GPO No 12A Multimeter circuit - UK Vintage Radio Repair and Restoration Discussion Forum

Richard675 · 24th Nov 2014, 10:38 am

I have been given one of those nice GPO test meters in the leather case. I was wondering if anyone has a circuit diagram please?

I wonder what range does the 15v battery activate? I currently only have the 1.5v installed and only the 0.2M Ohm range shows signs of life but won't zero. Do you need both batteries in place for the resistance ranges to work at all?

The passive ranges all appear to work OK.

AC/HL · 24th Nov 2014, 5:21 pm

You shouldn't need both batteries, the 15V one is only for the 20M range.
It's probably been damaged by reading voltage whilst being set to resistance. Have you looked inside for damage.
You probably realise that this is a customised GEC Minitest.

Richard675 · 24th Nov 2014, 5:49 pm

Thanks for the info. I didn't know about the GEC connection. In fact I have never seen one of these meters before!

Have yet to look at components, just been cleaning it up, gluing the glass back which was fouling the movement.

The one working resistance range varies with the adjustment pot but only moves about 10% of scale so no chance of getting it to zero ohms yet. (1.5v battery is OK)

AC/HL · 24th Nov 2014, 8:46 pm

TBH I only use the resistance ranges of Analogue meters for continuity testing. Resistance measurements are one thing that Digital meters are much better for.

24th Nov 2014, 10:38 am	#1
Richard675 Pentode Join Date: Jun 2009 Location: Braintree, Essex, UK. Posts: 170	GPO No 12A Multimeter circuit I have been given one of those nice GPO test meters in the leather case. I was wondering if anyone has a circuit diagram please? I wonder what range does the 15v battery activate? I currently only have the 1.5v installed and only the 0.2M Ohm range shows signs of life but won't zero. Do you need both batteries in place for the resistance ranges to work at all? The passive ranges all appear to work OK. Attached Thumbnails

24th Nov 2014, 5:21 pm	#2
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: GPO No 12A Multimeter circuit You shouldn't need both batteries, the 15V one is only for the 20M range. It's probably been damaged by reading voltage whilst being set to resistance. Have you looked inside for damage. You probably realise that this is a customised GEC Minitest.

24th Nov 2014, 5:49 pm	#3
Richard675 Pentode Join Date: Jun 2009 Location: Braintree, Essex, UK. Posts: 170	Re: GPO No 12A Multimeter circuit Thanks for the info. I didn't know about the GEC connection. In fact I have never seen one of these meters before! Have yet to look at components, just been cleaning it up, gluing the glass back which was fouling the movement. The one working resistance range varies with the adjustment pot but only moves about 10% of scale so no chance of getting it to zero ohms yet. (1.5v battery is OK)

24th Nov 2014, 8:46 pm	#4
AC/HL Dekatron Join Date: Nov 2003 Location: Heckmondwike, West Yorkshire, UK. Posts: 9,642	Re: GPO No 12A Multimeter circuit TBH I only use the resistance ranges of Analogue meters for continuity testing. Resistance measurements are one thing that Digital meters are much better for.