Time to repair a Heathkit IM-2202 Digital Multimeter - Page 1
Forum section: electronics / testgear

author: slburris
author_rank: Frequent Contributor
timestamp: August 09, 2014, 10:38:39 pm
content: I'm a sucker for Panaplex displays, so when a Heathkit IM-2202 Multimeter showed upon eBay, I snagged it. Here's what I got:It was listed as "doesn't power up". That can mean any number of things from "I never tested it" to "so many parts are missing from the inside that there's nothing left to power up". I plugged it in, and from the outside at least, it appears to be dead.It came with, uh, probes:I think these can go immediately into the trash can.So I opened it up.What's this? Batteries? And a Panaplex high voltage display? This should be interesting....Let's remove those two screws and see what we have to work with.Lots of discretes, a (presumably) custom wafer switch, and a bunch of wiring. With Heathkits, there are a lot more things to go wrong depending on the skill of the original builder. Whenever I re-open some kits I built when I was 10 or 11, I'm amazed they actually worked! Luckily, this assembly job doesn't look too bad.The brains appear to be a Mostek MK6013P. I'll bet this chip is essentially unobtainable these days. Hopefully I won't have to replace it.I can't see the Panaplex displays. I guess I'll have to take the front panel off. I at least want to make sure they are still here. Even if I can't repair the meter, the displays can probably go into other projects.Yuck, the display should be white. There appears to be a protective coating over this panel which has yellowed. It's slightly peeling off near the bottom.I'm not sure how to restore this. Should I try to peel off the protective layer completely then spray a clear UV coat on it? Is this a case of brominated flame retardants applied to plastic slowly turning things brown? I've always wanted to mix up a batch of that retrobrite stuff I've seen on the net, maybe that's the best approach here? Well, I'm going to punt on this for now until I can fix the meter.Looking underneath the front panelAha, finally, the displays. They appear to be undamaged.Next steps, find whatever documentation I can find on this meter, at least the schematic, and start work on the power supply.

author: slburris
author_rank: Frequent Contributor
timestamp: August 09, 2014, 11:26:19 pm
content: Time to break out the trusty Keysight Technologies multimeter and see if this is something simple, like a blown fuse.No such luck, full line voltage here.Well, I've wanted an excuse to try out my thermal imaging camera, so let's see if something looks amiss. Here's the real shot.And now the thermal.Looks like heat's coming from the transformer, a power resistor, and the diode which provides the charging for the batteries. I stupidly check how hot the resistor is getting with my finger, ouch!So we're not completely dead. Looks like there's a DC-DC converter in here to change 5V to +-12V and +-170v. I suspect much of this isn't working as there's no heat coming from anywhere else on the two PCBs. More debugging ahead.Scott

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2014, 11:52:53 pm
content: Scott and the group,Here is the schematic for the input section. I believe the resistor that is getting hot is R1. I suspect that batteries are shorted. This is a common problem with NiCad batteries. The batteries in this design are also the smoothing capacitor.Try replacing the batteries with a 4700uF or so, 25V electrolytic.Regards,Jay_Diddy_B

author: slburris
author_rank: Frequent Contributor
timestamp: August 10, 2014, 01:56:22 am
content: Shortly before I saw your message I removed the batteries and the meter sprang to life! Well sort of, there's a lot of flickering in the display. The 5volt supply is reading 4.5v and isn't very stable.But looking at the schematic this makes sense. A half-wave supply going directly into the batteries with no capacitor. So on AC you depend on the batteries to charge up during the forward wave and hold up the supply on the negative wave.I'd say that's a ridiculous design, but I'm sure this was a "value engineering" decision by Heath to hit a price point. I gotta think the ripple currents would be pretty punishing to the NiCads. And there's no regulation so if your line voltage was on the high side, you'd probably overcharge the batteries.So now what to do. Putting a cap in place of the batteries should work, although it would see a lot of ripple as well. I don't think dropping in NiMH cells would be a good idea either. I'm tempted to bodge in a 3 more diodes and a cap and make it a full wave bridge. Will there be enough headroom for the 5v supply? I don't know.I'm not seeing a simple way to preserve battery functionality and cure the design issues without making a lot of changes.Scott

author: edavid
author_rank: Super Contributor
timestamp: August 10, 2014, 04:58:53 am
content: It seems ridiculous now, but this is basically what Fluke did in the 8000A, and they sold a ton of those.The first thing to do is measure the transformer AC output voltage and see if you can just add a simple regulator, or possibly an LDO.Also, you can hook up a 5V DC supply and see if the meter works.

author: Vgkid
author_rank: Super Contributor
timestamp: August 10, 2014, 05:13:41 am
content: How much current does the diode bridge need to handoe? If small how about germaniums(never tried it)

author: SeanB
author_rank: Super Contributor
timestamp: August 10, 2014, 06:16:29 am
content: You could up the value of the charge resistor to around 12R and get a lower charge current into the battery, and this will actually work well to keep them alive. The cells if they are not leaking are easy to revive ( no white powder at the positive end) simply by discharging a very high current pulse into them, using a large value capacitor charged to a high voltage. I made a rejuvenator using a 470uF 250V capacitor charged to 200V and discharged into the cell via a large industrial push button switch. About 4-5 zaps and the shorts are gone for a few months.

author: slburris
author_rank: Frequent Contributor
timestamp: August 11, 2014, 04:10:04 am
content: Well if it was good enough for Fluke I guess I shouldn't complain....What would you think about putting in NiMH D-cells (not NiCd) and also putting a smoothing capacitor across wires DI/DH to cut the ripple on the batteries?Or is there some very subtle/clever point about the pulse charging of the batteries that I'm failing to understand that a smoothing cap would mess up? Why didn't Heath put one in?Scott

author: SeanB
author_rank: Super Contributor
timestamp: August 11, 2014, 05:52:55 pm
content: The ripple current ensures a smoother coating on the electrodes, as the whiskers of Cd and Ni tend to get eroded preferentially so they do not grow so fast. the only drawback is that you really have to time the charge, and have to have the peak current into the cell with the instrument off ( and charging at max rate) to be under C/10 so the cells do not go beyond their recombination rate and vent. If you make it charge at C/20 then you can leave it charging indefinitely as the cells are able to recombine the gas generated back into water and get a little warm, but do not vent and will survive this indefinitely.Unfortunately with NimH cells this is not going to work as the cells do not do prolonged overcharge well at any rate, so there you need either a charge controller or some other charge circuit to disconnect the cells when fully charged by either looking at terminal voltage dip or temperature rise when fully charged.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 11, 2014, 06:36:30 pm
content: Hi,In addition to SeanB's comments. I think that the design is counting on the battery pack behaving like a 4.8V Zener Diode to regulate the 5v supply.Regards,Jay_Diddy_B

author: septer012
author_rank: Regular Contributor
timestamp: August 17, 2014, 06:46:25 am
content: D112 failed short?