[time-nuts] HP-5245L Parts

Christopher Hoover ch at murgatroid.com
Thu May 25 00:51:41 EDT 2006

> Thanks Allan for the response. I have the HP-5245L and the HP-5262A
> 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

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.


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