[time-nuts] HP-5245L Parts
Christopher Hoover
ch at murgatroid.com
Thu May 25 04:51:41 UTC 2006
> Thanks Allan for the response. I have the HP-5245L and the HP-5262A
Time
> Interval Unit, but it is not working. Problems in the PS
One thing you can do is to replace any blown TO-3 pass transistor and
associated support circuitry with a TO-3 three-terminal regulator of the
appropriate polarity. It is often easier and cheaper than trying to
find a NOS transistor or decent equivalent. I did this in my 5246L, and
it worked great.
The trouble with a bad power supply is that the problem can cascade,
especially if the supply doesn't include a crowbar. If the ambient
temperature gets too high -- perhaps because the fan seizes (my counter)
-- the pass transistor can go into thermal runaway and saturate before
giving up its smoke, frying a bunch of things on the associated power
rail.
Things to look for include:
-- bad electrolytics tied to the rail beyond the pass transistor.
-- resistors tied to the rail in series with other circuit elements
(typically transistor legs). Series resistors offer some protection to
the other elements with a runaway supply. Blown resistors are usually
plainly evident from inspection.
-- transistors legs tied directly to the rail. Carefully check over the
schematic for these -- series resistors weren't used with some of the
transistors in the sampling circuit of the 5246L: several have their
collectors tied directly to -15V. They all fried in my counter.
Hope this helps.
-ch
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