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GPO test meter fault

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mick H14/06/2021 07:30:52
753 forum posts
28 photos

My SA9083 model ex GPO test meter has developed a fault on the 5 amp DC current sector. The 500 mA and 50 mA sectors work OK. Can anyone with experience of this meter offer any advice on remedying this situation.

Mick

Anthony Knights14/06/2021 07:59:28
532 forum posts
230 photos

I am not familiar with this particular meter, but I have several "cheapish" digital meters, which all have a fuse in the high current measuring circuit. Current is usually measured in test meters by actually measuring the voltage drop across a small resistor, in series with the supply. It's possible there is a fault with the 5 amp range resistor or it's associated circuitry. (i.e. open circuit joint or faulty switch contact. ) The problem is, you will need another test meter to check it out. Hopefully, someone familiar with this item will be along soon.

Regards Anthony

Jon Lawes14/06/2021 08:03:55
avatar
596 forum posts

Current measurement faults on meters of this style are often an internal fuse blown as Anthony mentions first.

mick H14/06/2021 08:06:04
753 forum posts
28 photos

Thanks Anthony. This meter is an old analogue type and whilst I also suspect a fuse problem I can't find a fuse.

Mick

John Haine14/06/2021 09:31:42
3940 forum posts
227 photos

A quick Google suggests that service manuals are available. The meters are I think still made!

Dave Halford14/06/2021 10:03:24
1590 forum posts
16 photos

My old meters multirange no12 (no one used the makers id) and somewhat earlier than your SA9083 had a fuse in the battery compartment.

SillyOldDuffer14/06/2021 10:30:46
Moderator
7341 forum posts
1617 photos

As the meter works on other ranges, I suspect the rotary switch. When the meter is switched to the 5A range it connects the movement and a shunt resistor. If the contacts are dirty, worn, or bent the selected range won't work.

This circuit is typical, note the rotary switch uses two poles for current ranges.

multimeter.jpg

Not familiar with the GPO meter. Some meters I've been inside are easy to trace and test, others hide stuff behind layers of other stuff. Rotary switches are often built-in, part of the PCB, and not replaceable. However, my experience is mostly with inexpensive meters, designed on the assumption they won't be maintained: glue, rivets, plastic clips, non-standard parts, no circuit or service manual etc, etc. A GPO meter built for solid reliability is more likely to be fixable than a cheapo meter, but much depends on the way it's built.

Can you post a photo or two of the innards?

Dave

Baldric14/06/2021 10:49:17
169 forum posts
16 photos

It looks like this could date from the 1930-now, so an idea of age would be good...

**LINK**

Baldric

mick H14/06/2021 13:50:15
753 forum posts
28 photos

img_2498.jpgimg_2497.jpgimg_2496.jpgimg_2495.jpgimg_2494.jpgimg_2492.jpgPhotos are now attached. They don't show a lot though because the circuit board has soldered tamperproofing by the look of it. (good thing when I am wielding a screwdriver).img_2493.jpg

mick H14/06/2021 13:51:32
753 forum posts
28 photos

Made a bit of a hash of the photo upload. Hope they are of some use.

Mick

Howard Lewis14/06/2021 18:08:34
5036 forum posts
13 photos

Having a PCB, looks to be within living memory.

Don't know the meter.

From what you say, the fault could lie inn the switch contacts for the faulty range; or that particular shunt may have gone open circuit.

Being ever the optimist, could it be a dry joint somewhere?

A broken track could be repaired by soldering a solid wire acros the crack.

Howard

john fletcher 114/06/2021 18:47:00
693 forum posts

Hello Mick, if you go to "Vintage Radio" there several ex GPO employees who will most likely have a circuit diagram and I'm sure, have the answer to your meter problem. Also there are some AVO meter experts there as well should anyone need it.. John .

SillyOldDuffer14/06/2021 19:10:39
Moderator
7341 forum posts
1617 photos

I believe Mick's meter is still in production, about £150 retail.

Looking at the photos, good news is the meter uses conventional components rather than SMD, bad news is the need to dismantle everything to get at the other side of the board. It's necessary to see what's what and make replacements.

If it were mine, I'd be inclined to put up with the missing range unless the thing is obviously easy to dismantle and get back together. Tamper-proofing, oh dear... The rotary switch contacts are my chief suspect and the photo shows the switch is fully enclosed. If contacts have failed, the switch will have to be replaced, which won't be cheap, and involves a shower of moderately tricky soldering.

Doing without the 5A range wouldn't worry me. I rarely use the big amp ranges on my meters and inexpensive digital ammeters are available on ebay & amazon etc. Buy one of them to do amps!

Dave

Jouke van der Veen14/06/2021 20:35:27
95 forum posts
8 photos

Approx. 5 years ago it appeared that the 2A (highest) range of my at least 30 years old digital multimeter Circuitmate DM20L did not work anymore. It appeared that not only the fuse was blown but also the low ohmic resistor belonging to that switch range was destroyed. Possiblibly, the fuse had been to slow. I was able to replace that resistor.

The multimeter is still going strong.

peak414/06/2021 21:23:44
avatar
1431 forum posts
159 photos

I've just had a look at the PCB on mine, and there's nothing that looks like a fuse, not even a solder in one.
For folks who aren't familiar with this tester, it's not quite like a conventional multi-meter, as it's designed specifically for testing phone lines.
N.B. from what I understand, the MegOhm range has an internal generator, which tests at about 150V, so a bit more akin to a megger, rather than a multimeter MegOhm range.

Sorry, can't find a diagram/schematic, just user manuals.

Bill

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