I’d added Mad Phil’s trusty Circuitmate to the tool kit I carry along to Squidwrench:
Over the last few months it became increasingly erratic, eventually got to the point where slight pressure on the case would blank the display, and finally didn’t turn on at all. Yes, I replaced the batteries.
So I took it apart:
Nothing seemed particularly broken and, even after resoldering all the joints, it continued to not work at all:
If you want to try your hand at instrument rehabilitation, let me know.
[Update:It’s back from the dead !]
The Smell of Molten Projects in the Morning 
I had something similar recently and it ended up being one of the slider switches themselves. But that was one where I took it apart, it all worked, put it together, stopped, took it apart, repeat until it dawned on me that the main slider was the culprit.
The “main switch” is those three parallel sliders captured by the large sliding cap: figuring out which one went bad would involve a major soldering project.
Thanks for the suggestion, though … I’ll balance the remains atop the to-do heap.
Yep, not worth the time, but it’s good to hear other possible culprits. I had another strange one, my travel multimeter is one of the China knockoffs and when I was measuring resistance it would occasionally start beeping and switching back and forth between voltage. I dismantled and cleaned it, no joy. I then thought it was due to the main dial selector having too much play, so made a Kapton bushing to keep it more centered. This helped a bit, but problem was still there. I finally found that the dial had worn just enough to allow some of the sliding copper tabs to lift far enough away to lose contact and making a kapton washer to hold it down solved the issue. Just in case you run in to that particular issue.
Mmmph, somewhat like the rotary switch in a white-UV-laser flashlight around here. The three tabs sat flush with the plastic surface and the rotating contact was also flat. It made the most gentle & intermittent contact imaginable, which I fixed by putting a distinct bend into it. Sheesh and similar remarks.
When you initially posted about the DM73, I was pleased to see that I wasn’t the only one still using one – it lives in my backpack. Since comparing against a known good unit is a valid diagnostic technique, I tried turning this one on in each function with various combinations of the Data Hold and AC/DC buttons pressed, but could not provoke a no-display condition. I did notice that it always beeps once when turning on the power switch, and the beep stutters if it is set to voltage and you turn it on while holding down the AC/DC button, as if the power-on beep is being interrupted by the AC mode change beep. I powered it up in continuity mode with the probes shorted, and discovered that pressure on the power switch affects the tone volume – sounds like the power switch is going in this one.
OK, so I must work on those slide switches. Perhaps a blast of contact cleaner through an end of each steel frame will encourage Proper Behavior, without having to unsolder / dismantle / rebuild all three.
Thanks for the diagnostic comparison!
Electronics repair (despite the name) sure tend to be on mechanical side :) OK, with the exception of power electronics that trend towards caps :)
FWIW, my cheap eBay VC99 multimeters failed intermittently because of faulty crystals – 4.096MHz if memory serves.
I really have to make my own repair blog just so I can remember stuff as well :) Ed, how much time does blog stuff knock out from your day? Not the work itself but writing it up, making it pretty and publishing? Thanks
A bad oscillator? Those are tough to debug: congratulations on finding it!
I take pix of what I’m doing to remind me what I did, then batch-write the posts maybe once a week. The simpler posts don’t take much effort and have a very high ROI when the same gadget breaks down a few years later. The more complex projects / posts generally become CC and DM articles, so I can kinda-sorta justify them on that basis; working through the topics, marshaling the references, and sorting out the data definitely helps.
Probably too much time, I suppose, but it’s a whole lot more rewarding than Sudoku … [grin]
You give me way too much credit :)
There was no real debugging involved. If it’s digital and misbehaving I assume a bad crystal. Lesson learned from a TV repairman who did a lot of IR remotes. Seems the crystals don’t like it when people drop them on the floor for years :)
It did save my cousin from replacing a 700$ A.E.G. built-in oven. The only clues were: “I found it with stuck clock and unresponsive front panel and after resetting it the clock disappeared as well”. Power rails seemed fine so I went for the crystal. He was very lucky when we (correctly) guesstimated what frequency it might be from markings on 4x2mm SMD package, for the second time when he found the actual part online for 40 euro cents with free shipping promotion and for third time when I soldered it to the board with the iron and didn’t botch it (I had no hot air gun at the time) :).
I suppose “misbehaving” means it powers up and does something, unlike the defunct junk around here with corroded batteries. Good work!
The bad caps can be in several places. Several years ago I had to fix a 20″ Princeton monitor that went toes up: no display, blinking pilot light. This was an early 2000s unit, and it had the bad electrolytic caps that were an issue back then. (https://infogalactic.com/info/Capacitor_plague) I don’t recall any electrolyte residue, but several had a severe bulge in the back.
A place on line did kits of caps for LCD electronics; had to buy a second kit, because Princeton did two versions of the electronics, and I guessed wrong when I ordered. The fix lasted a year or two, at which point the colors went wonky. (Maybe a bad oscillator or crystal, but I’m not set up to debug such.) That monitor ended up getting recycled, and replaced by a LED Dell. FWIW, the 16″ Princeton (same vintage as the 20″) is still going strong in the shop computer system.
(The kit place is still on line; lcdalternatives dot com It’s handy for this purpose. I recall having to replace about 7-9 caps, only a couple were the same value.)
Oh yeah, also lost a circa 1991 Mac Classic II to leaking surface mount electrolytic caps. Those dumped residue all over the mother board, and I gave it up as a lost cause. Similar situation with a 1950s church organ. (The power transistors predated the 2Nxxx numbering system) The power capacitors had some fearsome shorts, and it looked like they took the rectifier diodes with them.
I recently found my old one buried in my tool / parts bin. Given to me almost 30 years ago when I was a tech on an earlier career path.. It somehow survives. I just changed the batteries and works fine. Will keep it on my workbench until it gives up the ghost and then toss to the recycle bin like others. I am recently retired and miss the tech role I had that long ago. Life seemed a lot simpler – before wife and kids and the need to build career to keep up with the expenses of all that….LOL.
You must be the Perfect Master of Clean Living to find uncorroded batteries in there: well done!