GWInstek GOM-801 repair attempt - Now have documentation (Solved)

[guswin95]

Hello all,

I have been wanting a milliohm meter for a while and most DIY are low current, and most high-end meters are above the aficionado budget. Although I have nice collection of meters (mostly Fluke), for most values above say 1 ohm, the sub ohm is difficult to test unless you have a good milliohm meter. I was encouraged by the spec mentioning a current of up to 1amp in low settings. After quite a while found this one in eBay an as-is condition for a reasonable price. The owner stated it was used shortly long time ago, and then shelved for years. He sold it in as-is condition. It is a GW GOM-801 as the title describes. Cosmetically it's in excellent condition but it does behave strangely. Came only with User Manual. No power cord nor test leads.

Initial testing would show certain ranges working approximately to what it should be, but other scales are completely non-working. I needed a 4-wire test cable that took a bit to get, before decided to open the unit and giving it a try to repair. BTW, quite familiar with electronics repairs and have most equipment needed to attempt this.

It is well constructed, and nothing appears blown. A classic diode testing (using a meter in diode test) of all diodes found what I believe is an open diode. Furthermore, I found another Zener diode or more precisely an avalanche diode to be suspect. All voltage regulators and transistors show OK in passive test.

I am asking for help in this regard. The strange diode is labeled FR02-30 and believed to be a high reverse voltage high speed diode as indicated below:

https://www.alldatasheet.com/datasheet-pdf/pdf/460104/FRONTIER/FR02-30.html

The other diode marked ZNR1 and is a SA10CA as bellow.

https://pdf1.alldatasheet.com/datasheet-pdf/view/122748/LITTELFUSE/SA10CA.html

Both appear open with a simple meter. But unfamiliar with a diode with a forward voltage of 6.0VDC and a bidirectional device with 10VDC protection. Any practical ways to test? Hate to place an order in Mouser or similar for just 2 components.

Any help on anybody that has worked on these would be appreciated.

Note: Have not reached yet to GW Instek for support. Pictures will be posted next.

[Martin72]

Hi,

Initial testing would show certain ranges working approximately to what it should be, but other scales are completely non-working.

First of all, do you now have a 4-lead testcable and connected ?
Without this, most things won´t work as expected.
Then:
What´s the meaning of "other scales are completely non-working" ?
Pls more precise explanations...

[J-R]

A few thoughts:
You can get by with just using 4 normal test leads, no Kelvin clips specifically needed.

This model has quite a few different test currents, can you check them all out with your other DMMs and narrow down the issue more?

Also, be sure to try out calculating low resistances yourself using R=V/I.  Sub-10mOhm accuracy is easily reached, for example.

[bdunham7]

I wouldn't start on this by randomly testing components.  The thing should be simple enough to figure out.  It appears to have a 2000-count meter with fixed 200mV range and then 10X gain somewhere for the 20mR range, then a constant-current source with a pretty wide range from 100µA to 1A.  Is there an ICL 7107 or similar chip in there somewhere?  Can you post a good photo of the circuit board?

So tell us which ranges don't work and what they do.  If you have other meters, just connect an ammeter in series with one of the leads and a voltmeter across the inputs so you can keep track of what is going on.  Measure some wires or resistors and see what your other meters tell you--that will show you what direction to go.  And before you even go to that bit of trouble, try working the switches a few dozen times.

[guswin95]

Picture 1

[guswin95]

Picture 2

[guswin95]

Picture 3

[bdunham7]

What is the big IC that is partially shown on the top right of your first picture?

To reverse-engineer and fix something like this, I'd start with the largest and most unusual ICs and look up their datasheets and pinouts.  The LTC1052, for example, is a chopper amp and might be the 10X gain I referred to earlier or it could be part of the current source circuit.  In any case, once you determine how the meter behaves in each range (output current, voltage display) you can use the datasheet to determine which IC pins to measure at to see what it is actually doing.  You can also start tracing circuits visually.  I presume the tall heat sink is for the pass transistor for the 1A range--and maybe others--current source.

Edit:  I see an LM723 lurking below the LTC1052 in an area that seems separated from the upper part of the photo.   That lower section is going to be your current source circuit.

[guswin95]

Hello bdunham7,

You are correct. It has a 3 1/2-digit red LED ICL71XX based meter in it. Per some of the basic diagram it has a 10X gain op amp and several fixed current depending on the scale. The 200 ohm and above appear to work mostly. Did not fully check as I care mostly on the lower ranges.
The 2 ohm and 0.2 Ohm scale don't show any current on the circuit nor does the meter measure OL or similar. It fluctuates randomly. The 20-ohm scale shows a known 15-ohm resistor as about 16.2 ohm. And a 0.500-ohm resistor as 0.5 ohm if memory is correct.

The block diagram also shows a +15, -15, +12 and +5 and -5VDC supply. Did not fully validate all those voltages.

Have not tested the 0.02 range as I did not have a known resistor of adequate value at this time, although I think I have some and need to find them.
I bought a new nice Kelvin test lead that came in yesterday from Amazon which I used with my Keithley to validate a few resistors of above 1 ohm. My made test leads work as well. So that part is solved.

Very low resistance the wiring gets in a way of precision with normal multimeters. Therefore, the idea of having this unit. Need to record the current in the 2.0-ohm scale. That was one thing I did from the very beginning.

I carefully validated the switches, and they operate properly and are quite unused, look in pristine condition. I initially suspected them but was clear nothing changed after several cycles of changing the range switches.

Will document some values for tomorrow. Still the open diodes remain an issue.

Thank you all.

[guswin95]

Hello again,

The big chip half shown is the ICL7107 meter IC. The big heatsink is for a Toshiba 2SC5197 power transistor. And good eyes catching the LM723 regulator.

Tomorrow I will take detailed measurements on the working range and the non-working range. Both V and I.

Will post better photos also as I did quick phone pics to start.

Good night

[bdunham7]

Still the open diodes remain an issue.

Neither of the diodes you mentioned are testable with a typical DMM diode check function.  I don't know what the high voltage one does (where is it) but the other will be a protection device and even if it is blown it shouldn't stop your device from working--but whatever blew it might have caused other damage.  However, there isn't any evidence they are bad.

For testing purposes you really don't need to use resistors, at least at first.  You can just measure the current as shorted by your DMM and see if it is correct.  If the current is correct and the voltmeter part reads correctly, then barring some internal connection problem the milliohm meter should work.  I suppose it is possible that the compliance voltage might not be high enough, but that's not a very likely issue in the 20/200mR 1A range.  Your DMM is probably near the upper end of that anyway.

[Ugur]

Hello,
Take your meter to the ampere measurement stage and connect it to the outputs of the device, check if you read a constant current value in all ranges, for example, if it is the lowest stage, you need to read 1A.

[coromonadalix]

Are the switches  clean or have been cleaned up, you could have oxydation on them who would explain some problems ??

[ledtester]

What revision of the GOM-801 do you have?

fwiw I found this use manual for the GOM-801G which includes a schematic.

It's possible yours is the 801H. Still the block diagram might be the same and some of the blocks might not have changed too much.

[guswin95]

Hello all,

Work and life have delayed the continuation of the troubleshooting. As ledtester posted the manual, I found it independently. My unit has no letter after the 801 but the schematic is quite close, or almost identical. Components listed don't completely match but very close.

Today I concentrated on validating the power supplies, to ensure the basics are covered. From the 18-0-18 VAC supply rectified by BD202, after all the regulation it is quite close to the +15.0 / -15.0 VDC expected.

The supply for the 6-0VAC is converted to aprox +5.0 VDC at the emitter of Q204 which varies from +5.13 to +4.27 VDC depending on the scale selected.
R211, 212, 213, 214, 215 and 216 are exactly correct.

The supply 9-0-9 that eventually provides the +12 and +5VDC is pending. Hope to look into later today or tomorrow.

I am lost on some of the functionality of the LM723 variable regulator function. I remember using those to fix an output voltage with external resistors and if needed a power transistor to allow more current. Still trying to understand that part of the circuit.
I estimate the 3140 are the X10 and X100 amplifiers.

To some that suggested I measure the current at each scale with shorted leads, here it goes.

Scale       Current (A)
2K           0.000         Meter might not be sensitive enough
200          0.001
20            0.01
2              0.1
0.2           0.000        Something wrong here
0.02         0.046

I never got the expected 1A in the lower ranges. I tested D401 outside the circuit and appears OK, BTW. 1.46VDC forward drop.

I will continue investigating, so please keep up the suggestions. Work and life sometimes don't allow a more fluid communication lately.

Thank you all.

[bdunham7]

The current circuit is exactly the same for the two lowest ranges except that they use two different sets of switch contacts.  So getting two results, even though both are wrong, is a clue.

What you can do is set it for one of those ranges with the FORCE terminals shorted or your ammeter in place.  Then take a voltmeter set for ~10V and put the negative lead on the negative FORCE terminal, then use the positive lead to test the voltages at the other FORCE terminal, at both sides of R211 and then both sides of Q204.  The problem will probably be obvious at that point.

[Ugur]

There could be many reasons for this malfunction.
I suggest you check this first. LTC1052 (or ICL7650)
If this part is not defective, you can check other op-amps.
you can also check 2n2219 and 2n2905.
Maybe 2SK40!

[guswin95]

good evening, all:

Following bdunham7 suggestions, I shorted the FORCE terminals with the amp meter, and then with the switch selection in 0.2 ohms position measured across R212.

If memory serves me, I get 0.000 Amp and 5v at each end. The transistor Q204 has 8.87 V and about 5V at emitter.

With the switch at 0.02 ohm and same shorted FORCE terminals I get 0.026 Amp and 0.000V at both ends of R211. Plus, I get 0.00V at emitter of Q204.
I am not sure what it means, so your help is welcomed. Sorry it does not come to me the reason...

I managed to get a drop of Caig Deoxit in all the switches to help clean in case it helps. Also validated it is a GW-Instek 801H version (need better glasses

[guswin95]

Hello Ugur,

I have diode checked all semiconductors that I could. that is diodes and transistors. Have not looked in detail to the op amps. Will do that next.

Thanks for all the help.

[bdunham7]

With the switch at 0.02 ohm and same shorted FORCE terminals I get 0.026 Amp and 0.000V at both ends of R211. Plus, I get 0.00V at emitter of Q204.
I am not sure what it means, so your help is welcomed. Sorry it does not come to me the reason...

The problem is the switches.  Put the unit in the 0.2R mode then run two jumpers from the bottom (on the diagram) of R211 to the bottom of R207 and to the hi-side force terminal.

Edit:  alternatively, try engaging both the 0.2R and 0.02R switches at the same time, holding them in manually if need be

[guswin95]

Hello all,

And of course, bdunham7 (and others) are correct. The switches are flaky. Some resoldering, lube and cleaning; and everything works properly. Will do a bit more cleaning and check calibration is possible and slap together to join the bench.

I am grateful for the support this forum provides to people like me when questions arise. And sticking with me till the end.

Happy Holidays to all.

[coromonadalix]

Told ya  loll

[Brian of Romsey]

I'm a bit late to the party here.  I bought at GOM-801F at a HamFest in March 2024.  After a bit of physical cleanup and a small repair to a broken wire to the front panel PCB with the two ten-turn trim-pots it all seems fine.  All the power rails are OK, maybe the +12 Volt rail is a little low at +10 Volts, but it's an unregulated rail so I'm OK with that.  I haven't looked at ripple yet, not expecting any problems.  The six constant current test currents are also about right.

I have the schematic from the manual that 'ledtester' posted on 2022-12-13, thanks so much for that.  What I'm worrying about are the diodes that 'guswin95' mentioned in the original posting.  The schematic has loads of 1N4148 diodes and several simple half Watt Zener diodes (D201 & D202 16V; D301 9.1V; D203, D205, D301, & D303 5.6V) but I don't see either an FR02-03 or an SA10CA.  Where are these in the circuit and what are their Ref Des names please?  I'm worried that my unit has been messed about by a previous owner and doesn't have these.

For those who come after, the kelvin clips can be obtained cheaply from AliExpress, just search for "kelvin clips".  They also do leads that end with probes rather than clips for tracing shorts on PCBs (not quite a ToneOhm, but pretty good).

I haven't run through the calibration procedure yet.  I will need to source some precision resistors or get a loan of similar before I can do that.  Any thoughts on where I can get my hands on something like that in Melbourne Australia (not Florida)?