GW Instek GDM-8034 bench DMM: looking for schematic or calibration notes - Page 1
Forum section: electronics / testgear

author: dmmartindale
author_rank: Contributor
timestamp: July 23, 2024, 02:38:51 am
content: I recently purchased a used GDM-8034 bench DMM. It's a basic DMM with 2000-count LCD display, manual ranging, and 0.5% basic DC accuracy. It seems to be fully working, and the functions I've been able to test seem to still be in calibration (though I only have an even older Fluke 4.5 digit meter as a reference). Not bad for $25.However, it has a slight DC offset error. For example, measuring a 0.250 V reference on the 2 V range, it reads +0.251 or -0.249 depending on how I connect the leads. I'd like to trim out this offset if I can. I have downloaded the manual for it from the Instek web site, but it seems that the manual includes just end-user instructions - nothing about how to calibrate it, no circuit diagram, not even a board layout that identifies what the adjustment pots and cap actually control.This is in contrast to the manual for the older GDM-8135, which has tables for performance checks and calibration, and a drawing of which calibration controls are where. Now, the GDM-8034 is obviously simplified somewhat compared to the GDM-8135. The 8135 has 14 calibration adjustments (11 pots and 3 trimmer caps) while the 8034 has only 5 adjustments (4 pots, 1 cap). But I don't want to go changing the adjustments blindly while trying to figure out what they do, particularly when its calibration seems OK at the moment and I don't have the procedure to recalibrate it properly.So I would appreciate it if anyone can send me (or point to) a schematic for the 8034 (which would let me figure out what the adjustments do. Or, even better, the real calibration procedure for this meter. Note that I do have the 15-page users' manual already, but I need something beyond that to start changing calibration adjustments.The adjustment points on the 8034 are:VR401: left side near U401VC401: near U402 and J401, near left end of U606VR501: left edge of board, behind input jacksVR502: adjacent to U501 and near VR501VR601: between U606 (large chip) and R515 (power resistor)

author: Poroit
author_rank: Frequent Contributor
timestamp: July 23, 2024, 04:07:45 am
content: The accuracy for the DC Volts ranges on that model are:+/- 0.5% of reading + 1 digit.It is in spec.

author: dmmartindale
author_rank: Contributor
timestamp: July 23, 2024, 11:48:18 am
content: Yes, I know that the one count error is within the published spec. And there may not be any way to adjust this anyway; the GDM-8135 does not have any adjustment for this. But I would like to adjust out this offset if it is possible, and having a circuit diagram would tell me if there is an adjustment.Plus, having a circuit diagram and/or calibration procedure is useful to keep the meter running in the long term, particularly if some component eventually needs to be replaced.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 23, 2024, 11:58:14 am
content: have you tried at GWInstek ??other options is to measure the trimpots values and try fiddle them ... witch is a last course of action ...or trace down witch pots go the each ic's, search ic's data sheets and try to figure out the circuit ...since this is a basic 0.5% meter 3 1/2 digits, should not be hard ...

author: nctnico
author_rank: Super Contributor
timestamp: July 23, 2024, 12:12:47 pm
content: I'd leave it as is. Likely the offset will vary with temperature & humidty as well so trying to adjust will be futile. Fiddling with the adjustments is more prone to breaking something than making an improvement.

author: J-R
author_rank: Super Contributor
timestamp: July 24, 2024, 12:36:42 am
content: Can you post some internal photos?I say tear into this DMM and have fun. It makes no sense to try using it given the epic limitations: 2,000 count, manual ranging only, not true RMS, you must use the holes in the front to measure capacitance (can't use the test leads) and even then it can only measure up to 19.99uF... Then there is the iffy LCD. So it just doesn't have any redeeming qualities IMHO.A fun mod could be to add some batteries and a backlight to the display.

author: dmmartindale
author_rank: Contributor
timestamp: July 24, 2024, 01:48:10 am
content: Main circuit board from above, front panel on the left:Left side of the board, where most of the pots and the one trimcap are located:

author: coromonadalix
author_rank: Super Contributor
timestamp: July 24, 2024, 01:54:02 am
content: ok an classic intersil 7106you can find many schematics on the web about it, you should understand a bitvc401 is a frequency compensation, i would never touch itas other wrotes it's a very basic meter, how far do you want to let it be that way or you still have the urge to fiddle the trimpots ... i would not play in it ... too much mechanical switchs etc ...you seem to expect a lot, out of this meter for 25$ ...only 2 caps in the psu would need to be replaced if the meter is used 24/24 ... if not let it be

author: Poroit
author_rank: Frequent Contributor
timestamp: July 24, 2024, 05:32:23 am
content: Keep it as a back up and invest in a 5-1/2 digit or better.

author: J-R
author_rank: Super Contributor
timestamp: July 24, 2024, 07:02:27 pm
content: I see it doesn't have fuse for the 20A input, so avoid mistakes...As mentioned by coromonadalix this appears to be a popular DMM IC and I see lots of DIY projects using it with various schematics and pinouts available going back maybe 40 years. So you should be able to figure this out by doing some research and tracing this specific PCB layout.I think it's safe to say there is going to be only one DCV adjustment, probably on the 200mV range. There may or may not be a zero adjust. The other ranges are going to be fixed based on the input divider resistors. So making this perfect may not be possible, and not necessary as it's in spec, but fine for tinkering. If you're able to fully document the adjustments, it could help others down the road.

author: floobydust
author_rank: Super Contributor
timestamp: July 25, 2024, 02:39:54 am
content: I don't think it's calibration to blame.OP I think your issue is called "rollover error" which is a difference in reading between +ve and -ve values. It should be less than one count. Check Reference capacitor C603, it should be a 0.5-1uF part. 0.1uF is a bit small and cheap or maybe it is bad with leakage current. Beyond this, for the ICL family, a "rollover resistor" and diode can be added in series between INTegrator OUT and analog COM. Like 1N4148 (cathode to INT OUT) and 100k to analog COM at ICL7106 pin 27. I don't see it on your board. It's in the old Harris/Intersil datasheets.The multimeter is likely the same front-end as the LED version GW-8035, and GW Instek did white label under a few other brands like Metermaster, Amprobe, Wavetek etc. in your search for a service manual or schematic. It's just hard to guess their model numbers, use image search maybe.Failing that, it is not hard to figure out the calibration trimpots by tracing out some of the circuit.

author: mqsaharan
author_rank: Regular Contributor
timestamp: July 25, 2024, 06:24:13 am
content: Quote from: floobydust on July 25, 2024, 02:39:54 amHi floobydust,Could you please upload the old Harris/Intersil datasheet here that discusses/presents this solution. I downloaded a few old datasheet from different sites but couldn't find it in any of them.Thanks.

author: floobydust
author_rank: Super Contributor
timestamp: July 25, 2024, 06:47:35 pm
content: I can't find the one Intersil datasheet or app note with schematic for it I found last night. It's mentioned as text in others. Was it here... long, slow download over 1,000 pages:https://usermanual.wiki/Document/1981IntersilDataBook.1668550145/viewhttp://www.bitsavers.org/components/harris/1991_DB301.1_Harris_Data_Acquisition.pdfOP on second thought I think you have no rollover problem. "... measuring a 0.250 V reference on the 2 V range, it reads +0.251 or -0.249 ..."You are looking at the bottom of the 2V range.Def'n: "Rollover Error -difference in reading for equal positive and negative voltagenear full-scale." It's usually 0.5-1 count as a spec.Intersil attributes problems there due to leakage currents on a dirty PCB or an integrating or ref capacitor with higher dielectric absorption.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 25, 2024, 07:10:04 pm
content: The roll over error has in the ICL71xx has mainly 3 components. One is an offset, e.g. from input bias and protection resistors or the divider. A 2nd is the ref. capacitor from leakage and pump out from parasitic capacitance - so rel. high capacitance in a not too large case is wanted. Today possibly even an X7R of maybe some 2.2 µF. The voltage is constant and DA is thus not an issue.The final part is from the difference in the direction for the comparator. The 4.5 digit version 7135 has some hint's on this with an suggested adjustment for the steps around 0. For the 3.5 digit version this was less of an issue. While the error is still part of the turn over the bigger issue is the DNL around +-0.It is a very basic meter and chances are it will work for quite some time with no issues. If there is an accuracy problem this would be more like a defect (e.g. old electrolytic capacitor, cold solder joint, dirty PCB or spiders home under the chip). In this case a calibration alone would not fix the issue, but it would need a full repair. Chances are one would than ditch the meter or than look for a schematics or do a reverse engeniering with the help of the typical circuits. Nothing to really worry now as the meter may fail in different ways, like failing display, blow up from a lightning stike, get lost in a flood or fire or get smashed beyond repair.

author: coromonadalix
author_rank: Super Contributor
timestamp: July 25, 2024, 08:11:07 pm
content: some sheetsbasic circuit, woth to readhttps://electronics-diy.com/ICL7107_volt_meter.php

author: floobydust
author_rank: Super Contributor
timestamp: July 25, 2024, 09:35:11 pm
content: Towards calibrating the GDM-8034, I would need clearer pictures and a pic of the PCB bottom to trace out things, the DIP IC markings.It is generally one trimpot to cal 7106 input, looking like VR601 for 200mV full-scale but I will instead cal the 2V range with it depending on how good the divider resistors are.One trimpot for Current function (shunt amplifier). Is R606 a shunt?One trimpot for Ohms source (maybe in conjunction with Q501, it will connect to PTC R517).One trimpot for AC converter (plus trimmer cap for freq. response).The beeper I'm assuming the threshold is fixed.The "300Hz Point" I guess it has a RC oscillator for something? A bit low for a beep.Make sure U101 (maybe 7815) does not run too hot and drift. Not sure if it needs a heatsink, the tab is discoloured.I find it strange they went LCD display it could easily run off batteries. Note these do have a little capacitance to PE ground, the cap by the mains fuse C504?.

author: dmmartindale
author_rank: Contributor
timestamp: July 26, 2024, 02:55:55 am
content: Some quick replies for now. I can likely look more closely on the weekend.- I like the idea of treating this as a secondary meter. I recently acquired a couple of 4.5 digit DMMs (Fluke 8600A and 8050A) one of which will likely become my main bench meter.- I looked at the LCD. It sits pretty close to the display board, connected to it via a pair of rubber zebra connectors. I don't think there's any place to mount a LED backlight behind it without adding a spacer, which would require a source for longer zebra strips. On the other hand, a couple of LEDs as a side light looks plausible.- I found a couple of 7106 datasheets. That should be enough to trace circuits functionally. (Though that might have to wait until I retire).- Yes, 0.25 V and 2.5 V are at the low end of their respective ranges. That's because the voltage reference I happen to have is based on a 2.5 V LM4132, which is pretty good for 3200-count and 4000-count meters, but forces going up a scale on this 2000-count meter. However, I've also tried switching input polarity with inputs of about 190 mv, 1.9 V, and 19.9V (near the top of the 3 ranges) and the difference between positive and negative readings is usually 1 count there. The 7106 datasheet specifies a rollover error limit of +- 1 count so it's within that spec.- Yes, the 20 A input has no fuse. I think this was once common; I also have a couple of handheld meters with 10 A or 20 A inputs and no fuse. Most of the time, I use this to measure AC line current, so there is a 15 A breaker upstream somewhere, but I understand that the unfused input is a real risk with an unfused high-current source (e.g. car battery). The 200 mA/2 A current input is fused; there is a 5 x 20 mm fuse inside the blocky-looking red input jack housing.- I found a photo of the interior of GW-8035F, and it seems to be an older generation of instrument. It has nothing larger than a couple of 16-pin DIP ICs in it - no 40-pin DMM building block chip. The analog signal circuitry is quite different too.- On the other hand, the 8034's circuit board has a bunch of unpopulated locations near the right edge (bottom of the first photo): U603 and U604, plus R113-R120. And there's a 7107 meter chip which has the same functions as the 7106, but its outputs are designed for driving LEDs instead of a LCD. So I wouldn't be surprised to find a version of this meter with LED display.- 300 Hz is the frequency used for capacitance measurements in this meter. Thus the test point labelled "300 Hz POINT" is probably part of the capacitance to voltage converter. The capacitor jacks are connected to the main board by 4-pin J401. So VC401 right next to it could be part of the capacitor measurement, or it could be a high-frequency adjustment for the AC converter.- U101 (main voltage regulator) is a 7808 regulator. It may look a little discoloured, but it was only slightly warm to touch when I checked it. The whole meter draws only 0.9 W/1.3 VA when operating. Of that, 0.6-0.7 W and 1.1 VA is drawn when the power switch is turned off. The power switch is electrically between the transformer secondary and the rectifier diodes, so the transformer draws magnetizing current even when the meter power switch is off.

author: mqsaharan
author_rank: Regular Contributor
timestamp: July 29, 2024, 03:40:55 am
content: Quote from: floobydust on July 25, 2024, 06:47:35 pmfloobydust, thank you for the links. I already had these databooks. So, I searched them again.The required information is under ICL7135 4-1/2 digit AD Converter. For Intersil databook at the above mentioned link, on page 395 under heading "Rollover Resistor and Diode" and schematic on the next page. These details are also present in new datasheet of ICL7135. Perhaps these components are needed for rollover prevention in ICL7135 and they are not required for ICL7106/7.Quote from: Kleinstein on July 25, 2024, 07:10:04 pmKleinstein, thank you for an in-depth look, as always.coromonadalix, thanks for the datasheets. I already had these but the info I was searching for was not present in them.dmmartindale, as others have already said it, better not to touch the adjustments. This is not a high specs meter. It looks well within specs. What I am suggesting below are only guesses, so take them with a grain of salt.Usually, ICL7016 based meters are adjusted with an input of 190mV for on screen display reading of the same at 200mV range. And from the look of it, VR601 seems to be the adjustment for DCV. Is there any voltage reference present in this meter? If not then the internal reference is obviously being used.As per the datasheet of this meter and your last message 300Hz area seems to be for the capacitance measurement circuitry. So, I guess, VR401 and VC401 will be for the adjustment for capacitance function.VR501 and perhaps VR502 are for ACV adjustment. I don't have any idea for ACV adjustment for this meter. As per specs, its ACV function goes up to 500 Hz. So, I doubt there will be any adjustment for HF ACV.

author: J-R
author_rank: Super Contributor
timestamp: July 29, 2024, 06:50:22 am
content: I have the 8600A and 8050A as well as quite a few other similar vintage models that arrived in non-working order and while they were fun to fix and calibrate, I don't see the point of buying them for doing real or hobby work on a daily basis. They are just more of the same thing you already don't want in the GDM-8034. I would have suggested a BM235 handheld before getting all these relics!One reason these don't make much sense to me is that while they have slightly more counts than a low-end handheld, without a valid calibration you can't put any trust in them. So the couple extra digits are not worth all the other major shortcomings. And even then the digits are still weak compared to the 6.5 digit go-to bench DMMs.Maybe the cheapest/oldest I'd be able to live with would be the Fluke 45 (no capacitance) or the Fluke 8840A (also missing some features but still a bit of a powerhouse). The Fluke 45 has the handy dual display and 10A, the 8840A has 4-wire resistance mode and both have adjustable rate vacuum fluorescent displays for some truly fast measurements.

author: dmmartindale
author_rank: Contributor
timestamp: July 31, 2024, 03:28:50 am
content: My main reason for wanting an AC-powered DMM is to be able to set up something on the bench and have a meter monitor it constantly, without having that consume the battery in a handheld meter, and without having the handheld automatically power off (to save the battery) after a period of no input. Almost any AC-powered meter would provide that, including the 3 that I named.The other attraction of these meters is that they were all available locally. I was able to power each one on, see that the display worked, and check calibration on a few voltage ranges before buying them. And they were inexpensive enough that if they turned out to have problems, I didn't waste much money. The 8600A actually did end up having a 5 V regulator problem, which I fixed, but otherwise they all seem to be fully working. I am less comfortable buying something electronic from a random person on eBay without any chance to look at it in person first.Although the Fluke 8600A and 8050A likely haven't been calibrated in 40 years, there are applications where their greater precision (4.5 digits) is useful for making differential measurements even if the absolute measurements are not within spec. I have access to a 8840A at work and I intend to check my Flukes against it, so then I will know how well they are calibrated (and the Fluke manuals have circuit diagrams and cal procedures, so I can adjust them if necessary). Also, in limited DC volts testing, all 3 of the meters agree within their respective accuracy limits, so either they are still in spec, or they've all drifted by about the same amount in the same direction.Other replies have convinced me not to touch the calibration adjustments on the Instek meter unless I find it obviously wrong. At that point, the main chip datasheet and the simple circuit board should let me figure out which adjustments are involved. Maybe that day will never come.The BM235 seems like a nice handheld, but it's still battery powered, with slightly worse basic DC accuracy than the two Fluke meters, 6000 count instead of 20000 count, and stupidly expensive in Canada. The Fluke 45 and 8840A both seem like good meters, but fully functional ones seem to sell for considerably more money on eBay and I haven't seen them for sale locally. Maybe once I'm retired I'll find that I use a bench meter frequently and I'll be willing to spend that kind of money, but at the moment I might use a bench DMM once a month if I'm lucky.

author: J-R
author_rank: Super Contributor
timestamp: July 31, 2024, 09:02:03 pm
content: OK, it seems you just like these older DMMs. It's fine... Is the older 4.5 digit Fluke handheld you mentioned in the first post the 8060A?To counter some of the points raised, there are higher count budget Brymen handhelds above the BM235, such as the BM785 or BM867s, and I think it's money well spent compared to the vintage stuff. Why have three (or four?) old DMMs when you could have one newer one for similar total money? (Excluding sentimental/collector reasons.) Many Brymens have the optional IR interface for short-term logging.Psychologically it's true that the auto power off and battery life on the handhelds can be a concern, but in practice you're typically talking 15-30 minutes of complete inactivity before APO and 100-300 hours of battery life. And you said you only use a bench DMM once a month? (Is it an all day session?) Also, on many Brymen models using the Min/Max function disables APO and specifically the BM86x has a dedicated APO-disable mode you can use by holding down Select during power on.The Fluke 45 and 8840A do come up for sale regularly for closer to $100. At the upper end of the price range, I bought essentially a new open box Fluke 45 recently for $150 delivered as a test to see how much it had drifted from new based on a discussion in another thread (it was just out of spec on a couple functions). For eBay you definitely have to be willing to search and wait at least a little bit for something good to come up for sale.

author: dmmartindale
author_rank: Contributor
timestamp: July 31, 2024, 10:53:20 pm
content: I will admit that I do like to keep old electronics in service, including test equipment. The "older Fluke meter" I mentioned is the 8600A, a LED-display bench meter. The only Fluke handheld I own is the Fluke 113, which is probably theleastaccurate DMM I have (2% spec). But it cost $0, and the 6000-count display and low-Z ohms are sometimes useful.The 8600A cost me C$50. I recently added a Fluke 8050A which adds true RMS, relative measurements, and dB. It cost me C$45. In comparison, the BM325 is C$157 (plus shipping) on the EEVblog store, which seems quite reasonable. However, no one in Canada seems to sell it. On Amazon.ca, the lowest price is C$285, and all the sellers are US-based. They import the meter into the USA, mark it up, then ship it to Canada where additional taxes and duty are likely to be added. So the cost of a new BM325 is about 2 or 3 times the cost of both of the older Flukes depending on where I purchase it.One example of where auto power off is annoying: I sometimes charge a lead-acid battery using a bench power supply, configuring it for constant-current output initially and constant-voltage later. This takes 4-8 hours to the point where I want to switch to float voltage. Sometimes I record battery voltage and charge amps every 15 or 30 min so I can graph them later, and calculate Ah needed to recharge. A bench meter will just stay powered on. The Brymen ability to disable auto power off is a nice touch, but I don't think my handhelds have that feature.I know people recommend the Fluke 45 and 8840A, as well as HP 3468A, as good value for used 5.5 digit meters. Maybe someday I will want something better than the 8600/8050. But right now, the 4.5 digit Flukes are significantly better than all the handhelds I have, and I'm happy with them at the moment.

author: floobydust
author_rank: Super Contributor
timestamp: July 31, 2024, 11:07:10 pm
content: Would not go near Brymen due to the lack of support in North America. This is my preference right now. Sorry fanboys.I have many older DMM's, some have weaknesses, some have features that beat other DMM's - so I have fun keeping them going instead of the "buy new" mentality.Newer is not always better and lately Made in USA multimeters I kinda like the quality. My Taiwan-build multimeters constantly need repair, a step above the mainland for quality but still disappointing.

author: J-R
author_rank: Super Contributor
timestamp: August 01, 2024, 12:11:49 am
content: Welectron is the go-to place to buy Brymen DMMs in my opinion. They have 9.99 EUR DHL shipping. The Brymen-edition BM235 is 127.84CAD ($92.59 USD) delivered, for example.Brymen resellers do provide a limited amount of support, but it's typically only 1 year. Given that fact and the price point, I consider all my Brymen DMMs disposable. It's just how it is.Dave's blue-baby-buggy-bumper stuff is cute and unique, but the price is going to be a bit higher of course. He does sell the BM235 on Amazon, but the price is $150 USD. It's fine; in a way you're buying "merch".Greenlee has the license to rebrand Brymen here in the US and they offer lifetime warranties. But essentially you are paying almost double for that. So it's not really worth it for a hobbyist in most cases.The buy used vs. buy new is a huge conversation of course. I have quite a selection of 4.5/5.5/6.5 DMMs (and a Keithley 2010). I could have just bought a used calibrated 34470A from the start for less money.

author: dmmartindale
author_rank: Contributor
timestamp: August 01, 2024, 02:08:39 pm
content: (Warning: this message is veering away from DMM technical info and into DMM buying economics)That's very interesting. Welectron is currently out of stock on the regular BM235, but they also sell the blue EEVblog version for EUR 91.60 (without VAT). Add EUR 10 shipping makes it EUR 101.60 and C$151.38. Then there's probably C$20 in tax due on top of that, making the total around C$170.Amazon.com sells the EEVblog-branded BM235 for US$150. Amazon will charge an additional US$27.51 for shippingand estimated import duties. So it should arrive at my door for US$177.51, or about C$245. That's more convenient than Welectron, but about $75 more expensive.Meanwhile, on Amazon.ca, the EEVblog BM235 is being offered at C$283, C$295, and C$305, by 3 companies that all have ~60% positive ratings (which is pretty bad). The lowest-price seller ships from the USA, so there are likely to be additional duties and taxes added on delivery. The other two don't say where they will ship from. One says that their price includes duties and taxes, but the buyer has to pay them initially and then send the receipt to the seller to be reimbursed. The third doesn't have a stated policy. Total cost is likely to be about C$300 - if I get the item I ordered at all. This is riskier, less convenient, AND $50 more expensive than Amazon.com. (This is why I commented that the BM235 is "stupidly expensive" on Amazon.ca)All 3 of these should be exactly the same meter, and probably the same package of accessories, but the price variation is amazing.

author: dmmartindale
author_rank: Contributor
timestamp: August 01, 2024, 02:40:15 pm
content: I forgot to include the EEVblog store in the list. Their price for the blue BM235 is C$157. Cheapest shipping is $26, probably by boat (3-5 weeks), but DHL is only $31. Add taxes on arrival of about $20, for a total of about C$210. That's still more than Welectron, but better than Amazon USA.