[time-nuts] Re: Faulty HP 10811D

[David G. McGaw]

Great article.  I remember reading it with great interest when it came 
out.  Bought an early 10811A as a house reference and poured over the 
extensive manual.

Have had either and both the thermal fuse and the thermistor fail on 
various copies.

73,

David N1HAC

On 3/29/23 6:16 AM, Denis Dowling via time-nuts 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?
> https://nam12.safelinks.protection.outlook.com/?url=http%3A%2F%2Fhparchive.com%2FJournals%2FHPJ-1981-03.pdf&data=05%7C01%7Cdavid.g.mcgaw%40dartmouth.edu%7C0d22f32fcd854208d05808db3077c596%7C995b093648d640e5a31ebf689ec9446f%7C0%7C0%7C638157063203629937%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=%2BPtZs%2FBPdPjBCjRTmf9z3cQ4UAjLAaemtMJlh2n8x%2BQ%3D&reserved=0 
>
>
> 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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