And now the Marconi 2955B is on the blink again!

Well, this just about takes the biscuit!

Not only do I discover that my understanding of the use of my spectrum analyser is probably woefully inadequate, and that my oscilloscope is playing up and needs a service, but now my 2955B test set is on the fritz! Again!

This time, I know im at serious risk of screwing the firmware if I dont fix it. So, its time to open her up.

The 10MHz Oven Compensated Crystal Controlled Oscillator (OXCO) unit, is mounted, horrifyingly, on the same board that generates the EHT for the CRT!

Rear of the Reference/EHT board

I very much dislike playing near EHT supplies! I know from experience what a belt from the anode cap of a brand new never been connected TV tube feels like - and dont fancy experiencing the same from an actual supply! So I very carefully ensured (with a pair of long nose pliers) that the reservoir capacitors were properly discharged!

Inside the Marconi 2955B. Note the warnings!

The actual OXCO's six pins desoldered very easily, and the whole module came out smoothly. I opened it up to see if there was anything obviously wrong inside

The 10MHz OXCO...

...is bloody complicated inside!

But other than the trimmer capacitor (which I seem to remember being told could be the problem) it all looks ok, albeit extremely complex! So, I connected it up on the bench. 12v at 120mA steady (450mA initial) for the heater, 5v via a 22ohm current limit for the oscillator. 

On the bench...

 And the damn thing seems to be working perfectly!

...and seemingly working perfectly!

 I have had it powered now for about an hour. Ive even abused it - heating it with a hot air gun until it was too hot to touch, whereby it crept up to 10.00015MHz, and freezing it until the heater current maxed out again, and it barely dropped below 9.99998MHz!

Im now at a loss, but my suspicions center around either a power supply fault, or something as insidious  as a dry joint or leakage due to old flux! Where I go from here, im not sure, and ive a ton of work piling up that requires a working test set!