[time-nuts] Repair of 5370B input channels

Randy Smith randy.c.smith at gmail.com
Wed Jun 14 23:05:51 EDT 2006

I got a dead 5370A from Ebay to work by taking the input board (and
high-stability oscillator) from a dead 5345.  The 5370A arrived with blown
fuses, and I traced it to a dead short on the input board marked 5345-60138
41903L.  The short was because the board had a completely different
electrical layout than the bridge board (5345-60004 3903L).  The input board
was likely fried from the short.

I figured someone had tried to repair it with the board from a 5345.  Since
the boards were incompatible, they couldn't both be from the 5345, so
5370A's must have used the input structure from the 5345.  I eventually
found a 5345 dead in some non-input-related way.  It had the same input
board, but a different (single-ended) bridge board.  There's a differential
input board with start/stop signal amps (DIP chips with heat sinks) on it,
and a single-ended board with just traces for the signals.  I think both
things together were <$200 shipped, which is pretty decent with the

The two DIP chips bolted to the level pots are supposed to be the fragile
parts, though I haven't confirmed this first-hand.  I don't know how much of
this applies to the 5370B, but I think it's similar.

On 6/8/06, John Ackermann N8UR <jra at febo.com> wrote:
> Jim Miller said the following on 06/08/2006 07:33 PM:
> > Hi,
> >
> > I recently acquired two 5370B counters with bad input channels.  One
> > has a bad start channel,
> > the other a bad stop channel.  I have traced the problem to the input
> > board A3, which is the board directly behind the BNC connectors on
> > the front panel.
> >
> > Has anyone had any luck repairing the input circuitry in these
> > units?  Is there a common failure mode that occurs when signal levels
> > are exceeded?
> Jim, I'm not certain but I think that there are some magic parts on the
> input board that are pretty much impossible to obtain -- and the inputs
> are relatively fragile and won't survive excess voltages.  Sorry for the
> bad news; you might be able to merge the two blown boards into one
> working one, though.  I have the service manual if you want to borrow it.
> John
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