Quote:
Originally Posted by Stefan_O
Thank you very much. It seems that IC2 is broken. While at pin 1 I´m able a measure a decent input level of about 100 mV, output is only about 20 mV (measured with an uncalibrated TF 2603, so this values are only relative).
All others seem to be fine. Is that plausible with the problem I described?
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IC2 looks like it acts as a buffer amp ahead of a Plessey prescaler in the 160MHz PLL section.
So if you don't see an accurate (locked) 160MHz at IC2 then I guess the level is too low to drive the prescaler. Obviously if this happens, the 160MHz PLL will go out of lock.
The 160MHz signal is used as part of the downmix to get the LF to 62.5MHz range in the generator so I guess this does stack up with your fault symptoms.
The usual drive level for the prescaler would be about 500mVpkpk although they are usually much more sensitive than this. If you are desperate you could try replacing the amp with a 1nF series capacitor because it might just lock OK even with zero gain and get the generator 'working' whilst you look for a replacement for IC2.
You don't really need the full performance of the OM345 here because it only has to buffer a 160MHz signal. So even a basic home made clone of the OM345 circuit (with a low spec transistor) would work here.