Yamaha g50-112 II troubleshooting

[twilightofthedogs]

Howdy folks, I am working on a nice old Yamaha G50-112 that turns on but makes no sound. TR203 was hanging from the board when it was found, so naturally I looked there first. Got it hooked back up and found a voltage of -9 instead of +15 coming off the emmiter of TR203. I have attached a schematic to show what I mean. I swapped out TR203 just to see what would happen, but got the same results. Wondering if anyone with more experience than I can look at this and recognize a likely culprit. Thanks in advance!

[Jazz P Bass]

What voltage is at the base of TR203?

What voltage is at the base of TR201?
The WZ162 is a 16 volt 1/2 watt zener diode.

[twilightofthedogs]

Thanks for the reply Jazz P Bass.

The voltage on both the bases is +17.5.

What voltage is at the base of TR203?

What voltage is at the base of TR201?
The WZ162 is a 16 volt 1/2 watt zener diode.

[Jazz P Bass]

Can you disconnect the +15 volt rail from the rest of the amp.

If the +15 comes up, then someting downstream is pulling it down.

If it stays low, you may have a bad cap, 470uf/16V

[oc disorder]

I was going to suggest simplifying the power supply diagram so you can see what your looking at.
Reminds me of my old maths teacher "simplify the equation" where we often complicated it.... but i digress

There are two identical supplies and if I take JPBass's suggestion "Can you disconnect the +15 volt rail from the rest of the amp"
and suggest disconnecting both 15v rails from the rest of the amp so we can see if both power supplies, free of their loads,
are performing identically.

With one probe on the common ground point there should be similar readings (although one positive and one negative) from
both supplies.

The 16v half watt zener would be my first suspect.

[g1]

Any fault in the supply itself should only allow the +15V rail to drop to zero volts. As it is at a negative (-9) voltage it implies to me that the fault must be somewhere downstream, I'm guessing a shorted IC somewhere fed by the +/- 15V supplies.

[twilightofthedogs]

Thanks a lot all of you. I detached both 15v rails and found the following:

+15 volt rail: 0 volts VDC
-15 volts rail: -15 VDC

So, seems like the problem is not likely downstream? Upon light of day, I noticed that the 22uf/16v cap has a bunch of bluish crud on the negative lead. my suspicions are now leaning towards that cap. There is also a "rotten egg-ish" smell that I am unfamiliar with emanating from that general area.

[g1]

I don't think that cap could cause your problem, but if it looks bad, easy enough to just replace it.
It sounds like TR203 is bad, did you check it out of circuit? Also, double check your work where you reconnected it and make sure none of it's traces are broken.
And before you reconnect the wires you detached, check that there is not a low resistance from the +15 wire to ground. It is possible there is a problem with the supply AND the circuit it is powering.

[twilightofthedogs]

I have a couple different transistors that I am subbing in and out for TR203 and they seem to be OK. Weird thing is, I changed the 22uf and the zener diode 206 (wasn't getting any voltage drop across) and I was still not getting the right voltage from the emitter on tr203 (was about 1v), then I accidentally touched the emitter to the middle prong and I saw a little spark and voila the reading at the emitter was +15, as it was supposed to be. However, when I hooked the +15 rail up again, it went back to a bad reading. I am a little out of my league here.

[oc disorder]

Have you checked the 10k resistor ?

[Jazz P Bass]

I think you fried that transistor when you saw the spark.

That regulator circuit is called 'series pass regulator'.

Here is a tutorial link: Series voltage regulator

There is nothing esoteric about it.

If the transistors, resistors & zener diodes are good, then it works as advertised.