Protek 506 DMM Problem/Fault

[Krisby]

Hello badcaps.net Forum People,

I have a Protek 506 DMM which has a problem with the Diode Test Function and with the 4V range of the Voltage Measuring Function. I have posted this on three other forums, but have not been able to find out the source of the problem.

It seems that the Diode Test problem is linked with the 4V range Voltage Measuring problem, so I shall just focus on the 4VDC range to simplify things.

When I select the DC voltage function on the rotary switch of this DMM, the display alternates between showing 3.999V and 0V. If I test a 1.5V cell, the display alternates between 3.999V and 1.5V. If I test a 9V battery, the display shows a steady 9V. So that indicates to me that the fault is with the 4V range.

The Diode Test Function is stuck on displaying the word "OPEN" and the accompanying voltage of "3.999V". It should be showing 3V or less, which is the voltage used to do the diode test with.

I've checked out the components leading up to the A/D Converter IC, which is a MAX314CMH, in a MQFP-44 case (Metric Quad Flat Pack 44 pins). So it seems like the fault may lie within this A/D Converter IC. I have attached the data sheet for the MAX134, the schematic diagram of the Protek 506, and a pin configuration diagram for the MQFP-44 version of the IC, because it is not shown on the datasheet.

My limited understanding of this IC is that the ranges are determined via attenuator switches inside, which are linked to an external resistor network at pins 25-28 (RN1 on the DMM schematic diagram), and the voltage to be measured entering the IC at pin 24. When there is no voltage to be tested at the probes, pins 24 to 28 should all be reading 0V. However, I find the following voltages:

pin 24: 1.85V and 0V alternating
pin 25: 2.95V
pin 26: 0V
pin 27: 0V
pin 28: 0V

So, I am wondering if the problems I am experiencing are caused by a fault in the MAX134 A/D Converter IC (U2 on schematic diagram) or in the microcontroller (U1) that tells the ADC what to do, or some other componentry on the board?

Any ideas?

[megaraider]

Re: Protek 506 DMM Problem/Fault

The 1st thing i've noticed was a discrepancy in pin layout between the datasheet & the schematic.

Reading the datasheet "Ohms and diode measurement" section gives some hints on what voltage to expect depending on the range selected.

Looking at the schematic my polite guess will fall on:
-the zener diode(s), being ZD8 the 1st candidate... but not only (check all VRefs)
-the PTC protection resistors (likely to fail over time).
-slightly adjust VR2

[Krisby]

Re: Protek 506 DMM Problem/Fault

Yes, the datasheet for the MAX133-MAX134 shows a pinout of the 40 pin DIP version, not the 44 pin MQFP (Metric Quad Flat Pack). The datasheet also shows a 44pin chip on page 18, but that version starts its numbering from the middle of the top, not at the top of the left hand row of pins like the one used in the Protek 506.

I have tested all 15 Zeners, and they all test OK.

Both PTCs read 500 Ohms, as per the schematic.

You have suggested to "slightly adjust VR2". VR2 is connected to the VREF at pin 34. I shall give that a try, and test C31 which hangs off it.

[Krisby]

Re: Protek 506 DMM Problem/Fault

Have slightly adjusted VR2, and also turned to full lock in both directions, but there was no change in the symptoms.

I have ordered a new MAX134 from China, and will simply swap it for the old one. If that doesn't fix the problem, then it may well be the MCU.

[megaraider]

Re: Protek 506 DMM Problem/Fault

My limited understanding of this IC is that the ranges are determined via attenuator switches inside, which are linked to an external resistor network at pins 25-28 (RN1 on the DMM schematic diagram), and the voltage to be measured entering the IC at pin 24. When there is no voltage to be tested at the probes, pins 24 to 28 should all be reading 0V. However, I find the following voltages:
pin 24: 1.85V and 0V alternating
pin 25: 2.95V
pin 26: 0V
pin 27: 0V
pin 28: 0V

If pin 25 measures 2.95V then pins 26 to 28 should not read 0V since they're connected thru RN1 to a common point.
Take RN1 out and check if it's the MAX134 that is (wrongly) pulling that pins to ground.
Likewise, pin 24 should not be alternating, it should remain in a stable value.
Thus trace it back to see who is responsible for that voltage switch.

Also this evaluation kit schematic can shad some light...

As a final advise:
Rotary switchs are well known to fail over time!
In the event it will mess up the ADC settings (MAX134) and the common connection signal to RN1 (see the evaluation kit schematic).
.

[Krisby]

Re: Protek 506 DMM Problem/Fault

Yes, we seem to find ourselves at the end of the road....

Thanks for the Evaluation Kit download.... it would be an interesting tool....

OK, I might pull out RN1 and see if it has any effect on pins 25-28. But pin 24 should not have any voltage on it when there is no voltage being tested at the meter's probes, alternating or otherwise. When I manually select the next range up (40V), the meter performs normally, so the problem lies with the 4V range. I just hope that the origin of the alternating voltage is not the effect of some faulty commands from the CPU.

The meter has only had moderate use over the years, as I have been using another meter for daily tasks, so I can't see the rotary switch as being a problem. I have cleaned all the contact surfaces with isopropyl alcohol.

[megaraider]

Re: Protek 506 DMM Problem/Fault

... let's hope replacing the MAX134 will fix it.

In the mean time you can go thru the calibration procedure to check if all trimmers (VRx) are set to the correct value.
Found the manual online here (see last page).
.