[time-nuts] Re: Faulty HP 10811D

[Bob Camp]

Hi

Looks like it’s time to head off to eBay and find a $40 replacement.

Bob

> On Mar 30, 2023, at 2:03 AM, Askild via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> 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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