Question About Amp Schematic

[bostonman]

If I get time tomorrow, I'll power it and take more measurements. If not tomorrow, this weekend.

I appreciate the assistance so far as this would baffle me due to not seeing any obvious failures; or reason for the "pop".

[bostonman]

I took more measurements.

I noticed the op-amp on the schematic is OPA2604AP but the part is marked OPA2132P. Also, if my measurements have a discrepancy in the area of tens of milli-volts (i.e. the collector on the power transistors versus another component connected to the rail), I noticed positive and negative rails changed as the unit heated during my measurements - uncertain if this could be due to old main caps.

The measurements are:

Positive power transistors (BCE): 0.563V, 50V, 3mV (but increased to 8mV when I remeasured after the unit was on for >15min)

Negative power transistors (BCE): -0.559V, -50V, -4mV (but increased to 8mV when I remeasured after the unit was on for >15min).

Also, now that I'm looking at my measurements for the negative rail power transistors, maybe I should recheck base and emitter because I didn't write negative, but would assume all three legs would be negative. I was focusing on not shorting anything or getting zapped, so I may have accidentally forgot to include the negative. I'll recheck tomorrow if need be - but trying to avoid too much live measuring to avoid accidentally shorting something.

Q17 (BCE): -51mV 51mV, 1.11V, -1.08V

Op-Amp (1, 2, 3, 4, etc...): 10mV, 0V, 0V, -15.2V, 0V, 0V, -45mV, 14.92V

Pos 15V regulator: 19.94V input, 14.92V output (as expected)

Neg 15V regulator: -18.25V, -15.20V output (as expected)

Q18 (SGD): -8mV, 8.45V, -23mV

Q16 (BCE): 1.07V, 49.26V, 0.574V

Q2 (BCE): -1.04V, -49.25V, -0.559V

[bostonman]

Just an FYI, I remeasured the negative power transistors. As expected, the voltages are negative.

Also, when modifying my previous message, I noticed Q17 had the incorrect polarity on the base (I also fixed it).

[floobydust]

Those voltages look fine to me. No smoke, no short circuit etc. It should play music OK. Nothing to pop the fuse. Unless something is later heating up, or there is a bad connection which tapping a plastic pen here and there can flush out.

Bias current rising to 8mV/0.22R is 36mA per pair which is reasonable. You can compare other channels as well (mV across the emitter resistors), it's important to look at each one separately in case one of the paralleled output transistors is fungoli.

What about Q14 the protection transistor, and the PNP it connects to? Balanced input U1 SSM2143 voltages OK? I'm not sure if it was being run balanced or unbalanced input.
Or even a bad (oxidized) switch contacts on S1 would give quite a pop, according to the schematic.

[bostonman]

Q7 (not marked on the schematic - the one that connects to P4) from BCE: 49.20V, 20mV, 49.30V

Q14 BCE: 0V, 49.55V, 0V

U1: all pins 0V with the switch in both positions except 14.92V and -15.2V on the two power input pins.

At this point I'll order a replacement fuse (2.5A SB) and the 10ohm resistors that connect chassis and Earth; you mentioned wire wound and 2W, is this correct? The schematic shows 1W, but not sure if that's correct and I'd like to go larger if possible.

Two technical questions about this schematic regarding the unit not working. I can measure all the emitter resistors including the other channels to compare, but what voltage should I expect across the resistors while idle?

Also, I don't get that feedback/shutdown circuit. As mentioned, the unit immediately turns off the main power relay when trying to power the amp. I thought the feedback circuit was telling the main board to shut down because (in this situation) the fuse was blown. Looking at that circuit, the only time it should send a signal is when the speaker output has DC and/or the thermal fuse opens (as you previously mentioned). Does this board quickly send a signal to the main board at power up to tell the main board it has power or can something else be wrong with the unit?

Perhaps maybe another board (i.e. channel) is sending DC voltage due to an issue to the main board (something I'll have to investigate), but wanted to ask if I'm overlooking something in that feedback/protection circuit.

[bostonman]

I took measurements across the 0.22 ohm resistors at the emitter of all the power transistors simultaneously (see attached) using 55V DC on the rails.

The owner of the amp was over yesterday and I was explaining to him how things didn't make sense and no reason the fuse to that board would have blown. Apparently he completely forgot one channel hasn't worked for well over a decade which began shedding light on things (he was only using two channels and switched to the center channel when the right stopped working). The one confusion was: if the right channel board was bad with a blown fuse, how was it sending a signal back to the main board to immediately shut down.

Turns out, with a fuse powering the center channel and right channel (the one that had the blown fuse but nothing was wrong with the board - i.e. the one I've been looking at this entire time), the unit turns on fine. When I put a fuse in to power the left channel, that's when the unit goes into shut down.

This entire time I was troubleshooting the right channel because it had the blown fuse, but turns out I was chasing the wrong issue.

Now I've confirmed, the left channel causes the unit to go into shut down immediately. All three boards have two red LEDs (I guess they are on when the board is working) and they both light for the brief moment at turn on before the unit goes into shut down mode; and the fuse isn't shorted blowing. This tells me it's not a hard short, but for some reason the feedback circuit is telling the main board to shut down.

The interesting thing: immediately when I opened the unit, I noticed yellow dust around the positive filter cap on the left channel and it has a slight bulge on the top (the negative one and the ones on the other boards are quite flat). The caps have black wrapping, so I can't tell if the bulge is due to the cap being bad or just poor wrapping. As for the yellow stuff, I thought it was carpet dust or whatever, but I've never seen yellow dust from a bad cap.

Since the fuse to the right channel was blown, I figured the right channel was the culprit and ignored the bulge in the cap in the left channel, but now I'm second guessing whether that bulge in the cap in the left channel is causing issues and/or if that yellow dust is worth paying attention to.

These are Phillips 3186 series capacitors (P/N 3186DC213M070HPA3) with solder leads. They are 21,000uF, 70 VDC, and 95 V surge.

I don't have a plan at the moment. I'm waiting to see if my friend wants to continue having only two channels. If I attempt to repair this board, most likely trying to power it with a VARIAC isn't safe since the board doesn't want to be powered. I may resort to a DC power supply, but this may not help figure out if the cap is bad.

Has anyone seen yellow dust from the bottom of a cap? In my experience it's usually brown crud.

[bostonman]

I have an update and somewhat able to close this topic.

I removed both capacitors since if one is being replaced, both should be replaced. The one with the yellow stuff is cracked on the bottom with liquid leaking.

After removing the capacitors, I checked for shorts on the board; although I didn't suspect any since this board wasn't blowing its fuse. Feeling the board was safe to power, I applied 40V DC (20 per channel) and found the feedback circuit was failing.

The transistors in the feedback circuit seemed to work correctly, but I saw what appeared to be issues with the resistor arrays, and I suspected the fluid that leaked out of the capacitors was causing conductance. After several minutes probing, I decided to dunk the corner of the board in rubbing alcohol, cleaned all the gunk (lots was built up under the resistor arrays and around the transistor leads) with a brush, and air blew the debris. Reconnected power and the feedback circuit was working perfectly.

Now the issue: I could only find one capacitor that would fit in the unit. Every other capacitor was either too tall and/or too fat. The capacitor in the attached datasheet is the only one that matches, however, I need to confirm whether the pins will fit.

This board has extra holes where the caps go (maybe they used a different brand at one time), so I need to check whether I can rotate the capacitor to fit in the holes. Assuming the pins align with the holes, one fear is the three unused pins on the cap will go through holes that are connected to the input voltage; something that I read on other datasheets isn't permitted and need to be disconnected from voltage.

Update: I created a 3D model (and printed) of the replacement cap pins and they appear to align, but, unfortunately, one of the unused pins is on the same pad touching the voltage rail. The datasheet didn't mention whether a pin can touch a rail (other datasheets said it's not permitted) so I called the company. They stated not to connect it. My only option is to modify the PCB a bit by cutting the traces, but I'm waiting to see if my friend wants to invest in new caps and take such an approach.