VR voltage issue

[nosaj]

I've been trying to understand this been fooling with it for a while.
It's from a Baldwin gps2500 electric piano. http://www.shema.ru/cd/us/2/data/ps2500.pdf Service manual
.
Was getting a loud hum on the output. I could not bring everything back with me so I figured I could take the power supply board and rig up input power.

5v regulators were good.

!2 v regulators no so much. At c40 -3.6 and c39 16.18v
-12v rail was -12.01 and +12v rail was 2.39. I don't suspect the regulator because the +18 rail is low but I'm not sure what to check.
16VAV is going into Bridge rectifier. Also to note I am only inputting the 16vac input, not 8.5 or 27.5VAC but I don't think that could of issue.
Thanks for any help,
Jason

[TimmyP1955]

12 v regulators no so much. At c40 -3.6 and c39 16.18v
-12v rail was -12.01 and +12v rail was 2.39. I don't suspect the regulator because the +18 rail is low but I'm not sure what to check. 16VAV is going into Bridge rectifier. Also to note I am only inputting the 16vac input, not 8.5 or 27.5VAC but I don't think that could of issue.

You should have 32VAC hitting the bridge: 16VAC with respect to ground on each AC input of the bridge (and the rails should be A leaky cap (C20 through C23) might be able to pull the transformer down on one side. Pull the fuses (F4, F5) to check the transformer's no-load voltages. If that is good, put the fuses back and carry on. (I suspect that the spec is unloaded and the voltage during operation is more like , as otherwise the 18V rails would be a fair bit higher.)

A leaky cap, a bad regulator, or a load issue could pull the DC rail down on one side. Disconnect CN8 to see if it is a load issue. If the 12V rail is bad with no load, I'd pull the regulator and see if the 18V comes back up. If not, pull the caps (C31, C32). And don't forget the monkey business around the BC173 - an issue there could pull down the 18V rail.

[nosaj]

You should have 32VAC hitting the bridge - 16VAC with respect to ground on each AC input of the bridge. A leaky cap (C20 through C23) might be able to pull the transformer down on one side. Pull the fuses (F4, F5) to check the transformer's no-load voltages. If that is good, put the fuses back and carry on.

A leaky cap, a bad regulator, or a load issue could pull the DC rail down on one side. Disconnect CN8 to see if it is a load issue. If the 12V rail is bad with no load, I'd pull the regulator and see if the 18V comes back up. If not, pull the caps (C31, C32).

Guess I misread the schematic. To be sure you understand I have only the power supply board at hand. Using a variac to input the power so I'm only doing a section at the time. Input voltage was 16VAC I'll go back and input 32VAC and see how things go. CN8 is not connected as I don't have it here. Will follow up tomorrow.

Thanks,
nosaj

[Enzo]

Did you verify that your loud hum was not DC on the speaker? Like from a blown amp output?

Whatever teh transformer is doing, it has at least one of the 18v supplies right, so I assume the transformer is OK.

I don't suspect the regulator because the +18 rail is low

Oh? The regulator has a ground terminal, an internal short to ground would certainly load down the 18v supply.


All you have is a couple caps on the 18v side, and a couple on the 12v side, and a regulator IC. other than a solder short somewhere, that is all there is. Lift the regulator, does the 18v come back? Lift the output caps on the IC, were they holding it down? Lift the input side caps of teh IC, tack in some other cap to filter. 18 come back?

[nosaj]

Did you verify that your loud hum was not DC on the speaker? Like from a blown amp output?

Whatever teh transformer is doing, it has at least one of the 18v supplies right, so I assume the transformer is OK.



Oh? The regulator has a ground terminal, an internal short to ground would certainly load down the 18v supply.


All you have is a couple caps on the 18v side, and a couple on the 12v side, and a regulator IC. other than a solder short somewhere, that is all there is. Lift the regulator, does the 18v come back? Lift the output caps on the IC, were they holding it down? Lift the input side caps of teh IC, tack in some other cap to filter. 18 come back?

I pulled IC6 7912 an IC9 7812. j29 32.50vdc is the input to IC6 and j40 30.14vdc is the input to IC9
c40 we have 27.50 and c39 we have 26.75. So replace the regulators. I've also got a few jumper wires to replace that had some green corrosion on them. Recycled resistor wire should be ok replacement I believe.
nosaj

[nosaj]

You should have 32VAC hitting the bridge: 16VAC with respect to ground on each AC input of the bridge (and the rails should be A leaky cap (C20 through C23) might be able to pull the transformer down on one side. Pull the fuses (F4, F5) to check the transformer's no-load voltages. If that is good, put the fuses back and carry on. (I suspect that the spec is unloaded and the voltage during operation is more like , as otherwise the 18V rails would be a fair bit higher.)

A leaky cap, a bad regulator, or a load issue could pull the DC rail down on one side. Disconnect CN8 to see if it is a load issue. If the 12V rail is bad with no load, I'd pull the regulator and see if the 18V comes back up. If not, pull the caps (C31, C32). And don't forget the monkey business around the BC173 - an issue there could pull down the 18V rail.

Disconnecting D2 should allow me to see if that section is pulling it down, correct?
Thanks,
nosaj

[TimmyP1955]

Disconnecting D2 should allow me to see if that section is pulling it down, correct?
Thanks,
nosaj

Disconnecting D2 and D3 will remove the mute circuit as a load on the transformer.

[nosaj]

Disconnecting D2 and D3 will remove the mute circuit as a load on the transformer.

Definately something in the mute circuit. lifted d2 and d3 regulator voltages came back up. Crossover board is not connected. So looks like 5 transistors to check out.

Thanks,
nosaj

[nosaj]

Definately something in the mute circuit. lifted d2 and d3 regulator voltages came back up. Crossover board is not connected. So looks like 5 transistors to check out.

Thanks,
nosaj

Checking the transistors did not show any shorted. With no input to the amp I went ahead and checked voltages on the board.
CN8 Values looked inline except I didn't know what I should see on pins 2 labeled muting and pin1 labeled VALCD showing -20.14

CN10 Values looked right except pin4 AGND showing -.4vdc

CN5, CN6(no DC) showed expected values.

The disk drive connector Showed +5 on pin1 I'don't think it supposed to show on pin4 also.(it doesn't)
Power IC's with no power I verified pin14 was not shorted to ground.

Just kinda of concerned about the muting circuit.

STK ICs
Left ch Right Ch
pin1 -.1 -.2
pin2 -.2 -.2
pin3 0 0
pin4 -30.5 -30.2
pin5 -31.4 -32.01
pin6 -.7 -.8
pin7 1.2 1.2
pin8 31.2 31.3
pin9 -1.3 -1.2
pin10 -1.3 -1.2
pin11 31.5 31.74
pin12 -32.9 -33
pin13 -.0 .02
pin14 33.03 32.9
pin15 32.2 32.16

Thanks for any insights,

nosaj

[nosaj]

Disconnecting D2 and D3 will remove the mute circuit as a load on the transformer.

Lifting D2 and D3 makes the speaker relay no longer click. Soldered it back. Lifted TR4 no relay click resoldered back in Lifted TR3 no relay click resoldered back in, Lifted TR2 yes relay click resoldered back in Lifted TR1 no relay click resoldered back in.

Pins 13 on the stk's register .004 and .005 VDC
So now I'm thinking this is by design. Will check some signal sources to find out where it's going.
thanks,

nosaj

[nosaj]

In the muting circuit TR1 C is -.001 B is .792 E is .007
TR2 c is 12 B is .006 and E is -.001
TR3 C is 5.53 B is 11.68 E is 11.83
TR4 C is 11.80 B is 11.06 E is 11.83v
TR5 C is .023 B is .763 E is -.001
tr15 c is .016 B is .737 E is 0
R2 has 11.8 volts at pin1

Something is pulling down the +12 regulator and not allowing the R2 to latch. Still being very Green my eyes say something is wrong with tr3 and tr4
I did pull them and they tested ok on a bk520 buuut. IDK.

nosaj