Author Topic: Testing Varactor Diode for HP 8640B  (Read 314 times)

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Offline _rushin_or_draggin_Topic starter

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Testing Varactor Diode for HP 8640B
« on: November 19, 2024, 02:58:11 am »
Hey all,
I am currently troubleshooting a phase lock issue on my HP 8640B. I have gone through service sheet 4 where step 12 led me to service sheet 7. Reading through 7 it says a low varactor anode voltage at A7TP2 could mean a bad varactor assembly or a bad PLL filter. I have checked all the voltages around the PLL filter and they seem okay for the most part. I decided to check the anode voltage and it is around -0.9V when it should be -13.6 to -16V.

Checking service sheet 5 it says the resistance from varactor anode to ground should be 61.9k. When I check it I get nothing at all, full open. I am already expecting the worst and have started looking at replacement oscillator cavities. Looking at the schematic though, is it possible that the inductors on the buffer amp and filter between the varactor diodes could be blown? It's a long shot but it's currently next on my list of things to check.

I know it says to not take the cavities apart, but does anyone have any experience with it? Or maybe any knowledge on why I would be getting an open between the varactor anode and ground? I have attached a screenshot of the schematic. Thanks!
 

Offline Dave Wise

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Re: Testing Varactor Diode for HP 8640B
« Reply #1 on: November 23, 2024, 01:17:50 am »
My own 8640B came to me with an open metal-film resistor on the FM board.  There was no sign of violence, the resistor was simply dead.  You may have the same type of failure.

HTH,
Dave Wise
 


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