AVO VCM Mk3 - UK Vintage Radio Repair and Restoration Discussion Forum

Heatercathodeshort · 22nd Feb 2020, 2:48 pm

About six years ago I was presented with this valve characteristic meter/tester.
It was discovered beside a skip full of old test equipment complete with five volumes of the valve data books. It was grubby but otherwise in good cosmetic condition.
Back then I gave it a good check over to discover the meter was O/C. It was packed in a box and stored away waiting for the day when a meter would turn up.
Well it happened. A good mate presented me with a boxed NOS meter discovered in a pile of junk cleared from a long redundant sound movie projector repairers.
The AVO was pulled from it's resting place and to cut a long story short that involved valve holder, switch and general cosmetic cleaning it was ready for test.
With the aid of the service manual it was easy to set up and to my surprise it tested new valves to the data in the AVO book.
A batch of much used EF50's and six PL81's soon sorted out the useless ones reducing the quantity to manageable proportions!
I am delighted with the results but for one odd facility or rather the lack of it. There does not appear to be any way of selecting 9V or 19V heaters with the dual selector switches. Valves such as the PL33 must be tested slightly under run at 17.5v but valves such as the PCF80 with a 9V heater do not appear to be catered for. OK a good valve should read satisfactory with a slightly reduced heater supply but it's not very good practice with a valve tester! It's rather odd as the AVO is obviously aimed at the more professional end of the trade and I would have thought they would have included all heater voltages that were common at the time of manufacture.
Any comments about this?
Other than this annoying omission I am very pleased to own this fine instrument. It has cleaned up like new! Regards, John.

chriswood1900 · 22nd Feb 2020, 3:26 pm

John
It is the same with the CT160 and the Mk4 as far as I can see and does not appear to be fixed until the VCM163 perhaps it was a case of timing of the development cycles or there was not the demand for precision with TV valve!
Interestingly the instructions for some Hickok testers say use 10V for 9v valves which I suppose is within 10%, does anyone else have more info on this?

Heatercathodeshort · 22nd Feb 2020, 5:09 pm

Thanks Chris.Very odd, the PL33 arrived around 1949 with the PCF80 1954. There is a whole range of popular radio and TV valves manufactured before the tester was manufactured with 9V-19V heaters. TV valves particularly timebase are somewhat critical enough to make reliable testing doubtful even with correct heater voltages. Thinking about it the very best of testers are no guarantee of a good valve. Most service departments just tried a new valve. After all, a very large valve stock of the popular types cost a lot less than the AVO.

It certainly took them long enough to solve a simple problem. In retrospect it would probably have been a better idea to dispense with the .625v position and substituted 9V!
All water under the bridge now and maybe I should be thankful for what I've got. Regards, John.

David Simpson · 22nd Feb 2020, 5:12 pm

John, Chris,
For Ia & mA/V tests you could switch off the coarse & fine heater switches & feed an external heater source to the H+ & H- wee sockets on the top of the MK3. Many vintage radio folk own bench DC PSU's capable of up to 30V & 2A metered, and they often pop up cheap at Swapmeets, Auctions, and Junk Sales.
AVO VCM's are usually over generous, ac heater voltage-wise, for many small & medium sized valves, but then drop several tenths of a volt for the thumping big pentodes & beam-tetrodes. Both extremes do actually have an effect on the Ia & Gm of the valve, in many cases.

Regards, David

Heatercathodeshort · 22nd Feb 2020, 10:02 pm

Thanks for that David. Never thought of that one! I have a suitable external source. Problem solved and thanks for your advice. John.

linescan87 · 22nd Feb 2020, 10:18 pm

Great stuff John, wonderful bit of luck getting a NOS meter movement. I could do with something similar, I've got boxes of pulls!

Radio Wrangler · 22nd Feb 2020, 10:20 pm

Jam piled on jam.

Well done!

David

Heatercathodeshort · 23rd Feb 2020, 10:52 am

I always had a sweet tooth! J.

22nd Feb 2020, 2:48 pm	#1
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	AVO VCM Mk3 About six years ago I was presented with this valve characteristic meter/tester. It was discovered beside a skip full of old test equipment complete with five volumes of the valve data books. It was grubby but otherwise in good cosmetic condition. Back then I gave it a good check over to discover the meter was O/C. It was packed in a box and stored away waiting for the day when a meter would turn up. Well it happened. A good mate presented me with a boxed NOS meter discovered in a pile of junk cleared from a long redundant sound movie projector repairers. The AVO was pulled from it's resting place and to cut a long story short that involved valve holder, switch and general cosmetic cleaning it was ready for test. With the aid of the service manual it was easy to set up and to my surprise it tested new valves to the data in the AVO book. A batch of much used EF50's and six PL81's soon sorted out the useless ones reducing the quantity to manageable proportions! I am delighted with the results but for one odd facility or rather the lack of it. There does not appear to be any way of selecting 9V or 19V heaters with the dual selector switches. Valves such as the PL33 must be tested slightly under run at 17.5v but valves such as the PCF80 with a 9V heater do not appear to be catered for. OK a good valve should read satisfactory with a slightly reduced heater supply but it's not very good practice with a valve tester! It's rather odd as the AVO is obviously aimed at the more professional end of the trade and I would have thought they would have included all heater voltages that were common at the time of manufacture. Any comments about this? Other than this annoying omission I am very pleased to own this fine instrument. It has cleaned up like new! Regards, John. Attached Thumbnails Last edited by Heatercathodeshort; 22nd Feb 2020 at 3:08 pm.

22nd Feb 2020, 3:26 pm	#2
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,481	Re: AVO VCM Mk3 John It is the same with the CT160 and the Mk4 as far as I can see and does not appear to be fixed until the VCM163 perhaps it was a case of timing of the development cycles or there was not the demand for precision with TV valve! Interestingly the instructions for some Hickok testers say use 10V for 9v valves which I suppose is within 10%, does anyone else have more info on this? __________________ Chris Wood BVWS Member

22nd Feb 2020, 5:09 pm	#3
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: AVO VCM Mk3 Thanks Chris.Very odd, the PL33 arrived around 1949 with the PCF80 1954. There is a whole range of popular radio and TV valves manufactured before the tester was manufactured with 9V-19V heaters. TV valves particularly timebase are somewhat critical enough to make reliable testing doubtful even with correct heater voltages. Thinking about it the very best of testers are no guarantee of a good valve. Most service departments just tried a new valve. After all, a very large valve stock of the popular types cost a lot less than the AVO. It certainly took them long enough to solve a simple problem. In retrospect it would probably have been a better idea to dispense with the .625v position and substituted 9V! All water under the bridge now and maybe I should be thankful for what I've got. Regards, John.

22nd Feb 2020, 5:12 pm	#4
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: AVO VCM Mk3 John, Chris, For Ia & mA/V tests you could switch off the coarse & fine heater switches & feed an external heater source to the H+ & H- wee sockets on the top of the MK3. Many vintage radio folk own bench DC PSU's capable of up to 30V & 2A metered, and they often pop up cheap at Swapmeets, Auctions, and Junk Sales. AVO VCM's are usually over generous, ac heater voltage-wise, for many small & medium sized valves, but then drop several tenths of a volt for the thumping big pentodes & beam-tetrodes. Both extremes do actually have an effect on the Ia & Gm of the valve, in many cases. Regards, David

22nd Feb 2020, 10:02 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: AVO VCM Mk3 Thanks for that David. Never thought of that one! I have a suitable external source. Problem solved and thanks for your advice. John.

22nd Feb 2020, 10:18 pm	#6
linescan87 Heptode Join Date: Mar 2014 Location: Meath, Ireland Posts: 551	Re: AVO VCM Mk3 Great stuff John, wonderful bit of luck getting a NOS meter movement. I could do with something similar, I've got boxes of pulls! __________________ All the best, John Joe www.youtube.com/c/ojnojirishvintagetvandradio

22nd Feb 2020, 10:20 pm	#7
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,868	Re: AVO VCM Mk3 Jam piled on jam. Well done! David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

23rd Feb 2020, 10:52 am	#8
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: AVO VCM Mk3 I always had a sweet tooth! J.