Help with an AVO two-panel tester please! - UK Vintage Radio Repair and Restoration Discussion Forum

SKEXIS1 · 25th Sep 2008, 9:42 pm

I'm attempting to use an AVO two panel tester for the first time, but am struggling to interpret what it is telling me!

I'm testing an EF9 pentode side contact valve; I don't know whether it is OK or not and have never seen it working.

It is all plugged in, the top cap is connected and all the rollers have been set to the correct numbers as per the valve manual. Heater voltage is set to 6.

Anode selector is set to normal, screen to 100v, anode to 100v. Following the instructions, I adjust the ma\v selector to 100, adjust the meter to 0 on the scale, then do the same when the selector is at "ma\v".

In theory, when I move the switch in the centre of the meter to the left (i.e towards ma\v), I think the meter should register the "goodness" or otherwise of the valve.

However, when I do this, the meter seems to want to go in the opposite direction, i.e negative.

I don't understand - can anyone help?

Cheers

Andy

Leon Crampin · 25th Sep 2008, 10:40 pm

As a general principle, if the tester is new to you, don't use a valve of unknown characteristics first time. In electronics, as in many other fields, chasing two unknowns will cause unnecessary head scratching.

Double check the roller switch settings and that the top cap lead is in the "G" socket for an EF9. Loose metallising connections on these valves will sometimes cause oscillation whch can give negative readings. Ensure you can get and maintain a reasonably stable zero with the "set MA/V" (sic) control adjusted to MA/V - ie max sensitivity.

I keep a "calibrated" 6K7 to check my 2 panel tester - these valves are stable, worth nothing and don't oscillate.

Finally, if you can't get meaningful results on the tester with a good valve, check the selenium rectifier for the offset supply. It can be replaced with a silicon diode without upsetting the calibration.

Leon.

David Simpson · 26th Sep 2008, 3:01 pm

Andy, As we speak - I'm also in the middle of re-juvinating my 2 Panel. Have just experinced the same -ve movement on the meter. Am using a "New" 12K7GT just out of the box. I've found that a wee tweak of the "set zero" pot, + a waggle of the 3 way central swich resolves the problem. I've applied contact grease to all the wiper swiches & pots, but that 3 way switch is awkward to get to - shall remove the meter movement & clean the contacts.Check the three wee sockets on the valve base panel.One of mine (anode) had fractured & was intermittently shorting out to the case. Have replaced both anode & screen sockets with new RS ones.
Shall also fit a mains fuse holder with a 100mA A/S fuse in it in the main bakelite panel, and also B7G & B9A valve holders in the valve unit. The neon & its holder also needed a good clean & grease.

Regards, David

SKEXIS1 · 26th Sep 2008, 3:36 pm

Thanks Gents, I'll give it a good clean and check the points above!

David Simpson · 26th Sep 2008, 8:31 pm

Andy, with my Avo8/5 metering supply current, and a new KT66 under test - I'm reading 120mA. Therefore I'll revise my proposed fusing rquirements to 200mA,I think.Or prahaps higher for bigger power valves. Just a precaution really. After all, these 2Panel testers we're using are between 50 & 60 years old probably.Also, an analogue meter will indicate spurious wee current changes - which might indicate a possible insulation or component breakdown.
Another hint - check all the soldered joints on the valve bases. On mine - someone had been a bit heavy with the good old 60/40, resulting in occasional s/c when the pins were waggled slightly.

Regards, David

SKEXIS1 · 29th Sep 2008, 10:48 pm

It's almost as though the three-position switch actually has 2 positions on the ma/v (i.e left) side, i.e just a little to the left (which appears to work and give a sensible reading), then a little more to the left (reading ceases and meter needle falls quickly back to zero).

I've taken off the back and looked at the switch mechanism and am almost sure it's probably something to do with that; something is not quite making or breaking contact somewhere along the line. As mentioned earlier in the thread though, it is a bit of a pain to remove without de-soldering existing connections.

I'm not sure I fancy doing this job for fear of making it worse! Are there any companies . individuals out there who can service and check this tester?

Cheers

Andy

25th Sep 2008, 9:42 pm	#1
SKEXIS1 Retired Dormant Member Join Date: Sep 2008 Location: Redditch, Worcs, UK Posts: 66	Help with an AVO two-panel tester please! I'm attempting to use an AVO two panel tester for the first time, but am struggling to interpret what it is telling me! I'm testing an EF9 pentode side contact valve; I don't know whether it is OK or not and have never seen it working. It is all plugged in, the top cap is connected and all the rollers have been set to the correct numbers as per the valve manual. Heater voltage is set to 6. Anode selector is set to normal, screen to 100v, anode to 100v. Following the instructions, I adjust the ma\v selector to 100, adjust the meter to 0 on the scale, then do the same when the selector is at "ma\v". In theory, when I move the switch in the centre of the meter to the left (i.e towards ma\v), I think the meter should register the "goodness" or otherwise of the valve. However, when I do this, the meter seems to want to go in the opposite direction, i.e negative. I don't understand - can anyone help? Cheers Andy

25th Sep 2008, 10:40 pm	#2
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Help with an AVO two-panel tester please! As a general principle, if the tester is new to you, don't use a valve of unknown characteristics first time. In electronics, as in many other fields, chasing two unknowns will cause unnecessary head scratching. Double check the roller switch settings and that the top cap lead is in the "G" socket for an EF9. Loose metallising connections on these valves will sometimes cause oscillation whch can give negative readings. Ensure you can get and maintain a reasonably stable zero with the "set MA/V" (sic) control adjusted to MA/V - ie max sensitivity. I keep a "calibrated" 6K7 to check my 2 panel tester - these valves are stable, worth nothing and don't oscillate. Finally, if you can't get meaningful results on the tester with a good valve, check the selenium rectifier for the offset supply. It can be replaced with a silicon diode without upsetting the calibration. Leon.

26th Sep 2008, 3:01 pm	#3
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: Help with an AVO two-panel tester please! Andy, As we speak - I'm also in the middle of re-juvinating my 2 Panel. Have just experinced the same -ve movement on the meter. Am using a "New" 12K7GT just out of the box. I've found that a wee tweak of the "set zero" pot, + a waggle of the 3 way central swich resolves the problem. I've applied contact grease to all the wiper swiches & pots, but that 3 way switch is awkward to get to - shall remove the meter movement & clean the contacts.Check the three wee sockets on the valve base panel.One of mine (anode) had fractured & was intermittently shorting out to the case. Have replaced both anode & screen sockets with new RS ones. Shall also fit a mains fuse holder with a 100mA A/S fuse in it in the main bakelite panel, and also B7G & B9A valve holders in the valve unit. The neon & its holder also needed a good clean & grease. Regards, David

26th Sep 2008, 3:36 pm	#4
SKEXIS1 Retired Dormant Member Join Date: Sep 2008 Location: Redditch, Worcs, UK Posts: 66	Re: Help with an AVO two-panel tester please! Thanks Gents, I'll give it a good clean and check the points above!

26th Sep 2008, 8:31 pm	#5
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	Re: Help with an AVO two-panel tester please! Andy, with my Avo8/5 metering supply current, and a new KT66 under test - I'm reading 120mA. Therefore I'll revise my proposed fusing rquirements to 200mA,I think.Or prahaps higher for bigger power valves. Just a precaution really. After all, these 2Panel testers we're using are between 50 & 60 years old probably.Also, an analogue meter will indicate spurious wee current changes - which might indicate a possible insulation or component breakdown. Another hint - check all the soldered joints on the valve bases. On mine - someone had been a bit heavy with the good old 60/40, resulting in occasional s/c when the pins were waggled slightly. Regards, David

29th Sep 2008, 10:48 pm	#6
SKEXIS1 Retired Dormant Member Join Date: Sep 2008 Location: Redditch, Worcs, UK Posts: 66	Re: Help with an AVO two-panel tester please! It's almost as though the three-position switch actually has 2 positions on the ma/v (i.e left) side, i.e just a little to the left (which appears to work and give a sensible reading), then a little more to the left (reading ceases and meter needle falls quickly back to zero). I've taken off the back and looked at the switch mechanism and am almost sure it's probably something to do with that; something is not quite making or breaking contact somewhere along the line. As mentioned earlier in the thread though, it is a bit of a pain to remove without de-soldering existing connections. I'm not sure I fancy doing this job for fear of making it worse! Are there any companies . individuals out there who can service and check this tester? Cheers Andy