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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

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

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

Thanks to you both for the tips.

I was already wondering about those transistors, because the heatsink on one channel that covers all 3 had shifted out of position, and left Q2 dislodged (the amp is closed up right now, I could be wrong about which transistors). I popped it back into place, but it sounds like they might all be bad anyway. I will try to find and install replacements, and I plan to put together a list of caps to see if I can afford to do those also.

It's a PC-36 board.

John