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

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

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.

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

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

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

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