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This is a topic I've never been quite sure of. I have some SS power amps that have not been operated in 20 years. Should I just plug them in and hope for the best or is there something I should be doing to bring them up slowly with a variac? What's the consensus? Any recommended procedure to minimize damage?
Thanks,
John
Follow Ups:
I appreciate all the good comments. All very helpful. Any more are certainly welcome if anyone wants to weigh in.
I have looked at the schematic of one of the amps and don't see any voltage regulators. If I can easily do it, I think I will interrupt the plus and minus DC rails at the supply and just reform the main supply caps slowly. Then discharge everything and hook the rails back up. Then Dim bulb with the circuit whole. That should allow the circuit to come up to voltage quickly if there are no bad shorts. I think I will do this without load on the amp. Then check DC offset and go from there. Probably overly cautious, but I have the time.
Thanks,
John
Thanks for all the responses.
It sounds basically the same as what I do for tube equipment. I probably go much slower in raising the variac voltage than most, maybe 10V/hr.
My concern with transistors is that they are relatively fragile compared to tubes and I don't know if bias being off because of reduced voltage might cause problems. I just don't have much experience with it. I guess if doing it monitoring emitter current as TK suggests, that might take care of most of it. And maybe I should be going faster to raise the voltage.
dim bulb. they are the only pieces of gear that I have read a warning about.
the risk is if the voltage is regulated anywhere because it might not turn on until it reaches a threshold. I've monitored current and voltage with a variac and meter before and turned it up within maybe a second or 2 to 90vac to 100 and monitored results that way with dummy loads on the speakers attached.
If I'm wrong someone please chime in and correct me
This will drop the voltage and act as a current flow regulator. If there are issues the bulb will glow brightly as the short circuit tries to get full power, the dbt keeping stuff from burning up. Then evaluate for that problem.
If the bulb stays dim after the initial turn on when the caps fill up, leave it for a period of time and then repeat with a larger bulb...allowing a higher voltage for a few steps. I would probably use 25W, 40W, 75w and then 100W unless it is a big power unit in which case I'd continue to 150W.
All these 20min-1+ hour steps allow the old, unused in years electrolytic capacitors to reform which takes time.
Since I have a variac, I would use a 100W bulb and variac the unit from 20V on up in steps. If all it good do to line level power.
The usual stuff, bulging/leaking caps, burnt resistors, etc. and a thorough cleaning. Then slowly bring up with a variac, or dim bulb tester, while monitoring current draw and watching for smoke.
nt
Nt
Magic smoke is what makes it work!
"Then slowly bring up with a variac, or dim bulb tester, while monitoring current draw and watching for smoke."
Do this without a load, and monitor the current across the output emitter resistors if possible. This will reveal problems before excess current shows on the variac.
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