Tek DM502A multimeter, can't zero out. - Page 1
Forum section: electronics / repair

author: JohnTinkers
author_rank: Contributor
timestamp: July 24, 2025, 05:57:26 pm
content: I'm calibrating a DM502A multimeter. While the readings taken with measurements seem close, the zero adjustment pot can't zero out in DC (-17mV and -1.6mV with shorting strap), AC (80-100mV dancing display and 0.1mv with shorting strap), ohms and milliamps, the temperature readings are just haywire blink and don't change regardless if I plug in a probe (that perfectly works on my DM44).I replaced all of the electrolytics in my power supply, and am able to perfectly tune the clock, the 15.75v supply [thanks for pointing up the previous typo], 5V is 4.95-96v for both + and -, 12V seems high at 12.305. There is one hot polystyrene cap (!!!) at 125-135 Fahrenheit, (0.02uf 200V) and I've ordered a film cap with the same spec and 400V (cheaper by 80% than exact replacement).Wondering if I should be replacing the sockets under the LD110 and LD111 DIP chips, which are the only socketed ones on the board. This is a 1979 vintage, so I don't know if I have the naughty TI sockets. The LD110 runs at about 115F. the LD111 does not even register heat.The only other things I did when I got this were to really zap the push button switches with Deoxit D5, and let it drain for a few hours before reassembling.Would appreciate any general pointers. I have two more to cal in order to fill a TM503 with meters that will give me the ability to measure three rails at once.EDIT!!! As I wrote, the meter won’t zero out and the 12v rail is high in both poles. There is a 12v zener in circuit that is hot and lighting up the Flir like a meteor. I’m thinking a swap. Thoughts?Thanks!

author: m k
author_rank: Super Contributor
timestamp: July 29, 2025, 07:23:48 pm
content: Glowing zener can't be very healthy, but LD110 is counting numbers, so it has at least a possibility to be more sane.

author: JohnTinkers
author_rank: Contributor
timestamp: August 04, 2025, 10:52:43 pm
content: Quote from: m k on July 29, 2025, 07:23:48 pmHi. I replaced the Zener with a 1W 12v zener and it runs warm to slightly hot now. I have recapped the unit and that has had minimal effect on zero out with no strap. However with shorting strap I can BARELY zero out the meter as specified in adjustment. So I can get all the way through adjustment within spec. I thought of replacing the zero trim pot (100k) with a 10-turn Bourns but decided not to do so. At this point since it measures loads OK I decided to put it into service and revisit it next year.ThanksJohn.

author: m k
author_rank: Super Contributor
timestamp: August 05, 2025, 01:07:29 pm
content: For 17.5V supply do you mean 15.75V?Where's the zener?I found one setting L111A V1 pin 16, it's fed through 160R from 15.75V and is a 0.4W thing.So 17.5V gives it a bit over the limit and 15.75V comfortably below.

author: JohnTinkers
author_rank: Contributor
timestamp: August 05, 2025, 02:25:21 pm
content: Quote from: m k on August 05, 2025, 01:07:29 pmHi -- it is 15.75 volts exactly; sorry for my typo. Fingers were working faster than brain.ThanksJohn

author: m k
author_rank: Super Contributor
timestamp: August 06, 2025, 01:48:34 pm
content: Much better then.Next time you're there check what you have between 5V and M/Z of chip pair.The pin is also connected to 74LS00, display blanking, sheet 4 bottom, maybe it's disturbing.