[time-nuts] HP 8566B repair

[Darrell Harmon]

Jeff Mock wrote:
> Does anyone have any suggestions or advice about 8566B repair?
>
> I've had an 8566B for 7-8 years, it's been a faithful tool, never had a 
> problem with it.  It's reasonably clean unit that's never seen a tough 
> life.  It sits in the storeroom 90% of the time and is on my bench maybe 
> 10% of the time.
>
> So, this week, after a 4 month hiatus, I've been using it on the bench 
> and it starting producing a "YTO UNLOCK" message, the trace seems to 
> basically work properly but is clearly unlocked and the trace will 
> randomly slide to the left or right.  The Yttrium oscillator is clearly 
> unlocked.  I'm hopeful that the problem is in one the control loops 
> outside the oscillator module.
>
> I read the repair guide that Agilent kindly keeps on their website, they 
> have some good instructions and narrow the problem to 3-4 modules, but 
> I've never opened up the instrument and am a little apprehensive about 
> making things worse.  My hunch is that the problem is an electrolytic 
> cap that has dried out over the years and has failed.
>
> Any suggestions for either self repair or outside repair and 
> re-calibration is appreciated...
>
> In full disclosure, I also have a 70908A based spectrum analyser that 
> that has much better specs, especially in microwave. It can completely 
> replace the 8566B, but I'm quite fond of the 8566B, I can operate it 
> blind folded, it's like an old friend and I would really like to fix it.
>
> jeff
>
>
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This document is very helpful in dealing with YIG oscillators.

http://www.vhfcomm.co.uk/pdf/A%20Simple%20Approach%20tyo%20YIG%20Oscil.pdf

YIG oscillators should be quite reliable, and I would expect that it is 
just an electrolytic cap or similar as you mentioned. The oscillator 
portion clearly works, so the only thing that could be wrong with the 
oscillator itself is a burned out FM coil which is unlikely. The 
oscillator would not oscillate at all without main coil current assuming 
it is not a PMYTO, which is extremely unlikely in an older unit. I would 
expect that the FM coil (fine tune, driven by the PLL) current is pegged 
to either side or the FM coil is not being driven. It could be as simple 
as adjusting a pot on the main coil (course tune) driver. The PLL and FM 
coil may have as small of a tuning range as 10 MHz.

Darrell Harmon