Quote:
Originally Posted by Boulevardier
Well, I finally managed to remove the original AA112 diodes, and they tested OK as follows:
Diode 1 - Ir = 1.3 microamps, Vf = 244mV
Diode 2 - Ir = 4.3 microamps, Vf = 561mV
Out of interest, I tried loosely wiring in a couple of Schottky 1N5819s (off the same bandolier, so probably same batch - both had exactly same Vf of 246mV, and Ir's of 1.1 and 1.2 microamps), and the audio was just audible but extremely distorted. Maybe these weren't suitable replacements anyway, but I decided to stick with the AA112s. (At this point in procedures the tuning drive-cord decided it was time to snap - I hadn't touched it or been working anywhere near it, so just gratuitous malevolence). After nearly a day's time tracking down some cord and fitting it, I switched on - to silence. All the mechanical disturbance had broken a power connection (just a solder bridge) between the vertically mounted IF board and the main pcb (terrible construction method). The joint was unrepairable, so I had to drill a small hole in the main pcb and run a wire for a new connection.
This restored operation - still with the sibillance and now with no AM (something I've decided to revisit later, since I rarely use it).
Re the sibillance, I still wonder about the de-emphasis circuitry. Is this a likely cause? I can't isolate the components (see attached) - can anyone help?
Mike
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Just realised that my previous post (above) was made nonsense of by incorrect figures I gave for D1.
It should have read:
"D1 - Ir = 3.1 microamps, Vf = 581 mV
D2 - Ir = 4.3 microamps, Vf = 561 mV"
Sorry about that!
Mike