I acquired this Advance DMM2 about six months ago on the UK vintage radio forum, as received it's completely untested, a little corroded in places inside, but nothing too bad. Also all the button caps have had the fronts cut or filed for some unknown reason, they are glued on too.
It's been in the repair queue for a while and I've been working on trying to resurrect it for the last two weekends.
The button caps are a non standard size these days and I couldn't find anything to match from the usual sources for new parts. Eventually I found some Isostat button caps from a seller in Bulgaria, they are a excellent fit and have the same offset hole for the switch rod, as the old ones, one is shown fitted to the DMM2 in the above picture.
https://www.ebay.co.uk/itm/133257327505Here are some pictures of the boards, the power supply is a mains stepdown transformer, feeding a simple switching supply with step-up transformer, this allows allows operation from low voltage DC too. The analog input circuitry uses a μ709 Op-amp, with some transistors and a SL701 amp IC for the AC/DC converter (this is on the small vertical board). It uses a custom IC for the multiplexed display counter & storage functions and a single Fairchild/SGS μLogic 960 BCD display decoder IC, with transistors/diodes for the multiplexing.
The first test had I no display, this was due to low supply rails from the secondary of the switching supply, I eventually found it was being overloaded by the 8uF 350V Erie cap on the +190V rail, it was getting warm, it got a temporary replacement with 4uF 250V (all I had), I quick check of the other caps with the Peak ESR found another that was high ESR, the positive lead fell off on removal (corroded), this will get a full recap
if I can get it working.
I now had a display of sorts, but with double numbers showing in the display choobs, a test with an old 3.5V back up cell from one of the Tek scopes, did show a change in the display, the front end is not dead at least. The double digits lit in all displays fault, the manual tells you to investigate IC4 output, then IC3 outputs. These are the 996079 display decoder IC and custom counter IC.
The BCD decoder outputs of the 996079 are +V for 0 & 0V for logic 1.
Picture of the inside of the fancy custom IC, from the Uni of Queensland Australia;
https://physicsmuseum.uq.edu.au/cross-section-display-transistors-and-icsLast weekend I scoped the BCD outputs from the custom counter IC, two are faulty with some weird negative output pulses (the counter IC does use -12V too for the storage function?). The two faulty output are the lowest, i.e. BCD 1 & 2. BCD 4 & 8 seem to be working OK.
Earlier in the week I disconnected the BCD lines between the ICs, hooked up some test clips and went through the BCD code & see what was displayed. This proved the 960 decoder IC was working fine & for the first time I've got the separate "1" neon to light. The OL lights with the BCD code for 2. The BCD code used is 0V for logic 1 and +V for logic 0.
Yesterday I had a play with some LM741 Op-Amps, to see if I could correct the faulty BCD outputs.
I've had some success, but it's not 100% there. I can get it to Zero now, but my 3.5V battery measures near double it should and using a decade resistor I'm getting good readings on the first two digits, but it goes wrong with strange readings & some double digits with a higher input. I need to draw out the circuit later, comments would be good as I'm not a designer and I'm probably doing it all wrong.
David