Since I wait for new transformer to arrive, how can we test inguard supply to confirm it's OK?
Glad you ask, I have a solution.
From previous Keithley Model 2001 repair projects I have one extra power transformer, which happen to have same voltage outputs, as HP one, just at lower power rating. But since we connect only inguard supply A4 board, we don't need full power to check LDO's operation. So let's replace blown zeners and test A4 PCBA with properly wired Keithley mains TR-280 transformer. Never know which parts could ever come handy, eh?
But first, replaced original CR11,CR12 with fresh zeners (got 50pcs from eBay, just few dollars with free shipping).
Cleaned board both sides, everything else look intact and good.
Now can connect input IEC plug socket to transformer's primary (since I need 110VAC, I used WHITE + BLACK wires).
Keithley used same type of 5-pin connector, but with different pinout. Secondary connector need rewiring as on photo above. I powered transformer separately and measured VAC voltages on secondary, without A4 board connected, to make sure all voltages correct.
If you see schematics of A4 board above, you may noticed MH1,MH2,MH3 connections. These are connected directly to guard frame in 3458A's chassis, which is acting as mecca star point for GND potential. Since I testing board separately, not mounted to chassis, I needed to connect these MH* points together with copper wire. This will be our ground point for measurements as well.
Apply power to transformer, and if nothing smokes, test output DC voltage.
There are nice labels near test points, telling us voltage test points location.
All voltages are OK, also 0.325VAC 60Hz signal to read mains frequency is OK too.
Repair of A4 inguard power supply now complete.
I also got used bezel off eBay in good condition, to save some money on this project, as it's just mechanical part to keep meter in one piece.
New one on bottom side, old busted rear bezel is on top.
Original unit's bezel had three out of four screw mounts destroyed, so it was not holding well.
Stay tuned...