DM502 repairs: four at a time!

[worsthorse]

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 DM502 supposed to function. I will try to document my process as I go along, until I have all four either fixed or labeled parts mule.

More to follow...

[worsthorse]

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 problems

These 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.

[worsthorse]

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.
 

[worsthorse]

Might want to fix the title?

fixed. hopefully that's not an indication of things to come. 

[texaspyro]

Most common problems:

1) shorted tantalum caps
2) bad LM741 op amps in voltage regulator circuits
3) dirty rotary switch contacts

[worsthorse]

#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.

[BruceM740]

Was this project dropped?  I’m troubleshooting a DM 502, looking to see where your trail leads.  No DC volts.  DC Power +5, -12 & -12 are uhh close, each a bit ‘high’. (5.33) but not suspecting that’s the culprit.  I can go re-clean contacts and measure Tants..  😊