Quote:
Originally Posted by pdr-fml
When I aligned the sound IF I did as you said and turned tuner to blank position like the instructions say to do in the manual. When I hooked convertor box to mixer grid I then set the tuner to channel 3. Wouldn't this enable the local osc?
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The problem is that the local oscillator can't generate it's proper frequency when you inject your channel 3 rf directly into the grid of the mixer(see below edit). If you feel your rf amplifier is the defective stage, then I would suggest you make voltage and resistance checks around that area per the SAMs info. I'd like to point out that starting at the antenna end is sort of working the problem backwards, as it creates a lot of unnecessary confusion. The logical way to troubleshoot these things is to inject a signal as close to the output stage as possible. Once you have a picture or sound, you work your way back through the stages towards the antenna input. When you lose the signal, it's at that point that you have found the problem area.
[edit] After looking at your sets tuner circuit more thoroughly, I believe your injecting the rf channel 3 or 4 directly into the mixer grid will load down the oscillator signal feeding that same point through the 1.5 pf cap. I believe you would need some type of isolation or impedance matching circuit between your signal generator and the grid of the mixer. Even then I think without the gain provided by the rf amp, it would not mix correctly with the oscillator, or at best it would provide a greatly reduced amplitude IF signal. Keep in mind that your HP generator likely has a 50 ohm output impedance.
It would be great if one of the EE engineering types hanging around here could explain these type circuits so that everyone(myself included) could better understand their operation.
I only know from experience that injecting an rf signal at a critical mixing point tends to load down the circuit and create errors.