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Old 19th August 2019, 06:01 PM   #11
indianajo is offline indianajo  United States
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You've got three electrolytic caps up in the op amp area that might be dried out. I'd replace the 2 uf NP with a 2.2 uf ceramic cap, But the others are going to have to be in kind. The 10 uf tant on the left edge I'd use electrolytic, they are smaller now. You can get 10000 hours service life in the small values, but then this isn't your amp is it?
The rail caps will only limit full power if dried up, but the 1000 uf on the +-15 might need updating. Those go to the op amps.
If this amp has ever had a meltdown with the rails burning back through to the op amps, the 0.1 uf ceramics peavey uses on the +-15 near the op amps in the 90's were only 50 v rated and can be shorted or open. Op amps go weird if the power supply is not bypassed properly locally.
Of course check all the op amps that the two inputs and the output are all at the same DC voltage quiescent. Don't short to the power pins with your probe. A pamona dip clip and grabbers on the clip pins help prevent burnups from wiggly hands.
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Last edited by indianajo; 19th August 2019 at 06:11 PM.
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Old 22nd August 2019, 04:49 AM   #12
CapnDenny1 is offline CapnDenny1  United States
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I replaced the electrolytic caps. So far it hasnít blown up. Iím afraid to remove the light bulb. But Iíve connected up the preamp and run signal all the way thruand drove a speaker.

Iím still not convinced itís fixed, but we shall see.

When I removed the caps I measured their capacitance and their ESR, and all seemed ok. I suspect one of them was breaking down at higher voltage, hence the instability at higher voltages.

Thanks for the help. If you donít hear from me, no news is good news!
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Old 22nd August 2019, 07:04 AM   #13
JMFahey is offline JMFahey  Argentina
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Do you have the one with the output autotransformer or not?

Please check and answer that because it changes everything.
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Old 22nd August 2019, 07:21 AM   #14
CapnDenny1 is offline CapnDenny1  United States
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Yes it does have the autotransformer on the output. I have it disconnected. Clever bit of work there.

It also has a phaser control built in.

So how does the transformerchange things?
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Old 27th August 2019, 01:02 AM   #15
JMFahey is offline JMFahey  Argentina
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The transformer primary is a short to DC (actually some 0.2 ohm DCR, same thing) so if you have it "almost" repaired but still have , say, 1 or 2V Dc at the output or more, amp will blow fuses unnecessarily.

First thing to do when repairing SS amps is to disconnect speaker.
In this one you do so ... and you still have 0.2 ohm to ground, much worse than any speaker voice coil.

So you *must* disconnect transformer primary until amp is fully repaired.
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