Fender Stage 112 SE power supply

[Enzo]

I don't see anyone has mentioned it, but this is a "flying rail" amplifier. Some call it grounded emitter. The power supplies bounce around with the signal when measured to ground. R113 should always have 90v across it, the sum of 45 twice.

If the five 0.22 ohm current sense resistors at the speaker terminals are not open, I don't see how you could still have 45v on the speaker - terminal CP14.

[Helmholtz]

I don't see anyone has mentioned it, but this is a "flying rail" amplifier. Some call it grounded emitter. The power supplies bounce around with the signal when measured to ground. R113 should always have 90v across it, the sum of 45 twice.

If the five 0.22 ohm current sense resistors at the speaker terminals are not open, I don't see how you could still have 45v on the speaker - terminal CP14.

True, but he can have DC on CP13 which connects to C.T.
As the whole power supply including the C.T. is floating, CP13 could in principle have any voltage between +45V and -45V wrt ground in some fault condition.

[Enzo]

Yes, he certainly can get 45v on CP13, but early on he reported this:

What I did notice is that I have -45v on the positive speaker terminal and +45 on the negative speaker terminal.

One of those is CP14. Hence my post.

[Helmholtz]

I see, thanks.

We really have a problem with inconsistent measurement data here.

[Enzo]

inconsistent measurement data

Oh believe me...ALL.the.time.

[Steelwitch]

Oh believe me...ALL.the.time.

True! But I stated this before I realized that the amp connects the CT to ground through the speaker at which time I did not have a load plugged in.

[Enzo]

I am sorry, the "all the time" was not directed at you. It is just a daily occurrence on the forum in general.

[Steelwitch]

I am sorry, the "all the time" was not directed at you. It is just a daily occurrence on the forum in general.

I know I have contributed to that daily occurrence in the past but just making my case that there was a reason this time... haha.

[Steelwitch]

No dice with the op amp. I’m gonna pull these current sense resistors and see if any of them are open. I’m in the hole for this repair now anyway, I’d love to know what the problem is for my own sanity

[Enzo]

Oh, don't go tearing the amp apart, any tiny change in current feedback is not going to blow fuses or swing your power rails over to one side.

[nevetslab]

No dice with the op amp. I’m gonna pull these current sense resistors and see if any of them are open. I’m in the hole for this repair now anyway, I’d love to know what the problem is for my own sanity

A quick ohmmeter check across any one of them will either yield near-short, or open circuit. I'm guessing open circuit.