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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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Thread Tools |
17th Apr 2019, 5:24 pm | #1 |
Diode
Join Date: Apr 2019
Location: Hayling Island, Hampshire, UK.
Posts: 2
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Avo 7 mk2 - reading low on AC ranges
I have recently purchased a nice condition Avo 7, I believe it is circa 1949, looking at some pencil writing on the back of the dial. The case was still sealed, I have opened it up and after a bit of "fettling" it is working fine. Except for the AC ranges that are reading low. I've checked the connection behind the Q pot, it's fine. The rectifier seems to be the older type, a blue stack, not separate diodes. I'm not sure how to check it. Any ideas would be much appreciated.
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17th Apr 2019, 8:14 pm | #2 |
Octode
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,813
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Re: Avo 7 mk2 - reading low on AC ranges
A rectifier fitting the description of "a blue stack" doesn't seem familiar to me. Could you post a photograph please?
The standard rectifier up until the late 1960s would have been a Westinghouse copper oxide instrument rectifier which housed four diodes in a short cylindrical enclosure with radial connection tags to which were attached tinned copper braids in plastic sleeving. PMM |
18th Apr 2019, 5:07 pm | #3 |
Heptode
Join Date: Feb 2009
Location: Rotherham, South Yorkshire, UK.
Posts: 559
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Re: Avo 7 mk2 - reading low on AC ranges
Hello
I have had the same problem, it turned out to be poor contact on the wafer switch. Kevin. |
18th Apr 2019, 5:19 pm | #4 |
Octode
Join Date: May 2016
Location: Oxfordshire/Bucks borders, UK.
Posts: 1,604
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Re: Avo 7 mk2 - reading low on AC ranges
I had an issue with one of mine, but it affected all readings, so my issue remedy may not help you, but if you want a read, here is the thread for my repair.
https://www.vintage-radio.net/forum/...d.php?t=145635
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Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW. |
18th Apr 2019, 6:47 pm | #5 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,108
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Re: Avo 7 mk2 - reading low on AC ranges
Low AC readings can indeed be down to a lazy rectifier, but more often it is a switching clearance issue with the leaf switches (as per post No.3.)
I cannot be specific regarding a Model 7 MkII as i have only had this particular switching problem with a Model 8 MkIII. On these, the top stack of leaf switches on the DC rotary selector can get out of kilter and if both sets of contacts close simultaneously AC readings are (generally) 60% low. The picture illustrates this and the white arrow indicates the force exerted on the leaf to correct the problem! I have swapped out at least one rectifier in an avo that was perfectly good before i came across this. Grasping the rotary selector at north and south positions and manipulating it whilst taking ACV readings can expose this type of switching problem. Dave |
20th Apr 2019, 12:56 pm | #6 |
Diode
Join Date: Apr 2019
Location: Hayling Island, Hampshire, UK.
Posts: 2
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Re: Avo 7 mk2 - reading low on AC ranges
Thank you for the replies, I have "buttoned up" the Avo for the present time. I'm not allowed to play with my toys till I return from our holiday in a few weeks time. I will then look at the contacts Dave and if OK, then take a pic of the "blue stack". I love my Avo, I first used one in 1960 and then at various places of work until the Digital took Over. I've never owned an Avo 7 or 8, couldn't afford to at the time. But I did acquire a "Digital Avo DA116" which I believe is quite rare. That still works fine but the Display is a bit cloudy and misses a segment on the last digit occasionally. The Avo7 had never been opened, the seal was still in place. When I did open it the smell of the varnish etc was fabulous, just like the old days when I used to haunt the Army Surplus stores. Cheers all.
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20th Apr 2019, 6:17 pm | #7 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,108
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Re: Avo 7 mk2 - reading low on AC ranges
Always best to button up, i am apprehensive that once the dust is in there you can't get it out again! I find i can just about judge leaf switch movements under magnification if i slot a strip of white paper behind them as background. Avometer odour has to be up there with steam engine smell and the petrol/oil/grass cocktail from motor mowers. Have a good holiday.
Dave |