eyes on a problem is all that is needed. Take care and let me know what the tech finds.

Re: Dynaco ST-150: can't adjust bias

And a fresh set of eyes was all that was needed! Not what what my pride wanted to see but it explains a lot - another Rookie mistake but glad it is over. Turns out the bent pin on the PNP Q was shorting against the chassis - Doh!

See: https://i.postimg.cc/QNHyF6jC/57464874-371207100161239-27345186226710773...

At 1:16 AM this morning the Tech (who is a night owl) said by text "It balanced and biased perfectly and is sitting here warming up for test." I think the news is all good. Once I get it back I will certainly post a little ditty video of it operational - Yipeeeeee!

Re: Dynaco ST-150: can't adjust bias

Tim,

I thought I had seen it all by now, but you just provided me with another "how the heck did that happen" entry for my repair diary. I'm glad it was not a costly mistake for you; those driver and output devices can drain your wallet when you are not careful. Where did the smoke come from?

Re: Dynaco ST-150: can't adjust bias

Smoke came from R24 which was as crispy as a marshmallow dropped in a campfire. Missed it because it was way down near the bottom of the board and a tiny one I guess. I am tickled pink right now though. Here is a short run-down:

https://youtu.be/96TzFiRkaEc
:-)

Re: Dynaco ST-150: can't adjust bias

Thank you again Andy. You will no doubt see me again as I have a dead Dynaco Stereo 120 sitting on my basement shelf - on deck for a Updatemydynaco.com rebuild. After this is all done I plan to check the power supply and if it is ok will order the full rebuild kit. Didn't want that one to get in the way of this one. One thing is for certain I learned a lot which for me is what this us all about.
;-)
Best wishes
Tim

Re: Dynaco ST-150: can't adjust bias

Thank you again Andy. You will no doubt see me again as I have a dead Dynaco Stereo 120 sitting on my basement shelf on deck for a Updatemydynaco.com rebuild. After this is all done I plan to check the power supply and if it is ok will order the full rebuild kit. Didn't want that one to get in the way of this one. One thing is for certain I learned a lot which for me is what this us all about.
;-)
Best wishes
Tim

Re: Dynaco ST-150: can't adjust bias

Here is the situation with one (ST-150) amp I am working on. An old unit with PC-36 Board. You can watch the entire upgrade I did from start to finish here (with descriptions at each section and timelapse - long vid): https://youtu.be/HF4exNBSaGI

I was given three kits to install for a fellow.

Kit 1 was this: very last item on this page: https://www.updatemydynaco.com/storeindex.html which was two X 22,000uF 80 Volt Capacitor for Stereo 400 and Stereo 150 power supplies (22KUF80V) - $16.00 each

Kit 2 was this one from Ebay: https://www.ebay.ca/itm/DYNACO-ST-150-QSA-300-PC-36-43-POLYPROPYLENE-CAP...
It was a DYNACO ST-150 QSA-300 PC-36 OR PC43 POLYPROPYLENE CAPACITOR UPGRADE KIT

Kit 3 was this: https://www.ebay.ca/itm/DYNACO-ST-150-QSA-300-PC-43-SEMICONDUCTOR-REPAIR...
It was a semiconductor replacement kit for ST-150

Installed everything and it is working good except sound was off a smidgen - this was before I attempted to set the Bias as per the instruction guide for the semiconductor kit which I saved for last. So I proceeded to do that as per page 21 of the assembly Dynaco manual as it recommended. I did everything as per the test up to the Bias adjust and got a reading of 3.29 (L) and 3.24 (R) mA for Bias after turn on. adjusting the pots for each channel (yes I used the right pots) and nothing changed at all on either side. I sidestepped this after a couple of attempts (only small changes made so as not to take it entirely out of whack but the trim pots have absolutely no bearing on the new configuration (thinking this is key with the new transistors and Caps). Nothing rewired so I don't believe it was that which could have changed things other than the effectiveness of the pots. Voltages at the test points (rails etc) as per the manual were all correct. At this point I moved on to DC Offset and found that indeed after the upgrade they were reading up around 80 mV + and - when spec calls for + or - 20 mV with no load and nothing on inputs after 10 minutes warm up. These I had to dial counterclockwise all the way. One I got to 20 mV before the pot would turn no further and the other I got down to 6.9 mV. Went back to Bias and no change. Does this sound right to folks? After the DC offset adjust it sounded waaaaay better as one would expect I presume. Just need a measure of confidence I did the right thing here before I button up the case and give it back to the fellow I did this for. Don't worry, the fellow who I am doing this for knows up front that I did the upgrade of his PAT -5 and ST-150 as a learning experience. Here is a run down of the PAT-5 build (way more complicated): https://youtu.be/o04tB0UOEiY
Thanks
TMF

Re: Dynaco ST-150: can't adjust bias

Check transistors Q4-Q5-Q6 and Resistors R9,10,11,12,13,14 and 15. Those components are the ones that control the amp bias. R10 would open if there was any kind of overload in the drivers section. The bias for this amp should be set for 75ma to 90ma depending on the amps temp.
Was this amp working before the upgrades were installed?

Re: Dynaco ST-150: can't adjust bias

I know the optimum settings but trim pots do nothing to adjust the Bias after upgrade. This is the same for both channels - don't you find it odd that both channels are exhibiting the same behavious - I could see one channel but both??????

Re: Dynaco ST-150: can't adjust bias

Q4, 5 and 6 do not exist in the parts list or the PC-36 board diagram arrangement so hard to correlate what you mean. Amp worked beforehand. Did not do Bias or dc offset measurement in advance becuase I figured upgrading to modern transistors and caps would throw everything out anyway. Did not pull resistors as I thought it would be too traumatic for the board - the old solder was tough to remove even with Hako on highest heat. I measured all transistors before install so all were registering as good. I made triple sure EBC orientation was correct.If what you say R9 thru 15 would I assume correlate with R109 thru R115????
Should I pull R110 to measure to see if open as a likely culprit? Amp is sounding good and doing the R110 would involve major surgery given its location now that it is all re-assembled....
Could it be bad Trim units as a more likely culprit?
Thank you
Tim

Re: Dynaco ST-150: can't adjust bias

Yes, just add 100 to all the dynaco part numbers. You should be able to see if R110 is open by a quick in-circuit measurement. It's only 2.7K, so you should see a value in that ballpark; it won't be exact. If you get a reading over 15 to 20K, then you would want to pull one lead of the resistor and recheck. You should reverse the meter leads and take the lowest reading you find.

I doubt that both trim pots are bad; that would highly unlikely. You can also test those in circuit; they are only 1K and you can adjust them with your meter hooked up and watch the reading go from 1K down to 0 ohms.

All of Dyna's PC boards were wave soldered. Working on those type boards can be a pain when trying to remove components. The trick is to melt some new 60/40 solder into the joint. When mixed together, the solder should melt easier for you.

Re: Dynaco ST-150: can't adjust bias

Thank you Andy - will investigate. I was thinking more in terms of both resitors being open than the trim pots but 6 of on 1/2 a dozen of the other I guess. Great tip on the soldering - will definitely try it out next parts pull I do (maybe the resistors)...

Re: Dynaco ST-150: can't adjust bias

Fixed it. I got a transistor kit from a guy who sells Dynaco fixup stuff on eBay (Ed, sells as Qua-Co). First I replaced Q1, 2, and 3. No-go. Then I did 5, 9, and 10. These are the ones mounted to a small heatsink. Bingo. No more pops or strange noises. At one point I blew out R23 (10 ohm). Ed suggested checking to see if one of the collectors was shorted to the heatsink. Sure enough, 2 of them were. Those tiny old stepped rubber washers are hard to seat properly after 40 years of time to expand or deform. I used nylon screws on the second try. I will need to monitor temps, teflon would be better if i can find it, but I think the nylon will hold up.

The bias and DC offsets adjust OK and it's running cool and clean.

John

Re: Dynaco ST-150: can't adjust bias

PC-36 was used on the first production run of ST150's, so you have an early one. I should at least have a copy of the schematic for PC-36 units if you need one.
There is no heatsink covering any of the 3 transistors I mentioned above. They are grouped together very close to the DC offset pot on the top edge of PC-36. Also, check the value of R10, it should be around 2.4k. It is connected to one leg of the bias pot so it is physically close by. It can be adjusted + - 500 ohms either way to get the bias down to 75ma with the bias pot centered. There have been a few cases where the wrong value was installed on some early boards.

Andy

Re: Dynaco ST-150: can't adjust bias

My buddies PC-36 board does have a heat sink linking Q109,Q110 and Q105. Gosh that will involve a huge amount of work to even access R110 I think. Will pull the top and see how accessible it is...

Re: Dynaco ST-150: can't adjust bias

duplicate post removed

Re: Dynaco ST-150: can't adjust bias

Replace ALL the capacitors, not just the electrolytics. I recommend checking resistors; sometimes the values change to the point that bias can't be set properly.

Re: Dynaco ST-150: can't adjust bias

John,

Take a look at transistors Q1,Q2,and Q3 on the circuit board. There are 2 sets of these on the board, one set for left and the other for the right channel. If they have the number 562889 on the flat side that takes up the entire flat area, they must be replaced. There are no other numbers. There was a factory change notice issued for those parts. They would go into early secondary breakdown and cause all sorts of issues with ST-150, QSA-300, ST-400 and ST410 amps. Main issue was LOUD hissing, crackling and popping thru the speakers even at low listening levels. They will affect biasing since the circuit is direct coupled. They are PNP 2N4889 types. BTY what PC board are you working with, PC-36 or PC-43. This info is the best I can recall; I worked for Dyna many moons ago.

Andy

Re: Dynaco ST-150: can't adjust bias

Q101 to Q103 was replaced with 2N5401 on my kit

Re: Dynaco ST-150: can't adjust bias

2n5401 is used in place of the 4889 parts because the originals are no longer made and are hard to find. That is not causing your bias problem.
Also, the EBC layout of the 2n5401 is the same as the original 4889 parts, so they are installed with the same orientation.

Andy

Re: Dynaco ST-150: can't adjust bias

Affirmative - I have included the substitute parts kit list in a separate post. Most of the transistors in the kit were the same EBC orientation except one pairing from a different chipset which I triple checked before installation. So I went back as suggested and started measuring up R9 to R15 last night paying special attention to R10 and got the following values: https://i.postimg.cc/m2ny1cXy/20190327-212210.jpg (left column is left channel and Right is...)

This is how she looks splayed open for measuring: https://i.postimg.cc/SNRGKLrK/opened-ST-150.jpg

That being said I found that R10 on the left side plus the other resistors shown with values all read good resistance values (2 way measurements on each). R10 on the right channel however only measures 100 ohms (both directions) - tonight when I get home from work I will pull that one and get a value to confirm it is a bad resistor. It of course should read 2700 ohms like its counterpart on left channel. So, having gone that far (and because I found no problematic resistors on the left) I decided to try and do the Bias for left channel only again. It still only reads roughly 3.3 mA and adjusting the Trim pot makes absolutely no difference again. Next thing I will try to do is measure the trim pot to see that they are functioning. As there are three leads going into the board, which leads do I put the tester on to measure the 1k to 0 values while adjusting. Do I just go around the horn and test all three pairs? I have never tested them only replaced them like I did with my Quad 303 upgrade kit...

As part of my homework tonight I plan on measuring all the remaining resistors on the board while I have it splayed open just to get a warm and fuzzy that the remainder are good...

I did replace the two Zeners with good ones but did not install the 8 regular diodes because of their tricky locations and the difficulty removing the crappy solder from such minute holes. As they are essentially one way valves can I measure those up on the board like the resistors to get a feel for them operating correctly. Wish I would have know the wave soldering trick back then. Will be putting that top tip into my future arsenal for sure.
Thank You
Tim

Re: Dynaco ST-150: can't adjust bias

I doubt R10 is your problem; normally resistors will increase in value not decrease. I bet when you pull one of the R10 leads and recheck, you will find that it measures OK. That resistor is basically in parallel with junctions in transistors Q5 and Q6. One of those transistors may have high leakage or maybe one of the junctions shorted; could be either transistor. Make sure its mounted the right way. Also, make sure the first color band on R10 is red.
You would need to pull P1 to test it fully since all 3 terminals are used. But P2 the bias pot, only uses 2 terminals and is technically called a reostat not a pot. If you look closely at the solder side of P2, you will see that the center terminal and one of the end terminals are connected on the same solder pad; in other words they are shorted together. That can be quick checked by putting your meter lead as follows: one meter lead on the center terminal and the other meter lead on the end terminal that is NOT connected with the center terminal. When you adjust, you should see your meter reading go from 0 ohms when turned all the way in one direction, and read about 1K ohm when turned all the way to the other extreme.
Did either or both pots have some type of glue on them? Sometimes when you turn them without first scratching/digging off the glue, you will break the carbon element inside the pot and it will need to be replaced.

Re: Dynaco ST-150: can't adjust bias

At wits end - I pulled Q5 and Q6 on the right side. They were both oriented correctly and reading fine. I verified that right R10 was in fact working within tolerance (in my haste earlier I had measured the wrong resistor to get 100 ohm thinking it was in the same spot on the right side which of course it was not since they are not completely mirror image (close but no cigar). Went back and verified all my other readings as correct. As suggested I measured both left and right Pot2 (reostats) and they are functioning correctly between 0 and 1K Ohms as per spec so they work OK. Did not pull the left side Q5 or Q6 simply because I think it stresses them too much to pull them from the board in its current installed configuration where I did test to make sure each were reading correctly prior to the install and double checking EBC orientation. They are oriented the same config as the right side as per the markings on the board. I simply do not want to risk breaking the remaining transistors trying to pull them and have to wait a long time for mail order replacements when I believe them to be correctly installed and the symptoms are exactly the same on both sides with no changes. I have to believe it may somehow be the combination of new components (semi-conductors and cap kits) installed that has somehow conspired to make the bias pots ineffective in-circuit because I am unable to find a faulty component anywhere and the fact that after I set the dc offset with the p1s it is actually sounding quite good. I have also gone through the polar caps to make sure they are oriented correctly and I believe they are. I then went back and tried to adjust the Bias as per page 21 of the manual and of course I get the 3.4 mA on AC and it reads less when switched to DC (manual does not specify setting). DC Offset measurement is pretty self explanatory (I think). One of life's little mysteries I guess unless there are other suggestions or possibilities I am completely missing out on???
Thank you for you patience and all your help by the way. It has been very much appreciated!
Tim

Will do up an HD vid this evening of all I have done to trouble-shoot this issue. I did read somewhere (think it was the Boak/Jung mod that states when you do a major component replacement that the manual setting should be thrown out the window because it changes the dynamics of the circuits to such a degree that do not line up with the original design intent. I find this hard to believe as well since, component types did change to more modern ones but I would have assumed that equal values were utilized (certainly for caps but I can't speak for the new semi-conductor flavours used). Hmmmmmmmm!

Re: Dynaco ST-150: can't adjust bias

I can assure you the replacement caps/transistors that you changed did NOT alter the circuit configuration and is the cause of the low bias readings. When was the last time you measured current with your meter? It's possible the low readings are caused by your meter being "off"; if in fact the amp sounds fine as you say. That still wouldn't explain why you can't vary the readings with P2 though. Many meters have a current range protection fuse; check to see if your meter does and that it is OK. Some fuses are integrated into the positive probe socket. I've worked on dynaco equipment for the past 40 years and I can never recall an amp that had a bias issue like you have and the amp sounded fine. Try this, with no input or output connections to the amplifier, let the amp sit turned on for about 15-20 minutes. Then feel the heat sinks on the rear and compare side to side. Both should be warm but not overly hot that it would burn you, and they should both be about the same. If the 3ma reading is correct, then I would expect the amp heat sinks would be hardy warm at all.

If you have access to (2) 10 ohm resistors and an AA battery, you can check the accuracy of your meter. Just connect your meter leads to one end of each 10 ohm resistor. Connect each opposite end of the resistors to the battery terminals. That circuit will draw 75ma. I'm assuming your meter is digital. If not and it is an old analog type you will need to observe correct polarity when hooking the meter up. If it's digital, the reading will display a negative value if the polarity is reversed. Basically you are making a series circuit from the + battery terminal to a terminal on the first 10 ohm resistor; thru the 2 meter leads, to one lead on the 2nd resistor and finally the last terminal of the 2nd resistor to the - battery terminal.

Re: Dynaco ST-150: can't adjust bias

Well, I have now had my fill of humble pie. It turned out I was completely mis-using my multimeter like a true dough-head. I neglected to move the leads over to the Amperage measuring side so big apology for going through all of this. Big lesson learned here so hopefully someone else can learn from my bone-head mistake. So left channel is now dialed into 120mA initially as per instructions. I then shifted over the mm leads to the right side B+ fuse connections and R116 270 ohms started smoking so I have cooked that one. Of course this is the side I pulled the two Q5 and Q6 transistors, measured them and put them back. No easy feat in the tangle of wires. So I tried to clean things up and lacking a 270 ohm resistor to replace I series connected a 220 ohm and 47 ohm resistor to get a combined resistance of close to 270 Ohms and that pairing fried too. Any suggestions as to what may be causing this particular fuse to smolder and fry other than too much current? If only I had left enough alone. Pretty darn sure the two trannys I put in were installed the same orientation. Tried to see if any traces were shorted and looks clean from my perspective. This amp has been soooooo bloody frustrating I simply can't speak. Going ocean boating in the AM until Sunday evening as a time out but really would like to button this thing up for the patient fellow I am doing this for in Vancouver. He knows I am a noob - was forthright on that regard. Nobody can't say I didn't learn some valuable lessons here that is for sure which was my principle motive for attempting this. Building a tube amp was far easier me thinks.
Thanks again for all the sage advice. Maybe I should bring this thing along as my emergency boat anchor for the weekend

Video explanation link follows:
https://youtu.be/Od9CCSU1Bnw

Re: Dynaco ST-150: can't adjust bias

Clarify something for me. R116 was OK and not smoking before the last time you pulled out Q5 and Q6? If so, its possible you have a solder splash causing a short across a circuit trace. I would use your DMM and check Q4-Q5-Q6 again in circuit. You are looking for something that is shorted; any reading under say 20 ohms or so would be cause to investigate that area further. If they check OK, then its time to pull the board and get it out in the open where you can really see things from different angles. Get some masking tape and rip about (30) 1" pieces and unsolder each wire from the board. Tag each wire you remove with a piece of tape and mark the board eyelet number on the tape. Actually, with the scope of the work you were doing, it would have been much easier to just remove the board before you started the work.

Re: Dynaco ST-150: can't adjust bias

Thank you Andy
Yes R116 had no issues before I pulled the Q's. Will have a fresh look at it when I am back from boating Sunday - the good news is the wires are all still tagged from when I first pulled the board if I have to go that route. Hopefully when I look at it again with a fresh mindset I will be less inclined to throw it over the nearby balcony

Re: Dynaco ST-150: can't adjust bias

Since R116 was OK before you removed the Q's to check them, then you are down to a few items to check. A solder splash is the easiest thing to rule out, since it doesn't require removing components from the board. I would give a very detailed look around Q5-Q6 and nearby traces. I saw on your video that the small metal clamp that secures Q5 to the heatsink is tilted badly to one side. Make sure the metal edge is not touching one of the transistor leads causing a short.
Another possibility is too much heat from removing than reinstalling the Q's may have damaged one or both of them. In circuit testing should reveal any problems there.

Re: Dynaco ST-150: can't adjust bias

Man, this thing will not give me a break! But I am not a quitter! Anyway the heat sink was skewed that way because the transistor was so low it was the only way I could get it to contact the transistor to dissipate heat. Nothing shorted there. Have it removed for now. I have explained what I found and the new issues here as it is easier to explain than type:
https://youtu.be/vd144P-Ka2A

Re: Dynaco ST-150: can't adjust bias

OK, I did see a possible problem area on your video. In the video, on the physical left side of the board (actually the right channel) it looks like one of the mica insulators on the rearward driver transistor is twisted very far from where it should be. It was a quick couple frames of video and would have been easier to see if the whole transistor was seen straight on instead of at the angle of your video. In any event, measure resistance from the screw head on that driver transistor to the aluminum heat sink it is screwed to. You should see a 0 ohm short. Next measure from the metal tab on the driver transistor to the heat sink. You should NOT see a short here; probably 50K more more.

If the any driver or the output transistor is shorted, you will blow fuses. This is a direct coupled circuit, and any shorts will cause excessive current to flow and pop the fuses. The best thing to do now would be to remove both output transistors from the sockets and test them out of the circuit, looking for very low or shorted junctions. Once the outputs are removed, you can also check the 2 drivers transistors for shorts. You may find 1or all 4 transistors are bad and need replaced.

BTW, the large center post on the output transistor sockets is the collector circuit connection and has 75 volts on it. THIS TERMINAL IS NEVER GROUNDED. The wires from the 4 B+ B- fuses go directly to those terminals.

Re: Dynaco ST-150: can't adjust bias

Andy
Thank you very much for giving me some more hope. Heh, Robin Williams certainly paraphrased the hope concept best in his golf skit at 1 minute 51 seconds here: https://youtu.be/pcnFbCCgTo4
;-)

When you say driver transistors, I assume you are you referring to the the two Q9 and Q10 that are bolted to the heat sink, is that right? Looking at the schematic on page 12 they appear to be the ones in direct line with the output transistors so I assume these ones would bump up the output to useable levels by the output transistors? Perhaps as an afterthought I should simply pull the heat sink entirely and try that first as I have heard that some of the PC-36 boards actually did not have the heat sinks on them. As long as I don't run the amp for very long it would at least rule out or confirm the idea of shorts as being a contributing problem? I had thought the mica sheets did cover the entire transistors (I know one was twisted quite a bit but I thought the whole tranny backside was covered). Will confirm when I get home from work. but I did not change anything on those heat sinks from when the amp was actually working. All I did was unscrewed the bent metal piece that was covering Q5 to pull and check that one. Right now that cover is off.

Thank you for the safety observance and explanation on the transistor sockets - I do "always" bleed down the large caps with the DIY cap discharge device I built that taps into my multimeter and also has LEDs to verify all voltages are at safe levels before I go poking around inside. I also observe the hand in pocket rule when doing risky stuff like measuring the transformer secondary windings with alligator clip probes. Next time I plug in as well I think I will also pull out my dim bulb tester me thinks...

If the right output transistors (1 X PNP 2N6030 and 1 X NPN 2N5630) are toast, I found this article to find suitable replacements for these obsolete transistors: http://audiokarma.org/forums/index.php?threads/substitution-for-2n5630-a...
Will pull and measure them tonight.

As per the recommendation in post#2, I found the NPN MJ21194G and PNP MJ21193G. If after I pull and test the old output transistors I am assuming I should order two of each so as to properly match left and right channels? Also I saw some articles on transistor matching. Is that really all that necessary and if so how would I get Mouser to match them for me or is that just a smoke and mirrors type thing?

reading about direct coupled circuits now to get a better grasp of how they work: https://en.wikipedia.org/wiki/Direct-coupled_amplifier

Once again thank you for all your help Andy.
Tim

Re: Dynaco ST-150: can't adjust bias

Yes, Q9 and Q10 are the driver transistors. You must make sure both the driver pair and the output pair are OK before you reapply power again. Right now, at least 1 or maybe all 4 of them is shorted; that's why you are blowing fuses. If you just replace the outputs and you have a shorted driver, you will blow the outputs again the instant you flip the power switch on. I would also square up that driver insulator so it doesn't become an issue for you later on.
At this point, I would check around and see if you can rent or borrow an AC Variac with a current meter. This is a variable AC voltage supply that allows you to slowly raise the AC voltage while watching how much current is being drawn by the ST-150. With that model, you should be able to get to a normal 120 volts and the current would be about 1 amp as the amp is sitting at idle. If you adjust the voltage up from 0 and find a point where the current is rising past 1 amp and you are way below 120 volts, then you still have a major issue. Stop raising the voltage once you see the current go above 1.5 amps. As long as you keep the current below 1.5 amps, you don't have to worry about destroying more driver and or output devices.
The correct devices are as follows:

Output devices Q201 (npn) = MJ21196 and Q202 (pnp) = MJ21195

Driver devices Q10 (npn) = TIP41C and Q11 (pnp) = TIP42C

Those are the latest devices that are available on the open market.

Not really necessary to change the working channel outputs or drivers.

Re: Dynaco ST-150: can't adjust bias

Andy
All great points which I shall absolutely heed. Will pull and measure all those devices. I do have a variac that I use to make DIY Acrylic Turntable Dust Covers with. You can see it here: https://youtu.be/Bo7astZvWu8 (at 1 min 2 seconds in). It only has a Voltage indicator on it - How can I actually hook in an AMMETER to measure the current flow with what I have? Would I just hook inline near the power switch somewhere or rig an extension cord with the ammeter inline and set on 10 Amp for the multimeter?
Thanks Again
Tim

Re: Dynaco ST-150: can't adjust bias

I would get an old extension cord and hook up your DVM in line and set it to measure on the 10 amp scale. If you see the meter go above 1 amp, I would stop unless your are at 120volts on the Variac voltmeter. The normal current draw of the ST-150 is right around 1 amp or so when sitting at idle. Good luck with your measurement. Be careful as you will have 120 volts on your meter leads and possibly on your workbench. Make sure you use tape to insulate the exposed probe ends and any extension cord ends.
You can make a safer and permanent hookup by using a plastic dual gang AC outlet box and a receptacle. You drill 2 holes in one side of the box and mount banana plugs in the holes. You then connect the white wire from the extension cord directly to one of the receptacle's silver screws. Then you connect the extension cord's black wire to one of the banana plugs. Another wire connects from the 2nd banana plug to the brass colored screw on the receptacle. You would then plug the extension cord into your Variac and the DVM leads into the banana plugs. At that point you are ready to measure your current.

Re: Dynaco ST-150: can't adjust bias

Great idea Andy, will try when I get the new output transistors.

Result of Transistor tests:
Test 1 NPN Output Transistor: https://youtu.be/TQAqFQxLxwk (Fail)
Test 2 PNP Output Transistor: https://youtu.be/LcyLa84DDJI (Contrary to what I say in the Vid I believe this is a fail)
Test 3 NPN Q9 Driver: https://youtu.be/LwCl7J6_FVw (Pass)
Test 4 PNO Q10 Driver: https://youtu.be/Q5kYs1siK10 (Pass)

Re: Dynaco ST-150: can't adjust bias

Tim,

Looks like the drivers are OK; you just need to replace the outputs. Remember in an earlier post you asked about matching the output devices. This is the case where you would want to do that. Get both the npn and the pnp MJ2119X parts for replacements.
You wouldn't want a 35 year old transistor amplifying half of the output signal and a new one amplifying the other half of the signal. Plus, the MJ transistors are more rugged devices.
FYI, the output transistors you have are the factory original devices; they are marked with the dynaco in house part numbers. The 2N numbers listed on the parts list are the industry numbers you can use for replacements if needed. Those 3 digit numbers tell the date of manufacture so dynaco could keep records of any problems with certain parts runs.
BTW, I noticed on your DVM that you have a special range for measuring diodes/transistors. Do you know how to use that function?

Re: Dynaco ST-150: can't adjust bias

Andy
Here is my setup after installing the two output Q's (have another pair for the other side if it all pans out) - thought I would run this by some extra eyeballs before I start giving it incremental amounts of juice tomorrow:

https://youtu.be/5TfXlXVG3Ow

Does it look like a reasonable arrangement?
Tim

Re: Dynaco ST-150: can't adjust bias

Your setup looks OK to go Tim. Just keep a close watch on the ampmeter as you S L O W L Y raise the voltage. If there is still a problem somewhere, the current may rise slowly then jump up with very little increase in voltage. Your amp may draw more or less than 1 amp; what you are looking for is a slow, steady increase in current as you get to full AC line voltage. Most problems usually show up when the AC voltage gets around the 35-50 volt area.

Re: Dynaco ST-150: can't adjust bias

A further bit of sleuthing:

https://youtu.be/waku4X07Skc

Re: Dynaco ST-150: can't adjust bias

Andy
Here are the results.
https://youtu.be/yA01_IhQ-i0

Does the amperage seem low?
Tim

Re: Dynaco ST-150: can't adjust bias

Looks like you are good to go. The 1 amp I mentioned in previous posts was an absolute maximum; typically a random sample amp would draw less than that. The fact that you didn't have any fireworks from the board and all the fuses held is a very good sign.

The 2 caps you were measuring in the other video are fine. They are NOT wired in a way such that 1 cap is used for the left channel and the other cap is for the right. Rather, that is a split plus minus supply; meaning one cap develops the + 50 volts for BOTH channels and the other cap supplies the -50 volts for BOTH channels.

Carefully measure the DC bias current again, and see if they are back to a normal 75ma per channel and can be varied up and down. If so, then you are finally done. Good luck.

Re: Dynaco ST-150: can't adjust bias

Arghhh!
0 mA on both sides across the B+ fuse. Perhaps the fuse in my meter on the mA scale is toast? Gonna see if I can open it up. A few days ago I did measure all the voltages as per page 21 from the various holes and all were correct.

Re: Dynaco ST-150: can't adjust bias

No need to open up your meter; normally the current fuses are located behind the colored (not black) front panel MA banana jack. If the ma/current jack has 2 slots in the plastic, use a flat blade screwdriver and turn it 90 degrees and the fuse should pop out. Remember to remove the B+ fuse for the channel you want to measure and connect the meter leads in place of the fuse; not across the holder with the fuse still installed. You are measuring DC current so make sure you have the meter leads in the correct jacks on the meter and the range/mode switch is set correctly.

Re: Dynaco ST-150: can't adjust bias

Insert many many expletives here!

This is two part show and tell. Here is the first part ramblings:
https://youtu.be/XOCBz2rKfms

Here is the second part ramblings. On this one I said to hell with it and connected it up to speakers, its preamp mate that I rebuilt from an bunch of updatemydynaco kits and a turntable source. Surprising outcome but still need some suggestions if possible:
https://youtu.be/R3MsuOoelAk

Re: Dynaco ST-150: can't adjust bias

In the first video you mentioned that the right channel heatsink was getting very warm in relation to the left channel. I can tell you that at a normal idle, the heatsinks will be warm to the touch after about 20 minutes, but nowhere near hot enough to cause a burn. This is without any sound being played. In any event, the fact that it was heating up and then later it didn't, has me concerned. You may have some type of loose connection or short issue in play here.
Those DC offset voltages are normal in that they will vary from -10mv to +10mv all the time. It is a function of the thermal junctions inside all of the transistors used in the circuit. You can never adjust them to 0 volts and have the reading stay at 0. If you can get the readings to bounce between -10 and +10mv, then you are fine. Also, the pots should not be turned all the way to the end of rotation positions. From the quick glimpse I saw in the video, it looked like both offset pots were about 30 degrees off center which is fine.
That fuse blowing is a bad sign. You need to recheck the drivers and outputs again for shorts. I would also double check all the driver and output transistor mica insulators. Something is moving around and causing the channel to heat up more than normal at times.

Re: Dynaco ST-150: can't adjust bias

Here is a weird one. Soooooo I used my multimeter to measure continuity across the B- fuse in place. Worked good so no micro-breaks there. For shits and giggles I measured across the obviously blown right channel B+ fuse and holy crap I had continuity. Pulled the fuse and measured across the fuse and of course the fuse is deader than a post. I then measured top of the fuse holder to the bottom and low and behold I get intermittant continuity. Sometimes it reads strong and other times it reads open - more often than not it reads strong continuity on one side of the fuse holder prongs. Here is the thing. I have never touched the PC-37 board in any of the work I did here:
https://youtu.be/HF4exNBSaGI

I only applied all the components to the PC-36 board with the exception of installing the two 2200 uF caps adjacent and attached to the PC-37 and those were merely screw on - no solder near the board. any idea what may be shorting the two fuse terminals together as I have not even soldered near this board before. Failed components somewhere? Visual inspection shows large gaps and no solder splashes etc between the trace planes. If I believed in ghosts I would say that this machine is truly haunted - Doh!