Help Fixing a DMM

Thread Starter

Krisby

Joined Mar 13, 2020
15
Hello Everybody,

Is this the right place to ask for help fixing my Protek 506 Digital Multimeter? I don't see any repair or troubleshooting forum at allaboutcircuits.com, so I don't where to post my issue. Please advise.
 

SgtWookie

Joined Jul 17, 2007
22,221
Yes, you found the right place!

As SamR asked, what problem(s) are you having?

Does the display turn on in any mode? If not, your battery may be low or dead.

Had you been trying to measure resistance, and actually put the test leads across a voltage? If so, you've blown a fuse. **WARNING** USE ONLY EXACT SAME REPLACEMENT FUSE

Were you trying to measure current, and accidentally went across too high a voltage? Blown fuse. **SEE ABOVE WARNING**

Are your test leads old? Try putting the meter on the 2k Ohms scale, and measure the resistance of the test leads, one at a time. If either reads open, you can fix it or (better) buy a new set.

Check the soldering around where the panel jacks meet the circuit board using a magnifying glass. Any cracks? If so, unless you are REALLY VERY GOOD at soldering, send it in for servicing.

Multimeters are safety items and need to be maintained by people trained in their repair. You can do things like replace the battery, test leads, fuses, etc. But really, anything more than that needs to be done by a pro, and calibration needs to be performed afterwards.
 

Thread Starter

Krisby

Joined Mar 13, 2020
15
Thanks for the responses. No worries about poking around inside my multimeter, as I only use it for low voltage DC testing. In truth I must disclose that I have posted this issue on two other forums, but have not got the right answer yet, so I am casting the net farther, in hope of catching the right information.

I do have a lot of information about this Protek 506 DMM, and can post it if requested.

The problem is that the Diode Test Function is not working, and there is an issue with the DCV and ACV Measurement Function, but everything else works OK. The Diode Test is supposed to indicate "OPEN", "SHORT" or "GOOD", along with the voltage detected. However, it only displays "OPEN" and 3.999V. When switched to DCV Measurement, it alternately displays "3.999V" and "00.00". If a 9V battery is being tested, the display alternates between "3.999V" and 9.00V. The fact that the meter is displaying "3.999V", indicates perhaps an issue with the 4V range. In the Diode Test Function, when "OPEN" is displayed, it should be indicating a test voltage of 3V, not 3.999V.

I've tested all the likely component suspects leading up the the MAX134CMH+D, identified as U2 on the attached schematic diagram, and they all seem OK. The MAX134 is an Analog to Digital Converter, and has a MQFP44 case (Metric Quad Flat Pack with 44 pins).

I measure the following voltages at U2:
pin 24: alternating between 1.84V and 0V
pin 25: 2.987V
pin 26: 0V
pin 27: 0V
pin 28: 0V

I have attached the datasheet for the MAX134, and I would draw your attention to page 12 which shows the waveforms of the Integrator Output, and a photo of what my oscilloscope sees at pin 20 of the MAX134 on my Protek 506 DMM. There seems to be some similarity with figure 8 on page 12, but there is a lot of resonation/oscillation.

Any ideas?

MAX134CMH_MultimeterChipset_Pinout.png
 

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Thread Starter

Krisby

Joined Mar 13, 2020
15
Strange: There appears to be a time limit of 10 minutes after a post, in which editing can be done! In my post above I had deleted the photos of my multimeter because of excessive size, but, there they are! Anyway, in the two photos, the meter is showing what appears on the display when on the Diode Test Function without a diode between the the probes, and the second photo shows what is displayed when the probes are shorted together. It should read "SHORT".
 

Phil-S

Joined Dec 4, 2015
195
Looks as though this meter is discontinued so you might have had your money's worth.
Unless there is something obvious on the board, it might be time to let it go.
I don't think I would trust it other than for very approximate readings.
The only things I've bothered to fix on a DMM (Fluke 83) were a slightly iffy display that just needed to be reseated and kept in place with epoxy, cleaning up the elastomer keypads or replacing fuses.
It's a real workhorse and is the only DMM I have that I trust.
 

Thread Starter

Krisby

Joined Mar 13, 2020
15
Looks as though this meter is discontinued so you might have had your money's worth.
Unless there is something obvious on the board, it might be time to let it go.
I don't think I would trust it other than for very approximate readings.
The only things I've bothered to fix on a DMM (Fluke 83) were a slightly iffy display that just needed to be reseated and kept in place with epoxy, cleaning up the elastomer keypads or replacing fuses.
It's a real workhorse and is the only DMM I have that I trust.
Yes, the Protek 506 is discontinued, and Protek no longer gives support. Because it was my "good" multimeter, I kept it in reserve, so I was much miffed when it developed a fault after so little use. Because of that fact, I heroically soldiered forward, much like Don Quixote! Sigh.......

Anyway, I came to believe that it was the MAX134 analog to digital converter which was at the heart of the problem, so I have just received a new ADC IC and have soldered it into place. Yes, the original problems that I complained of initially have been fixed by replacing the MAX134 ADC, but now there are two new problems!

Problem No. 1 is with the Continuity Test Function: When that function is engaged, the display reads "OPEN", and "OL" under it. If I short out the test probes, it will briefly display "SHRT" and make a brief buzz from the buzzer, before returning to displaying "OPEN" and "OL". Alternatively, if I engage the Continuity Test Function, the display reads "SHRT", and the buzzer is on continuously, even though the test probes are not shorted together! A third fault is that sometimes the buzzer will sound when "OPEN" is displayed, and the buzzer stops when the test probes are shorted together, and the word "SHRT" is being displayed.......... functioning exactly opposite of what it should be doing!

Problem No. 2 is with the Diode Test Function: Several faults alternate on this function. Without testing any diode with the test probes, the display will initially read "OPEN", with a voltage of 1.2V, and the voltage will continue to fall until it reaches 1V, and then the display reads "GOOD". The display should initially read "OPEN" with 3.029V (tested on my other meter), and either "GOOD" with the voltage displayed, or "SHRT" with 0.000 displayed. Sometimes when this function is engaged, but the test probes not touching anything, the display reads "SHRT" and a voltage of near 0V, which steadily rises to 5V, where the display will switch to "GOOD". I note that if I connect another meter to the test probes, I see a voltage of 3.040V is present, even though the display on the subject Protek 506 is showing "GOOD" or "SHRT" with a rising or falling voltage, but there is nothing being measured, and the voltage at the test probes is a steady 3V!

Perhaps the two problems above are linked?. Is the new MAX134 ADC IC functioning perfectly, and this is new fault? One strange thing is that after I put the new IC in, I found that the small .5A fuse had blown, so maybe I had touched the connector to the 9V battery the wrong way around? If so, maybe some diode or zener has blown?

Or maybe the fault is primarily with the main microcontroller IC, and has been all along? I am thinking/hoping that there may be some discrete component that is causing the current problems with the Diode and Continuity functions. Any Ideas out there?

Update: Just had an idea of swapping the Printed Circuit Boards (PCBs) between my well-working Protek 506 and the Protek 506 which is the subject of this Technical Repair post. By that means I could isolate the fault to one of the two PCBs in the subject meter. The lower board, described as "MAIN" board, has on it the 100 pin microprocessor IC, which contains the digital multimeter software program, and the top board has the MAX134 ADC IC.

Is there any danger in swapping boards around? I'm concerned that some "glitch" may happen with the microcontroller, because it was connected to a top board of a different version number. Compatibility issues?
 
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Phil-S

Joined Dec 4, 2015
195
There are too many things going on here to pin the fault down.
There probably is some microprocessor or microcontroller involved with software or firmware that might be corrupted.
Have you identified the IC?
My Fluke has several fuses in it a large cartridge fuse for Amps current range and a smaller one for the milliamp range.
Fuses can go accidentally, but are a warning that all is not right.
Of course there is a danger of swapping boards.
If you have one good working meter, then personally, I would leave it alone.
I can fully understand that you want to get the other meter running again and I do the same.
In fact I am replacing many components in an old Thurlby bench power supply because it is still useful and I couldn't throw it out.
The difference is that it is very simple, all the components apart from the display chip are passive.
I also have a lot of dead PCBs, with not a hope in hell in getting them working again. Most are not even worth trying to get components off. They are just awaiting their appointment with the rubbish bin, one day.
 

Thread Starter

Krisby

Joined Mar 13, 2020
15
Thank you Phil-S for your response.

After determining that the two top boards were of the same version (although there were some differences to be seen) I bravely connected the top board of the "good" meter onto the bottom board of the subject meter, and Voila! .... everything works perfectly! So, my problem has now been effectively halved! The bottom board has on it the Samsung microcontroller/microprocessor IC, which contains the DMM software/program/coding, and if that IC was bad, then I would have to consider what the best recycling options for the subject meter would be.

The further advantage of having a duplicate meter is that I can check each component and pin of the ADC IC, of the subject meter by comparing it with the good one.
 

Phil-S

Joined Dec 4, 2015
195
Well done, nothing ventured nothing gained.
It will be interesting to hear if you can pin down the fault on the fourth board.
I'm at a disadvantage of not having the boards in front of me, but if the dodgy board has less complex components on it, it should be fairly easy, even if you replace components one by one, to get there eventually.
Be quite sure it's not down to the process of disconnecting and reconnecting boards.
DMM manufacturers like to use elastomeric connectors and keypads that can be troublesome.
If you have access to a magnifier, check every detail, damaged tracks, signs of over-heating on components.
It goes to show that the uP is pretty robust.
Sometimes a good search on Google can throw up something where someone has had a similar problem and I was surprised to find a very old item on recalibrating the very old Thurlby PSU that I had refurbished (all the pots had dodgy tracks)
 

Thread Starter

Krisby

Joined Mar 13, 2020
15
I have been trying to map out the relevant circuit of the Diode Test Function, but that is no easy feat! I've noticed a difference in voltage between Meter 1 (M1), which is subject ill-working meter, and the well-working Meter 2 (M2), to do with the RMS input, pin 36, on the MAX134 ADC IC, That pin 36 gets it's input from pin 2 of the AD737 chip, which I think has a dedicated RMS function. Also attached to that pin 2 is a pair of IN4148 diodes. The Anode of one is connected to "Digital GND" and its Cathode is connected to pin 2 and the Anode of the other diode, which in turn has it's Cathode connected to the battery positive. On the schematic diagram they are named D1 and D2.

Curiously, I get different measurements of these diodes between M1 and M2: With M1, D1 is forward biased (FB) at 0.624V (1.7 M Ohms) and reversed biased (RB) at 2.968V (8.3 M Ohms) and more or less ditto for D2. With M2, D1 is FB at 0.63V (1.775 M Ohms), and RB at 0.634V (1.83 M Ohms), and again more or less ditto for D2. Of what relevance this is I don't know, nor do I know why there is the difference between the Meters. Is there a problem with Diodes 1 & 2 on M1, or is the problem relating to the AD737 chip which they are connected to, and out of which is a connection to pin 36 on the MAX134. With M1, the voltage at pin 36 is 0.28V and with M2 it is 0.005V.

Given that these meters are no longer manufactured or supported, if I was running Protek in Korea, I would publish all relevant diagnostics/troubleshooting info to the web!
 
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