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8590A with small drift and instability

Question asked by rayp on Jan 18, 2015
Latest reply on Jan 20, 2015 by rayp
Hi folks,
I have  been working on a faulty 8590A.
*Symptoms:*
It is about -700 Khz off frequency at power on.
After 30 min it is about +300 Khz off.
The displayed signal drifts about + 10 Khz per second after power on.
This rate of drift slows to about +10 Khz in 30 seconds after 30 min on time.
There are No error codes.

*Tests carried out:*
The LO out viewed on another analyser is also drifting when set to zero span which should freeze the LO right ?
The Cal out 299.9Mhz is correct and stable.
Analyser tests all look good, Main Coil DR, FM Coil Dr, FM Span, Binary span etc…
No jitter and all levels look as per manual.

Gentle hairdryer heating of A7 near the YTO connector _instantly_ affects the frequency.
The YTO  is driven from A7, heater, coarse and fine and Vcc.
But the service manual does not provide a detailed A7 schematic diagram (from Keysight website).

So perhaps the YTO is fine and A7 is the problem...

Can anyone advise next steps?
Should I try to measure the control and voltage lines to the YTO?
It’s hard to get as without an extender card for A7.

Thanks in advance.

Ray

Edited by: RayP on Jan 18, 2015 10:52 AM  

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