Keithley 192 DMM power supply issue - Page 1
Forum section: electronics / repair

author: kronk
author_rank: Newbie
timestamp: July 26, 2020, 10:39:10 pm
content: Howdy,My question is if the damage shown below is worth trying to fix. The parts aren't expensive, but if the board is questionable I think I would pass on the repair and just salvage the thing.I picked up a Keithley 192 DMM for cheap with a sticky note that says "does not work" a year or so ago but never even tried to power it on until last week. It does turn on but the display stays at 18888.8 and occasionally does a slight blink. According to the manual this means there's either an issue with the MCU/ROM or a power supply problem. Following the manual for diagnosis I struck out with the first test - the analog 5V is dead. On further inspection, I noticed some browning around a nearby full-bridge rectifier (Pic 1). I pulled out the board and on flipping it over found more signs of failure under that FBR and under the 7805 5V regulator (Pic 2). The traces look intact, but the surrounding board is a bit toasty looking. Is it worth popping these out, testing them then replacing one/both if needed?Ive never done a repair like this before but its kinda fun! Still, I might be more interested in the parts I can get from it since I have access to newer, nicer meters.

author: wn1fju
author_rank: Frequent Contributor
timestamp: July 27, 2020, 12:10:28 am
content: I've seen a lot worse. I had one piece where the board underneath some power resistors actually melted away. Slight discoloration of power supply boardsdue to heat happens a lot.Might as well give it a go since the rectifier and regulator are inexpensive if they are bad. I would also check the filter caps before and after the regulator, andalso check that the 5V rail supplying the rest of the board isn't shorted to ground. You could also verify all the traces are intact with an ohmmeter.The only question I have with your first photo is whether or not there is something sticking out of the hole (in the middle of the photo) and touching a nearbytrace? Or is it an optical illusion?

author: kronk
author_rank: Newbie
timestamp: July 27, 2020, 12:20:55 am
content: It certainly didnt seem terrible to me, but figured I'd ask since I dont have much experience with PCBs. Thanks for the suggestion to test the filter caps and the rest of the rail, good points.Yes, there is something there in that picture, but it seems to be an elastic rubber plug underneath the large filter cap, and the part touching a trace is a smear of that rubber. Any ideas on what that would be?

author: coromonadalix
author_rank: Super Contributor
timestamp: July 27, 2020, 12:37:28 am
content: you could or can use solder wick to remove carefully the big capacitor, the regulator and the rectifier bridge check them outside the board, dont over heat the pcb traces ...The pcb traces seems good at first sight ?For the rectifier bridge, for the same foot print you could use a new one, find one with a hole in it, attach an small aluminum plate, keep the leads longer tooFor the regulator, if it heats a lot, sometimes with insulating hardware you could put it on a metal frame when its availableAnd finally the two gray axial capacitors would need to be checked too, if they aged well or their capacitance droped too much over time ?

author: wn1fju
author_rank: Frequent Contributor
timestamp: July 27, 2020, 01:22:30 am
content: Well, if there is damage to the seal on the electrolytic capacitor, it may very well be that the electrolyte is gone and the capacitor is open. This would cause thebridge rectifier to work much harder than it is supposed to (it will conduct longer) and heat up. Less likely is that the capacitor is shorted - easily tested with anohmmeter.If you don't have a capacitor tester, you could power up the meter and hook up an oscilloscope across the capacitor. If it is open, you probably will see a 120 Hzsinewave of several volts. This can cause the regulator input voltage to be too low on every cycle and the regulator won't work correctly.Electrolytic caps in power supplies fail far more often than bridge rectifiers and regulators. Incidentally, if there is a rectifier problem, the scope usually shows a60 Hz sinewave, not 120 Hz.And if it is a capacitor problem, don't think you necessarily have to replace it with one that is the same dimensions. As long as the capacitance and workingvolts are the same, you can usually replace those old large caps with much, much smaller (cheaper) ones. I've done it a zillion times!

author: kronk
author_rank: Newbie
timestamp: July 28, 2020, 07:06:22 pm
content: Thanks to the folks that replied!I pulled out the rectifier, 5V regulator, and the huuuuuge 11,600uF cap right next to them. I didnt get a capacitance value from my handheld meter with it in-circuit, but the resistance was pretty low (100-300 Ohm). Turns out, there was significant adhesive that made it very hard to remove. I did get it though, and confirmed that the cap was bad, some signs of electrolyte.For fun, I tested the 5V regulator and rectifier and found both to work. I have now learned that:Quote from: wn1fju on July 27, 2020, 01:22:30 amI have ordered a new cap, here's hoping that does the trick!