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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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5th Aug 2020, 4:36 pm | #1 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
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Avo 8 Mk. 11
Over the last 60+++ years I have dismantled and generally repaired almost everything, but strangely, never a Avo 8. Today I was asked if I could fix a fairly tidy looking 8 Mk. 11. The cutout button was UP and would not latch down, I connected a battery (sorry, single cell) which showed 1.52v on my decent DVM. Holding down the button, is indicated 2.0v (on the 2.5v range of course.
I switched to the 25v range to check its 15v battery. The pointer reached about 3v, and stuck. Summing up therefore, cut out not latching, over reading by 30% on 2.5v DC range, and pointer sticking. I know we have plenty of AVO experts here who can no doubt tell me exactly what to do. Hoping it is a sticking pointer not ferrous shards in the movement's gap. I had that with my VCM Mk.4 if I remember, but that must have been 40 years ago. For the record it has two Cal labels. One in the UK from Aug '78, another in May '86 by the Kallab Haakonsvern, so it must have travelled somewhat. The scale say Model 8 No. H3010-C-1060 for those who like to note these things. Les. |
5th Aug 2020, 4:37 pm | #2 |
Octode
Join Date: May 2016
Location: Oxfordshire/Bucks borders, UK.
Posts: 1,604
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Re: Avo 8 Mk. 11
They only went up to mark 7.
__________________
Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW. |
5th Aug 2020, 4:40 pm | #3 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,940
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Re: Avo 8 Mk. 11
That is over-pedantic. It is perfectly clear what the OP meant.
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5th Aug 2020, 5:23 pm | #4 |
Octode
Join Date: May 2017
Location: St Austell, Cornwall, UK.
Posts: 1,018
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Re: Avo 8 Mk. 11
Hi Les,
The Cut out will need adjusting if it doesn't stay in. With the Mk II the Cut out sits on a Circular Brass Plate, held in place with a Screw and a Brass Standoff (Guide). Slacken both, so the the Circular Plate can rotate. Adjust the plate such that the Cut out will just latch and stay in. It would need to be tested to confirm correct operation. (ie Current required for Tripping). The Model 8 MkI & II Service Manual is to big to post here, but if you haven't got one, I could extract the Pages you need. It is possible the Cut out Pawl is worn and needs replacing, but I have only ever come across one that ever needed replacing. The 30% high reading will almost certainly mean the Universal Shunt is broken. The Shunt of several Resistances, sits in Parallel across the Meter Movement at all times. Due to age and abuse, it is often the Case one of the Wire wound Resistances is O/C. Do you have a Schematic ? Regarding the Pointer Sticking, this is probably the most difficult to repair. It could be something as easy as slackening off a Tight Jewel, or the pointer has got bent and is rubbing against the Scale, but is most likely, Debris in the Air Gap of the Moving Coil, or worse, Worn Jewels or Bearings. Let us know what you want to tackle first, and we can go though the Fault finding. Ian |
5th Aug 2020, 5:27 pm | #5 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
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Re: Avo 8 Mk. 11
As i recall two screws need to be loosened to allow the cut-out table to be rotated slightly for adjustment.
The sticking pointer is indeed (almost certainly) going to be ferrous shard/s blocking the movement. As it's a Mk II some of voltage shunts could well be winding type not component type, which might explain the voltage overread. With a DVM across the terminals on the DC 2.5v range check for a reading of 50kohms as a start point. (Crossed with Ian's response!) Dave |
5th Aug 2020, 6:31 pm | #6 |
Guest
Posts: n/a
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Re: Avo 8 Mk. 11
Here you are a little more information
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5th Aug 2020, 8:35 pm | #7 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
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Re: Avo 8 Mk. 11
Sinewave, yes, pedantic (as often am I). However, if I had written Avo 8 Mk 2, you would surely have commented. I don't have simple access to Roman numerals, nor a simple way to add under and overlining as did Avo.
Reality time. Superscope, thanks for info. Cutout info should help. I can normally sort these things, but a bit of advance info is always worth having. I thought I had read about the 30% over reading and the swamp resistance, your confirmation is good to have. I had not thought of tight pivots, but an obvious possibility, though I think it would be tight everywhere. When it was around the 2V area, it did seem free; I will soon find out. The only info I have is Attor's circuit (thanks A.E.) Not sure when I get to strip it, we are promised good weather here tomorrow, and I have a big stone wall to continue creating. Thanks all. Les. |
5th Aug 2020, 8:59 pm | #8 |
Octode
Join Date: May 2016
Location: Oxfordshire/Bucks borders, UK.
Posts: 1,604
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Re: Avo 8 Mk. 11
No, wasn't being pedantic (it either is or it isn't). Saw 11 and not 2, so thought you may have had something else, as other meters are often referred to as an Avo when not made by Avo.
__________________
Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW. |
5th Aug 2020, 11:03 pm | #9 |
Nonode
Join Date: Dec 2012
Location: Kirk Michael, Isle of Man
Posts: 2,346
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Re: Avo 8 Mk. 11
Sinewave, OK, explanation accepted.
I have just looked again at the serial number having been asked to check. Model 8 No.113010-C-1060 I had an H where the 11 should be. When I rechecked under my Ledu, all four of those "1"s in the number had what looks like a dot, just halfway up on the leading side. Without the magnifier, they really were an "H". Les. |
6th Aug 2020, 7:32 am | #10 |
Octode
Join Date: May 2017
Location: St Austell, Cornwall, UK.
Posts: 1,018
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Re: Avo 8 Mk. 11
Thanks Les,
Let us know how you get on when you get round to looking at it. Would be particularly interesting to know how the Movement Fault pans out. Whilst your doing that, start clearing some shelf space for the Avo Meters you didn't know you wanted until now! Ian Last edited by Superscope; 6th Aug 2020 at 7:33 am. Reason: Should have used "you" instead of "to" |
6th Aug 2020, 8:34 am | #11 |
Octode
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
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Re: Avo 8 Mk. 11
Les,
Please see private message. PMM |