[time-nuts] HP10811 losing EFC

Neville Michie namichie at gmail.com
Sat Jan 16 04:07:09 UTC 2010


Hi, thanks for the constructive comments.

I took both units out of their cases and gave them a visual inspection.
I tested the varicap diode as a diode, and checked the values of  
surrounding resistors.
When I re-assembled them one of them now works OK. It will take some  
time to see if it is stable.
I have an instrument that I can run them in which has an EFC lead  
that I can easily lift,
so I am going to apply about 20V through 1megohm to the EFC terminal  
and see how high the voltage measures,
that should give me the 6.8 volts regulated plus a diode drop. The  
diode and the capacitor should stand 20V.
I dont like trying to run it while dissasembled, that can lead to  
disasters, but this test should show if
the 6.8 volts is a problem. Failing that I will unsolder the varicap
and measure its capacitance on a bridge with changing bias voltage.
Thanks again for the suggestions,

Neville Michie


On 16/01/2010, at 11:54 AM, Rick Karlquist wrote:

> No one has mentioned the fact that the cathode of the varactor
> is connected to an internal 6.8V Zener diode voltage reference.
> This is another source of trouble.
>
> Rick Karlquist
> N6RK
>
>
> Bob Camp wrote:
>> Hi
>>
>> I sure would do some quick continuity checks on the EFC line before I
>> started swapping varicaps out. A bum solder joint opening up is a  
>> lot more
>> likely than the varicap going nuts in a fashion that still lets the
>> oscillator run.
>>
>> Bob
>>
>>
>> On Jan 15, 2010, at 7:08 PM, Neville Michie wrote:
>>
>>> Hi,
>>>
>>> I have two HP10811 oscillators that do not respond to EFC voltage.
>>> The latest to fail had been quite accurately set against a TBOLT,
>>> one morning (after being on standby in a HP5328A) it was off  
>>> frequency
>>> and would not shift with EFC but would adjust with the coarse  
>>> adjust.
>>> I have bought in two Phillips varicap diodes, BBY40 and BB149A,
>>> but before I did some damage fault finding in the HP10811s  I  
>>> thought
>>> it wise to ask for any advice that was available.
>>> While I am playing with them I may try to readjust the temperature
>>> setting
>>> of the oven to get on top of the turnover. Can anyone point me to  
>>> that
>>> article?
>>>
>>> cheers, Neville Michie
>>>
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>>
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>
>
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