Author Topic: GB GDT-3190 Reads low in all modes... (Read 15157 times)

knockbill · « **on:** October 19, 2016, 11:10:02 am »

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...

knockbill · « **Reply #1 on:** October 19, 2016, 12:58:29 pm »

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...

ModemHead · « **Reply #2 on:** October 19, 2016, 03:47:22 pm »

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?

knockbill · « **Reply #3 on:** October 19, 2016, 04:56:28 pm »

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...

The Soulman · « **Reply #4 on:** October 19, 2016, 05:34:48 pm »

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!

knockbill · « **Reply #5 on:** October 19, 2016, 05:50:16 pm »

Sounds like a plan.. not really sure what to read... are these readings to ground? Are we looking for an out of value resistor?

The Soulman · « **Reply #6 on:** October 19, 2016, 05:53:06 pm »

Quote from: knockbill on October 19, 2016, 05:50:16 pm

Sounds like a plan.. not really sure what to read... are these readings to ground?

Yes, black probe goes in the com terminal of the faulty meter.

knockbill · « **Reply #7 on:** October 19, 2016, 06:04:11 pm »

OK,, I'll give it a try... Thanks...

knockbill · « **Reply #8 on:** October 19, 2016, 06:49:03 pm »

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..

The Soulman · « **Reply #9 on:** October 19, 2016, 09:02:41 pm »

Quote from: knockbill on October 19, 2016, 06:49:03 pm

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...

Your 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.

Quote

Just updated pic to include trace to chip... (.14VDC)... Is this where 1VDC should be?

Thanks for the input..

Well, 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 10K $\$\Omega\$$ resistor and the measured voltages seem to support that, but measured 2020 $\$\Omega\$$ ?, could be a in circuit measurement error.
Replacing R22 with a 20K $\$\Omega\$$ (2002) resistor should fix it, a standard 1/4W bodged in should also work.

knockbill · « **Reply #10 on:** October 19, 2016, 09:19:24 pm »

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..

ModemHead · « **Reply #11 on:** October 19, 2016, 10:31:07 pm »

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...

knockbill · « **Reply #12 on:** October 19, 2016, 11:13:27 pm »

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?

ModemHead · « **Reply #13 on:** October 19, 2016, 11:49:18 pm »

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.

knockbill · « **Reply #14 on:** October 20, 2016, 07:44:46 am »

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...

The Soulman · « **Reply #15 on:** October 20, 2016, 10:12:29 am »

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 way
down. (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 unlikely
2. 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.

knockbill · « **Reply #16 on:** October 22, 2016, 11:35:30 am »

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...

ModemHead · « **Reply #17 on:** October 23, 2016, 01:44:59 pm »

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 the data sheet for the original ICL7106, and the schematic for a generic DT830B or MAS830B, 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.

knockbill · « **Reply #18 on:** October 23, 2016, 02:10:36 pm »

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

knockbill · « **Reply #19 on:** October 23, 2016, 02:23:47 pm »

First pic is COM to Batt +,,, 2nd pic Com to Batt - ... This is the procedure I used before...

knockbill · « **Reply #20 on:** October 23, 2016, 03:11:37 pm »

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...

ModemHead · « **Reply #21 on:** October 23, 2016, 03:20:15 pm »

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 pm

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...

Sounds plausible. Use your Wavetek and measure the current draw from the battery. Normally it should only be a couple mA or so.

knockbill · « **Reply #22 on:** October 23, 2016, 03:43:02 pm »

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,,,

ModemHead · « **Reply #23 on:** October 24, 2016, 11:28:48 am »

Quote from: knockbill on October 23, 2016, 03:43:02 pm

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...

That 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 pm

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,,,

Something odd here. 30uA is not enough to run the GB meter, much less bring a (good) 9.25V battery down to 8.12V.

knockbill · « **Reply #24 on:** October 24, 2016, 11:54:54 am »

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!


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Author Topic: GB GDT-3190 Reads low in all modes... (Read 15157 times)

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