Re: Help needed with - Racal Dana 9081
Hello Mark,
Your first picture shows that crystal is oscillating, with a slightly higher than 5MHz signal, which is good. The second one shows something completely different, a pulse waveform at about 500kHz, possibly with something of a higher frequency as well.
The crystal is capable of oscillating at somewhere near to the right frequency, so you need to check around the oscillator circuit to see what is going on.
The metal cased 2N2369 transistors are a help here, as the case is connected to the collector of the transistor. Set the tuning to wherever it needs to be to reproduce photo 1. Next measure the waveform on the collector (case) of TR4. You should see a waveform that is about 15 times greater in amplitude than you saw at TP6, so you will need to adjust the input attenuator of the oscilloscope to suit. If there is still a lot of RF seen, you will need to change C19, which should be removing the RF. In fact, it is probably worth changing it anyway if it is the original component.
Another thing you could try is to lift one end of C18. That will break the ALC loop. The voltage on the collector of Q5 should rise to at least 8V. It should be lower when operating with C18 in place.
Paula
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