DM502 repairs: four at a time! - Page 1
Forum section: electronics / testgear

author: worsthorse
author_rank: Super Contributor
timestamp: January 22, 2020, 08:31:19 pm
content: I've been collecting and fixing TM500 plugins for a couple of years. Along the way, I accumulated a pile of DMMs, mostly the 502 and 502A. It seems that any time I bought a more than one plugin, a DMM was in the pile. I don't have them in my every day mix on the bench, so they've been stacked up out of sight. A couple of days ago, I decided to pull all the 502s together and work on them:The TM stuff is good project if you are a repair beginner: the manuals include complete schematics along with a pretty good theory of operation section, the devices are single purpose and generally easy to understand, and they use mostly common components. They aren't usually terribly expensive, so building up a functional set of basic equipment - DMM, function generator, etc - doesn't require a ton of money. Plus they are fun and easy to work on using relatively simple test equipment.I have two with the temperature option and two without. A quick function check (with a DMMCheck Plus) revealed that all but one have problems. I am going to start with the "working" unit and go through the manual check out and calibration procedure. It will be handy to have one working if I want to be sure I understand how a DM502supposedto function. I will try to document my process as I go along, until I have all four either fixed or labeledparts mule.More to follow...

author: worsthorse
author_rank: Super Contributor
timestamp: January 30, 2020, 08:48:19 pm
content: I did a quick check of all four the the DM502s, though I didn't check the temperature function on the two unit so equipped.#340 - broken ground jack, one digit of the display not working, no apparent internal damage, current measurement problem#140 - broken ground jack, no apparent damage, DC voltage and resistance measurement problem#617 - good physical condition, resistance and current measurement problem#748 - good physical condition, no apparent problemsThese four units came from four different sources. The first two, with the temperature option, are the most beat up. Going to go straight to #748 for check out and calibration as described in the manual.

author: worsthorse
author_rank: Super Contributor
timestamp: January 30, 2020, 08:51:02 pm
content: Before going further, I decided to check the fuses on each unit.One fuse was blown (#617), the rest had slow blow fuses ranging from 2 to 6 amps.The spec calls for a 2.5A fast blow fuse. That might explain why a couple of the units have current measurement problems.Put the proper fuses in all four units.

author: edavid
author_rank: Super Contributor
timestamp: January 30, 2020, 09:21:22 pm
content: Might want to fix the title?

author: worsthorse
author_rank: Super Contributor
timestamp: January 30, 2020, 11:46:44 pm
content: Quote from: edavid on January 30, 2020, 09:21:22 pmfixed. hopefully that's not an indication of things to come.

author: texaspyro
author_rank: Super Contributor
timestamp: January 31, 2020, 05:38:49 am
content: Most common problems:1) shorted tantalum caps2) bad LM741 op amps in voltage regulator circuits3) dirty rotary switch contacts

author: worsthorse
author_rank: Super Contributor
timestamp: February 03, 2020, 03:30:39 am
content: #748 needed only a general checkup and calibration. So it is done except for cleaning the front panel.I have figured out how I can reliably generate a "good enough" current source for testing the low end of the AC and DC current ranges and now I have a unit I can use to test against. Going to start on the next 502 tomorrow; probably the other unit that doesn't have the temperature option.