Author Topic: Protek 506 DMM not performing diode test (Read 4236 times)

Krisby · « **on:** September 30, 2020, 06:47:01 am »

Hello eevbloggers,

I have a Protek 506 digital multimeter, and the diode test function does not work. I have checked the fuse and the leads. When I switch to the diode test function, the screen displays the normal word, being: "OPEn", because there is no connection between the probes. When I short the probes together, the word "Shrt" should appear on the display, but instead the screen still displays the word "OPEn". I have the schematic diagram of the circuit, and I see there are a couple of components that could be checked. It would appear that the microprocessor is not getting an input signal from the leads, or that there is some fault in microprocessor program. Any ideas how to proceed?

Cheers,

Krisby

coromonadalix · « **Reply #1 on:** September 30, 2020, 10:23:50 am »

check input fuses if they are any ?? is the dial contacts okay ??

shakalnokturn · « **Reply #2 on:** September 30, 2020, 12:07:34 pm »

+1 with coromonadalix.
How does it behave on other functions?

I have one of these DMM's around, if you're really stuck after some troubleshooting and are good enough at keeping the thread alive I may go to the bother of making some measurements for you.

retiredcaps · « **Reply #3 on:** September 30, 2020, 07:41:36 pm »

The fuses are usually only used to protect the mA and A functionality and will not affect diode mode.

When you put the meter in resistance mode and short the probes, do you get 0 ohms?

MosherIV · « **Reply #4 on:** September 30, 2020, 07:46:18 pm »

I you have another dmm, test the voltage of the protek 506 when in diode mode.

It could be the rotary switch is not making good contact.

Krisby · « **Reply #5 on:** October 01, 2020, 01:47:07 am »

Thanks all,

Yes, some good ideas. Fuses are OK, have swapped probes.
Testing the voltage output when on the diode test function, I read: 2.991V. My other Protek 506 reads just over 3V, so I guess that is OK.
When on the resistance measuring function, shorting the leads shows 0.000V on the display, and the continuity tester works OK. The ammeter function and voltage testing are working, and the other functions display what they are supposed to when selected, but I haven't actually tested their functions.

That leaves the idea of cleaning the rotary switch contacts, and I think there may be relevance there, because I have a second Protek 506 which has an intermittent fault with the diode test function, sometimes displaying "Shrt" when selected, even though the test probes are not shorted. When this second meter shows "OPEn" upon selection, it will test diodes correctly.

So, cleaning the rotary switch contacts is what I shall do next.

helius · « **Reply #6 on:** October 01, 2020, 02:37:04 am »

Use care when you disassemble the switch, the contacts therein have a strong desire to emigrate into the carpet.
Another possible suspect is zener ZD8, part of the current source for the diode mode.

Krisby · « **Reply #7 on:** October 01, 2020, 02:55:24 am »

Yes, helius, the schematic diagram gives a value of 6.2V for that Zener, and above it there appears to be 2K resistor labeled "RB", so I'll check them when I open the meter up. When taking apart the rotary switch, I'll put the meter on a white sheet so I can find any bits that go "sprong". I'll test the Zener and the resistor first, which may fix the problem.

Cleaning the rotary switch contacts will have to be done later when I have more free time. I've just had some thoughts about the rotary switch idea, and I would like to know how a DMM actually performs its diode test function. I understand the theory and practice of testing a diode, but I am assuming a DMM has a program in the microcontroller that actually performs the test and displays the results. If that is the case, then the rotary switch is only turning on and turning off the microcontroller input for the diode test function, and therefore it seems unlikely that the rotary switch can be causing the problems associated with my two DMMs. Perhaps there is a "bounce" "de-bounce" issue with the switching which affects the program functioning?

shakalnokturn · « **Reply #8 on:** October 01, 2020, 06:29:14 am »

Basically a constant current is output to the V/ohm socket, voltage is measured for the test, anything close enough you 3V will display as "open", close enough to 0V as "short" in between should read the diode's forward voltage.

You could also check that both meters output similar current on diode test.

Krisby · « **Reply #9 on:** October 01, 2020, 12:23:44 pm »

In regard to testing ZD8, which is part of the Diode Test circuit, I have read that a Zener Diode can be tested in circuit using the Diode Test of a multimeter, and there should be voltage reading of between 0.5 and 0.8 when forward biased and 0.0 when reverse biased. So, using my other Protek 506 on diode test mode, ZD8 tests 0.7V, and ZD4 near it also gives that result. So, should I conclude that ZD8 is OK, or should I remove one lead from the circuit board, and test it that way?

The rotary switch contacts look OK, but I shall clean them with isopropyl alcohol.

Krisby · « **Reply #10 on:** October 01, 2020, 01:05:25 pm »

Have cleaned rotary switch contacts with alcohol and put meter together, but, no change!

I have noticed something, though.... If I have the rotary switch turned to the Ohm/Continuity tester, and hold the probes together (short circuit), then very slowly turn the switch to the Diode Test function, there will appear on the display for a fleeting moment, the word "Shrt", before it displays "OPEn". The display should read "Shrt" when the probes are shorted in the Diode Test function. So, it is very curious that it displays the correct word "Shrt" for just a few milliseconds, before it displays the unchanging "OPEn", even though the probes are shorted out. So, it appears that the meter wants to display the correct indication, but then it is quickly overridden by the false indication. So, is it some faulty component, or is it a problem in the microcontroller? I think I may try and bend the spring contacts on the rotary switch so that they press more firmly on the circuit board contacts.

Krisby · « **Reply #11 on:** October 01, 2020, 11:51:51 pm »

I think it is relevant that the voltage displayed along with "OPEn" is 3.999V, given that it is a 4,000 count meter, maybe it is indicating that the voltage in the test circuit is over range, because it should be no more than 3 volts. That suggests a component failure rather than a programming issue.

floobydust · « **Reply #12 on:** October 02, 2020, 05:58:48 am »

Awww it uses the MAX134 I never saw the DMM IC make it into a multimeter til now.

In diode-test, the DMM IC is just reading voltage on the 3.999V range (at the V jack) and there is a current-source (R8 2k ohm pull-up switched in to +9V & 6.2V zener and PTC2). The MCU seems to be the one deciding what voltage is open/good/short when P50 gets switched to let it know it is in diode-test mode.

I would see if the 4V range works on DCV, and then on diode-test put a 10k potentiometer across the input and dial in (can measure with another DMM) say 1V and see what it reads. It seems to be stuck on OPeN but also the voltage displayed is wrong, why is that also stuck at full-scale.

Krisby · « **Reply #13 on:** October 02, 2020, 12:49:51 pm »

Wow, thanks for all the info.... I can't digest it right now, but I wanted to add that the subject meter, while indicating 3.999V in the "OPEn" display, I actually measure 2.99 to 3V, which is what it should be. Have removed ZD8 as well as ZD4 from the PCB, and will put them in a test circuit to see if they are up to the 6.2V rating. Testing them with my other meter's diode test function, indicates that they are good.

Krisby · « **Reply #14 on:** October 03, 2020, 06:38:53 am »

floobydust: I checked the DCV function, and it alternates between 3.999V and 0V (maybe a couple times a second). When I put the test probes on to a 1.5V cell, it alternates between 3.999V and 1.49 or 1.50V. In the higher voltage measuring ranges, it is functioning normally.

I tried the Diode Test Function with the 10K pot and 1.2K resistor in series between the probes, as you have suggested, but the display is unmoved from "OPEn" and "3.999". On my other Protek 506, that test delivers the desired results, cycling through "OPEn" ..... "Good" ....... and "Shrt" at the appropriate voltages.

So, it would appear that there is some issue with the 4V range, and that is ultimately delivering a high 3.999V to P50 (pin 67 of the MCU).

Do you have any general idea of how I can track down the fault? Presumably the voltage reading picked up through the diode being tested goes through the ADC (Max 134) before it is fed to the MCU. I can trace the circuit through PTC2 and past R8 and ZD8, but there I lose the trail.

MosherIV · « **Reply #15 on:** October 03, 2020, 09:57:41 am »

Dmm usually route the signal being measured thruogh the rotary switch through to resistor divider network before reaching the ADC.

If it is not the rotary switch (try wiggling and pushing it while taking reading to see if it stableizes) then it is most likely a dry joint somwhere in the circuit for the 4V range.

Probe components in the 4V range with something non conductive if you can trace it.

Freezer spray and heat gun on spot area of the pcb will help.
NOTE: Avoid area around ceramic resistor divider, it is delicate and will not like temperature difference across it.

Krisby · « **Reply #16 on:** October 03, 2020, 12:32:08 pm »

I think I see on the schematic diagram where the signal is coming through the resistor divider network before reaching the ADC, but I don't see where the relevant signal exits the ADC. Where would I find the components for the 4V range?

Krisby · « **Reply #17 on:** October 04, 2020, 03:42:01 am »

Using the continuity function of my other DMM, I have traced continuity from the Positive test probe socket to pins 25-28 on IC U1 (Max 134), which I assume covers the path that the Diode Test current is flowing. The four resistors in RN1 (resistor divider network) all show the $\$\Omega\$$ that they are supposed to. I can't figure out where the circuit goes from U1. There is a connection from the rotary switch to the diode test pin of the MCU U2, being P50 (pin 67), but that might be just to switch on the diode test function, and not to process the current/voltage from the test leads on a diode. I have attached the Schematic Diagram for the Protek 506, and would immensely appreciate if someone could make some relevant sense of it. Perhaps a knowledge of hieroglyphics is needed.

robert.rozee · « **Reply #18 on:** October 04, 2020, 12:24:10 pm »

have you checked to see that PTC1 is not open-circuit? it looks like it is shared with the mV range, as well as (i think) the capacitance check).

cheers,
rob :-)

floobydust · « **Reply #19 on:** October 04, 2020, 09:41:56 pm »

Just on the 4VDC range (not diode-test), readings are not stable?

MAX134 4V and up input is the "10MEG" resistor to 10M pin 24 and it would switch-in divider resistor 1.11MEG to scale 0-4V down to 0-400mV for the A/D. There is a separate input pin just for 400mV range.
With 4.00V coming in, I would expect 0.363V on the other side of the 10MEG (R1+VR5) at pin 24 and also across the 1.11MEG at pin 25. Your multimeter will load it down a bit so I used a DMM with 10MEG input resistance for that number.
Check what the divided down voltage looks like. If it is noisy or weird, I would suspect Q5, C58. The 1.2V reference is OK?
I think PTC2 gets connected to GND which lowers the 9V battery along with R8 to the diode-test voltage. Check the PTC is not noisy as well.

Krisby · « **Reply #20 on:** October 05, 2020, 02:00:19 am »

robert.rozee: PTC1 is OK, I measure 492 $\$\Omega\$$ across it, and the spec is for 500 $\$\Omega\$$ .

floobydust: Yes, I think it is only on the 4V range that there is an oscillation between displaying 3.999V and the true reading. When I test a 9V battery, it is rock solid, but I haven't measured any higher voltage ranges.

How would I test if a PTC is "noisy"? In this case, could I replace either of the two PTCs with a 500 $\$\Omega\$$ resistor, and see if there are any changes?

I'll go ahead now and test the various components and circuits you have suggested.

Krisby · « **Reply #21 on:** October 05, 2020, 12:20:48 pm »

floobydust: When on the 4VDC range, I get a voltage at VR5, SG1, PTC1 and PTC2, which alternates between 0.985V and 0V continuously. Pin 24 at the MAX134 alternates between 1.858V and 0V. Pin 25 is steady at 2.988V.

On the Diode Test Function, I get a steady voltage at VR5, SG1, PTC1 and PTC2 of 2.990V. Pin 24 is at a steady 1.857V and Pin 25 is steady at 2.986V.

At the Voltage Reference Pin 34, I get a voltage of 0.542V.

At the combined base and emitter of Q6 (that is what is printed on the PCB, but I think it is the Q5 on the schematic diagram) there is a voltage of 1.236V. I tested Q6/Q5 by putting the positive probe of a DMM set on Diode Test to the collector, and the negative probe on the base/emitter, and I got a voltage of 0.657V. Reversing the probes gave a reading of 0.981V.

You have suggested that I test C58, and so I got a reading of 290 uF (micro Farads). On the schematic, is C58 the cap that is joins pin 25 and the ground? The schematic diagram has a crease that runs right through it, so I cannot read the I.D. nor the value. Does it say 50 pF?

floobydust · « **Reply #22 on:** October 06, 2020, 09:16:42 pm »

Pin 25 should be about 1/4 of the input jack voltage, when the DMM IC is on the 4V range. Careful it will autorange and switch out (pin 25) and go to other ranges hunting unless manually on the 4VDC range or in diode-test.

With the multimeter input leads shorted, all mux divider pins on the IC should read around 0V - pins 24-28 unless the IC has a fault, assuming Q6 is not leaking. Or I'm wrong in not understanding how the MAX134 works.

If you're getting 3V there on pin 25, then the DMM IC mux switches might be damaged or check the resistor pack RN1 common leg is grounded OK "ATT-B" by the rotary switch contacts at CN15 (in diode-test, DCV etc.). It might be an open-circuit so the divider is lifted.

The ref voltages are fine 1.236V pin and 0.542V pin 34. C58 (on pin 25) looks like a 100pF or 1000pF but would make the meter read low if it was leaky or bad. Leave it for now.

Krisby · « **Reply #23 on:** October 07, 2020, 01:39:34 am »

Thanks floobydust. I shall have a think about what you have written, but in the meantime, what does "mux" stand for?

What is labelled as "Q6" on the PCB (Q5 2n3905) on the diagram), might be problematic. Tested in circuit with a diode test, it reads "good" and .655V with positive probe on the collector, and the negative probe on the conjoined base/emitter. When the probes are reversed, it should read "OPEn" and show the 3V in the test circuit, but instead it is reading "good" and .981V. Maybe that is caused by being in the circuit and would test OK if removed. Another strange thing is nearby on the PCB, is something in a TO92 type case with one of the three legs cut off. It is labelled on the board as "ZD12", and maybe it is the 1.2V ZD6 in parallel with the emitter/base of Q6/Q5. Testing it in circuit with a diode tester, it gives a voltage both directions, and again that may be a quirk of being in circuit. Below is a photo of the component, but a search of what is printed on it's flat side has produced no results.

floobydust · « **Reply #24 on:** October 07, 2020, 02:33:00 am »

I think you could temporarily pull out Q6 or lift a leg, and see what happens. It is suspect, and just acts as a diode-connected transistor.

Multimeter IC's have an array of CMOS switches inside, first to switch-in or out the range voltage-divider resistors. The MAX134 is really a 0-400mV A/D converter, so on the 4VDC range it needs to see 1/10 of that by using the 1.11MEG resistor along with the 10MEG input resistor to divide it down. So internally the IC switches in the 1.11MEG, and the multiplexer or mux as I call it are also CMOS switches that direct the measurement between ACV or Current or Ohms functions. "Mux" isn't really the correct term for the CMOS switch array.

ZD12 looks like a LM385B 1.2V reference diode with the Feedback pin internally connected to (-). I guess they saved a pin.


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Author Topic: Protek 506 DMM not performing diode test (Read 4236 times)

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