thandar TM351 dmm wierd readings

[tekguy]

Hi all,i have a 1980's thandar TM351 multimeter,it seems to read oddly on the volts range,for example on the 2v dc range it reads -1v,on the 20v range it reads -8.2v,200v range reads -0.76v,2000v range reads -011v,heres the service manual attached,it seems ok on the restistance ranges also the diode range seems somewhere near,any ideas?,cheers.

[tekguy]

can anyone tell me how to zero the dc volts range?,cheers Paul.

[tekguy]

would c19 be the reference capacitor mentioned in the sm text,would that cause the issues if bad?.

[tekguy]

anyone!.

[robert.rozee]

hi,
    i have tried to repair a similar one, although branded as "sansei". the problem is unlikely to be in the 7106 and associated components, far more likely those push-button switches. in the case of my one the contacts had oxidised over the years (it had been in storage) and there had possibly been battery leakage as well. the end result was that none of the switch contacts were reliable, and electrical leakage between pins. the only solution would have been to unsolder the whole switch assembly, dismantle, and clean. i didn't bother proceeding.

sorry to be so negative   


cheers,
rob   :-)

[tekguy]

HI rob,i have sprayed the switches with contact cleaner and reflowed all the solder joints,also the battery compartment is spotless.

[Andy Watson]

Do you have another meter to test voltages? If so, take a look at the reference voltage across C19 - it should be about 1.2V

[robert.rozee]

if you short between pins 30 and 31 of the 7106 (IC3), and the displayed reading on the display drops to 0.00 (or close thereto) then the fault almost certainly lies in the switching circuitry.

attached is a marked-up copy of the schematic for when the 2v DC range is selected. RED is the positive input path, passing through the 9M dropped resistor (the upper half of a 10:1 divider), and through various switches to R23 and then into pin 31 (IN+) of the 7106.

BLUE is the ground path, going to pin 30 (IN-). pin 32 (COM) is also tied to ground, which is an OUTPUT from the 7106 that it holds a (precicely regulated) couple of volts or so below the +ve battery terminal. ground also connects via a string of resistors adding up to 1M that runs up to the bottom end of the 9M resistor. the string of resistors RP1, R3, R4, R5, R6 form the input voltage divider.

YELLOW is the 200mV reference input to the 7106.


the only other thing that could possibly cause problems is IC4. it controls the values of integration components attached to pins 28 and 29 of the 7106, essentially changing between 200mV full scale and 2V full scale. this is likely something to do with the ohms ranges, and even if not switched correctly would not affect zero on volts or amps ranges.

(addendum) however, if IC4 were defective and injecting odd voltages at its pins 1,2,3,4,5,13, then this could cause non-zero display with no input to the 7106. but i'd consider it an unlikely fault.


cheers,
rob   :-)

[floobydust]

I would take a few voltage measurements to see if the 7106 A/D is damaged or not. Normally it auto-zeros, so there is no zero-adjust trimpot etc. Only check C27 10uF might cause these symptoms as well, I'd outright replace it for its age.
OP, never power this from a bench power supply or wall wart etc., only use the 9V battery. Most multimeters have Input(-) floating at a potential, it is NOT the circuit ground which causes a lot of confusion and a blown A/D.
I have no idea why this DMM uses a reverse-pinout ICL7106RCPL unless Sir Clive got a good deal on them, which he was known for doing.

In DCV say 2V range, measure these voltages (using 9V battery(-) for ground) and post:

IN(+) Pin 31
IN(-) Pin 30

REF(HI) Pin 36
REF(LO) Pin 35

COMMON Pin 32

+V Pin  1 (should be ~9VDC)
-V Pin 26 (should be 0VDC)