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Old 19th Aug 2014, 9:14 pm   #1
sparkymike
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Default PO 12D AVO 81/1 Meter Multirange

A good friend has asked me to look at a non working AVO multimeter, PO (Telecom) model 12D 81/1. Has anyone got the circuit for one of these and or the handbook, or could supply a link to either.?
Mike.
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Old 19th Aug 2014, 11:59 pm   #2
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Default Re: PO 12DAVO 81/1 Meter Multirange

I believe this is a slightly modified AVO71, that may or may not help.
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Old 20th Aug 2014, 9:27 am   #3
Gi4CZW Cliff
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Default Re: PO 12DAVO 81/1 Meter Multirange

Hello Mike, thread title nearly missed me, we knew them as "PO meter multirange No 12D". Had trouble fixing mine but try the internet for AVO73. The AVO71 is more a 12A with internet look up as Taylor127A. Cliff.
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Old 20th Aug 2014, 11:51 am   #4
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Default Re: PO 12D AVO 81/1 Meter Multirange

Thanks for that info Cliff. Did you work for Telecom?
I had found the instructions for type 71, but the circuit does not resemble mine. I was surprised how many diodes there are in it, over 12 at last count !! When PO is mentioned, youngsters will not realise that BT was all part of the post office in the 50's/60's and before I guess.
Well I got it working, but now it is not working !! It seems to have an intermittant fault somewhere but at the moment I can not locate it. I did look closely at the movement but could not see any stray bits of metal anywhere, but that could still be the case. The tracks on the switch were discoloured and I have cleaned them up somewhat, but still no joy..
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Old 20th Aug 2014, 12:51 pm   #5
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Default Re: PO 12D AVO 81/1 Meter Multirange

I had a look at the circuit for the 73 but that does not resemble this one.
There are several 1N5400 and 1N5401 diodes numbering around 21 plus around four resistors and two minature diodes. One large 9 turn coil and 1 printed circuit resistor and a long chip which has around six resistances in it.
The face is very similar to the 71/73 type, but does not have the extra socket for amps . I will post some photos later.
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Old 20th Aug 2014, 1:01 pm   #6
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Default Re: PO 12D AVO 81/1 Meter Multirange

Thumbnails now included.
Mike
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Old 21st Aug 2014, 10:53 am   #7
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Default Re: PO 12D AVO 81/1 Meter Multirange

Hello Mike, this is the meter I have, it's marked GMS 87-88/3 on the back and has a Yaxley type switch inside. It's one of the lugs on the central ring of a wafer that has broken off on mine ( so leaves it with no current ranges ). They are a nice unit, hope you get that one sorted. Cliff.
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Old 21st Aug 2014, 11:49 am   #8
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Default Re: PO 12D AVO 81/1 Meter Multirange

Well today it is working up till now. yesterday it was start stop start stop for no apparent reason.
If it does it again, I can only think that it might be a deposit on the rotary switch tracks or the wipers of the switch. Can't see or think of any other reason for the problem, unless there is a minute hair in the air gap of the movement which is sometimes locking the movement. I did find that the movement spindle has loads of side play which was causing it to stick, but I managed to cure that by turning in the centre end float screw, which needed several turns. I would think that someone " got at it " in the past. I would not think it was due to natural wear on the end of the spindle.
DC volts seem about right, but AC volts are reading around 200 for 235 mains.
Ohms get fsd when probe terminal are linked on X1 and X100 but X10k only gets half fsd deflection.
Anyone any idea why so many diodes in this meter and what purpose do they serve ?
Mike.
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Old 21st Aug 2014, 11:52 am   #9
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Default Re: PO 12D AVO 81/1 Meter Multirange

Hi Cliff, yours seems to be a variant of mine, slightly different and probably yours is a later version.
Any chance of posting a photo of the insides ?
Did you ever find a circuit for it ?
Mike.
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Old 21st Aug 2014, 5:27 pm   #10
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Default Re: PO 12D AVO 81/1 Meter Multirange

Cliff's is based on the GEC Minitest.
It would appear that these meters, although being externally similar to their AVO and GEC siblings, can be quite different internally.
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Old 3rd Jul 2016, 9:01 pm   #11
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Default Re: PO 12D AVO 81/1 Meter Multirange

Mike
I have the same meter and it had an intermittent fault that drove me mad for a year. One day it would work; another day it would be faulty. The fault showed as insufficient and varying deflection when testing the low-ohms range. But, on removing the meter from its leather case, all would be well again. The change occurred because I unplugged the meter leads to make it easier to remove the meter from its case. This caused the PCB to bend slightly and disturb a dry solder joint. In my case, the dry joint was between a flexible wire and the PCB – the grey wire between the terminals in the left-hand bottom corner of your picture. This fault was impossible to see, but I discovered it by pressing on the joints one after another. Suddenly the needle deflected properly again. All is well now that I've resoldered the joint.
So, if that meter is still sitting on a shelf waiting to be repaired, you could check the various connections made using flexible wire, which happens to be difficult to tin properly.

Keith
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Old 7th Jul 2016, 6:43 am   #12
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Default Re: PO 12D AVO 81/1 Meter Multirange

Hi Keith,
Thanks for that, will have to have another go when time permits.
Mike.
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