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Old 6th Oct 2014, 9:43 pm   #65
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,351
Default Re: Roberts RMB Problems

Okay, Dave, glad you got the test gear up and running. It's worth checking first, with your oscilloscope, that you can see a strong sine wave at the top end of C35. If there's nothing, or just a mish-mash of signals, the local oscillator isn't running.

The problem then is that you are trying to inject the 472kHz intermediate frequency modulated RF at the wrong place. You can inject 472kHz at the grid of V3 (pin 3) and you should hear the tone. Then move back to the grid of V2 (pin 6) and the tone should be much louder. This proves that the IF amplifier (V2) is working. Reduce the output from the signal generator to get a comfortable level of sound.

The area around C1 handles modulated RF at signal frequency, so inject the signal into the grid of V1 (pin 4). Tune the radio to 300 metres MW and set the signal generator to 1,000kHz. Rock the radio's tuning either side of 300m and at some point you should hear a signal. Work back from pin 4 to the aerial side of C1, then back to the aerial socket. Keep the generator signal as low as possible.

As you work backwards in this sequence, the point where the signal vanishes shows where the fault lies. But do check that the LO is working first.


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