Please help with Sunn SL-260 repair. Exploded cap and blown output transistors.

[QdB]

ok.
I've got all the components installed. I replaced the bad diode and blown cap.
Plugged it into the light bulb limiter and turned it on for about 5 - 10 seconds. 100w light bulb lit up and stayed lit.
If I understand correctly it should have gone dim after a few seconds.

[The Dude]

It should indeed dim. Please, if you can fire it up again, measure DC on the speaker output and tell us what you get. If the bulb doesn't dim, it's best to leave the amp on only long enough to make the measurement.

[QdB]

It should indeed dim. Please, if you can fire it up again, measure DC on the speaker output and tell us what you get. If the bulb doesn't dim, it's best to leave the amp on only long enough to make the measurement.

Ok. Done.
~ -7.5vdc at output.

[The Dude]

Something is obviously still wrong. Next, I would check both of your supply rails and make sure they are balanced at +&-45V.

[QdB]

Something is obviously still wrong. Next, I would check both of your supply rails and make sure they are balanced at +&-45V.

I am not entirely sure on how yo check the supply rail voltages. So I took a shot a in the dark and measured from a terminal of each fuse to a screw hole in the chassis. ( I honestly have no idea if that is even anywhere near what you had in mind).
Regardless, the measurements that I got were ~ +8Vdc and ~ -8Vdc.

[J M Fahey]

OK, then your lightbulb is pulling down hard those voltages, (it´s doing its job) which means your amp is pulling too much current at idle.
Possible causes: either way over biasing or still bad parts.
Too busy now but later will suggest a couple tests.

[QdB]

Went home at lunch time and had a couple of minutes. On a whim, (actually something I saw in a different thread), pulled output transistors Q204 and Q205. Turned it on with the LBL and the bulb didn't light. Prompted me to check the transistors yet again. Q205 (2955) showed diode voltage drop ~5.7 for B-E and B-C, however, E-C and C-E the meter started screaming at me.
So, transistor is shorted E-C/C-E?
I didn't have time at lunch to put it all back together at lunch, so this is where I have left off for now.

[The Dude]

If any of the transistor leads read shorted to each other, the transistor is shorted and needs replacing. That is why your light bulb limiter is dim and supplies are low.

[QdB]

replaced the 2955. LBL is very near completely dark (tiny red glow). Showing no DC on output.

[The Dude]

That's good news! Next, fire it up without the LBL and recheck to confirm no DC on ouput. If that's good, you can hook up a speaker and try it. Bring the volume up slowly until you know it works. If that's good, the only thing left is to check/set the bias. Instructions are on the first page of the service manual you uploaded. Be sure to let us know how you come out and good luck!

P.S. Be extremely careful your probes don't slip when you bias it or you'll be back to square one!

[QdB]

That's good news! Next, fire it up without the LBL and recheck to confirm no DC on ouput. If that's good, you can hook up a speaker and try it. Bring the volume up slowly until you know it works. If that's good, the only thing left is to check/set the bias. Instructions are on the first page of the service manual you uploaded. Be sure to let us know how you come out and good luck!

P.S. Be extremely careful your probes don't slip when you bias it or you'll be back to square one!

Late response. Out of town for a couple of days.

Ok. Fired it up sans LBL. still no DC on output. Played a little bit through it with bedroom volume. Sounds nice

So here is where the newbieness kicks in, On checking/setting the bias, the manual says;

1. put a voltmeter across 235 & 237
2. With no input and no load
3. wait a minute
4. adjust for 6mvDC

Sounds easy enough. But, my inexperienced self is a little confused on the orientation of the +/- probes. When I check one direction I get ~6mvDC but if I flip the probes I get ~ -5mvDC. I know that I don't want to throw reverse bias into the thing. Seems to me that if I do that then there should no output? Best case, transistor doesn't turn on? Worst case, something more sinister?

EDIT: Per the instructions, I was able to get this thinged showing 6mvDC across r235 & r237.
~ 47Vdc and ~ -47Vdc on the +/- rails.
No DC on the outputs.

Everything looks like it is working as it should.

Thank you so very much to everyone here for helping me and more importantly for being patient with me.

...now onto the next one

[g1]

You are measuring DC across the resistor. So with probes one way it will say +, reverse the probes the meter will read - .
The same will occur measuring any DC. As you suspect, if the wrong polarity got to the base of those transistors, they would not turn on.

And as a side note, try not to use ~ symbol to mean "approx." when talking electronics. It also is used to denote AC rather than DC.

[QdB]

thanks @g1. I was only concerned about the polarity of the probes since it seems if I turned the variable resistor too far, it would switch polarity of the voltage and I wanted to be sure to get it the right way around. also thank you for the heads up regarding the tilde (~) being used for AC, I hadn’t made that connection yet. I can see where it could lead to confusion.

[Enzo]

The bias adjustment control has no way in the world to reverse the polarity of the DC in the circuit.

If you adjust it too far one way, it just results in more crossover distortion in the sound. If you turn it too far the other way, the amp starts to draw excess current.

If they want 6mv across those emitter resistors, them more ma is bad, less is just cooler settings. Cooler settings might not sound as good, but they are totally not stressful for the amp.

[QdB]

The bias adjustment control has no way in the world to reverse the polarity of the DC in the circuit.

Wow. Very good to know. Thank you.