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Old 5th Jul 2019, 6:06 am   #21
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Armstrong 626 problem at high volume.

Quote:
Originally Posted by jubilado View Post
A problem still persists...


1) I've checked the capacitors I replaced for polarity and they are OK.

2) Is this a problem with the power supply?
1) Have you ?

2) Almost certainly.

Electrolytic capacitors can be difficult to test properly even for the experienced engineer. They require 4 tests: Capacity value, ESR, electrical leakage, and max applied voltage electrical leakage.

The main filter capacitor (3300uF) on the output of the bridge rectifier is responsible for storing charge there, that keeps the main power supply voltage stable, when current is drawn at intervals from the supply by the audio output stage amplifiers. The current peaks increase with volume.

This capacitor is topped up with charge via the power transformer and rectifiers at 10mSeC intervals (on 50Hz full wave rectified voltage).

If this capacitor has gone high ESR or low capacity, then on load volume peaks you would for example, see the pilot light flicker and you would have a common resistance in the supply that would cause feedback to early amplifier stages and instability.

I think your 3300uF capacitor needs replacing, most likely. There could be other electrolytic caps that need replacing too.

I would "guess" if you had a scope and monitored the voltage across this capacitor, you would see the voltage jumping up and down when the problem occurs, but , since you don't likely have one, or are familiar with using one, then your next best bet is exactly what you have observed, with the pilot light pulsing with the fault......well done.
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