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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