[time-nuts] HP10811 losing EFC

Bob Camp lists at cq.nu
Sat Jan 16 04:12:59 UTC 2010


Hi

Diodes rarely fail half way. Either a bond wire goes out or they melt down. If the varicap looks like a diode at DC it should be ok at RF. I would look elsewhere for the problem.

Bob


On Jan 15, 2010, at 11:07 PM, Neville Michie wrote:

> 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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