Fender Stage 112 SE power supply

[Steelwitch]

Hey y’all.

i have a Fender stage 112 SE where I have negative but not positive 45v. I have replaced the 6amp diodes and filter caps for the 45v supply. So something maybe be pulling the voltage down to ground I think. PT winding voltages are normal. Any suggestions? No shorted output transistors from what I can tell. A little confused. Thanks!

[Steelwitch]

Here’s a schematic https://schems.com/bmampscom/fender/stage_112se.pdf

[TomCarlos]

Where are you measuring? Are your probes across C52 and C53 when checking the voltage? You checked to make sure C52 is facing the right way? And I take it you checked R113 (100K) to make sure it is not damaged and that all traces on the connection point for the +45v are ok?

[Steelwitch]

Hey Tom, actually I’m reading a direct short across R113 shortly before it creeps up indefinitely. Just taking voltage from the test points on R113. Maybe it is damaged

[TomCarlos]

Lift one side of R113 and then check it for resistance. Otherwise, you would be measuring everything else that is in parallel with that resistor.

I am thinking if you had a short somewhere else in the circuit, you would be blowing the main fuse. Check the fuse - pull it out. Do you have the right value fuse in there?

[Enzo]

Anything that would load +45 down to zero would blow fuses in an instant.

Are you measuring to ground? Note the center tap - CT of the main power rails DOES NOT go to ground. It goes to the + terminal of the speaker. Notice both TP22, TP23 have an asterisk. See where right below TP23 it says "Read TP22 and TP23 with respect to CT."

You are reporting a voltage, but what is WRONG with the amp? Why is it in your shop?

With the amp running, what voltage appears across R113? Not to ground, but from end to end?

R113 is a resistor. It could go open, and that would not affect your voltage. It could drift high, and again, no effect. Resistors do not short.

Your transformer is fine if it makes 45v aanywhere. The same winding makes both positive and negative.

[Helmholtz]

Surely not something pulling voltage down. Would mean a short causing huge currents and dissipation in the PT, also would pull down other voltages.

Must be an open path somewhere. Check correct polarities and connections of rectifier diodes (CR33,34,35,36).

Do you have DCV at the speaker terminals?

[Steelwitch]

Anything that would load +45 down to zero would blow fuses in an instant.

Are you measuring to ground? Note the center tap - CT of the main power rails DOES NOT go to ground. It goes to the + terminal of the speaker. Notice both TP22, TP23 have an asterisk. See where right below TP23 it says "Read TP22 and TP23 with respect to CT."

You are reporting a voltage, but what is WRONG with the amp? Why is it in your shop?

With the amp running, what voltage appears across R113? Not to ground, but from end to end?

So, I have 89.5v across R113 which seems possible. I was mistakenly reading voltage with reference to ground. Thanks for pointing that out!

[Steelwitch]

Surely not something pulling voltage down. Would mean a short causing huge currents and dissipation in the PT, also would pull down other voltages.

Must be an open path somewhere. Check correct polarities and connections of rectifier diodes (CR33,34,35,36).

Do you have DCV at the speaker terminals?

So the amp came in humming 120hz. Looks like there is 45vDC on the speaker terminals. The rectifier diodes are the correct polarity. What would lead DC to be on the speaker terminals in this case?

[Helmholtz]

So the amp came in humming 120hz. Looks like there is 45vDC on the speaker terminals. The rectifier diodes are the correct polarity. What would lead DC to be on the speaker terminals in this case?

With around 90V across R13 you have both plus and minus 45V supplies. But somehow the positive supply doesn't seem to get to the power amp.

[Steelwitch]

I believe the open connection happens before then. With reference to ground I can see -45vDC at the rectifier but not +45vDC. Only 1v which is present on the power supply. So that voltage is getting through. There’s -45v on the speaker terminals to be clear.

[Steelwitch]

Oh I guess reading with reference to ground isn’t telling me the right thing. But I also can’t see 45v with reference to the center tap?

EDIT: Sorry I was wrong. I do have +45v in the power supply and in the output section. What I did notice is that I have -45v on the positive speaker terminal and +45 on the negative speaker terminal.

[nevetslab]

If you're seeing +45VDC on one side of the speaker and -45VDC on the other side (90VDC total), then the speaker must be open circuit. Check the five resistors R83 thru R87 (0.22 ohm 1W). All five are in parallel, yielding 40 milliohms. If you had 45V of either polarity across those to ground, it would result in 1125 Amps flowing (for an instant before all five blew open). I'd suspect those five resistors are open, along with your speaker. Where were you hearing the 120Hz hum from?

See what you have for voltages on U7A & U7B (NJM 4560D). Could be that IC failed. Output current flows thru the C.T, then thru the speaker thru resistors R83 thru R87 to ground. Not a conventional design, but we've seen this sort of design before. Enzo could no doubt quickly describe the basic mode of operation better than I can at the moment. Not sure just what failed first to yield the present predicament, but you are for sure looking at the result.

One of the weaknesses of this circuit is there's no Voltage Gain Stage following the op amp U7A. So, you're limited by it's maximum voltage swing and output current of U7A to drive the three pairs of output xstrs. Granted the outputs are Darlington power xstrs, but you're still limited to the max voltage swing of U7A, which is +/-20V pk. I don't see how Fender gets 28.9VAC output from this circuit, as there's no additional voltage gain stage following U7A. Am I missing something with what U7B is doing?

Also, TP 23 is incorrectly marked +45VDC. It should read -45VDC.

[Steelwitch]

Not open from what I can tell. I have to get the board out and test them out of circuit to know for sure. That makes sense. I was running the amp unloaded since I didn’t want to damage the speaker and the hum was crisping up my dummy load. It sounds like some parts of the amp won’t work unless it’s under load. It’s not the speaker as I’ve tried another just now.

edit: unloaded there is 45v across R83-87 and 45v on the other side, but not 0 if they’re connected to ground. Is this due to the path from the CT to ground being open?

[Steelwitch]

Thanks for the help so far y’all. I usually work on tube amps so solid state is confusing to me at times