Fender Stage 112 SE power supply

[Helmholtz]

Please always specify if you mean plus or minus 45V wrt ground. The difference is 90V!

No difference between electronics' principles with tube or SS circuits.

[nevetslab]

So, the five 0.22 ohm resistors in parallel are measuring near 0 ohms? You'd have to zero out your ohmmeter probes, or account for what they contribute in series with R83-R87. If you have a Relative mode on your DMM, and if it can read down to around 10 milliohms, you'd get a reading near 40 milliohms.

And yes, as I recall, you need a dummy load (or speaker, if all is working normal) to test the circuit. I've never had one of these that failed on the bench, though we do have one in our Rental Inventory, I think. No....I was mistaken. Company must have sold it off, not seeing it anymore in our inventory list.

[Steelwitch]

I know, but it’s a lot more challenging for me to figure out! So, what should I be seeing on R83-87. I have a scope so if anyone could recommend anywhere to look to track down the hum, I’d be very appreciative!

[Steelwitch]

So, the five 0.22 ohm resistors in parallel are measuring near 0 ohms? You'd have to zero out your ohmmeter probes, or account for what they contribute in series with R83-R87. If you have a Relative mode on your DMM, and if it can read down to around 10 milliohms, you'd get a reading near 40 milliohms.

And yes, as I recall, you need a dummy load (or speaker, if all is working normal) to test the circuit. I've never had one of these that failed on the bench, though we do have one in our Rental Inventory, I think. No....I was mistaken. Company must have sold it off, not seeing it anymore in our inventory list.

Yeah they do measure close to 0 ohms across them. My fluke is giving me .1 to .2 ohms ish. Not saying one or more of them can’t be open, but it seems like if there was an inrush of current it would have opened them all. With reference to ground I am not reading 45v on R83-87. Looks like that means they are working normally through the speaker when under load.

[Helmholtz]

It is impossible to exactly measure DCR below 0.1R with only 2 wires because of unavoidable (and varying) contact resistance.

R83-87 are used to provide some current feedback to increase the output impedance.
The amp should work even if some of them were shorted (which is very, very unlikely) or open.

[Steelwitch]

It is impossible to exactly measure DCR below 0.1R with only 2 wires because of unavoidable (and varying) contact resistance.

R83-87 are used to provide some current feedback to increase the output impedance.
The amp should work even if some of them were shorted (which is very, very unlikely) or open.

Makes sense. I believe the problem is somewhere else. I checked U7, but not sure if that’s failed either. Still have 29v on the positive lead of the speaker now with it loaded. It may be possible that the connection to CT where L1 is no good.

[nevetslab]

In Note 4 on Fender's Schematic, they state an 8 ohm resistive load must be installed in place of the speaker. You've already mentioned your dummy load was getting 'cooked' from the potential across it (if it was an 8 ohm load, and you have 90VAC across it @ 120 Hz, that's 1025 Watts dissipation! So, I could certainly see why your dummy load was getting cooked. What to do?

I think before doing more AC mains-powered testing, I'd check your Zener Diodes in the power supply which source the +/- 16VDC supplies to the op amp circuits of the entire amp. Can you unplug the +/- 45VDC supplies to the output stage? If so, that would allow checking the other components in the output stage. See if current limiter xstrs Q1 & Q2 are ok, along with the associated zeners and diodes around that circuit. Also the diode string feeding the bases of the output stage CR13 thru CR16. Also the series diodes in the power supply lines to U7 CR11 & CR12. Also see if all emitter resistors of the outputs are ok R106-R111.

Do you have a Variac? I don't know what wattage your dummy load can handle, but, to get the circuit complete with a load, if your dummy load can handle 100W, I think you could get the circuit fired up at a fairly low AC mains voltage. If you brought up the AC mains so you only have +/- 14VDC (28VDC total across R113), you'd have about 28VDC across your 8 ohm dummy load, which is close to 100W. Might need to keep the supply below that 28VDC total.

In correct operation, there should be 0VDC across the speaker terminals (CP13 & CP14).

The AC voltage you'd see across resistors R83-R87 would be a small fraction of that voltage across CP13 & CP14. It's a loss of 0.00498. Meaning, if you had 10V across the 8 ohm load, you'd have 49.8mV across the resistor bank. U7B is sampling this voltage for feedback to send to U7A.

You also need to see what the DC voltage is on the op amps U7A & U7B. Usually, if an op amp fails, it's latched up to one of the power supply potentials. If working, you'd see near 0VDC (+/- 25mV or less).

[Helmholtz]

It's not clear to me if there is +45V at the collectors of Q3,4,5.

[Steelwitch]

In Note 4 on Fender's Schematic, they state an 8 ohm resistive load must be installed in place of the speaker. You've already mentioned your dummy load was getting 'cooked' from the potential across it (if it was an 8 ohm load, and you have 90VAC across it @ 120 Hz, that's 1025 Watts dissipation! So, I could certainly see why your dummy load was getting cooked. What to do?

I think before doing more AC mains-powered testing, I'd check your Zener Diodes in the power supply which source the +/- 16VDC supplies to the op amp circuits of the entire amp. Can you unplug the +/- 45VDC supplies to the output stage? If so, that would allow checking the other components in the output stage. See if current limiter xstrs Q1 & Q2 are ok, along with the associated zeners and diodes around that circuit. Also the diode string feeding the bases of the output stage CR13 thru CR16. Also the series diodes in the power supply lines to U7 CR11 & CR12. Also see if all emitter resistors of the outputs are ok R106-R111.

Do you have a Variac? I don't know what wattage your dummy load can handle, but, to get the circuit complete with a load, if your dummy load can handle 100W, I think you could get the circuit fired up at a fairly low AC mains voltage. If you brought up the AC mains so you only have +/- 14VDC (28VDC total across R113), you'd have about 28VDC across your 8 ohm dummy load, which is close to 100W. Might need to keep the supply below that 28VDC total.

In correct operation, there should be 0VDC across the speaker terminals (CP13 & CP14).

The AC voltage you'd see across resistors R83-R87 would be a small fraction of that voltage across CP13 & CP14. It's a loss of 0.00498. Meaning, if you had 10V across the 8 ohm load, you'd have 49.8mV across the resistor bank. U7B is sampling this voltage for feedback to send to U7A.

You also need to see what the DC voltage is on the op amps U7A & U7B. Usually, if an op amp fails, it's latched up to one of the power supply potentials. If working, you'd see near 0VDC (+/- 25mV or less).

So, the +/- 16v supplies look normal. And the low voltage supply is present on pin 1 and 5 of each op amp. On U7 I was running the low voltage supply at 2.7v and that voltage was present on pin 2 and 1 volt on pin 3, so that must be it! Does that sound somewhat on point and like it would make sense? U7B is operating like you’re describing

[nevetslab]

Normal operation of U7A/U7B (if supplies are +/-16VDC) would be near 0VDC. If either has failed, the outputs at Pins 1 (U7A) and Pin 7 (U7B) would be at or near the +/- supply potentials (latched up). Not sure what you mean by runing the low voltage supply at 2.7V. Unless this is what you got for supplies (+/- 2.7VDC) with variac set for low AC Mains voltage).

[Steelwitch]

Normal operation of U7A/U7B (if supplies are +/-16VDC) would be near 0VDC. If either has failed, the outputs at Pins 1 (U7A) and Pin 7 (U7B) would be at or near the +/- supply potentials (latched up). Not sure what you mean by runing the low voltage supply at 2.7V. Unless this is what you got for supplies (+/- 2.7VDC) with variac set for low AC Mains voltage).

thats what I mean. It’s 2.7v with the reduced mains voltage. Pins 1 and 7 are increasing in v with the power supply, definitely not 0v. So, I guess that would make sense with there being DC voltage on the speaker outs if the output is producing DC voltage.

[Helmholtz]

You can't have zero DC at the output if one of the power amp supply voltages is missing.

[Steelwitch]

You can't have zero DC at the output if one of the power amp supply voltages is missing.

Sorry, I meant to reply to your last comment before. I was reading the +45v supply incorrectly on my meter. It’s there with -45v on the other side.

[nevetslab]

thats what I mean. It’s 2.7v with the reduced mains voltage. Pins 1 and 7 are increasing in v with the power supply, definitely not 0v. So, I guess that would make sense with there being DC voltage on the speaker outs if the output is producing DC voltage.

That sounds like IC7 is bad, if both op amp's outputs are at / near one of the supply voltages as you adjust the AC mains. Swap out that IC and see what you get. The NJM4560D is a high output current op amp, so you will need to use that part, rather than make substitutes.

[Steelwitch]

That sounds like IC7 is bad, if both op amp's outputs are at / near one of the supply voltages as you adjust the AC mains. Swap out that IC and see what you get. The NJM4560D is a high output current op amp, so you will need to use that part, rather than make substitutes.

Cool! I have one on order now. Thanks for the help y’all, I probably would not have been able to figure that out on my own. I need to delve deeper into op amps and how they function. I know it’s almost like a triode. Will update the thread when the issue is resolved