Avo 2 panel valve tester - zero control. - UK Vintage Radio Repair and Restoration Discussion Forum

Guitarist28 · 1st Nov 2014, 6:37 pm

Hi All,
I have now restored by 2 panel tester to what appears working order. It does seem that the zero control is a little too sensitive and I not able to zero the dial correctly. Its either too much above the 0 level the on the dial scale or too far below it. Can anyone suggest what the problem might be (I have not replaced any internal resistors as those that I tested where not far away from their original values and within tolerance).

Many thanks

Rob

PS I've attached some pics of my rewiring of the valve selector switch.

Freya · 1st Nov 2014, 7:28 pm

Do you mean the initial adjustment of the meter with the set screw ?, if so then provided it has not been damaged or relocated incorrectly if the meter has been out it has to be a hairspring issue.

Guitarist28 · 1st Nov 2014, 10:17 pm

Hi Stephen,
Apologies, I should have been more specific. Its the set zero control. On my Avo VCM mk1 I can adjust the zero very easily although on the 2 panel tester it does not appear to want to set at zero, being either slightly above or below the zero indicated on dial measure irrespective to how much I adjust the set zero pot.

Any thoughts would be most welcome.

Kind Regards

Rob

Freya · 1st Nov 2014, 10:24 pm

Have you checked the unique resistance assembly under the control for worn through sections, have you tried a variety of types of valve, also try something like a double triode on each anode setting and see if its the same.
I do still wonder if there is an issue with the meter having a binding spot near zero

peter_scott · 1st Nov 2014, 10:36 pm

Hi Rob,

If the windings on wire wound pots are pushed along the track by the wiper then it makes adjustment very difficult. Look around the back as you move the wiper.

Peter

Harry40 · 18th Nov 2014, 2:15 am

I've had a look at this control on my tester. I was wondering if any one had come up with an alternative zeroing design using a single wire wound pot that could replace the dual track system should it fail. The AVO system has two tracks that are swiped together. As far as I can make out the 20V zeroing circuit has a 1.5K value to stop the Va supply bypassing the meter through the zeroing diode circuit and passing down through the valve under test.
Without the 1.5K and only having a low zero volts when testing a small signal valve the valve could pass a current from the Va supply that is bypassing the moving coil meter and thus give erroneous results.
A large drain valve would need more zeroing current from the 20V supply to zero the meter and this should swamp out any leakage from the Va supply circuit.

1st Nov 2014, 6:37 pm	#1
Guitarist28 Heptode Join Date: Dec 2009 Location: Hockley, Essex, UK. Posts: 575	Avo 2 panel valve tester - zero control. Hi All, I have now restored by 2 panel tester to what appears working order. It does seem that the zero control is a little too sensitive and I not able to zero the dial correctly. Its either too much above the 0 level the on the dial scale or too far below it. Can anyone suggest what the problem might be (I have not replaced any internal resistors as those that I tested where not far away from their original values and within tolerance). Many thanks Rob PS I've attached some pics of my rewiring of the valve selector switch. Attached Thumbnails

1st Nov 2014, 7:28 pm	#2
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: Avo 2 panel valve tester - zero control. Do you mean the initial adjustment of the meter with the set screw ?, if so then provided it has not been damaged or relocated incorrectly if the meter has been out it has to be a hairspring issue. __________________ Stephen _________"It`s only an old telly" ___

1st Nov 2014, 10:17 pm	#3
Guitarist28 Heptode Join Date: Dec 2009 Location: Hockley, Essex, UK. Posts: 575	Re: Avo 2 panel valve tester - zero control. Hi Stephen, Apologies, I should have been more specific. Its the set zero control. On my Avo VCM mk1 I can adjust the zero very easily although on the 2 panel tester it does not appear to want to set at zero, being either slightly above or below the zero indicated on dial measure irrespective to how much I adjust the set zero pot. Any thoughts would be most welcome. Kind Regards Rob

1st Nov 2014, 10:24 pm	#4
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: Avo 2 panel valve tester - zero control. Have you checked the unique resistance assembly under the control for worn through sections, have you tried a variety of types of valve, also try something like a double triode on each anode setting and see if its the same. I do still wonder if there is an issue with the meter having a binding spot near zero __________________ Stephen _________"It`s only an old telly" ___ Last edited by Freya; 1st Nov 2014 at 10:30 pm.

1st Nov 2014, 10:36 pm	#5
peter_scott Dekatron Join Date: May 2003 Location: Edinburgh, UK. Posts: 3,274	Re: Avo 2 panel valve tester - zero control. Hi Rob, If the windings on wire wound pots are pushed along the track by the wiper then it makes adjustment very difficult. Look around the back as you move the wiper. Peter __________________ http://www.nostalgiatech.co.uk https://www.youtube.com/c/PeterScott/videos

18th Nov 2014, 2:15 am	#6
Harry40 Retired Dormant Member Join Date: Oct 2007 Location: Plymouth, Devon, UK. Posts: 51	Re: Avo 2 panel valve tester - zero control. I've had a look at this control on my tester. I was wondering if any one had come up with an alternative zeroing design using a single wire wound pot that could replace the dual track system should it fail. The AVO system has two tracks that are swiped together. As far as I can make out the 20V zeroing circuit has a 1.5K value to stop the Va supply bypassing the meter through the zeroing diode circuit and passing down through the valve under test. Without the 1.5K and only having a low zero volts when testing a small signal valve the valve could pass a current from the Va supply that is bypassing the moving coil meter and thus give erroneous results. A large drain valve would need more zeroing current from the 20V supply to zero the meter and this should swamp out any leakage from the Va supply circuit.