Products > Test Equipment
GW Instek GDM-8034 bench DMM: looking for schematic or calibration notes
floobydust:
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?.
dmmartindale:
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.
mqsaharan:
--- Quote from: floobydust on July 25, 2024, 06:47:35 pm ---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/view
http://www.bitsavers.org/components/harris/1991_DB301.1_Harris_Data_Acquisition.pdf
--- End quote ---
floobydust, 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 pm ---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.
--- End quote ---
Kleinstein, 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.
J-R:
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.
dmmartindale:
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.
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