Wavetek meterman 27xt multimeter : Ac - voltage measurement too slow

[ELANDREAKIS]

Hello from Athens, Greece !

I have a Wavetek METERMAN 27XT digital multimeter some 10 years now.
It has brand new 9V battery and it is clean inside.
I put the meter selector to measure AC Voltage at the range 750 Vac and then I press the ON button of the instrument. No voltage connected yet ! Then,
the instrument display starts from a very large indication, i.e >1000 V, and slowly counts down to zero ! If I repeat this procedure shortly afterwards ,then it starts from a smaller initial value and counts down to zeroagain !
Similarily, when I connect to a voltage source,e.g 230V mains, we have both count up and, after disconnection, count down to zero slowly.
When I try to measure lower voltages e.g 15V ac , then instrument gets confused and at times it keeps the reading on the display after disconnection !!
Presumably it is in the comparator stage and in particular some capacitor which doesn't charge or discharge properly. Whatever is the problem, is it feasible to locate the faulty part and replace it ? It is a very compact PCB !
What is your experience on this ?
I look forward to hear from you,

Thanks a lot for your time !

 

[BradtheRad]

Your meter has developed at least one malfunction.
There is the counting circuit and the latch circuit. It counts up (or down), until the DAC matches the incoming voltage. Then the latch changes state, conveying the count to the display. (This is the simplest method I've heard of, although I don't know that all meters use it.) After a moment it executes a new count.

Your meter has:
* extremely slow counter
* latch always sending, never hiding the count
* inconsistent as to when it starts or ends a count
* comparator no longer comparing

Years of use can subject a meter to a lot of stress and vibration. You turn the dial, put pressure on it, drop it, etc. Look for microscopic damage on the pcb. Hairline fractures in copper traces. Don't forget to look for a bad ground. This should be tested for in several places. Connect wire bridges to ground points.

It seems odd that the number display still works. You might repair the meter if you could build your own ADC and install it.

 

[BradtheRad]

Questions:
Does the problem occur only on the ACV range?
Does the meter behave normally at other times?

 

[ELANDREAKIS]

Questions:
Does the problem occur only on the ACV range?
Does the meter behave normally at other times?

in the DC Voltage range , problem is similar ! In particular when I first switch on instrument after some hours. E.g I measure an 1.5 V Battery in the 20V DC range and I get OverLoad !! After repeating a few times , it measures stadily 1.48 V .
Also, when first switched on , with no voltage connected, it may converge to a Non - Zero Indication !!
Thanks for your interest !
Athens 10 a.m

 

[ELANDREAKIS]

Thanks a lot for your assistamce ! I will take it to a friends Electronic Lab and see what we will be ableto do !
Thank you !
Athens 10 a.m

 

[srizbf]

7107/7106 types of dig meter ics are widely used.For these types,the capacitor responsible for integrating seems to be the culprit.
Digital meters are based on integrating type with counting for zeroing the values internally.

But Wavetek may have different one . still the principle is same.

 

[KlausST]

Hi,

what about a bad solder joint in the input section. A high impedance somewhere in series to the measurement path.

Schematic?

Klaus