Quote:
Originally Posted by stonehopper
I am wondering if, apart from a short somewhere, one or both the replaced OC81's; TR6 & 7 are in fact being destroyed by this current flow. Their values are no longer similar when checked with the meter before I replaced them.
Haven't lifted any capacitors yet. I suspect the output transformer has an internal short - but that is guesswork.
Derek
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I think you are right on both counts.
It looks as if the transformer does have a short circuit turn and the OC81 emitter resistance doesn’t look quite right as the emitter voltage has risen meaning they are not conducting properly so may have been damaged. C32 getting warm could be a separate fault, this can be replaced. The squeal is probably just because the voltage is dropping and messing up the radio section, it is encouraging that this stage is working otherwise.
A more conventional transformer could be fitted, even a pocket radio type. I have made a sketch from another similar radio, the base bias resistors might have to altered and the feedback resistors from the speaker could be retained instead of how it is shown in my sketch. It does mean a fair bit of modification.
I would not turn on the radio anymore until this transformer has been sorted.
I wonder if the well known Eagle LT700 Output transformer would work here ?
The driver stage looks OK
Geof