[time-nuts] Noisy varactor diodes

John Miles jmiles at pop.net
Tue Dec 18 21:42:54 UTC 2012

> I was recently repairing a noisy A3 module for an HP 5065A.


> The phase modulator has 4 varactor diodes in parallel.


> Since I suspected a diode, I removed them one at a time.


> Of course it was the last one! Must be a universal rule there!


> Anyway here is a PIX of the noise with the diode reverse biased at 20

> volts.


> Anybody know where to get an MV840 ???


www.octopart.com identifies 23 of them in stock at a reseller called 4 Star
Electronics.  Bob's idea of trying a power-supply rectifier sounds
interesting, though.  Failing that, an NTE replacement part for AM radio
tuning might work, perhaps in a different series/parallel combination.


Just curious, what was the 5065A acting like with the bad diode?  Mine seems
to run quietly for days at a time, but then occasionally it generates excess
low-level random noise for a period of hours to days (see image).  Every
time I've tried to get serious about tracking the problem down, it stops
misbehaving.  Did yours look like this?


It's especially annoying now that I finally sprung for a second 5065A, which
seems to behave almost identically...


-- john

Miles Design LLC


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