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Originally Posted by kbmuri
The Sams chart expects 0.1 VDC on pins 1, 2, and 5 of V11 (schematic 3 posts ago). With VCR input I get 0.024 on pins 1&2, 0.054 on pin 5. With my signal generator as described earlier, I can tune a maximum of 2.81 VDC on pin 5, when the tone is at its loudest. It drops off abruptly as I tune the signal generator away from "dead on" point. Hangs out around 0.025V when the signal generator is tuned well away from the target frequency.
I can understand pin 5 having DC, and varying with signal strength. If there's an AC source on pins 1 and 2, pin 5 will be the rectified component of pin 2. Smoothed by C5.
But I don't understand why pins 1 and 2 should be at a positive DC potential. Where is the source? But without understanding why, it's still troubling that
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In a sense, both diodes are in series, and as the negative end of the cap is tied to ground, the IF transformer secondary coil would tend to be at about half the voltage of what the positive end of the cap has on it.
Maybe one of the resistors have gone bad? R46 or 51 going too high in resistance could do it. Also, C37, as shown on the schematic, seems rather small. I would have expected something like 0.02 or so.