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Rotel RA-310 advice required

I picked up this little amp for a few dollars not working a few weeks back. I'm a novice at repairs but want to learn so figured it would be a good piece of gear to learn/practice on. The DC fuse had blown and a few pokes with a multimeter told me the output transistors on one side had blown. So I downloaded the service manual and started prodding around and replacing things. I replaced all the transistors on the main pre/power amp board then fired it up with a dim bulb tester. The amp not runs but is badly distorted in one channel. Following the instructions in the service manual I attempted to set the idle current and found it to be far too high in one channel (the other channel can be adjusted to the reccomended 10mV across the emitter resistor, the bad channel is at 80mV minimum). So far I've done the following and have about reached the end of my limited fault finding capabilities.

Checked all input voltages, all are close to spec
Replaced all transistors with new ones as closely matched as possible, either original spec or newer ones with as close characteristics as I could find
Checked all visible solder joints
Replaced all electrolytic caps
Checked all resistors, all are close to spec and consistent across both channels, replaced a couple of carbon comps that were looking a bit scorched with new metal film resistors (Will eventually replace all of them)
Disconnected input signal wire to check problem wasn't being passed thru from there, nothing doing
Removed, cleaned and tested trim pots, all ok, reinstalled
removed diode and ceramic/mylar caps, don't have means to test properly so swapped them to opposite channel one by one to see if the problem migrated with any of them but had no effect.

As I said this is a practice/learning exercise as much as anything else so not too worried about how it ends up but I've spent a fair while mucking with this thing and would be good to get it going. But if nothing else my soldering desoldering skills have already improved immensely!



Re: Rotel RA-310 advice required

Sounds like you checked everything. One thing you might do is check for open traces on the boards; you may need to clean off any rosin with alcohol.
Next you should make sure you got all the components in with proper polarity; it's often easy to reverse a transistor and get the collectors and emitters backwards. You should leave the bias controls where they were originally before powering up.

Re: Rotel RA-310 advice required

I will add to Johnnysan's recommendation with the following:

Think about all of the parts that you found that are bad. Think about where they are in the circuit and what would have happened to the rest of the parts in the circuit. For example if the output transistors were shorted, they probably took out the emitter resistors.

Go back over all of the work that you performed. Sometimes a solder bridge is difficult to see, use a meter to verify. It is easy to create a new problem when trying to fix the original problem! Don't ask me how I know this! :)

After checking all of the components with the power off, the next step is to check voltages between the good and bad channels with the power applied. This will help to zero in on where the problem lies. However, be advised that a problem in one part of the circuit can cascade to other parts of the circuit. You will need to analyze why the voltages are different. But it will help to show you where there might be an open circuit or shorted component. Pay special attention to the Base and emitter voltages of the transistors. Look to see if they are biased correctly. Be careful that your probes don't slip and create more problems!

If you do not have an isolation transformer to use while probing with the power on, you should plug the unit into a GFI protected outlet.

My rule of thumb when replacing bad transistors is to always replace complimentary transistors even if one checks okay. If one fails shorted, it will have stressed out the other transistor and weakened it. I also replace the next set of transistors as well. So if I find shorted output transistors, I replace the driver transistos as well. If the output and driver transistors are bad, then I also replace the pre-drivers. Working as a bench tech, parts are cheaper than the labor to troubleshoot.

Also don't overlook the transistors that are mounted to heat-sinks. They are usually required to be insulated. If you used an old insulator it may not be effective. Use a meter to verify that it is still doing it's job.

Some circuits will not function correctly with a dim bulb tester. I don't think this is the situation with your unit, because one channel is working correctly. Just be aware that it can cause havoc with some units down the road. The dim bulb test is only to find short circuits and to protect newly installed parts from suffering the same fate that happened to the originals. If you are comparing voltages between the unit and those provided in a schematic, you will need to apply full power to the unit.

Don't overlook the grounds in a circuit. The current needs to go somewhere!

And last, check the adjustable pots, there are usually two for each channel, one for dc balance and one for idling current. They could be open or intermittent and affect the entire circuit.

-bob

Re: Rotel RA-310 advice required

Ok making some progress. Followed your advice, cleaned and checked everything. still same issue. Replaced the rest of the carbon comp resistors in the channel that seemed faulty. No change. started checking voltages following the schematic from the input. all good around the first transistor but got weird around the second (TR702/802) double checked component disgram and schematic. found an error- TR702 appeared to be oriented wrong in the component diagram. (802 was consistent with the schematic). I reversed TR702. Now BOTH channels had too much idle current and output noise. I remembered that one channel had worked previously, so I reversed both 702 and 802 to match the (supposedly wrong) orientation of TR702 in the component diagram. Now the idle current is good, and I get output at about the right volume to the headphone in both channels, albeit with some distortion. however the DC offset of way off (reading about 160mV on both channels) and the offset trim pots don't seem to have an effect. but at least I've got both sides behaving consistently now!
Next steps? I've pretty much replaced all the transistors, though the replacements have probably copped a bit of a beating in all the messing around.

Re: Rotel RA-310 advice required

Hi,

Good to see you are making headway. A couple of points to consider.

When replacing transistors, the best cause of action is to replace with the exact same type. If you need to substitute you need to keep several factors in mind. Obviously voltage and current ratings, but also bandwidth and gain. Some circuits do not tolerate changing the parameters and will not function correctly. Generic transistors such as NTE and ECG are a last resort substitution. In most cases they will "probably" work, but NOT ALWAYS. Best to stick to the originals.

As for DC offset, you do realize that this is an AC coupled design? It is not a dc offset control, but instead an AC BALANCE control. You set this while outputting a signal at maximum and adjust the control so that the waveform clips evenly at the positive and negative peaks. The reason you see a dc voltage at the output is due to no load being present to drain it off. The headphones may not be enough of a load to do this. Hooking up a speaker should lower it to an acceptable amount. If not, then the output capacitors are leaky and need to be replaced.

Since this is a new amp to you, you don't know the history of the unit. It may have more than the fault with the output stage. And definitely with the age of the unit a re-cap of the unit is probably in order.

-bob

Re: Rotel RA-310 advice required

Cheers Bob, very valuable info. As I said I’m a novice and learning as I go. I’ve attempted to replace with original spec components here whenever possible. Where unavailable I’ve tried to match the gain and bandwidth as close as possible while matching or bettering the voltage and current ratings.
Makes sense on the offset- unfortunately I don’t have an oscilloscope, it’s on the list of gear to obtain. I’ll source or borrow one and revisit.

Agree on the capacitors- I’ve already replaced all th electrolytics on the main board (Rubycons for the large main capacitors, just generic ones from the local electronics shop for the others, for now).
As for it’s history- I’d say it’s been rough- it’s obviously had plenty of work before, the output transistors had already been replaced with TIP31Cs and some of the others had been changed, and there are quite a few lifted, part broken pads- which is fine by me as it’s good preactice for repairin those sort of things!

Any idea re what’s going on with the orientation of those transistors? Shall I assume I’ve got them round the right way because it’s working now? It just makes me uneasy when it looks backwards compared to the schematic but the results are a lot better than when they were the “right” way!

Dave

Re: Rotel RA-310 advice required

Cheers Bob, very valuable info. As I said I’m a novice and learning as I go. I’ve attempted to replace with original spec components here whenever possible. Where unavailable I’ve tried to match the gain and bandwidth as close as possible while matching or bettering the voltage and current ratings.
Makes sense on the offset- unfortunately I don’t have an oscilloscope, it’s on the list of gear to obtain. I’ll source or borrow one and revisit.

Agree on the capacitors- I’ve already replaced all th electrolytics on the main board (Rubycons for the large main capacitors, just generic ones from the local electronics shop for the others, for now).
As for it’s history- I’d say it’s been rough- it’s obviously had plenty of work before, the output transistors had already been replaced with TIP31Cs and some of the others had been changed, and there are quite a few lifted, part broken pads- which is fine by me as it’s good preactice for repairin those sort of things!

Any idea re what’s going on with the orientation of those transistors? Shall I assume I’ve got them round the right way because it’s working now? It just makes me uneasy when it looks backwards compared to the schematic but the results are a lot better than when they were the “right” way!

Dave

Re: Rotel RA-310 advice required

Hi Dave,

Unfortunately you have discovered one of the pitfalls of repairing audio gear. The schematics and circuit boards often have mistakes. Since the manufacturer knows about the mistake, it's not a problem for them. Sometimes they correct it with a reprint of the schematic or an updated silk screen for the PC board, but most of the time they don't Always observe the orientation of the parts you take out. This is most important with capacitors. They are the ones that have the most mistakes. When re-capping I like to do one capacitor at a time, observing the way the old one was mounted. Occasionally you will find that the manufacturer put the part in wrong and it made it's way past Quality Control, but that is pretty rare.

As for the balance adjustment, it isn't super critical as it only affects the distortion near the maximum output. Until you get access to a scope and a signal generator, you can adjust this to the center point and then listen by ear while playing music. Put it at the point that sounds the least distorted.

-bob

Re: Rotel RA-310 advice required

cheers again. Gave it a test with some speakers and its definitely a long way from right- very quiet output and plenty of distortion. Gonna give this amp a rest for a while I think, its annoying me!

On the upside I'm winning with the NAD C340 I picked up the other week, another one with one dead channel which I quite easily isolated to the input switching IC which I've now temporarily bypassed by removing the IC and bridging the aux input pins to the outputs, its now working perfectly on that single input and doing a nice job with my phono preamp as my vinyl amp :) I'll order a replacement switching IC for it and might upgrade the preamp while I'm at it too. New gear might not quite have the charm and be fiddly to work on but sometimes its ha hell of a lot less frustrating!

Next on the list will be a Pioneer SX-450 with one blown output. It looks in better condition than the Rotel so hopefully it'll be less problematic!

-Dave

Re: Rotel RA-310 advice required

Well I’m an idiot... after leaving this one on the shelf for a while I finally went back to it. After realising a couple of transistors weren’t installed the right way and still having issues with distortion and far too much idle currrent I realised I hadn’t checked the DC offset (mainly as I don’t have an oscilloscope). I connected some speakers and tried adjusting it by ear and found I could get it to a point that actually sounds very nice and clean, and this also dropped the idle current into a range that was properly adjustable. I suspect I did a lot of unnecessary work on this unit, but the main thing is it finally works, and I’ve learned a lot!