Rotel RA 312

[Oldman79]

Hi Guys

I am having a couple of issues with my Rotel RA-312 Stereo Amplifier. When I received the amp it couldn't go above volume 2 with out breaking up, so I have replaced all of the electrolytic capacitors on the amp and for the left channel this has cleared up the noise and it can go nice and loud again with out distortion. But the right channel is not so lucky. When the right channel gets to about 3 on the volume knob it is distorting. I have put a single tone through the channel and taken a screen shot of the scope output showing the input tone and the output tone, you can see the bottom half of the wave is not right and I am unsure why or how to proceed in fixing the issue.



Is this perhaps a biasing issue?

The second issue is with the left channel, every now and then it just cuts out, no distorting or crackling just pure clear audio to nothing in an instant. I have wiggled all the controls and nothing seems to be related to the cut out of sound. Is there something I am missing or should be looking for?

Any help is much appreciated
Thanks

[floobydust]

There's 8 electrolytics for each power amp channel, and common C617. I would double-check the replacement went OK for soldering and polarity, whenever new issues come up after a recap.

As a wild hunch, stabistors are known to not last in vintage amplifiers. They go noisy, intermittent. These would be bias diodes D601, D602 SV-04 which are mounted touching the main heatsinks. They are a module that acts like a series string of four silicon diodes with maybe some zinc oxide in there.
I would measure the voltage on each side of them. You can take voltage readings to compare the good and bad channel. Don't bang them up, if they go open-circuit there will be smoke. So that would be 4 voltage readings.
Out of circuit SV-04 should measure 2.10-2.60V typ. 2.35V at 1mA using a multimeter on diode test, or 3.0V +/-0.3V at 70mA.

If the bias current is too low distortion is audible when playing quieter. If bias current is too high the amp will run hot, or blow output transistors if it is really high.

edit: I used RA-312 service manual

[Oldman79]

Awesome, thank you very much. Ill check out the diodes ASAP

[shakalnokturn]

To add to floobydut's good advice:

If you've already checked the PCB's for weak solders which should be done in amplifiers of this age, for just dropping out you may want to check the speaker fuse holders and fuses themselves for signs of oxidation.
This amp. seems to use a few carbon composition resistors (the cylinder shaped dark brown ones) definitely check their values.
After that I'd just replace both differential pairs (Q601 to Q604) even if they test OK.

[SilverSolder]

I would be very suspicious of switches and pots, they can cause both cut-outs and distortion and often give a false sense of security...  De-Oxit is your friend!

[mzacharias]

Awesome, thank you very much. Ill check out the diodes ASAP

The TO-220 package transistors used for outputs on these are notorious for going open circuit or intermittent on the emitter-base junctions.

Easy to troubleshoot, measure the DC voltage at the base of each output transistor on your affected channel while the volume is raised to the point of noticeable distortion. The bad transistor will show a higher DC voltage at the base, not the .6V or so you might expect.