Looking for a little help troubleshooting an issue I’m having with a vintage Onkyo A7 amplifier my father gave me. I’m having an issue with the right channel. When I play music, the right speaker sounds very thin, like there is no low end. If I use the balance control on the amp and switch from full left to full right, it’s obvious I’m not getting full sounds out of the right speaker. The tweeter seems to be coming through fine though. When I swap the output wires, the issue follows, so I’ve ruled out the speaker being the issue. I’ve tried new rca cables that I know work and the issue is still there.
I tried pulling out the main-in and pre-out jumpers to bypass the volume knob on the amp and now there is no low end in both speakers. Plugging the jumpers back in gives the left speaker low end again. At this point I don’t know if there is anything else I can do to try and identify the problem. I did open the amp to inspect the caps and they all seem OK (no leaking or bulging), but I’m no electrical engineer so I don’t really know what I’m looking for. Any suggestions would be greatly appreciated.
Just in case this matters, I am running spotify from my tv to an smsl su8 via spdif, rca to amp, speaker wire to speakers. Any and all help would be appreciated. Thanks in advance!
Make sure the polarity of the connections are correct, positive to positive, negative to negative etc. Reversing this in the chain can cause loss of bass…from out of phase etc…
Thank you for the suggestion! Just double checked the wiring and the polarities are correct. I was hoping it was something simple like that that was overlooking, but I’m starting to think it’s something internal to the amplifier.
When I initially got the amp, it had a scratchy volume pot that I cleaned up and all was well for a couple months of very light use. Could this be related to the volume potentiometer?
When I tried swapping the outputs (left output to right speaker, right output to left speaker) the issue was consistent with the right output. Therefore, I believe the speakers are OK. I have another amp I’ll swap in as a final check to rule out the speakers or possibly something else going wrong in the chain.
Having a schematic is nice, but you need to have the unit on a workbench with voltmeters, and oscilloscope and some other test gear to figure this out.
Process of elimnation without this stuff up to a point…if the other amp is ok with speaker on the right output then its something in the amp, possible a tone control or associated circuitry…
Try the other speakers with the same music or low freq test tone and see if its indeed the amp…
If it is then its back to the shop if you cant handle internally troubleshooting…I used to do this for a living in the past…I have no test gear here anymore…
Replace caps, or test them well with a high quality multimeter that can scale to the capacitance. They don’t need to bulge or leak, they can just dry out and look perfectly fine. One of the biggest issues with old amps, and one of the default “channel imbalance” fixes. If the problem persists after that, at least you eliminated one of the most common issues.
Thanks for the suggestions everyone! I really appreciate all the input I’m getting. I pulled the lid off today to get a quick peak at the bass control circuit as @MazeFrame suggested. I don’t see any compromised solder joins, everything seems intact. While playing music through the right speaker only, I messed with the bass control and could hear the bass being adjusted up and down, it’s just very light. Sounds like I may need to take some measurements on the caps and see if that leads me anywhere. The amp is from the late 70s and all original so it’s probably due for a service anyway.
Here’s an internal picture just for fun. Thanks again!
Not sure, but I think that’s glue. It looks like the components (thermocouples?) Are covered in a plastic wrap and glued to the aluminum. Here’s a better look at it…
After more poking around, found this capacitor with a substance below it. Is that a leaky cap or is that glue from manufacturing? I read sometimes components will be glued down to help keep them in place for soldering.
Glueing components down helps keeping them in place during shipping. Vibration from being on a plane or truck could otherwise snap the components of the PCB or break their legs.
Looks like some epoxy to stabilize that taller cap?
Amps that are old, when re-vitalizing folks replace ALL those electrolytics.
Even those BIG 25,000 uf ones…and like @MazeFrame states they might cost a bit.
Replacing caps means taking the boards out to get to the backside to de-solder. A pain.
Before this, I would make sure you can get all the caps replacements online at Mouser of Digikey etc
and that they will fit in the spaces they are in now…for the correct voltage ratings and pin measurements.
For me and others this would be a fun project, but one that wouldnt be done overnight.
