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Old 25th Mar 2018, 10:26 pm   #1
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default HP 5245L counter crystal oscillator

Way back in this thread:

https://www.vintage-radio.net/forum/...ad.php?t=58065

I documented getting the precision ovenized crystal oscillator going in my trusty HP 5245L counter. Acquired on the day of the 'great storm' of 1987, I'm still using the counter regularly. Or at least I was until last night when I turned it on to make a measurement and found it dead.

Cutting a long story short, the mains fuse had blown because the transformer supplying the crystal oven had developed shorted turns because a short-circuit had developed inside the wiring in the crystal oven.

To fix this I either need to sort out the crystal oven or replace it. I'd quite like to fix it because the crystal is a thing of vacuum-encapsulated gold-plated beauty and is one of the interesting features of the counter. Trouble is, the wiring is all buried inside the insulating foam of the oven and I can't see a sensible way of taking it apart. It looks like the foam was sprayed or moulded in once everything had been assembled. Of course, it's all crispy having been sitting at 60+ degrees C for the last 40 years. No amount of wiggling or tugging seems to free anything.

I've dug out the wiring as far as I can using a dental pick, but all that's done is make it clear that the short-circuit is further afield. The short is between the -36V rail for the heater and ground, which is why the mains transformer burnt out. Note that the four colour wires visible in the picture are the ones to the crystal. The heater wiring is just about visible down the side of the central aluminium cylinder, tucked in to the foam in the bottom left. Though it started life as a ribbon of coloured wires they're now all brown and crunchy.

What to do next? Any bright ideas for removing the foam? Does anyone have a spare 5243A crystal oven? Or should I abandon this crystal and replace it with a (better-performing but less characterful) 10544A or 10811A crystal oscillator?

Chris
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