DMM troubleshooting help - Page 1
Forum section: electronics / testgear

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 12:07:31 am
content: Hi guys,I found your forum from AK... I'm into building tube amps, but I have DMM question that I was told to post here...I have a Kingdom KD55C DMM with the manual and full schematic that started to read high in all ranges today... I checked the battery, as that sometimes makes em go wacky, but that didn't help... I'd like to try and fix it, for the experience if nothing else, but I don't have much SS circuit knowledge... It is built very nice, not like many others I've seen opened up...Heres the schematic, if anyone wants to venture into this with me...Thanks in advance...

author: lowimpedance
author_rank: Super Contributor
timestamp: November 02, 2015, 12:48:23 am
content: Interesting old timer, not much to it ! 3 IC's and a transistor. maybe some interanal pictures would be nice (prefrerably in focus!!).Reading high in all ranges ... is that also all modes ?.Anyway most likely the main 40 pin IC , which is almost certainly an ICL7106 (search the net, plenty of info there and these are easy to come by), is suspect.There is a dual opamp in there but its unlikly to be dead unless there is no AC volts reading.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 12:54:18 am
content: More info fromhttp://www.diyaudio.com/forums/equipment-tools/281879-another-dmm-problem.htmlIf the 7106 is socketed, take it out and clean the contacts.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 01:00:54 am
content: Thanks for replying,,, Yes it reads high in all modes,,, for instance iI just tested 3 1% WW resistors with it 100r, 1K, and 10K... as the DMM won't zero, if I add the starting reading to teh reult, it comes out exact,,,IE: reading 10k R, shorting teh leads starts at 10 so it reads 1.10... the resistors are all right on, tested with a different meter...Heres an inside pic, hope it helps...

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 01:08:38 am
content: Quote from: retiredcaps on November 02, 2015, 12:54:18 am

author: lowimpedance
author_rank: Super Contributor
timestamp: November 02, 2015, 01:09:56 am
content: Looks like you may have to unsolder the LCD to then remove the 7106, (need a pic of the top side to see if sockets are used).One other thing to do with the other meter is check the resistors in the KD55 just to be shure non have been damaged.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 01:17:27 am
content: There is a lot of solder flux residue over in the 7106 area? It wouldn't hurt to give the pcb an IPA bath and let it dry thoroughly. I had a Fluke 77 that had incorrect readings and wouldn't zero as described below. An IPA was all that it needed.https://www.eevblog.com/forum/chat/fluke-77-ii-incorrect-readings-on-all-ranges/

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 01:19:43 am
content: BTW, try not to have the tape holding down the 9V battery touching the pins of the 7106. They may be affecting the readings?

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 01:25:12 am
content: Quote from: lowimpedance on November 02, 2015, 01:09:56 amThe board is not out of the meter, however I can see the LCD and 40 pin chip are soldered in... if you think that is the problem, I don't think I can remove it, I can get by with P2P wiring, and have rebuilt a preamp board in a SS amp... but these chips are way too small for my old eyes!I read your Fluke thread,,, I got this DMM about a month ago, and it seemed to be very accurate til today,,, no dirt or grime, like you mentioned in the Fluke thread....

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 01:27:59 am
content: Quote from: retiredcaps on November 02, 2015, 01:19:43 amI'll move teh tape,, I just now put it there to keep the batt from dangling,,,

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 01:40:52 am
content: From the diyaudio thread ...Quote from: knockbillDid you have an oops moment when measuring the ACV? Was the meter set to ohms when you tried to measure ACV?

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 02:12:17 am
content: No,,, this meter is supposed to protect from that, it has the fuse, and a zener, according to the manual,... I was using it on the ACV mode, and it started reading high...I just checked it again, on ACV,,, if you start with what the DMM has on the display, and subtract it from the reading, its right on... does it on DCV also... I cleaned the solder flux around the 40 pin chip also, still the same...Is there a specific resistor, or votage regulator that is keeping bit from returning to zero?I found I can use the 500k pot and calibrate it on the 20VAC band, for instance and get it to match the reading on a different meter, but it still doesn't zero...

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 02, 2015, 01:54:17 pm
content: Never underestimate the conductivity of PCB surface contaminants and the power of a good cleaning with IPA. The A/D converter is 2000 count, 200.0mV full-scale. The input impedance is about 10Meg, which works out to 10pA per count. Obviously it doesn't take much leakage current to cause a significant error.Regarding oops moments: The fuse is too slow for anything other than over-current situations, when the current-measuring shunts are engaged. The ohms range of this meter is protected by a couple of zener diodes and a PTC thermistor. The value of the thermistor is not noted in the schematic, but we'll guess it's probably between 1K and 2K at room temp.Since the ICL7106 seems to be basically operating, except with offset errors, it might still be OK. Some checks you can do, with the meter on, set to 0.2V DC, and nothing connected to the input jacks:1) The voltage measured with another DMM between the COM jack and the positive battery terminal should be 2.80V. This is the only "voltage regulation" present in this design.2) The voltage on pin 36 of the 7106 should be very close to 100mV. This is the A/D reference voltage.3) Temporarily jumper pin 31 (A/D input) to the COM jack (ground). The KD55C display should indicate all zeros.4) Jumper pin 31 to pin 36, causing the A/D converter to integrate it's own reference voltage. The KD55C display should indicate "1000", plus or minus a couple counts. The decimal point is irrelevant.If (3) works to get a zero display, there may be current leaking into the front end. This usually comes from contamination on the board or in the switch. If (3) and (4) fail, the auto-zero cap (0.47uF) may be leaky. Replace with a good-quality film cap. I'd replace the 0.22uF integrate cap at the same time, since both caps are the same age and type.Calibration: This meter has two basic DCV adjustments, VR2 (300R) tweaks the A/D reference voltage, and VR3 (500K) tweaks the voltage divider. My recommended procedure to calibrate would be:1) Set meter to DCV 0.2V range. Connect the input jacks to a source of 190.0mV. Adjust VR2 for a display of 190.0.2) Set the meter to DCV 2V range. Connect the input jacks to a source of 1.900V. Adjust VR3 for a display of 1.900.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 03:12:42 pm
content: Thanks for taking the time to outline this procedure,,, I will print it out, make the tests and get back to you,,, When you mention Voltages as in Test #2,,, is that in reference to Battery negative?Is any DMM suitable for these tests? I've read where some meters have too high an operating V for some SS testing ...

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 02, 2015, 04:02:59 pm
content: The COM input jack is tied to circuit ground, so make voltage measurements relative to that. I usually just clip my ground lead onto the 10A current shunt. A DMM is appropriate, the higher the input impedance, the better.I suspect what you have read is that some DMMs have a fairly high open-circuit voltage on their ohms and/or diode test ranges, which can turn on semiconductor junctions and maybe in a rare case damage something, if you are poking about measuring in-circuit resistance values.Please be careful probing or jumpering nodes while the meter is on. There is some chance of damage from accidentally shorting adjacent pins and so forth.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 04:23:15 pm
content: I made the tests in order, with the only DMMs I have working.. not sure they are accurate enough for lo voltage readings, however 2 different meters got the same result... I am assuming pin 40 is on the same side of board as power switch, with meter facing flat down on bench, display to top so pins 1 and 40 would be towards right...1.. The 2.80V reading was 3.01, 3.04,,, as expected the rest were way out also...2.. 100mV was 549,,, 550mV3.. Display fluctuated,,, no zeros4.. pin 31-36 display reads 46.8 and doesn't change when jumped...That's as far as I got...I removed the board from the case, seems very easy to work on, if you can see better than me!!

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 02, 2015, 04:37:36 pm
content: The 3.01V reading may be OK, some 7106 variants do regulate at 3V instead of 2.8V.The 550mV reading on the voltage reference is way way too high to believe, please double-check your pin counting. I marked them on your photo in my post.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 05:04:42 pm
content: Ah,, I read the wrong end of 40 pin chip! the schematic shows pins one and 40 on same side as power switch,,, I should have figured the notch meant something!!!We may get this guy working,but I'm sticking with tubes and P2P wiring...Thanks again for all the help, I'll check again, and post results...

author: SeanB
author_rank: Super Contributor
timestamp: November 02, 2015, 05:12:49 pm
content: Clean the board with alcohol and dry thoroughly, then get some contact cleaner into those 2 switches, as it is likely one of the contacts is dirty. If the input pins of the converter are open it might still read but will read high. Those chips rarely fail as faulty, they either work or they cook, never reading high or with an offset unless the capacitors attaches to the IC are faulty. Check the solder joints on those 3 green mylar capacitors and the 2 brown mylar ones, if one of the leads has a dry joint (the 0.1uF green ones in the diagram) or the capacitor is shorted then the chip will not auto zero, and will read high or low depending on the particular chip.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 05:57:19 pm
content: Quote from: SeanB on November 02, 2015, 05:12:49 pmI cleaned the board w/ alcohol, and swabbed the switches with contact cleaner... I'll give it another round...

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 06:02:34 pm
content: Just tested again, on proper pins this time! Results much better...1... 3.03VDC2...A/D .993... Displays all zeros4... 31 to 36 ... 998If you think its capacitor time, I will need to get them, don't have any LV caps in stock... probably need to replace them to move fwd...Thanks again...

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 06:32:00 pm
content: Quote from: ModemHeadQuote from: knockbill on November 02, 2015, 06:02:34 pmIf I'm interpreting modemhead's tests and your results properly, your problem is somewhere on front end or switches (some contamination) since tests 3 and 4 are successful implying the 7106 and its capacitors are working fine.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 06:38:54 pm
content: See modemhead's method for "dunking" a meter in IPA.https://www.eevblog.com/forum/testgear/old-fluke-multimeters/msg633201/#msg633201I do the same and let it dry thoroughly before reassembly. A simple PC fan will help it dry faster.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 07:04:18 pm
content: Are you saying the 5 film caps are apparently working as designed? If so then a aggressive cleaning is in order?If so, I have no problem proceeding in that direction... I was going to try to replace caps, but don't have them on hand at 16V....

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 02, 2015, 07:11:18 pm
content: Yes, the readings indicate the 7106 is working as expected, and so is the integrate and auto-zero caps (the brown ones). But something is allowing a voltage to develop on the A/D input (pin 31) that isn't coming from the meter's input jacks, causing an offset reading.So we still have some stray contamination somewhere, or as Sean suggested, the switch contacts, the 0.1uF green caps, and bad solder joints. There isn't much else left! And I'd go with agressive cleaning first.I don't think you've mentioned yet, exactly what does the display read with the input jacks shorted, in the 0.2V DC range? Also, if you parallel another working DMM with the KD55C, does it read about the same non-zero value as the KD55C? In other words, can you actually measure an offset voltage at the input jacks of the KD55C?Edit: clarity.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 07:13:37 pm
content: Quote from: retiredcaps on November 02, 2015, 06:38:54 pmAppreciate the link...Modemheads work is truly impressive,... I think I'll get a little more aggressive cleaning the PC board...Thanks to all that contributed, so far....

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 07:16:53 pm
content: Quote from: knockbill on November 02, 2015, 07:04:18 pmSeanB asked you to check the soldering on those caps. I'm not sure if you have done that yet?QuoteAll three of us, modemhead, SeanB and I have recommended a complete IPA bath. SeanB and modemhead also recommended cleaning the switch contacts.All three of us buy used/abused meters for fixing up and many of the fixes are simple IPA baths (as I documented with my Fluke 77). Modemhead's blog only shows one or two IPA documented cases, but I know firsthand he has fixed many more with a simple IPA bath, but chose not to document them as they are repetitive.PS. I see modemhead has replied while I'm typing this so I will leave my comments above unedited.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 07:22:47 pm
content: Quote from: knockbill on November 02, 2015, 07:13:37 pmA clean used toothbrush is what I use to scrub the board with while it is soaked in IPA.Quote from: ModemHeadIt took me a long time to appreciate and understand the orders of magnitude in the above comment by modemhead.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 07:27:16 pm
content: Quote from: retiredcaps on November 02, 2015, 07:22:47 pmAFK... aquirring toothbrush Alcohol and contact cleaner... will return shortly!!!!

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 07:31:41 pm
content: Quote from: knockbill on November 02, 2015, 07:13:37 pmThis blog entry on the 7106 will help you understand how your meter works (and likely the Extech MN35) ...http://mrmodemhead.com/blog/dmm-basics-dual-slope-adc/BTW, in that same old fluke meters thread, you will see how the 7106 originated.

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 07:36:01 pm
content: BTW, if you have an ultrasonic cleaner, seehttps://www.eevblog.com/forum/testgear/fluke-187-reads-2-ohms-high/If not, I think Harbor Freight sells them. On sale for $32.99. Haha!http://www.harborfreight.com/ultrasonic-cleaner-3305.htmlDon't forget to get your free meter while you are there.

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 10:20:26 pm
content: Quote from: retiredcaps on November 02, 2015, 07:31:41 pmHA,,, that Extech is pure garbage compared to this one!!! The HF free DMMs are better than it was... To quote our friend, there truly are no "user serviceable parts " in it!!! I don't think it even had the same 40 pin chip, rather just a blob stuck to teh PCB, IIRC...I only go to HF to get a free meter, or return a defective one!!!!Modemheads work is way over my head!! Most guys I know are engineers of some discipline, I was just a electrician for 45 years... I guess they put up with me because I always start out saying " I can fix or build that"!!!!I want to get back to the twin triode amp I was building... I have it laid out on a test mule, testing turns ratio for an output transformer, when this meter went wonky... Hope to get it sorted out quickly, not that I need it but it will bug me until it gets fixed!

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 10:32:07 pm
content: Quote from: knockbill on November 02, 2015, 07:27:16 pmOK,,, I scrubbed and dried the PCB, (I keep wanting to say chassis!) over the last couple hours,,,, powered it up with the same results as before, it won't return to zero, but it sure is shiny... I can change the caps we talked about, but I'd need to get them... Just have 450-630V in stock... These should be 16V (25-35?)I believe? Is this the next step, or are there other thoughts?Thanks for hanging in this long!!!

author: retiredcaps
author_rank: Super Contributor
timestamp: November 02, 2015, 10:56:48 pm
content: Quote from: knockbill on November 02, 2015, 10:32:07 pm1) I think you should follow SeanB's suggestion about cleaning the contacts as he wrote earlier (see below).2) Check the solder joints on the caps mentioned by SeanB. If in doubt, you can always touch/reflow them with a soldering iron.3) Check to see if the capacitors are shorted using another meter.Quote from: SeanB on November 02, 2015, 05:12:49 pm

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 11:06:34 pm
content: Quote from: retiredcaps on November 02, 2015, 10:56:48 pmOK,,, I went thru the switches and contacts 3-4 times with contact cleaner,,, checked the cap joints under high magnification, figured rather than heat the joints twice, I'd just replace them,,,not real comfortable messing with PCBs too much... I can check em for shorts, tho....I'll measure them, and get back... thanks...

author: knockbill
author_rank: Regular Contributor
timestamp: November 02, 2015, 11:35:02 pm
content: Tested the 5, actually 6 film caps for shorts, at all R ranges on the DMM, all read open... so, I reflowed teh joints and no improvement...If the switches are still dirty, disassembling them to polish the contacts would be pretty hard,,, I flooded them with contact cleaner and exercised them as it dried many times... if one is open, I don't know how to test it...Guess we're running out of options?

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 12:01:50 am
content: Although anything connected to the 7106 analog section is of course suspect, I've seen nothing so far to actually indict any of those caps specifically. And since they are film caps, not electrolytics, they're probably rated for at least 100V maybe more.So if you're up for a little more trouble-shooting, let me repeat something I asked earlier that may have been lost in the shuffle:Exactly what does the display read with the input jacks shorted, while in the 0.2V DC range? And with input jacks open? Consistent, steady values?Also, if you parallel another working DMM with the KD55C, does it read a non-zero value as well? In other words, can you measure a real offset voltage at the input jacks of the KD55C?

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 12:16:31 am
content: Quote from: ModemHead on November 03, 2015, 12:01:50 amI'm getting old!!!! I took those readings the 1st time you ask, they are sitting next to the computer, and I forgot to post them!!!!.2 range, pins shorted... reads 41.9-42,,, pins open...reads 3.8,,,,, the test meter on the open pins read 2mV, however its a cheap HF meter, and they aren't known for real accurate low readings... It does read zero if left on the 20V range...Sorry for the wait....

author: retiredcaps
author_rank: Super Contributor
timestamp: November 03, 2015, 12:34:53 am
content: Quote from: ModemHead on November 02, 2015, 01:54:17 pmThis item also got a bit lost in the mix of things.The PTC, red component, right next to the COM jack and current shunt should measure between 1k and 2k ohms as modemhead pointed out. Please measure the PTC and report your reading.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 12:39:52 am
content: Good catch!! Reads 1.98 on 20k range, so just between 1-2K....Thanks for trying, tho!!! We'll get it....

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 01:11:16 am
content: A larger reading with the leads shorted? That's quite bizarre. I assume the reading is positive?The following may be a bit tedious, but it should help to isolate the source of the offset.There is a 1Meg resistor in series with the 7106 pin 31. I cannot locate it in the photo, but you can probably find it easily by tracing from pin 31. Remove this resistor temporarily. This breaks the input from the "front" section, ie. the switches, voltage divider and so forth. If you turn the unit on at this point, it may display just about anything because the A/D input is open, except for it's filter cap. Earlier I asked you to ground the pin hard, which did zero the display. But this time, use a 10Meg resistor to ground it temporarily. Arrange to keep your fingers off of it, maybe tack solder it between pin 30 and 31, since pin 30 is actually the input "low" pin.If it settles on zero, we know this pin is no longer influenced by some fault current, and the problem is ahead of the 1Meg resistor. If it will not settle to zero, we know the fault current has to be coming from some path really close to the 7106 A/D chip. Or worse, from inside the chip...

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 01:20:25 am
content: I'll retest the pins reading and post the results,,, if no change,,, try the other test tomorrow,,,I really appreciate your perseverance with this, I'd like to save this meter...

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 01:34:11 am
content: Retest... DCV pins open 39.8-40 shorted 3.7...I'll study the schem and try to locate the 1M... Post results after test...Thanks....

author: lowimpedance
author_rank: Super Contributor
timestamp: November 03, 2015, 02:10:36 am
content: Quote from: ModemHead on November 03, 2015, 01:11:16 amknockbillHave you removed the PCB from the case ?, does the soldering show the same amount of flux residue on the 'top' side'?. Photo of that other side!.I wonder if someone else has had a go given that large amount of flux left , and since the 7106 is buried under the LCD there still maybe some contamination there.Although given that it was working then went 'wonky' with the fairly stable fixed offset which has not been affected much by cleaning (?) my guess is more likely a component failure. hopefully not the 7106 but..Complete Modemheads test with the pin isolated and tied with the 10M resistor to see if the current path is external or internal

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 12:21:01 pm
content: Will post a photo of both sides as its out of case,,, Thats the plan, I will do teh 10M test today and post findings,,, If its teh main IC, I'll need to find help replacing it,,, I don't have teh tools or eyesighte at this point to tackle that job,,,,Thanks for the reply....

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 12:57:42 pm
content: Heres the latest pics after cleaning...

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 01:42:37 pm
content: Quote from: ModemHead on November 03, 2015, 01:11:16 amIn studying this post, is it possible that the greater reading is from something putting positive V the neg/ground lead, or resistors failing at lower range(which they never seem to do), or the caps leaking and passing V at idle (open leads)?Hate to think its the main chip after all the time we spent on this, but even so, would want to replace it, if possible...

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 02:26:54 pm
content: Quote from: knockbill on November 03, 2015, 01:34:11 amYour re-test showed the expected results (offset voltage lower with the input jacks shorted.)The meter has an input impedance of 10Meg with the leads open. A tiny current can develop a voltage across that 10Meg which is registered on the display. Dropping that 10Meg to zero pretty much has to decrease the voltage, unless there is some really bizarre thing going on that lead to an increase in fault current with the leads shorted.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 02:59:56 pm
content: I think I'm beginning to get punchy!!! The 1st test was correct (post 37), lasts nights retest was wrong, I must have transposed the figures,,,Retested (3rd time) just now and find open test reads 3.7.... However, I noticed a new "twist'... the measurement was made with open jacks, IE no leads plugged in,,, I plugged in leads to get short reading, and the display reading shot up, passed OL reading in range (.2), to settle on a "1" on display which relates to "out of range" reading on this DMM... In handling teh probes, the readings would fluctuate, from 60 to OL,,, so, I placed the leads parallel to each other on the bench and as they moved closer together, the readings would increase, did the same thing if I held leads in my hands... added capacitance, I guess?As I just now noticed this, I wanted your opinion before moving on with the 10M shunt test...This would be a lot easier if we were in the same room!!!! Forums are great, but its very rare to have the opportunity to meet and work with each other, very frustrating dilemma...Sorry I posted incorrect figures last night, too close to bedtime Iguess!! After all that's all you have to go on,,,Really appreciate the time and knowledge you offered, hope it comes to a positive conclusion,, Soon!!!!

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 03:41:54 pm
content: Yes I'm no good at remote trouble-shooting, and more than a little confused now on the evidence so far.If the DMM is in a fixed 200mV range, it would be quite normal for it register totally random values with the leads plugged in, after all it has a 10Meg input impedance and you've just attached two antennas.What is not normal, is for it to display a constant (or fairly constant) repeatable reading with no leads plugged in, or a reading that is not very close to zero with the input shorted. This is the situation I thought we had, but now I'm not sure.Do you have a shorting plug or a short banana-to-banana patch cord? Use that to short the inputs in a reliable manner, and if it is not within a few counts of zero, we still have a problem.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 04:05:58 pm
content: Sorry for the confusion,,, With meter in .2 DCV range and V and Common jack shorted we have steady 3.8 reading....

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 04:30:32 pm
content: Quote from: knockbill on November 03, 2015, 04:05:58 pmOk... Does this reading (in terms of counts, not volts) persist in the higher ranges?

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 04:44:15 pm
content: Readings DCV mode... .2= 38.0,,, 2=.070,,, 20=.38,,, 200=3.5,,, 2000= 035(note no decimal point),,, 20M=.35

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 05:16:11 pm
content: Quote from: knockbill on November 03, 2015, 04:44:15 pmBelieve it or not, those numbers actually make sense. I think I can say that you have about 3.5 to 3.8 nA of leakage current at the 7106 A/D input. Due to the 1Meg coupling resistor and the way the voltage divider is arranged, and with the input jacks shorted, the impedance to ground from that node will be 1Meg in the 200mV range, then it doubles in the 2V range, then goes back to about 1Meg in the higher ranges.The only question is where this tiny current is coming from. It is technically possible it comes from within the 7106 chip itself, but with just playing the odds it is more likely to be coming from an external source. I have seen these leakage paths persist through several IPA baths before. You really need full access to top and bottom of the 7106 chip, to scrub the hell out of the area around those analog pins. And the chip itself as well!

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 05:29:24 pm
content: Thanks for the info,,, I can soak it rather than just scrubbing it with contact cleaner and blow drying it,,, not sure how to clean under teh chip, without removing it, other than a soak... may be able to fish a pipe cleaner under it... is it safe to soak the display? I will give it another go, any special detergent to use, or not for that sake? I don't think it is actually as clean as it could be yet...I have been studying the 1M in series with pin 31, and haven't been able find it yet, the traces from the chip run under it,,, figure it connects to a .1 cap,so i may be able to find it with an ohmeter, tho...I'll give it another cleaning in the mean time...

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 05:38:16 pm
content: I personally would not use contact cleaner, I don't trust that stuff not to leave a film of some kind. Only 90% or better isopropyl alcohol. I think it's OK to get the LCD wet, if anyone else reading this has an opinion on that, please chime in.As for the 1Meg resistor, your numbers do support the idea that there is a leakage path to a positive voltage on the 7106 side of that resistor, so if it's location is far away from the chip, there is more area to cover.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 07:43:16 pm
content: OK,,, I read some about PCB cleaning, and took the plunge,,, seems water, alcohol and detergent is a viable combination... Not having any 90+ IA, I used the 50% I have and mixed in some Dawn,,, and scrubbed it very well, this time and rinsed it well with water...... It came out shiny, no blotches as before, the solder joints sparkle... I used a pipe cleaner, and got the chip pins and under the display as well as possible... blotted it dry with a cotton cloth and its been in front of a fan for over an hour... turned it a few times to get air thru teh switch "tunnels" Even cleaned the case, as I expect it to work, sooner or later!!! I'm sure any remnants of contact cleaner are gone now...I will assemble it later, and we'll see if the stray V changed, if not, I guess its not a high resistance short from dirt, so we'll do the test for a hard part failure...

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 10:01:04 pm
content: I air dried the PCB and rechecked it DCV .2 range,,, .2=82.3,,, 2=165,,, 20=.78,,, 200=7.4,,, 2000=72,,, 20M=.72 Seems numbers changed after cleaning, but stayed somewhat relative...Found and lifted 1M, readings fluctuated until reaching OL on DCV .2 scale....Tacked 10M to pins 30 and 31... no zeros, display reads 41.9-42...

author: lowimpedance
author_rank: Super Contributor
timestamp: November 03, 2015, 10:25:47 pm
content: Did the other side have the same leftover flux deposits when you took it out of the case (pre cleaning) ?.Does the offset reading change or fluctuate when the PCB is lightly flexed and twisted along its length ?.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 10:33:48 pm
content: The PCB looked about the same on both sides before cleanings,,, now it has no blotches or deposits on it after the alcohol/Dawn scrubbing, just squeaky clean... I will try to twist the PCB and look for changes,,, may be a false reading tho, with the high sensitivity of this DMM, touching it sometimes moves the readings around... I will try it and post back...

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 10:40:51 pm
content: Its in DCV .2 range reading 36.6,,, twisting it allowed it to move about a tenth,,, the board is very rigid, as the two switch tunnels run diagonally across it...

author: lowimpedance
author_rank: Super Contributor
timestamp: November 03, 2015, 10:51:03 pm
content: Okay , was just checking to see if there was a dodgy plated hole/joint.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 10:55:42 pm
content: Thanks fro thinking! Thats a trick I've used to find intermittent in tube amps,,, found a wonky slide switch that way... drove me crazy, til Iwent to move the amp by one corner, it flexed and found the problem, been doing it ever since!!I would like to find the gremlin in this DMM!!!

author: ModemHead
author_rank: Frequent Contributor
timestamp: November 03, 2015, 10:56:17 pm
content: Isolating the A/D input with a 10Meg load pretty much confirms the previous data. It takes 4nA to develop 40mV across a 10Meg load, and that current has to be coming from somewhere. The only other component on that node is the 0.1uF going to ground, so it can't provide the current. If you're convinced it's all clean, the one option left is to condemn the 7106.ICL7106 chips are still available if you're up for a replacement. It will not be easy for a novice at PCBs. Due to the construction it looks like the LCD has to come off too? Some practice on a piece of junk electronics would be in order.I have to mention that in monetary/practical terms, this unit is not worth the effort, but I'm in no position to condemn anybody for wanting to fix something.

author: lowimpedance
author_rank: Super Contributor
timestamp: November 03, 2015, 11:11:46 pm
content: Do you know anyone else in the same hobby etc that might have the tools (good iron and solder sucker) and might help?.removing the LCD without cracking the glass is the most tricky part and if that is successful the 40 pin IC would be easier.Perhaps putting it aside until you have got some more experience with PCB work assuming you dont want to scrap it.

author: knockbill
author_rank: Regular Contributor
timestamp: November 03, 2015, 11:24:15 pm
content: Well, first of all thanks for all the time and education you gave me... I wouldn't have had a clue how to look into this... I did notice the display readings started to come down since I cleaned it... I'll let it sit and try it again, as those switch tunnels may be still damp......As far as replacing the chip, I don't have the vision to do it now, even I had the tools,,,, altho, if I run into to someone local who is game to try, this would be a good candidate... The display would have to be raised for access, maybe just one side tho, if thats possible...I'll post back, if I get to do it,,, Glad to have found this forum... there aren't many test gear forums I've come across, and I try to rebuild/restore teh gear I have so its useable...Regards,John

author: knockbill
author_rank: Regular Contributor
timestamp: November 04, 2015, 03:43:41 pm
content: Quote from: knockbill on November 03, 2015, 10:01:04 pmPS:: numbers change because 10M was still in ckt...