sorry for the error, sentence to be corrected::? “and this problem would NOT affect other frequencies higher than 266.66... MHz since the MSB is already at 1 normally.”

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Hi,

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Just a quick comment that may have nothing to do with your problem.

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You say "The output signal from A18 (rear LOCAL BNC named J2) has to vary between 233.33 and 413.33 MHz. But it starts at 266.x MHz."

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If you look at the image below, it is perhaps just a coincidence, but the fact that the MSB would always be fixed at 1 for example (some defective register), the start freq would then be 266.66... instead of 233.33 ... and this problem will only affect other frequencies higher than 266.66... MHz since the MSB is already at 1 normally.

So, in the faulty location, there would be no detection and probably PLL unlock.

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Yves

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I have retraced my steps to check the signals passing from the upper unit to the lower unit.

The LOCAL signal, responsible for being the LO in the lower unit for both the Sources and Inputs, has improved its behavior. But he still has a problem. This signal is the product of mixing of a signal from A10 that varies between 5.83 and 10.33 MHz and is sweeped three times to complete the scan seen on the CRT. That signal has the appropriate frequency, although I cannot be sure of the amplitude because I don't have extenders for this boards and probing the signal is difficult and erratic. The output signal from A18 (rear LOCAL BNC named J2) has to vary between 233.33 and 413.33 MHz. But it starts at 266.x MHz. Hence the attenuation observed three times during the scan on the CRT.

I though in amplify the A10 signal with an external amplifier in case it's attenuation isn't enought to drive the first mixer in A18. But the sweep is too flat to be a problem with mixer input, imo

In the excerpt added you can see the procedure suggested to isolate the fault and then toss the entire board :D 1990 was the year to reject component level service, that's clear for me now...

Regards,
Ruben