Rocktron Velocity Issues

[gtr-fxr]

Well...here's what i got
U1
pin 1. -15
pin 2. -15
pin 3. 0
pin 4. -15
pin 5. -15
pin 6. 1.9
pin 7. 15
pin 8. 0

[J M Fahey]

Well...here's what i got
U1
pin 2. -15
pin 3. 0
pin 4. -15
pin 6. 1.9
pin 7. 15

You have normal +/-15 on pins 4/7
You have 15V difference between pins 2/3 (differential inputs)= the Op Amp is toast.
The 1.9V on pin 6, under these conditions, does not mean much.
Just for testing, try a TL071 there.
*Maybe* the amp wonīt meet its specs, even become unstable with speaker loas, dunno, but at least it should recover proper DC voltages, if everything else is fine.
*If* it works, maybe you can find a higher performance Op Amp, closer to the original one.

At least, thatīs what Iīd try, if it were sitting on my bench.

[gtr-fxr]

well i dont have a tl071 handy , but i did try a 4558 and the voltages seem to be stable. i think there was somewhere about 12v on pin 3. does that seem right. but still no out sound.

[J M Fahey]

No, it does not seem right.
RC4558 is a DUAL Op Amp, obviously pinout and, more important, function, will never match a 741 pinout single Op Amp.
And 12V DC on Pin 3 is not what should be expected.
Sorry.

[Jazz P Bass]

Why would you even expect any voltage to be on pin 3.
It is an input pin, tied to ground through a 10K resistor & isolated, Vdc wise, by C11.

[gtr-fxr]

well i ended up finding some ssm2131 online hopefully that will do me some good. they should be here by the end of the week. so i guess well see what happens then

[J M Fahey]

Agree (with jpbass).
Originally there was 0V on pin 3, what should be expected, specially because nothing feeds DC into it.
Now you have 12V there because you connected the wrong chip.
It must be a blob of molten silicon by now, shorting internally some DC voltage to pin 3.
Pull it at once and get a TL071 at least or better.
I bet you still have other bad semiconductors (Transistors and diodes) .
Sorry.
Donīt hurry up, go step by step, use *always* your bulb limiter until starting to read reasonable results.
Work on only 1 channel, disconnect the other for now.
When you get one running, it will be a reference for the other.
Good luck.

[Steve Conner]

Yup. Every time you turn the thing on with a wrong/bad op-amp, you run the risk of blowing any/all of the transistors, and possibly feeding 50V back into the preamp again. This one calls for a pretty cautious approach.

Did you check that the preamp is intact, and the 15V on pin 3 isn't coming from a blown op-amp in the previous stage?

Anyway, re the blown capacitor and diode. The output stage probably blew and shorted between the rails, then one rail fuse opened but the other didn't, maybe on account of being replaced with silver foil.

The diode is an antireversal diode to stop the rail getting dragged to the wrong polarity, but the silver foil was too much for it and it failed open, dragging the positive rail to -50 through the blown output transistors. The electrolytic cap failed from reverse voltage, and so did probably just about everything else.

I think if this suggested failure mode were true, shotgun replacement of all semiconductors would be justified, or even shotgun all of the guts and build a new amp with the case, heatsinks and transformers.

[J M Fahey]

Fully agree.
I didn't wantto suggest something so radical, but worst case you bought a cool cabinet , Big transformer and PSU components and beefy heatsinks.
Maybe you can easily adapt there a couple new power boards, there are thousands to choose from.and call it a day.
You already have what makes uo 70% ofthe cost of a working amp.

Yet, maybe itīs stillworth trying to revive it, but you now have a "Plan B" as backup.
Good luck.

PS: post a couple shotguts, so we see the space you have inside, the heatsink mounting, etc.

[Chuck H]

I'm printing this thread (as an exercise in troubleshooting)

[Jazz P Bass]

I'm printing this thread (as an exercise in troubleshooting)

Not troubleshooting.

[Chuck H]

Based on the advice given. his will be an exercise in trouble shooting by the time it's resolved. And I can use the tutorial from the long time members for future endevours.

[Jazz P Bass]

With all due respect to the OP, this repair is not a lesson in troubleshooting.
Troubleshooting, to me, involves an understanding of what you are trying to fix.
And it also requires a systematic approach to resolving the problem.
Rant: I see way too many posts by wannabees that are darn lucky they do not kill themselves.
I have nothing against learning how to repair equipment.
It is the willy nilly, lackadaisical (I had to look that one up), cavalier attitudes that just frost me.
(Oh, this is burnt, what do I do next)
End Rant.

[Chuck H]

I meant no disrespect to anyone. But I'm not here because I'm altruistic. I'm here for fun and learning. I am empathetic to gtr-fxr's problem but I don't have the repair chops to help much on this one remotely. Maybe I'll be a little better on the next one by following along and reading the advice given by you and others.

[gtr-fxr]

well i do appreciate all the help i have been given " especially being a new member here " i know my approach probably wasn't the best on this one, seeing that this was the first SS amp i have ever really worked on and given the sad condition it came to me in. i just want to say thanks. and maybe next time I'll just keep to myself .