Dear Time-Nuts,
I´m still struggling to get my 5370A to work reliable again.
It keeps showing error 04.
Sometimes it is working without any problems for hours,
sometimes it´s a matter of minutes until the error is
displayed, and sometimes I doesn´t come up without the error
at all. I don´t think it´s related to temperarure, the usual
procedures with a hairdryer and cooling spray didn´t show
any effects.
I dug into this a bit. It seems like the startable VCO on
one of the interpolator boards isn´t working reliable
anymore. The error stays with the board, no matter if I put
it into the slot for the start channel or the stop channel.
If the unit is in the non-working-state, there is no RF at
the output of the VCO. As soon the VCO oscilates, the PLL
locks and everything works fine, only the tuning voltage is
a bit outside the range given in the service manual. If the
VCO stops oscillating, the tuning voltage runs to it´s lower
rail, just as expected. I opened the PLL loop and tuned the
VCO with a external power supply, this works as expected -
no regions in the tuning range in which the oscillator stops
working or similiar.
I checked the signals going to the board, and they all look OK.
I fooled the logic that detects a tuning voltage outside the
allowed area, and as expected, the error 04 isn´t displayed
anymore, but also as expected the unit still stops working.
I pulled the board, removed some components and supplied
power to the VCO from an external power supply - same
behaviour here.
So I came to the conclusion that the VCO itself is
defective, although I have no idea what could cause a VCO to
behave that way.
Now for the strange part: A noble member of this list sent
me a spare interpolator board which was pulled from a
working unit. This shows exactly the same behaviour! Also at
this board the tuning voltage is outside the given window,
and the oscillator doesn´t start from time to time.
If a dead VCO module is a common problem, I´d expect that
someone on this list also experienced this problem before.
Otherwise, I think it´s unlikely that I found two defective
interpolator boards in a row. I stil have the feeling that
I´m overlooking something simple. So `d be happy to receive
any comments on this. Also, if someone wants to get rid of a
5370 that needs repair I might be interested.
I guess this is what you can expect if your´re using test
equipment which was introduced before you were born, but
it´s part of the fun...
Thanks,
Matthias
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