After cleaning off rather a lot of mould I decided it could enter the house and I could investigate further. Not being one to start things on a variac I plugged it in and it was mostly fine. The DM44 was displaying a single zero, the focus wasn't amazing and the focus control was very hard to turn. The 10x attenuator on channel 1 was intermittently providing x2 instead of x10. But we have a trace, the controls don't seem too noisy and the multimeter isn't totally broken so I decided to repair the multimeter first.
The DM44 manual is brilliant and the first question in its faultfinding block diagram, after checking power supply voltages, is "is there only a single digit on the display?". That was my fault, attributable to a missing clock pulse which is a symptom of a problem with a LM339 quad comparator. That was in a socket and maybe I should've just reseated it because it was one of *those* TI sockets, but I changed the IC instead and that restored the missing digits.
Once the digits were all lighting up I could see a missing segment in the second actual digit. Tektronix didn't cheap out here, the first digit is polarity. The middle segment only used to display either a - or not. The part number was the same for all the digits and the failed segment was not the middle one, so I desoldered digit 2 and the polarity digit and swapped them round. A zero parts fix is always good.
With the display now working I could revisit the channel 1 attenuator. These are weird hybrid-like boards in tiny plastic boxes, they have goldie-looking-pins and fit into those strange rubbery Tektronix sockets. The attenuators have a couple of variable capacitors so the frequency response can be adjusted, wiggling these caused the correct attenuation to momentarily appear but it didn't seem consistent. I removed the attenuator from its box, gave it a blast of compressed air, reassembled, cleaned all its pins with IPA and the problem seems to have vanished.
All that was left was to clean up more of the nasty black mould stuff and do a bit of "detailing" on the front panel. I suppose I should've fully dismantled it but it wasn't really that hard to clean, or that dirty. The hard front cover, still with a spare fuse, is present and did its job well. I would like to replace the knob on the DM44, though, as the plastic has departed.
It's a nice thing to use, but I've learned not to turn the intensity up too far, it can focus, but it's best with the intensity down. It will go absurdly bright, if required.
