Faulty MAX134CMH+D A/D Converter?

Question

I have been trying to troubleshoot a problem with my Protek 506 DMM, and I think that the problem may lie within its MAX134CMH+D A/D Converter chip, named U2 on the attached schematic diagram. The DMM has an issue with its diode test function and its 4VDC measuring range. These functions I believe are associated with pins 24-28 on the MAX134. Pins 25-28 are connected to a resistor-divider network component,RN1, which is connected to the rotary switch CN15 at "ATT-B", then finally to the positive test lead at "V" CN6. Pin 24 seems to connect directly to "V" CN6 via R1 and VR5.

Unfortunately, the datasheet available for the MAX134 does not include a pinout for the MQFP44 case version which this DMM has, but I found a hand-drawn image on a Russian forum.

Here is a screenshot of the Protek 506 schematic diagram:

Here is another pinout from the web:

I am not fully understanding how the relevant part of the MAX134 works, or how it is associated with the resistor divider network RN1 connected to pins 25-28, and the 10 Ohm input at pin 24.

Using another DMM, with its negative test lead attached to the "COM" CN5 socket of the subject meter, I detect the following voltages when the subject meter is on the 4VDC range:

Pin 24: alternating between 1.84V and 0V Pin 25: 2.987V Pin 26: 0V Pin 27: 0V Pin 28: 0V

Further, the common leg of RN1 shows a continuity with GND, at 0.2 Ohms.

On the display of the subject meter, when it is on the 4VDC range, it shows two continually alternating values: 3.999V and 00.00V. If there is a voltage applied to the test leads, it will display two continually alternating values of 3.999V and whatever the voltage being measured is.

When the meter is on the Diode Test mode, it will only display the word "OPEn" and the voltage of 3.999V, even when it has a diode between the test lead probes.

I've checked the componentry and rotary switch connections between the "V" CN6 socket and Pins 24-28, and everything seems within specs. MAX134's voltage reference Pin 34 measures 0.542V

Below is a photo of the display on my Oscilloscope, when connected to Pin 20 integrator Output, compared with figures 6 and 8 on the datasheet. The same results on meter's DCV or ACV inputs, and if there is no voltage being measured, or a 9V battery being measured.

Any thoughts, ideas?

Answer 1

Try to duplicate waveforms in figures 6,7 or 8. If you have no scope use your Aux-in Audio port and get the free scope S/W. DMM Clock is only 32kHz. (if you really can't find the SW, buy a new DMM.)

I suspect it may be related to the periodic Auto Zero Integration Phase which is showing all FFFFh as full scale 4.000 which you should see periodically on above. It matches the display change rate. Then dig deeper.

My SWAG debug methods include wandering my finger around all the traces with other hand grounded or open to add capacitance or crosstalk between two fingers or shunt stray noise or bridge cracked solder joints.