[time-nuts] HP 5370a

[Matthias Jelen]

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