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Old 5th Nov 2017, 7:22 pm   #1
Boater Sam
Banned
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,897
Default Oh No! Not another howler.

I seem to be plagued by howling Bush radios laterly.
The VHF41 is fixed now that I have found the poorly connected wire.
This one is only an AC41, possibly the simplest Bush chassis of the period.
Once the inevitable rotten wiring was replaced it works but when tuned off station it howls and rumbles.

Replacing the screen decoupler cap, AVC cap, oscillator anode isolator cap and audio grid cap leaves only the tone corrector caps that are waxies.

Its just the same. The twiddler has not been here, its a virgin, as it left the factory.

Valve voltages are about right, resistors are close enough to correct values.
Electrolytics are good, no HT hum, shunting all 3 does nothing.

It is fine in the gram position, so the triode in the ECH42 which is used as 1st audio amp in gram is OK and pulling the ECH42 doesn't stop the racket.

Pull the EBF80 and there is silence.

So it seems to be the EBF80 IF amp/diodes that is taking off.
Swapping V1 and V2 valves for known good ones also has no effect.
Reducing the volume to near to nothing stops it so it has to be before the EL41.
There are so few components in these sets that it is driving me mad.
My only thought is to split the combined V1 and V2 screen feed and decoupler and put a separate one in for V2 and see if that works, but this should not be necessary.

Please, any ideas?
Boater Sam is online now