Broken Wavetek DM15xl - Page 1
Forum section: electronics / testgear

author: extervision
author_rank: Newbie
timestamp: December 04, 2013, 12:18:32 am
content: Hello,I am trying to repair a wavetek dm15xl multimeter, it doesn't seem to display accurately. Just want to use it as my second/field multimeter (my protek 506 is to big to carry around and depletes the battery very fast).The story: I bought it used(and broken) at a local fair for a whooping 4usd, it had rust on the battery contacts (possible battery leakage). Clean it up and woohoo, it powered on.But at a closer inspection it didn't seem to display accurately so i started investigating.-On volts range(ac and dc) it doesn't zero, it adds a 0.2 - 0.6 volts to almost all ranges(at 1000v it adds 4-8 volts)-On ohms it adds a few least significand digits to my masurements(even when shorting i get about 3ohms on 200ohm range)(it also has a weird 2000Mohms range but it stays between 45Mohms - 250Mohms when not measuring)-The amps ranges seem to be a bit off to.After letting it powered on for a while the offset is lowering to a few tens of milivolts, but after powering off and then on it reads higher values again.The repair investigation(so far): Main IC is a tc7106, date codes 94-95, 2 board construction(Main board + Banana jack small board)-Current draw seem ok about 1,9ma in idle.-The protection ptc seems ok(measured 1.8kohm)-10Amp fuse was blown and somebody wound a copper wire around the fuse :-)-Tested a input resistors, and only 2 of them seem off (or in parallel). Also somone mouned 2 resistor instead of one but theyseem fine. Actualy he mounted 5% (crappy) resistors in place of probably 0.5%(all other resistors in that area are 0.5%,and the precision thick film resistor is there). I will investigate on this further.-There is an adjustable resistor near the main IC, played a little with it but didn't seem to affect measurements at all.The Multimeter: looks in very good condition despite the obvious battery leakage witch didn't affect anything but the battery contacts.The previous owner apparently had a repair attempt, blown fuse and replace of a precision resistor with crappy 5% ones;so something bad may have happened to it.Other than the user manual I coudn't find any else info or service manual to help me repair this mutimetter, so any info will be appreciated.It would be nice to have this working for field and second multimeter use, the continuity test is amazingly fast, can't even sense a delay(my protek takes about 1 second to beep), the measurements are pretty fast(unfortunately not accurate), and 2000Mohms range.Thank you

author: ModemHead
author_rank: Frequent Contributor
timestamp: December 04, 2013, 02:27:00 am
content: Offsets such as what you describe are often caused by PCB contamination. If someone's been soldering in there, undoubtedly flux residue is present.First order of business is to carefully remove the LCD assembly, then dunk the board in a tray containing about 1/4" of the strongest IPA you can find (91% or better.) Use a brush (toothbrush, acid brush, etc.) to scrub the board and work around components. Clean both sides, the rotary switch tracks look grungy. Allow it to dry in front of a fan for an hour or so before re-assembly.

author: lowimpedance
author_rank: Super Contributor
timestamp: December 04, 2013, 02:30:43 am
content: Do the clean Modem head suggested before all else. Then;If all other parts look okay, see if you can pick up an ICL7106 easily and cheap and do a swap, (just chop the old one out remove the pins and solder wick the holes clear). probably not worth any more time than that.

author: extervision
author_rank: Newbie
timestamp: December 04, 2013, 03:52:12 pm
content: Yes, thorough cleanning seems to be the best ideea.When i first opened the unit it was contaminated in some places especialy the rotary switch contacts,and because of that the beeper made strange noises when I was moving the rotary switch.I've cleaned the contacts with IPA solvent but not very thoroughly.I shall do that tonight and let you know how it goes.If it's not working I'll probably buy a 7106 replacement and replace it, it's about 4 USD localy.Thank you for the answers.

author: extervision
author_rank: Newbie
timestamp: December 04, 2013, 09:06:16 pm
content: Hoorayit works now, volts show a max of 2 milivolts, and ohms stay at 0,2ohm when shorted.excelent. ThanxThe contacts at the rotary switch, still look a bit imperfect, sometimes seems to recover slower after measutrment.Anyway it works very well, Thank you for the help.