[time-nuts] HP 8566B repair

John Miles jmiles at pop.net
Thu Dec 6 22:45:26 EST 2007

Never fear.  I have seen this a few times, in a couple of 8566Bs that I have
owned as well as in some others.  The YTO UNLOCK error always seems to come
down to an open electrolytic on the YTO driver or pretune DAC boards, under
the hinged plastic cover toward the rear of the RF section.

Check power supply voltages first, then check ESR of all of the axial-lead
electrolytics on all boards under that cover.  Then, if those areas are both
OK, go through the YTO calibration procedure in the manual until you either
successfully recalibrate the pretune/driver system, or reach a particular
step that you can't complete.

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Jeff Mock
> Sent: Thursday, December 06, 2007 7:38 PM
> To: Discussion of precise time and frequency measurement
> Subject: [time-nuts] HP 8566B repair
> 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.

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