Hi
denimdragon, not sure if you had success in repairing your 5440B. I'm ending in repairing mine, bought 2.5 years ago - no time, but this is something I would have never let go
. Issue of mine was just that .15R resistor on guard that was ... desoldered!
BTW, I had weird issues like yours, after repairing my unit. When it was cold, I had all sorts of errors. There was even a weird noise like a quiet sounding buzzer
. My 5440B had a bunch of these yellow Nichicon capacitors, but two had turned to brown on regulator board (I think that this calibrator ran for week/months or even years without being powered off...).
I've replaced all these capacitors by +125°C on all boards (c.a. the same price than +105°C), and I even increased values (most if not all 22uF were replaced by 47uF - exact same footprint). Since, no more issues, calibrator runs perfectly even if cold (apart stability, of course). I've checked stability after several hours against a 8505 from eBay ($50) and @ +10V, there is a difference of 10ppm between both. And both are rock stable
(of course, this is not a proof in itself).
Note about relays: I had issues too with relays, especially with smallest ones (2 form C). I cleaned contacts without any product, apart with very soft sandpaper (grain of 5000). For other relays, including sanded ones (total of 25 IIRC), I made a small board with an NE556, one half operating @ c.a. 7Hz and the other half being a monostable for c.a. 2 minutes. A kind of self-cleaning. That trick worked like a charm (same trick used with attenuator boards of Tektro TDS 700 series).
I've since dumped nearly all devices - PALs on display controller PCA are readable - and an interesting thing, my calibrator has software v2.1 (most 5440B I've seen are fitted with software v2.0). As I'm in Europe and have the habit of 6 and 9 with tails, I edited EPROM U61 to change display font (reason why I've always hated 4511's
) I will make a separate post about repair and mods of my calibrator when everything is solved (the four remaining things: being able to read 68701, solving 2x20 VFD issue - mine is readable, but not new, replace both leaky output 1uF capacitors, and add four feet).