Onkyo TX-SR501 speaker output problem

[alexdanis]

Hello everyone,

I recently became the owner of this Onkyo amplifier (for free by a friend moving out, cool right?). The amp is a home theater model, but I would like to use it as a stereo amp. Problem is as follows:
Plug the amplifier and turn it on. Everything looks ok, the screen is on, you can change the settings, choose a different source, adjust volume etc.. But when you connect a pair of speakers, in any of the two channels (A+B) for stereo there is no sound coming out of the speakers. The speakers themselves are ok, I have tried them with a different amp and the work flawlessly.

In order to locate the problem, feed the amp with a signal on the CD input and use the tape out exits to lead the signal to another amp that I own that works. With this setup there is sound, so I assumed that the pre-amp module of the amp works. I then opened the amp and thought, why not remove the fuses and check them thoroughly? I did and they were ok. After that I connected again a pair of speakers and oh! a miracle there was sound coming out of the Onkyo (actually sounds a lot better than my other amp too...). I tried all possible inputs and each one was working, volume control was working, bass/treble adjustments, everything. But after about 5 minutes it made a weird noise like somebody pulled the plug and then no sound again.

Volume control and input selection etc (pre-amp module) was again responsive. But but nothing coming out of any of the speaker connectors. If I power it off, unplug it and plug it again it plays for about a minute and then stops. The sound it makes before stopping is not like a scratching sound that would lead me to look for loosened wires, but something more abrupt. Puf and no sound sort of. The fact that it still functional makes me think that is not related to the power supply circuit. So I have narrowed it down to the power amp module. Because of the fuse incident i replaced the fuses thinking that they might be malfunctioning. Nothing changed though.

I can't seem to find any obvious problem with any of the solders or other connectors. The radio however doesn't seem to work either. Does anyone have any ideas as to where I should focus or what I should look for?It feels really bad to not be able to use the amp but what appears to be a minor issue and the budget is too low to send it for repair....any ideas?

Thanks in advance

[valveradio]

hmmmm. certainly sounds a weird one that. i would suggest the power amp is ok if it works for a while unless its going into some sort of thermal runaway. its not unknown for transistors or indeed ic's to develop an internal fault, but its rare. as your model has multi channels it seems that its unlikely that the fault is on another channel as the current sensing relay (if it has one) would trip and allow no sound at all. are your speakers 4 ohm or 8 ohm? sometimes if the speakers are too low impedance, this can cause the protection relay to operate as to protect the output stage.
it could be a simple thing like a bad connection, possibly even a dry joint on one of the internal plugs or sockets depriving the output stage of its power supply or somewhere else in the amplifier and causing the protection circuit to kick in. the fact the sound trips out sounds to me like the internal speaker protection relay is operating for some reason.

good luck!

kind regards, peter

[alexdanis]

Hey valveradio, thank you for your reply. What you indicate about the speaker protection circuit kicking in sound quite possible. My speakers are rated at 8 ohms by the manufacturer (bose 201, 1984).However there has been an unexpected turn of event to my problem....I unplugged the amp and dismantled all individual circuit boards. Then i got around just making sure all transistors and capacitors were firmly in place (by touching them and gently pushing/shaking them down). Then i plugged in a single spare speaker and played the amp quite loud (60-65%) for about an afternoon. Guess what....It has never stopped playing since then.....

I was determined to take to service, but in the end i didn't had to. Maybe it was sheer luck, or just some part was not making a good contact with the board, but the problem is solved now . Cleaned the battery acid from the remote and now that works as well. So I am quite happy with how this adventure concluded....

[valveradio]

im glad its working again lol. you might have just disturbed something when you took it all apart, the fault might recur but it sounds like it was a bad connection on a plug/socket on one of the boards. just see how it goes, if it does happen again, i think you will need to do some soldering.......

all the best!

peter