[time-nuts] Faulty HP 10811D

[Denis Dowling]

Hi All,

This is my first time on the list so apologies if I am going over an old 
topic.

I am repairing a HP 8662A Synthesised Signal Generator that I purchased 
from a government auction. The unit was purchased without any 
information about its working state and I have fixing various issues 
with it. Firstly there was a digital logic problem with the keyboard 
card that was causing the generator to periodically lock up. When this 
was fixed the unit came to life but the output signal level was down 5dB 
and this was traced to a bad step attenuator. I attempted a fix but 
ended up needing to replace this to get the output steps correct.

I have now turned my attention to calibrating the reference timebase and 
unfortunately this seems to be well out of spec.

The timebase is a HP 10811D 10MHz OCXO. The 10MHz output of the 
oscillator seems to be ~40Hz low. I have attempted to adjust the unit 
via the multi-turn capacitor adjustment but this seems to be at the end 
stop and the frequency is still low. So my questions are:

1) I assume this module is faulty as it is so far out of spec? The 
signal generator has been powered off for multiple months but even after 
having the unit powered on for 24 hours the frequency has not changed 
significantly.

2) How repairable are these modules? I ended up blowing $200 on the 
replacement step attenuator so will need to save up for any new parts. I 
can always use an external 10MHz reference but it is always nice to have 
equipment complete.

3) From searching the list it seems that the oven could be what is 
faulty. Would a cold over account for this much error?I have confirmed 
that the edge connector is supplying 25V on the over supply connections 
but I don't think the module is warming up.

4) Is it possible  the OCXO has just worn out and has aged beyond the 
point it can be adjusted back into spec?


Regards,
Denis