Avo 7 mk2 - reading low on AC ranges - UK Vintage Radio Repair and Restoration Discussion Forum

Walrus12a · 17th Apr 2019, 5:24 pm

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.

pmmunro · 17th Apr 2019, 8:14 pm

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

Kevin Hoyland · 18th Apr 2019, 5:07 pm

Hello
I have had the same problem, it turned out to be poor contact on the wafer switch.
Kevin.

Sinewave · 18th Apr 2019, 5:19 pm

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

The Philpott · 18th Apr 2019, 6:47 pm

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

Walrus12a · 20th Apr 2019, 12:56 pm

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.

The Philpott · 20th Apr 2019, 6:17 pm

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

17th Apr 2019, 5:24 pm	#1
Walrus12a Diode Join Date: Apr 2019 Location: Hayling Island, Hampshire, UK. Posts: 2	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.

17th Apr 2019, 8:14 pm	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	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
Kevin Hoyland Heptode Join Date: Feb 2009 Location: Rotherham, South Yorkshire, UK. Posts: 559	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
Sinewave Octode Join Date: May 2016 Location: Oxfordshire/Bucks borders, UK. Posts: 1,604	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 __________________ Avometer, vintage Fluke and Marconi collector. Also interested in vintage Yaesu and KW.

18th Apr 2019, 6:47 pm	#5
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	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 Attached Thumbnails

20th Apr 2019, 12:56 pm	#6
Walrus12a Diode Join Date: Apr 2019 Location: Hayling Island, Hampshire, UK. Posts: 2	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.

20th Apr 2019, 6:17 pm	#7
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	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