[time-nuts] HP 5370a

[paul swed]

Matthias
I am afraid I am not much help here.
I did look at the a19/20 interpolators. A lot going on in that small space.
I guess i would check the 200 MHz feeding into it. The VCO is fairly simple
in using a gate and a delay line. I will guess the delay line is coax.
But there are all kinds of gates to control the system and Flip Flop used
as a mixer. Not easy to deal with.
Regards
Paul.

On Sat, Jul 4, 2015 at 1:44 PM, Matthias Jelen <Matthias.Jelen at gmx.de>
wrote:

> 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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