Yamaha PSR-500 keyboard - no audio from speakers

[YurkshireLad]

I took my old Yamaha PSR500 apart to see if I could identify why I'm not hearing any audio from the speakers. I do hear a slight pop when I power it on/off though.

Referencing the attached circuit, I started at the speakers and couldn't register a signal there. I worked back to IC22, the power amp, and, while I'm not very good at debugging circuits/ICs, all I could see was mains hum when I put my scope between the ground pain and either input or output pins.

I went from there to both op amps (IC21), and had the same result. I then checked IC20, and couldn't find a signal there either. I did note that all of the ICs I checked had 26V when I put my DMM between ground and Vcc. (Edit - I need to re-check and note down which ICs showed 26V).

I didn't look at IC2, as I don't think I've found a datasheet for it, if one is even available. I will check all these ICs again, to make sure I was checking the right pins.

What is the best way to debug an op amp? I saw a video online that said to compare the voltages between the non-inverting and inverting inputs. I didn't see anything there, though I think I need to go back and check using a smaller V/division, just in case.

I also forgot to check the headphone jack, to see if that was getting a signal.

Anyway, I thought you might find this circuit interesting if nothing else. I don't really expect to fix this keyboard, but I'm enjoying the investigation.

[cncjerry]

If you have 26v on pin 8 of ic21, then you have the wrong power inverter plugged into.  Sounds like you have one from a PC hooked to it, because that's a common  unloaded voltage range for the older PC laptop models.  I regularly use old Sony and Lenovo adapters to power oscillators that need 20v.

You should have 9v on pin 8 of ic21.  ic22 pin 5 should have 9v and if you can find D5 that should have 9v on both sides, a little less on the circuit board side if it is floating around in there by the AC adapter plug.

The circuit is digital to the DAC, then into ic21 and the power amp, ic22. nothing else to debug.  If you put a scope on pin 12 of the dac, ic20, you should see a clock signal, a square wave.  Put a meter or a scope on the midi output pins and hit a key.  see if you have any voltage changes or if you have a scope, you should have some type of square wave that you can't trigger on as it doesn't repeat the same data, I mean you can, but you have to be setup for it.   You need to use the midi ground pins because IC7 is probably an opto isolator and the grounds might be lifted at the midi output.

If the midi is working, I would be amazed if you have 26v as you mentioned on those pins.  It's possible because the 9v is brought down by ic19, a regulator.  See if you have 5v out of pin 3 and 9v on pin 1.  Pin 2 is ground.  don't short that regulator because if you have 26v coming in where it should be 9v, that is the only thing protecting your digital circuitry.

My money is on the wrong AC adapter, then a defective AC adapter, that blew everything in the analog chain.  Or maybe you just misread the voltage on the meter.  Note that some AC adapters (bricks) don't output the correct voltage without a load, others can tell you why.  The unloaded output of the power amp can be an unknown voltage.  Just because it pops when you plug it in doesn't mean much.

If the midi works, run it into a PC with a USB to midi adapter, they cost less than $20, and then use the PC to drive the volume output.  That assumes your IC's are blow and it didn't propagate to the other circuitry.

If the opamps are blown, then the output amp is probably shot too, and maybe the DAC.  all that can be replaced inexpensively.

Jerry

[YurkshireLad]

Thanks for the very useful info. I realised after posting that I hadn't noted down which particular ICs measured 26V. I need to double check that for sure.

[YurkshireLad]

It's a 12V 1A power supply, but I'm measuring 29V across pins 4 and 8 of IC21! 

I added a picture of the supply I've been using for years.

[YurkshireLad]

I tried another 12V adaptor and the damn keyboard works. 

Haha, oh well. Now to see if I can put it together without breaking anything further.

[cncjerry]

great news, glad I can help.  I still don't understand the 26v between 4&8, maybe it was 26mv.

I wish my defective scope was easy like that.

Jerry

[YurkshireLad]

Now it's making noise again there is a constant hum from the left speaker. I thought I observed a 60Hz mains signal on one of the op amps, but at this point it doesn't matter so much. I put it back together and I may take another crack at it later. We're planning on moving house this year and I'll probably get rid of it. It's over 30 years old and has more than served its purpose.