HP 8920A TLC (proactive PS recap and more)

[kj7e]

After replacing the CRT with the NewScope-0Jr LCD (https://www.eevblog.com/forum/repair/hp-8920a-newscope-crt-to-lcd-retrofit-display/msg5217075/#msg5217075), I was motivated to freshen up the old 8920A.  Recapped the power supply, changed out the memory card battery and replaced the original OCXO for one with much lest aging drift.

I found some new old stock OCXO's as used in the optional reference board on eBay, see here: https://www.eevblog.com/forum/repair/ocxo-pin-out-ofc-1813-1106-aka-corning-mc857x4-004w-used-in-hp8920ab/

After letting them cook on the bench for a few weeks watching them age, I had one clear winner.   The warmup time was faster, less overshoot, faster settling and about 10% the aging drift as the others.  So I swapped this OCXO in.

The memory card battery is not crucial, if it dies you wont loose you cal data, just saved automated test programs and setup presets.  I took this opportunity to replace the memory card battery anyways, it will be good for another 10 years+

The weak link in these units is the power supply, which are impossible to find.  So I thought it better to recap it now before I ended up with an issue causing a more complicated repair.  The hardest part was lifting the PCB off the plastic pins on the aluminum frame.  There is a tab you press in on each pin while lifting the PCB up slightly, but have to do them all at the same time, just a little at a time.  I've included a cap BOM PDF attached here.  The power button rod can be very fragile and again unobtainable so take care here and follow these instructions;
Use the following sequence to remove the Power Supply:

1.Remove the Input assembly (see figure 8, "A23 RF Input Assembly," on page 77).
2.Pull the POWER switch shaft forward from the power supply, then pull it out back toward the rear of the Test Set.
3.Remove the screws and molex connector.
4.Slide the power supply out from the bottom of the Test Set.

I found using a long thin blade screwdriver to gently pry the power button shaft forward off the switch while pulling the shaft forward made removal easy.  Then slide the shaft back into the supply to gain room to remove the shaft up and away from the front panel.

So now with the new LCD display, Power Supply caps, Memory battery and a more stable OCXO, its good to go for another 20 years.