GB GDT-3190 Reads low in all modes... - Page 1
Forum section: electronics / testgear

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 11:10:02 am
content: Thanks for the continued interest, as I too would like to learn from these projects...This is another of the DMMs I got Sunday,,, seems its still a currently available model... but other than a simple instruction manual, no other paper work is available... will post interior pics after camera battery recharges...It reads a 9V battery at about 6+ VDC,, approx the same % low reading on ACV, DCV and R modes...So far, I've replaced with fresh battery, and swapped out RT1 which reads 1200R with lower value resistors,,,, that allowed the readings to climb incrementally, but would go past teh known reading and just run away (I put the original. thermistor back in).. my thinking here is it needs the limiting factor of a thermistor,,, but I could be wrong...More info to follow...

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 12:58:29 pm
content: Pics as promised... Other than the thermistor, rubber protective jacket, larger display(not pictured) and push button power switch... this board looks remarkably similar to the cheap/free HF meters flooding the market,,, I got it because I am familiar with the Gardner-Bender brand, and I only was able to check continuity...I can make sample readings with known V and R sources if wanted...Thanks for the input...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 19, 2016, 03:47:22 pm
content: Yes, this meter is essentially the same electrical design as any little rectangular "830" cheap-o such as the HF freebie. It is missing the current ranges, and thus the current shunts and glass fuse. Looks like they added an auto-off timer which is unique. We can probably all agree that the value in a repair effort on this unit would mostly be as a learning exercise. (Nothing wrong with that.)Your initial report on the symptoms is a bit confusing so some basic readings on known values would help. A 9V battery, and a 1K resistor, for example. And on DCV, does it zero out with nothing connected? Does it read zero or close to zero when the probes are shorted in 200-ohm range?Also, set the meter to 20V DC. Disconnect the black probe. Take the red probe and touch it to the meter's 9V battery positive terminal. Does it read 3.0V?

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 04:56:28 pm
content: Here's some test readings,,, 1st column is the test meter, 2nd the GB we're troubleshooting...NOTE: readings taken with GB are with adjustment pot set for max reading, they are lower with pot set at 1/2 turn...

author: The Soulman
author_rank: Super Contributor
timestamp: October 19, 2016, 05:34:48 pm
content: Have dealt with similar meters, on those (and 98,7% chance on this one) the trimmer is used together with r21 and r22 to form a voltage divider to get the reference voltage for the chip, most times 1 volt.Measure for that with another multimeter, if it is higher,follow around the pcb traces in that area, make a little schematic and measure the voltages at r21, r22 and the trimmer, write the numbers down, do the math and you'll find the culprit!

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 05:50:16 pm
content: Sounds like a plan.. not really sure what to read... are these readings to ground? Are we looking for an out of value resistor?

author: The Soulman
author_rank: Super Contributor
timestamp: October 19, 2016, 05:53:06 pm
content: Quote from: knockbill on October 19, 2016, 05:50:16 pmYes, black probe goes in the com terminal of the faulty meter.

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 06:04:11 pm
content: OK,, I'll give it a try... Thanks...

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 06:49:03 pm
content: I posted a schem and measurements of what I think you mean,,,reading taken to Grd connection of DMM being tested... Numbers in () are marked on the resistors...Just updated pic to include trace to chip... (.14VDC)... Is this where 1VDC should be?Thanks for the input..

author: The Soulman
author_rank: Super Contributor
timestamp: October 19, 2016, 09:02:41 pm
content: Quote from: knockbill on October 19, 2016, 06:49:03 pmYour schematic makes perfect sense, a typical voltage divider as aspected, "to display?" connects to a stable voltage source, could be inside the chip or made with some discrete components.QuoteWell, that is the spot but seeing this numbers it would be save to say it requries a 0,1 or 100mV reference voltage, not uncommon.R22 seems to be a issue, the code identifies as 10Kresistor and the measured voltages seem to support that, but measured 2020?, could be a in circuit measurement error.Replacing R22 with a 20K(2002) resistor should fix it, a standard 1/4W bodged in should also work.

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 09:19:24 pm
content: Thanks for replying,,, If the R code identifies it as 10k,, how would doubling (20k) be proper? Just trying to learn a little here, as this isn't a very high end DMM, but the lesson is priceless!!!Glad you said use a "regular" R, as the little square ones are really hard to see! I needed 2 pair of glasses and a magnifying glass to get the info I posted!!! By the way, what do I google to find the codes for those little square resistors?I really appreciate all the help I've gotten on this forum, thanks again..

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 19, 2016, 10:31:07 pm
content: I doubt there is anything wrong with R22, other than the code was read upside-down and it really is a 2K resistor (2001).The voltage reference on this meter should be 100mV (0.100V). I would use the working DMM and just set it to 100mV for now. It can be tweaked later if the GB is repaired.The "stable voltage" from which the reference is derived, is 3.0V generated by the COB blob, and appears between ground (the COM jack) and the positive battery terminal (V+). The 5.10V reading you made with the GB itself is way out, so please check that with the working meter before continuing. If the COB is not producing the 3.0V, this meter is probably toast.Otherwise I suspect the voltage divider resistors may be the root of the problem...

author: knockbill
author_rank: Regular Contributor
timestamp: October 19, 2016, 11:13:27 pm
content: I read the "stable voltage" again with 2 separate DMMs (COM terminal to battery +, with the GB meter turned on and in 20VDC range),, one reads 6.93VDC, the other 6.88VDC... so I guess this is the stopping point?

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 19, 2016, 11:49:18 pm
content: I suppose there is a chance that a fault external to the chip could screw up the voltage regulation, but I'd be hard-pressed to find it without having the meter in front of me.Or I could be wrong, but I've never seen one of those cheap ICL7106-clone COB meters that did not regulate 3.0V between COM and V+. The original 7106 uses 2.8V.

author: knockbill
author_rank: Regular Contributor
timestamp: October 20, 2016, 07:44:46 am
content: Thanks for the help,,, as was said going into this project,, this is a cheap meter in fancy clothes...I thought it would be of "useable " quality, given the name on it, but guess I was fooled!! I may run into one like it, in working condition, and be able to make "side to side" reading comparisons, to finally determine its problems...

author: The Soulman
author_rank: Super Contributor
timestamp: October 20, 2016, 10:12:29 am
content: The original measurements taken with this meter are all reading to low indicating a to high a reference voltage, even with the trimmer turned all the waydown. (as can be seen in the schematic: 0.14 Volts on the trimmer wiper and r21).Three possible causes:1. The (stable) voltage feeding the voltage divider (r22, trimmer, r21) is to high. could be but unlikely2. Value of r21 is to high. unlikely, a lower value would cause to much current to flow.3. Value of r22 is to low. likely:Hey Chung the machine ran out of those little black thingies!Chung: No problem let's use these, they look the same and the part number is even just only one digit of!Chang: Perfect!Seen it happen before.

author: knockbill
author_rank: Regular Contributor
timestamp: October 22, 2016, 11:35:30 am
content: Just got back to it,,, If "stable voltage" is 6.93-6.88VDC, against 3.0VDC as the goal... Reading back I see .1 VDC as reference... how/where do I read that, and if its off, what adjusts it...Other than that blob chip being damaged, it seems this should be pretty easy to trouble shoot, with its low parts count...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 23, 2016, 01:44:59 pm
content: Is it possible that you may have measured that wrong somehow? The voltage from COM to battery plus should be 3.0V, the voltage from COM to battery negative should be around -6.0V. The difference of the two voltages should of course be the full battery voltage. 3.0 - (-6.0) = 9.0V. Deviation from this suggests a major fault.Yes you can trouble-shoot the meter if you have the patience. There may be no schematic for this particular model available, but you can study thedata sheet for the original ICL7106, and the schematic for a genericDT830BorMAS830B, for insight into how these basic meters work. Your GB model doesn't measure current, so it is even simpler than these examples.Identifying the pin-out of the COB chip is possible with some effort. The order of the pins will probably be very close to the order of the 7106 40-pin DIP package. Below are two diagrams I worked out for a couple of variations of the famous Harbor Freight freebie meters. Notice they are different, but by buzzing out the V+, V-, and COM connected pins, you can work out the rest of the analog side. The rest of the pins just go to the LCD, and are generally uninteresting.The reference voltage would be measured between the RefLo and RefHi pins (in DCV mode), once they're identified.

author: knockbill
author_rank: Regular Contributor
timestamp: October 23, 2016, 02:10:36 pm
content: Thanks!!Very nice of you to take so much time on a project like this,,, the thing folks will get out of this thread is information/education, which to me is priceless,, as honestly, this meter isn't even as useful as a HF freeby,,, no current mode to set tube bias with... It does have a larger than average display tho, which is helpful to old guys like myself!!That said, I will try to confirm my Voltage readings and repost them, before I study your pics and try to sort out the blob chips...Regards, John

author: knockbill
author_rank: Regular Contributor
timestamp: October 23, 2016, 02:23:47 pm
content: First pic is COM to Batt +,,, 2nd pic Com to Batt - ... This is the procedure I used before...

author: knockbill
author_rank: Regular Contributor
timestamp: October 23, 2016, 03:11:37 pm
content: I noticed the readings in the last post don't add up to the Batt V reading,,, so I replaced the Batt in the GB, with one that read 9.25VDC,,, and got 6.80 + and (-1.32) which only add up to 8.12VDC,,, this indicates to me a short in the GB meter, or at least a component (resistor?) way out of range...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 23, 2016, 03:20:15 pm
content: I like your Wavetek 2015. Your measurement procedure looks correct, sorry for doubting you but I had to ask.For proper operation, the A/D converter in the chip must have a bi-polar power supply (negative and positive voltage.) So the chip contains a voltage regulator that sets the COM pin to 3 volts below the positive battery. Using COM as the analog ground, V+ will become +3V and V- will be -6V (fresh battery). If the A/D converter requires +/- 3V to operate, that means the 9V battery can discharge to about 6V before things stop working. You can read about this in the 7106 data sheet (although it uses 2.8V instead of 3.0V).So you can see why I would consider your readings to indicate a major fault. Either the COB chip regulator is not working (bad chip, full stop), or maybe something else is dragging the analog ground down towards battery negative. Or this meter uses a chip or a design totally unlike most others in its class...Quote from: knockbill on October 23, 2016, 03:11:37 pmSounds plausible. Use your Wavetek and measure the current draw from the battery. Normally it should only be a couple mA or so.

author: knockbill
author_rank: Regular Contributor
timestamp: October 23, 2016, 03:43:02 pm
content: The Wavetek was also from the "meter hoard" of last Sunday got it for a buck, along with the GB and Fluke!!! It seems to work fine...I appreciate your doubt,,, thanks, that's what make one think thru a problem...Batt current reading... .03 on 20 Ma scale with a DMM I use for bias setting... Wavetek wouldn't lock in, seems the it drew the battery down, however its the 1st time I used it for current reading, so I need to study the manual,,,

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 24, 2016, 11:28:48 am
content: Quote from: knockbill on October 23, 2016, 03:43:02 pmThat was quite a deal. I would happily pay $1 for a Wavetek 2015 and a Fluke 8024. You can keep the GB though.Quote from: knockbill on October 23, 2016, 03:43:02 pmSomething odd here. 30uA is not enough to run the GB meter, much less bring a (good) 9.25V battery down to 8.12V.

author: knockbill
author_rank: Regular Contributor
timestamp: October 24, 2016, 11:54:54 am
content: I'll give it another go round,,, not real sure I used the Wavetek properly or if the current mode is accurate,,, this is the problem with hamfest gear, there's probably a reason it was sold there!

author: knockbill
author_rank: Regular Contributor
timestamp: October 24, 2016, 12:13:42 pm
content: It reads .40ADC + or - on three different meters I'm familiar with...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 24, 2016, 01:33:06 pm
content: Oh wow. That's 400mA which would kill a 9V battery very quickly, and would probably cause something to get hot.

author: knockbill
author_rank: Regular Contributor
timestamp: October 24, 2016, 02:34:57 pm
content: I'm not sure we were on the same page!!! I read mA across the battery being used, not in series with the DMM... I thought you wanted to know the current available in the battery which is a dead short across a consumable power supply, explaining the decreasing current readings.Thinking it thru, seems you want the meter draw current, which now that I've gotten a few lingering business calls out of my head make much more sense!!!!Here is the accurate current draw thru the GB DMM, set on 20VDC range, test meter in series with battery and GB...Meter 1... .05 on the 20 mA scaleMeter 2... .05 on the 40 mA scale (Wavetek meter)Very sorry for the confusion, trying to accommodate your time/help and dealing with real world BS at the same time doesn't work!! Got the BS out of the way now, tho!!

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 24, 2016, 04:07:30 pm
content: Yes, a measurement of current in series with the battery and the meter turned on, to see what current it's drawing from the battery. All I wanted was a confirmation on what you suspected about a "short" (or really an overload) on the power supply of the meter. But now it appears you're reading 50uA of current draw on the battery, which does not seem like enough to even operate the meter to me. I would expect it to be around 1 or 2 mA.For reasons I've explained up-thread, I'm not sure the COB chip is OK. On the other hand, it is promising that the meter appears to be running and measuring "something", even if it isn't right.

author: knockbill
author_rank: Regular Contributor
timestamp: October 24, 2016, 04:37:58 pm
content: I agree with you,,, and sorry for not having more knowledge about SS circuits and testing them...I will continue to try to understand the diagrams you previously posted on the two different HF DMM chips, and see if I can at least confirm that one of them is similar enough to the GB to continue troubleshooting it...Also,,, as you stated, its curious that the meter seems to make readings on all modes, and they are all low, to a certain %... I think previous poster Soulman may be on to something with the resistors in the trimmer circuit... Remember, I tried changing the resistance feeding that ckt originally by replacing the thermistor with resistors to modify the trimmer voltage,,, it did allow the DMM to read progressively higher but wouldn't lock onto the proper reading...It can be repaired if the blob chip is useable...

author: retiredcaps
author_rank: Super Contributor
timestamp: October 24, 2016, 04:38:56 pm
content: Quote from: ModemHead on October 24, 2016, 04:07:30 pmI agree. The Fluke 70 series lasts 2,000 hours on a 9V battery suggesting a 250uA draw. Only a handful of meters are in this category of close to 2,000 hours runtime.At 50uA, that implies the GDT meter will last 10,000 hours. For a $20 meter?

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 24, 2016, 08:25:36 pm
content: Quote from: knockbill on October 24, 2016, 04:37:58 pmYour photo is too blurry to make out much, but I can see a few things. The pins for the COB's analog section appear to go across the top. Most of the bottom pins go to the LCD. C3, the large film cap, should be the integration cap (Cint). With the battery out and the GB dial set to any VDC range, you can probably buzz out which COB pins are connected to the COM jack. The kicker here will be the auto-power-off circuit. It likely interrupts either the positive or negative supply going to the battery, and may make it hard to find the V+ and/or V- COB pins.Quote from: knockbill on October 24, 2016, 04:37:58 pmThe PTC thermistor is normally only used in Ohms and Diode-check mode to protect from external voltage being applied at the input jacks. If in good condition, at room temperature it should have little to no effect on anything.Quote from: knockbill on October 24, 2016, 04:37:58 pmI agree, but I'm not convinced yet that this one is not toasted somehow on the analog side.

author: knockbill
author_rank: Regular Contributor
timestamp: October 24, 2016, 09:18:06 pm
content: I'll check the top pins against the COM jack and mark them, if Auto Off doesn't hold the ckt open...

author: knockbill
author_rank: Regular Contributor
timestamp: October 24, 2016, 10:11:27 pm
content: Here's some readings across the top of the blob chip, Left to Right, testing meter set on 200R range, COM lead connected to GB COM jack, POS lead to pins... Another pic of board, maybe clearer?

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 25, 2016, 12:04:33 am
content: Very good. Attached is my best guess at the pinout.You can now do some voltage measurements with the GB meter powered on, with the dial set to any VDC range.1) V- to V+2) COM to V+3) REFLO to REFHIP.S. 1.7 ohms from COM to V- is very troubling.

author: knockbill
author_rank: Regular Contributor
timestamp: October 25, 2016, 12:11:20 pm
content: Here's the readings...V- to V+ 6.00VDCCom to V+ 4.93VDCRef Lo to Ref Hi .07VDC

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 25, 2016, 12:52:02 pm
content: V- to V+ should be the full battery voltage less whatever small drop appears across the auto-power-off switching device.The reference voltage is way low, but is derived from the Com to V+ voltage which is way too high.So none of it makes the slightest bit of sense to me.

author: knockbill
author_rank: Regular Contributor
timestamp: October 25, 2016, 01:42:02 pm
content: We figured it wouldn't pan out,,,Full Batt V was 9.02VDC before starting the tests, it reads 8.55VDC now, indicating to me a major current draw... I checked readings with 3 meters, the Wavetek read ref Voltages at 111+ VDC on auto range, which seemed rediculous, the other two readings were about the same,,, must be a problem in that meter also, unless the GB is messed up enough to confuse the Wavetek auto range feature...However, we are basically guessing at the correct pin outs,,, Major lesson here for me was to ignore hamfest DMMs!!! Even the Flukes use hard to find parts... My eyesight isn't good enough to see the boards let alone the pads and traces!!! I've had much better luck rebuilding analog test gear,,,If I run across someone, probably another electrician, that has one of these meters, maybe I can get the readings from it and continue the exercise... Thanks for all the time and effort you put into this, maybe some others got info from it...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 25, 2016, 02:18:28 pm
content: At the risk of beating a dead horse...Quote from: knockbill on October 25, 2016, 01:42:02 pmI do agree that probably the current draw on the battery is probably too high, despite the fact that you seemed to measure only 50uA earlier, so that reading is very suspect. You do need to have reliable test equipment and be familiar with it to get good data for trouble-shooting. Is it possible the Wavetek was reading 111 *millivolts* and not 111V? Auto-ranging meters are not that easily "confused".Quote from: knockbill on October 25, 2016, 01:42:02 pmI am reasonably confident of the COB pinout, especially if the resistors and capacitors I noted on my drawing actually match the connections on the board, using eyes instead of a slightly fuzzy photo.Quote from: knockbill on October 25, 2016, 01:42:02 pmI still suspect the COB chip is damaged, but I don't mind going the extra mile to make absolutely sure. The 1.7 ohm reading you got from V- to analog ground is a big sign that something is terribly wrong. At this point about the only thing we haven't investigated is the auto-power-off circuit. I suppose there is a slim possibility a fault in that section could these problems.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 25, 2016, 04:13:13 pm
content: Quote from: knockbill on October 24, 2016, 02:34:57 pmHmm, I don't have the Wavetek meter and don't know what your #1 meter is above, but I just noticed that on the Wavetek meter you might/must have to press the yellow button to get DC mA.Is your 0.05 reading on AC mA or DC mA?@OP if you have a DC variable power supply that shows current draw, that is the easiest way to get a reading. Set your V to 9, and the power supply should show the current draw.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 25, 2016, 04:19:33 pm
content: Quote from: ModemHead on October 25, 2016, 02:18:28 pmReminds me of a person that I tried to help over at badcaps.net. OP reported readings of 162V DC on the secondary side of a lcd monitor PSU. I kindly suggested they measured it incorrectly and wrote detailed instructions on how to do it properly.OP insisted they did it right. I said the caps would have all violently exploded with some other things if there was really 162V DC on the secondary side. OP got pissed at me and decided to "prove" that I was an idiot and posted a picture of the reading.Sure enough, the multimeter showed 162 ....... mV. OP negelected to mention the "m" milli.When I pointed that out, never heard from the OP again on that forum.

author: knockbill
author_rank: Regular Contributor
timestamp: October 25, 2016, 05:17:28 pm
content: Well, I'm not real confident in the Wavetek I got from a hamfest for so cheap,,, however, I'm not really familiar with it, and didn't spend the time to digest the whole manual yet,,, best I can do is take a new set of readings and try to photograph them...Note... the battery lost approx a full volt during testing (4-5 minutes tops??), as some of the readings show a different value... I used the Wavetek, and did notice the last reading was in mV, sorry for the confusion,,, guess I'll have to get used to an auto ranging meter, I never was comfortable with them on the job...I hope these readings are useful,, I double checked them, and they came out close, but declining each time, accounting for the battery load... I don't have a low voltage DC power supply, as I'm normally working with tubes...2nd pic V- to V+ 7.11VDC3rd pic COM to V+ 5.76VDC4th pic Ref L to Ref H 129.8mV

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 25, 2016, 06:29:34 pm
content: On auto-ranging meters, just get used to the fact that the units symbol in the display is just as important as the numbers.--------------Back to the GB: OK, this is becoming clearer. The unit is obviously drawing excessive current. Some fault is causing analog ground to be pulled closer to the negative battery as a result. The question is whether this fault is internal or external to the COB.To answer the question, I can propose one more exercise with a club and the poor horse carcass. But only if your eyesight allows!The COM pin is the fourth one from the left on the top row of COB pins. It appears to be stitched to the ground plane on the opposite side of the board with a via (the tiny plated-through hole). If you run through that hole with a drill bit that isjust barely bigger than the holeit should destroy the copper plating, and thus the connection to the ground plane.The trick will be to leave enough of the rectangular pad so as not to break the connection from the pad to the COB.With power applied, check the COM to V+ voltage. If freeing COM from the ground plane allows it to come up to 3.0V below V+, the COB may not be damaged. If however, it still reads way high, the COB is beyond repair.

author: knockbill
author_rank: Regular Contributor
timestamp: October 25, 2016, 09:29:02 pm
content: HA,,,OK,,, I think I met my match with someone who shares my patience for trouble shooting!!! I am able to make my own small drill bits and have the proper tools to use them, as I spent a lot of time with an Uncle who was a 1st class watch/clock repairman,,, he left me his tools, and some worth while skills...I will give this "trace/pad cutting/drilling" test a go,, may not get to it till tomorrow tho... I'll make a bit just big enough to ream the plating out of that thru hole...Thanks for your patience,,, I've been building my 6AS7 PP amp from the test mules into its chassis, for some good Karma, maybe it will rub off on this cheap DMM!!!

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 25, 2016, 11:28:27 pm
content: Well I do like to keep going until I know what the failure is. The experience may make the next one go faster.Making your own drill bits must be a nice skill to have! But before spending too much time, make sure that my assessment of how the pin is connected is correct. We just want to disconnect it from everything else to see what it does without any influence from anything else...

author: knockbill
author_rank: Regular Contributor
timestamp: October 25, 2016, 11:42:48 pm
content: Will do,,, I have to disassemble the GB again, I put it back together after the last tests, don't want to lose parts... I'll study your post, and ream the pad/hole to get rid of the plating,,, get the reading and post back...Thanks again, at this point, this is the most expensive 10 buck meter of all!!! May as well see it thru,,, either fix or condemn it, but the learnin' will be the reward!!!Now I'm wondering if a HF freeby board will line up with the big display in the GB meter?? It would need the jacks relocated... It never really ends does it! I've already put a HF meter in the GB holster!

author: knockbill
author_rank: Regular Contributor
timestamp: October 26, 2016, 01:52:20 pm
content: Results...Reamed out "rivet" from the opposite side of the board, to not mess up the pad near the chip, I can see the plating is gone half way thru the board, but the 4th pad from top Left still has continuity to the ground plane on the other side,,, maybe a short in the chip?Test,,,Battery has 9.13VDC at start of test, GB set on 20VDC, and turned on... leads connected to COM jack and V+, (7th pin down on Left side of chip)... Reading taken with Wavetek, auto range= 6.00VDC...I'm about certain I disconnected pin 4 Top from the grd plane on the other side of board, but since it still had continuity the test was invalid, correct?Sorry for the flash in the pic, it reads 6.00VDC... 2nd pic is the clearest I could get of the reamed hole...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 26, 2016, 02:44:46 pm
content: To summarize: the idea was to *fully* disconnect the COM pin of the chip from everything else on the board, and then measure the voltage from the COM *pin* (not the COM jack) to the V+ pin, to see if the internal voltage reference would go back to the normal 3.0V.From looking at the photo, it seemed to me the quickest way to disconnect the pin was to ream the hole (it can be restored easily with a bit of wire and solder.) I can't see any other copper trace going to that pin in the photo, but you'll have to double-check since you have eyes on it.If there is still low-ohm continuity from the COM pin to the ground plane and no other copper traces to the pin, then it must be going through the chip internals. A short in the chip as you say. This is a bad thing, but not too unexpected I guess. I just checked an ICL7106 I have here, the lowest resistance I could measure from COM to any other pin was 4 meg-ohms.

author: knockbill
author_rank: Regular Contributor
timestamp: October 26, 2016, 04:30:51 pm
content: OK,,, sorry for the confusion,,, the battery dropped a volt and a half as soon as the meter was turned on... I just measured from the COM pin at the chip to the V+ pin and got 5.55V and declining,,, there must be a short in the chip, as something is dropping V as soon as the meter is turned on, I believe your original suspicion was proved... The chip isn't replaceable in this meter so I guess we figured it out...I really appreciate all your input, and time with this meter,,,, It fooled me as I am familiar with GB brand, and can't imagine them putting their name on this POS...I think I have plenty of meters for now, the Wavetek was a pleasant surprise, and I also got another CenTech meter about the size of the Wavetek with many features... I didn't mention it before, but it works very well, and is usable as is...I'll try to find the parts for the Flukes as I get the time to re visit them...Thanks again, I hope some others got some info from this exercise also...

author: ModemHead
author_rank: Frequent Contributor
timestamp: October 26, 2016, 05:03:27 pm
content: Quote from: knockbill on October 26, 2016, 04:30:51 pmNo problem, glad to try and help.Quote from: knockbill on October 26, 2016, 04:30:51 pmI've posted this before, but I'll post it again for posterity. If you see a manual-range 2000-count DMM with just two AC voltage ranges, with the lowest one being 200V, you are looking at a very simplistic, low parts count, ICL7106 clone-based design. It doesn't matter how good the plastic shell looks, or what manufacturer name is on the front, at its core it's the same as the Harbor Freight Freebie.

author: knockbill
author_rank: Regular Contributor
timestamp: October 26, 2016, 05:08:12 pm
content: That's a a good thing to know about the AC ranges,,, Never heard it before, but it makes sense...Thanks again...

author: pilobond
author_rank: Newbie
timestamp: December 01, 2018, 02:32:39 am
content: Problem solved...?Just for people who will be perusing in the future... I had same problem with GDT311. I am sure it's pretty much the same circuit board inside.R1, pot, R2 looked the same as the picture from OP. With the pot turned all the way to left, I was only able to read about 3.9V for 5.0V source.R2 was 1002 SMD, thus 10kohm. I replaced this with 22kohm through-hole resistor, as I didn't have any SMD in that range handy. Then I was able to adjust the pot so the reading matches my Fluke DMM. Since then all reading in DC voltage and ohm reads with in 1% of my Fluke 117. AC voltage from my main reads 120V (+ 1 decimal who value I forget) so it seems reasonable.