[time-nuts] Re: Faulty HP 10811D

[Askild]

Hi,

Generally I would agree that probably the thermal fuse has gone open
circuit when there has been no over temperature, more often than it has
protected the ocxo from a fault,
but the HP 8663A i found, seems to have had the thermal fuse replaced by a
wire. I hope my, not to good, pictures makes it through, so you can see the
result.
[image: P1010305_s.jpg]
[image: P1010306_s.jpg]
[image: P1010309_s.jpg]

Regards,
Askild


On Wed, Mar 29, 2023 at 8:37 PM Bob Camp via time-nuts <
time-nuts at lists.febo.com> wrote:

> Hi
>
> The use of the thermal fuse was very unique to HP’s specific needs. There
> is little or no
> evidence of it actually doing much good ( = protecting things). There is a
> *lot* of evidence
> of it failing for no good reason.
>
> Save your money for another $40 OCXO and replace it with a short.
>
> Bob
>
> > On Mar 29, 2023, at 6:16 AM, Denis Dowling via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > Thanks everyone for all of the advice. As predicted the fault was an
> open circuit thermal fuse. I have bypassed this (temporarily) and I can now
> trim the oscillator to be 10MHz to the precision of my PM 6672 frequency
> counter. I am plotting both 10MHz waveforms on a scope and there is minimal
> slip. I will order a new fuse and then reassemble and then attempt a more
> accurate calibration when everything has  been allowed to burn in for a
> while.
> >
> > On a related note, how good is the Hewlett Packet March 1981 Journal
> article on the HP 10811D?
> > http://hparchive.com/Journals/HPJ-1981-03.pdf
> >
> > There is so much information in this article including how they used a
> flex PCB to make the oscillator easier to assemble. They even discuss the
> issues they had with stray capacitance due to the thinner PCB and tearing
> on sharp edges. Unfortunately today I doubt any manufacturer would be as
> open about their design process.
> >
> > Regards,
> > Denis
> >
> >
> > On 28/03/2023 6:44 pm, Adrian Godwin via time-nuts wrote:
> >> I had a 10811 that didn't warm up. I'd never taken one apart before but,
> >> although a bunch of parts had failed, they were all common (if ancient)
> >> op-amps and the like. I didn't have much trouble fixing it once I'd got
> >> started. I'd encourage you to go ahead.
> >>
> >> On Tue, Mar 28, 2023 at 7:28 AM Askild via time-nuts <
> >> time-nuts at lists.febo.com> wrote:
> >>
> >>> Hi Dennis,
> >>>
> >>> The HP 10811 are serviceable. Just download the service manual and
> study
> >>> it.
> >>> I would expect the oven to be faulty with this much error.
> >>> Most likely the thermal fuse has gone open circuit.
> >>> The fuse can go open circuit without there actually being any fault
> with
> >>> the oven, but there might also be a fault in the oven circuit.
> >>> The fuse is on the first PCB you get to after opening up the 10811. All
> >>> described in the manual.
> >>> If its the thermal fuse, replace with correct thermal fuse. When
> replacing
> >>> make sure to not over heat it while soldering.
> >>> You can of course bridge the fuse with a wire for testing, but then you
> >>> need to monitor it closely, to be sure it does not over heat.
> >>>
> >>> I was lucky to find a HP 8663A in a electronic recycling bin. It was
> >>> missing PCB's in the PSU and the 10811 was totally burnt to a crisp.
> >>> Guess the fuse had been bridged. So I needed to get a replacement off
> ebay.
> >>>
> >>> Regards,
> >>> Askild
> >>>
> >>>
> >>>
> >>> On Tue, Mar 28, 2023 at 7:49 AM Denis Dowling via time-nuts <
> >>> time-nuts at lists.febo.com> wrote:
> >>>
> >>>> 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
> >>>>
> >>>>
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