Ampeg fault

Hi , everybody,Im new to this site. I have a Ampeg SVT150H and my problem is : when looking at the trace on the scope the positive half cycle clips when o/p
reaches certain level. All ic's, caps and output transstors have been changed but fault persists. Anyone got any ideas? Thanks in advace. A.S.
 
That's a great idea. Once in working order it will make you a nice amp.
But before changing anything, be sure it has to be changed. any way, it's to late now.
Next step is looking for bad solder joints if not done.
Then measure.
We say "Meten is weten". Translated in English: "Measuring is knowing"
Do you have a schematic? Are you confortable in reading a schematic?

Good luck.
 
opamps driving darlingtons so it is not like a discrete amplifier.

just curious what original fault was.
dont know if we are chasing missed issues from original fault.
or if now, all new work recently done needs to be checked for connections.


check opamp +16 -16 rail voltage on power board.
very common they use shunt regulators so
it sounds like it is working but one rail low on voltage.

5pin cable carry power from power amp to preamp
can measure voltage at connector. j23 on power amp j3 on preamp

Schematic Here
 
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OK here some pics
Pic no. ending650 Blue trace is collector of Tip 142 Yellow acoss dummy load
649 limit led
648 master vol. and pull for EQ
647 knob settings
646 volts across dummy load which is 8ohms
645 Blue trace base of TIp147 Yellow across dummy load

forgot to add if I turn vol any higher amp distorts and blows o/p transistors
 

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limiter light is on at 23.8 volts
sounds about right , about 140 watts if it was 4 ohms.

but louder makes outputs blow...hmmm.
what is bias at.
their is limit also set by 39 volt zener diode.

do diode check on all diodes.
there is VI limiters and threshold detect diodes.
worst case zener still work but threshold different.
r31 /r44 / r30 would be very hot all their lives.
 
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You found bad diodes, or all good

really all there is to cause external stability issues.
is diodes in VI limiter.
or if the amp did suffer a output failure
the flyback voltages might have exceeded the small caps
used in the VI limiter. C20 C21

the darlingtons have built in diodes but nothing tied over
the output, so the VI limiter components still susceptible to flyback damge