Protek 506 DMM not performing diode test

[Krisby]

I did another test for voltages on pins 24-28, and came up with different results from my previous test of pins 24 and 25.  Now:
Pin 24: alternating between 3.999V and .0__ (numbers jumbling)
Pin 25:  "OL"
Pin 26:  .000
Pin 27:  .000
Pin 28:  .000

Shorting probes makes no change.  I am measuring these voltages by putting the negative probe of my test DMM on the "COM" socket, and the positive probe on what I am testing.

[Krisby]

floobydust: I'm not understanding where you say, "... check the resistor pack RN1 common leg is grounded OK..."  Do you mean that I should check to see if there is continuity between "ATT-B" at CN15, and "ATT-B" at the common leg of RN1?

[floobydust]

RN1 is a four-resistor pack with a common bus that is eventually connected to GND (COM) through the rotary switch CN15. It's something to check. I would measure the resistance from input lead (-) to ATT-B node at the resistor pack and see if that connection is actually happening.

I don't quite understand what you mean by pin 25 "OL". With the input leads shorted (on the 4VDC range/diode-test), it should be 0V there at pin 25 and pin 24.
Numbers jumbling I think is due to auto-ranging, it should stop when you manually force the 4VDC range or the diode-test function? But we don't know what DMM's firmware does when it's on the 4VDC range (for diode-test) and thinks it sees past that. It might up-range then see low value, then down-range and repeat.

My theories right now:
Q6 leaky or open connection to resistor pack or DMM IC has blown CMOS switches inside. But other functions/voltage ranges would malfunction then.
Pin 25 seems to have some strange leakage currents to it, keeping it up when it should have about 1/10 of Vin, especially it should have 0V with 0V input to the DMM, like pin 24.

[Krisby]

floobydust:  When you say that the common bus leg of RN1 is "eventually connected to GND (COM) through the rotary switch CN15", are you meaning that it eventually connects to GND (COM) via the positive test lead "V" CN6?  I say this because everything from the positive test lead to the common bus of RN1 carries a positive voltage, and the only connections to GND along that path is via the zener diodes.  So you are asking if there is continuity between the common bus of RN1 and the CN6 "V" positive test lead socket (via rotary dial switch CN15)?  Am I getting tangled in semantics here?

I managed to destroy the SMD transistor Q6 when removing it, so I can't test if its OK or not.  I am endeavoring to replace it with a 2N3905 PNP TO-90 case transistor, by carefully drilling three holes through the PCB adjacent to the vacant solder pads.  That's the plan. 

[floobydust]

"eventually connected to GND (COM) through the rotary switch CN15"...
I mean it (RN1 common bus, ATT-B) connects to CN5 (COM jack) through the rotary switch for every function- except ohms. The switch is drawn a bit confusing.
To test that connection, you would check continuity from input jack (-) CN5 to RN1. You could also just measure across C32 (by RN1), it should be a few ohms at most- if the switch/traces are working.

Too bad Q6 is mashed, try run it without and see if anything has changed.

[Krisby]

floodydust:  Unfortunately I didn't read your last post until after I had replaced Q6, and so I have not tested the unit to see how it responds to Q6 being omitted from the circuit.  Replacing Q4 with the 2N4403 TO-92 case PNP transistor has made no difference, and it tests the same way in circuit.

On both diode test and 4VDC ranges, there is continuity between the common pin of RN1 and the GND at COM CN5, being 0.2   I don't know how or why that is happening.

Measuring with my other DMM, putting the negative test lead to COM CN5, and then probing the MAX134's pins with the positive test lead, I get the following:

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

Common Leg of RN1:  0V

[Kartika]

Hi Do you ever fix this 506 DMM. I find one but the opposite  problem diode test always short and  resistor section float... Ill try make short video to see...
Output signal also do not work.. until now after 25y i will try fix it..
Do you think problem is in Max134

https://quartermile.ws/avi/prote506diode_short.mp4

[Krisby]

Hello Kartika,

No, I haven't fixed my Protek 506 DMM......... Yes, I think the problem is within the Max134 chip.  I purchased a new Max134, but I needed to practice soldering these surface mount devices (SMD).  Actually, I have just started to work with SMDs, having purchased some surface mount technology (SMT) practice boards. I am practicing using solder paste and a heat gun.  So, I will soon be skilled enough to attempt to replace the Max134 chip.

I would be interested to see any short video you make of fixing the Protek 506 DMM.  In its day, it was a good quality meter with lots of features, so it is frustrating to not have it working as it should.

[Kartika]

Im recently give up---   

[Krisby]

Hello Kartika,  You've given up?  I just looked at the video you posted, and can only suggest that the contacts for the rotary dial might need to be cleaned with isopropyl alcohol.
I never did get around to soldering in the replacement MAX134 chip.  But, I found a brand new Protek 506 DMM for sale on the internet, from INDIA!  So I ordered it and received it in the mail.  My old one came with the software for computer display on a "Floppy Disk", but this new one came with the software on a little "Compact Disk" (CD).  I've never used the software anyway.
If your Protek 506 DMM still isn't working, I could pay for the postage, and you could send it to me, for me to use it as spare parts. 
Good luck!