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Old 19th May 2008, 08:46 AM   #1
PM650 is offline PM650  United States
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Default cracked el34 -> replacement squeals

I have been given an old radio to repair for a friend, a mid 50's Telefunken opus 6. The radio uses a pair of el34 outputs, one of which was cracked, the other still sealed. I replaced both with a new matched pair and the amplifier squeals as soon as it is powered on, I have not left it on long enough to tinker with the controls while this is occuring. The noise is not coming from the radio section, I have tried powering it with the input switched. I can only assume the radio worked fine before it broke, so I do not believe it is a microphonic preamp issue. All components appear to have been replaced within a few decades, film capacitors used throughout. No resistors appear burnt.

Any idea of where to begin?
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Old 19th May 2008, 02:47 PM   #2
kevinkr is offline kevinkr  United States
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Find a schematic first.. Then I would suggest replacing all of the power supply caps whether or not they have been replaced in the last decade or two.

Squeal may be indicative of feedback somewhere within the amplifier circuitry and the logical place to start to look for that is in unintended feedback paths through the supply due to bad bypass caps. Other possibilities include stability compensation components like caps that have drifted in value.

Don't assume that because the coupling caps are film that they are good. Philips mustard colored ceramic encapsulated film and paper caps are notorious for going open. I have had problems with these more than once.

Be careful about changing parts in the rf circuitry of the radio other than carbon resistors because they may have special characteristics you are unaware of. For example caps with specific tempcos to compensate for other drifting component values - this is often the case in local oscillators in the receiver front end. Don't disturb lead dress or component locations if at all possible - alignment will be adversely affected. Do not attempt to align the radio without proper service information and test equipment - you'll be sorry...

Good luck!
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