Power Supply for FM synth

[Milkmansound]

well, seems as if I lost yet another ebay battle. I just received a Yamaha TX81Z with a horrible power supply. Seems that either the 15 or -15 volt rails have it out for me, because I can never seem to get them both to work at once. I replaced some coupling caps and had about 20 minutes of FM fun before the hum returned to the output and the signal turned to poop.

I have a few quick questions:

is it worth putting a different power supply in there? It seems as if I just need 15, -15 and 5V to run the damn thing. I double checked every component on the board and even lifted the coupling caps out of circuit that I had once replaced to no avail. Also, I replaced the 15 and -15 regs but that changed nothing. Its starting to drive me insane because its such a simple power supply and I can not get all 3 voltages at once.

I was thinking of just making my own and putting it onto a bradboard inside and calling it a day.

Any sage like advice? Thanks!

 

[alk509]

[quote author="Milkmansound"]I replaced some coupling caps[/quote]

Why? If anything, it's the filter caps in the PSU and any bypass caps present in the circuit that you want to be looking at! Although that's probably not the problem...

[quote author="Milkmansound"]hum returned to the output and the signal turned to poop.[/quote]

Hum? How's the grounding in there? What exactly do you mean by "can't get them both to work at once"? One will be at 15V, the other at 0V? both at 2V, or something? Do they overheat?

[quote author="Milkmansound"]and the signal turned to poop.[/quote]

Again, this doesn't really say anything... please explain.

Peace,
Al.

 

[Milkmansound]

ok - thats a good point about the filter caps. I did not even think to replace them because they all checked good. Perhaps under load they are in fact in need of some replacement.

I think the hum is because of the unbalanced supply in the output - its not a grounding problem at all. Nothing overheats - and one of the voltages will drop to nothing shortly after power on.

signal turns to poop means that its breaking up. This also has to do with the imbalance in the voltages. When both sides were up (15, and -15 that is) the hum was gone and the output was normal. When one is missing the output becomes weak and there is a very loud hum. (output is based around a JRC4558DV opamp)

The nice thing about this is that there is a ribbon cable connecting the supply PCB to the main one - and I am positive the problem is in the supply because the voltages are still missing when the supply is isolated.

Tomorrow I will just replace all of the remaining caps on the board - there are only 5 or 6 left to replace, and I have some used ones in my shop. Then I can probably return the original regulators and call it a day.