[time-nuts] HP 5370A

Didier Juges didier at cox.net
Sun Oct 15 10:46:00 EDT 2006


Thanks for the explanation, and you were right on the money. It looked 
like one of the tuning caps was open, funny that it resonated so close 
to 30 MHz. I reflowed the solder joints on the two 500pF capacitors 
(even though they looked fine under the magnifier) and voila, Bob's your 

These look like silver-mica capacitors, and I have seen solder joint 
problems develop with those before. I may look inside the unit for more 
caps like that and reflow them too.

Picture is at

Output is now a clean 4V p-p at 10 MHz, instead of 900mV at 30 MHz.


Didier KO4BB

PS: The card edge connectors on this unit are extremely tight. I 
"easily" broke one of the extractor on the CPU card, and the ROM card 
was also extremely hard to pull (I did not even try to use the 
extractors). The A8 reference assembly, even though it has only a 
single, rather small connector, was the hardest!!!

PPS: The A8 card had quite a bit of hard dust on its solder side, but 
it's right in front of the fan, so it's not unexpected. I was actually 
surprised to see so little dust inside the unit, even though it does not 
look like it's been cleaned, or if it has, it was a while back because 
the dust was very much encrusted. The front panel had the same type of 
hard dust on it, but it cleaned easily with alcohol and now looks great. 
Maybe I have been lucky and got a low hours unit. It even came with the 
feet, handles and rack kit, and I only had to drive 40 miles to pick it up!

Dr Bruce Griffiths wrote:
> Didier
> The path from the external frequency input to the external 10MHz input is
> 1) single transistor buffer/multiplier with collector tank tuned to 10MHz
> 2) Single transistor amplifier/multiplier with collector tank tuned to 10MHz
> 3) ECL gate chain
> 4) longtailed npn transistor pair which has a capacitively tapped LC 
> tank circuit tuned to 10MHz.
> The path from the internal crystal is via a set of ECL gates
> The internal or external source is selected both by a set of ECL gates 
> and by a slide switch to minimise feedthrough.
> Since the only common circuit other than a few ECL gates is the 
> longtailed pair with a tuned collector tank, it is possible that the 
> collector tank components are tuned to 30MHz and not 10MHz. One of the 
> tank components may have failed or there may be a dry joint or 
> equivalent in the tank circuit. The collector tank is excited with a 
> squarewave current source which has 3rd, 5th, 7th harmonic components as 
> well as the fundamental.
> You should check the signal path through the A8 reference frequency 
> buffer assembly board.
> There may also be other problems but you need to isolate and fix them 
> one at a time.
> Bruce

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