AVO Mark IV Valve Characteristic Meter. - UK Vintage Radio Repair and Restoration Discussion Forum

scope515 · 7th Feb 2008, 7:39 am

I've recently acquired an AVO Mark IV VCM which appears to work fine but seems to have a 'stability' fault when I back off the anode current, using the course and fine controls, to do the gm test.
Once I've measured the anode current, with the meter switch set to 100mA, I back off the current to zero using the coarse control. All's fine at this stage, with the meter needle very steady on zero. I then drop the meter switch to 2.5mA and attempt to re-adjust the needle to zero. This is where I seem to have trouble. The needle is unstable and, after I do get it to sit on zero for a few seconds, it starts to move back and forth again. Once I get it as steady as possible, I shift the meter switch to mA/V to do the gm reading. Similarly, I then can't get the needle to sit still on the 1 mA/V line. It continues to waver about. Hence, I can't get an accurate gm reading. Even leaving the valve on for some time makes no difference. Can anybody advise me what could be the problem? Thanks, Neil

XTC · 7th Feb 2008, 10:47 am

Have you tested and had this problem with a few valves, or just one?

Some valves just won't stabilise because they're clapped out.

Other than that, I'd say it was a dirty or bad contact somewhere.

You shouldn't leave valves on a tester for any longer than it takes to do the test in reasonable time.

Pete.

scope515 · 8th Feb 2008, 10:43 am

Thanks for your response, Pete. The 'wavering' is more pronounced with output valves such as EL34 and 6V6. Hardly noticable with say, double triodes. The valves it happened with were new EL34s and new 6V6s. I have since cleaned the sockets but it has not improved things. I appreciate your thoughts and ideas. Has anyone else experienced this problem?

Alex · 9th Feb 2008, 6:22 pm

scope515, All the AVo's seem to freak with power valves but they calm down after awhile . Let it warm up for 15mins ,other wise thats all i know, but i have had em all, they all have their own personalties . Try Kevin Champ think he's in Cloverdale. (i used to live in Prerth spent1/2 my life there)
Or check out out goggle CROWTHORNE TUBES (Gerald Horrox), 65 Greenwood Road, Crowthorne, RG11 6JS .He may have something that will help. Jerry fix my 163, which came from Midland in Perth.
If the MKIV has it's original plastic cover i know it. Anyway have a can of black duck for me.............Alex

retailer · 11th Feb 2008, 10:10 pm

My ct160 was behaving in a similar fashion, I originally thought it was the wire in the wire wound pots moving around as the slider went past them. Looking into the inards one day a suspect solder joint on one of the pots caught my eye, a few minutes work with a hot soldering iron and problem fixed.

scope515 · 17th Feb 2008, 8:13 am

Thanks for that hint, Retailer. I must say I haven't had a really good look for dry/faulty joints so will do so. Hopefully, that's my problem too!

ampzilla · 31st Mar 2008, 9:29 pm

There can be problems with tester Mk IV and older
The prevention of Self Oscillation of valves under test
See page 12 in Working instruction manual MK IV

7th Feb 2008, 7:39 am	#1
scope515 Retired Dormant Member Join Date: Feb 2005 Location: Perth Western Australia Posts: 6	AVO Mark IV Valve Characteristic Meter. I've recently acquired an AVO Mark IV VCM which appears to work fine but seems to have a 'stability' fault when I back off the anode current, using the course and fine controls, to do the gm test. Once I've measured the anode current, with the meter switch set to 100mA, I back off the current to zero using the coarse control. All's fine at this stage, with the meter needle very steady on zero. I then drop the meter switch to 2.5mA and attempt to re-adjust the needle to zero. This is where I seem to have trouble. The needle is unstable and, after I do get it to sit on zero for a few seconds, it starts to move back and forth again. Once I get it as steady as possible, I shift the meter switch to mA/V to do the gm reading. Similarly, I then can't get the needle to sit still on the 1 mA/V line. It continues to waver about. Hence, I can't get an accurate gm reading. Even leaving the valve on for some time makes no difference. Can anybody advise me what could be the problem? Thanks, Neil

7th Feb 2008, 10:47 am	#2
XTC Retired Dormant Member Join Date: Oct 2004 Location: Bristol, UK. Posts: 837	Re: AVO Mark IV Have you tested and had this problem with a few valves, or just one? Some valves just won't stabilise because they're clapped out. Other than that, I'd say it was a dirty or bad contact somewhere. You shouldn't leave valves on a tester for any longer than it takes to do the test in reasonable time. Pete.

8th Feb 2008, 10:43 am	#3
scope515 Retired Dormant Member Join Date: Feb 2005 Location: Perth Western Australia Posts: 6	Re: AVO Mark IV Thanks for your response, Pete. The 'wavering' is more pronounced with output valves such as EL34 and 6V6. Hardly noticable with say, double triodes. The valves it happened with were new EL34s and new 6V6s. I have since cleaned the sockets but it has not improved things. I appreciate your thoughts and ideas. Has anyone else experienced this problem?

9th Feb 2008, 6:22 pm	#4
Alex Retired Dormant Member Join Date: Jan 2005 Location: Brighton Posts: 57	Re: AVO Mark IV scope515, All the AVo's seem to freak with power valves but they calm down after awhile . Let it warm up for 15mins ,other wise thats all i know, but i have had em all, they all have their own personalties . Try Kevin Champ think he's in Cloverdale. (i used to live in Prerth spent1/2 my life there) Or check out out goggle CROWTHORNE TUBES (Gerald Horrox), 65 Greenwood Road, Crowthorne, RG11 6JS .He may have something that will help. Jerry fix my 163, which came from Midland in Perth. If the MKIV has it's original plastic cover i know it. Anyway have a can of black duck for me.............Alex

11th Feb 2008, 10:10 pm	#5
retailer Heptode Join Date: May 2005 Location: Adelaide, South Australia Posts: 541	Re: AVO Mark IV My ct160 was behaving in a similar fashion, I originally thought it was the wire in the wire wound pots moving around as the slider went past them. Looking into the inards one day a suspect solder joint on one of the pots caught my eye, a few minutes work with a hot soldering iron and problem fixed.

17th Feb 2008, 8:13 am	#6
scope515 Retired Dormant Member Join Date: Feb 2005 Location: Perth Western Australia Posts: 6	Re: AVO Mark IV Thanks for that hint, Retailer. I must say I haven't had a really good look for dry/faulty joints so will do so. Hopefully, that's my problem too!

31st Mar 2008, 9:29 pm	#7
ampzilla Retired Dormant Member Join Date: Mar 2008 Location: Sweden Posts: 9	Re: AVO Mark IV There can be problems with tester Mk IV and older The prevention of Self Oscillation of valves under test See page 12 in Working instruction manual MK IV