AVO two panel valve tester - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Ed_Dinning · 15th Mar 2005, 6:28 pm

Hi Ken, tests should be as short as possible on most valve testers, but holding the lever over for 10 secs or so after the meter has settled should not cause problems. These are only likely with high power valves such as an 807, EL34 etc where the anode currents are high and may cause heating (and temporary drift of ) components. The tempory fuse you mentioned earlier is also a good safety measure. This may blow if you test too long with a power valve.

Ed

pmmunro · 15th Mar 2005, 6:57 pm

Ken,

On reflection, and prompted by the thoughts of other contributors, could I suggest that you check the meter of your valve tester by connecting a known good meter in series with the movement?

If you use a lower-power valve which will not cause heating concerns it should be possible to leave it on test long enough for complete thermal stabilization - for valve broadcast TV equipment the pre line-up period was at least 30 minutes. You could then check the current readings at full scale and as many intermediate points as you think necessary which would give you a calibration curve showing fsd and linearity.

I have found that cleaning with isopropyl alcohol reduces noise on wire would meter pots which show no visible contamination.

Ken · 15th Mar 2005, 9:55 pm

Hi Ed

Thanks for the advice on how long to leave the lever in the test position. I think for most of the valves I have tested the needle has come to rest within 10 secs. I will be wary of testing valves like the 807. I only chose to test it because at 6mA/V it gave a reading roughly in the centre of the scale. I had to uprate the fuse for whole tester from 200mA to 300mA fast blow. I did wonder about trying a 200mA time delay fuse but I seem to remember reading that they can allow a current to pass which is quite a lot higher than they are rated for before they blow. Is a 200mA time delay fuse a better bet than a 300mA fast blow? On reflection I think the 200mA fuse did blow when I tested the 807.

Hi pmmunro

I like you idea for testing the calibration and linearity of the meter although given what has already been said about time with the lever in the test position I would be a bit worried about keeping it in this position for 30 mins even using a low powered valve. What is the shortest time you think I could get away with? I think I will try the idea of "standardised" valves first just for peace of mind. The reason I cleaned the wire wound pots with switch cleaner was I thought it was supposed to evaporate to leave a residue of lubricant which I thought might be benificial. I found it very hard to see whether they looked clean or not. I have some isopropyl alcohol in spray form. I did not spray the switch cleaner but used one of the swawbs used for cleaning video heads as I did not want the residue from the switch cleaner all round the meter. The alcohol showd leave no residue and the pressure might help to clean the pots. Did you use a spray?

Regards,

Ken.

pmmunro · 16th Mar 2005, 9:13 pm

Ken,

I've never used isopropyl alcohol in spray form on meters or their components - I just use a cotton bud dipped in the alcohol. I would advise against a spray of any kind anywhere around a meter movement as there's no knowing where it may go.

One batch of Avometers which I repaired for a Further Education college had been "fixed" by someone who said he knew what to do. He had sprayed what seemed to be WD40 into the movements and many had turns of the hairsprings stuck together, entirely altering their characteristics. The only effective cure is to dismantle the movement and wash the moving coil assembly in isopropyl alcohol - not to be undertaken lightly unless you have the infromation, resources and experience.

In the service manuals Avo advised against using flux-cored solder on hairsprings as the flux could creep and cause adhesion of the turns. However, in the interests of getting a good joint, I believe it may be reasonable to use a modern low-residue flux solder and carefully wash off as much as possible, taking care that it is washed free and not inwards. Does anyone have detailed knowledge of the tendency for modern fluxes to cause such problems?

On the subject of time-on-test I don't really have enough experience of the two-panel tester to say exactly what is reasonable, but valves do take a long time to fully stabilise and it would be essential for accurate work on a VCM that they should be fully stabilised. I would be surprised if Avo had designed the two-panel tester in such a way that smaller valves could not be tested over a reasonably long period but I must take the advice of others here.

To decide it would be necessary to know what the limiting factor would be when using the two-panel tester.

Neil F · 16th Mar 2005, 11:05 pm

Quote:

Originally Posted by Ken

In the copy of the "Abbreviated Working Instructions" I have there is a section on accesssories which mentions adapters for testing valves with bases not already on the panel. The adaptor plugs into the octal holder. I have an octal "plug", taken from an old KT61, and a B9A base. If it is possible I would like to make an adaptor. The B9A adaptor was listed as No 7. I have looked at the pin outs for B9A valves and all nine pins seem to be used. Has anybody got one of these adaptors? Can any body tell me how they were made? How did they get round the nine to eight pin problem? Were the separate anode, grid and screen connectors used in some way?

Regards,

Ken.

Yes, I've got one of the adaptors, and the 'plug' is a British 9-pin. You can't do it on an 8-pin 'plug' because (as you rightly point out) on a B9A valve all 9 pins are potentially in use. It's simple enough to knock up an adaptor if you have the right base, but even easier just to mount a B9A socket in the panel. You simply wire it pin-for-pin to the nearest 9-pin socket (B9G, I think) and away you go. There's at least one blank space built in for just this eventuality (3 spaces if your panel doesn't have the heater voltage 'divide by seven' arrangement).

I've done 3 EF80s for you and will pop them in the post tomorrow.

Ken · 16th Mar 2005, 11:39 pm

Hi pmmunro

Thanks for the advice on using isopropyl alcohol to clean the wire wound pots. I noticed when I had the tester apart that even a small air movement was enough to cause the needle to move. This has also put me of using a spray. Even if I sprayed away from the movement I think the blast could be deflected by other parts towards the needle.

Hi Neil F

Thanks for the information on the adaptors. My panel does have the 'divide by seven transformer' and so I have just one 'free' space. I don't have anything I can use as a nine pin 'plug' but now I know there is nothing very clever going on and the connections are just pin to pin I'll make something up. I'll look out for the EF80s and will let you know how they test. I will send three more for you to test if thats still OK.

Regards,

Ken.

15th Mar 2005, 6:28 pm	#21
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,195	Re: AVO two panel valve tester Hi Ken, tests should be as short as possible on most valve testers, but holding the lever over for 10 secs or so after the meter has settled should not cause problems. These are only likely with high power valves such as an 807, EL34 etc where the anode currents are high and may cause heating (and temporary drift of ) components. The tempory fuse you mentioned earlier is also a good safety measure. This may blow if you test too long with a power valve. Ed

15th Mar 2005, 6:57 pm	#22
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: AVO two panel valve tester Ken, On reflection, and prompted by the thoughts of other contributors, could I suggest that you check the meter of your valve tester by connecting a known good meter in series with the movement? If you use a lower-power valve which will not cause heating concerns it should be possible to leave it on test long enough for complete thermal stabilization - for valve broadcast TV equipment the pre line-up period was at least 30 minutes. You could then check the current readings at full scale and as many intermediate points as you think necessary which would give you a calibration curve showing fsd and linearity. I have found that cleaning with isopropyl alcohol reduces noise on wire would meter pots which show no visible contamination.

15th Mar 2005, 9:55 pm	#23
Ken Hexode Join Date: Oct 2003 Location: Near Glasgow, UK. Posts: 309	Re: AVO two panel valve tester Hi Ed Thanks for the advice on how long to leave the lever in the test position. I think for most of the valves I have tested the needle has come to rest within 10 secs. I will be wary of testing valves like the 807. I only chose to test it because at 6mA/V it gave a reading roughly in the centre of the scale. I had to uprate the fuse for whole tester from 200mA to 300mA fast blow. I did wonder about trying a 200mA time delay fuse but I seem to remember reading that they can allow a current to pass which is quite a lot higher than they are rated for before they blow. Is a 200mA time delay fuse a better bet than a 300mA fast blow? On reflection I think the 200mA fuse did blow when I tested the 807. Hi pmmunro I like you idea for testing the calibration and linearity of the meter although given what has already been said about time with the lever in the test position I would be a bit worried about keeping it in this position for 30 mins even using a low powered valve. What is the shortest time you think I could get away with? I think I will try the idea of "standardised" valves first just for peace of mind. The reason I cleaned the wire wound pots with switch cleaner was I thought it was supposed to evaporate to leave a residue of lubricant which I thought might be benificial. I found it very hard to see whether they looked clean or not. I have some isopropyl alcohol in spray form. I did not spray the switch cleaner but used one of the swawbs used for cleaning video heads as I did not want the residue from the switch cleaner all round the meter. The alcohol showd leave no residue and the pressure might help to clean the pots. Did you use a spray? Regards, Ken.

16th Mar 2005, 9:13 pm	#24
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: AVO two panel valve tester Ken, I've never used isopropyl alcohol in spray form on meters or their components - I just use a cotton bud dipped in the alcohol. I would advise against a spray of any kind anywhere around a meter movement as there's no knowing where it may go. One batch of Avometers which I repaired for a Further Education college had been "fixed" by someone who said he knew what to do. He had sprayed what seemed to be WD40 into the movements and many had turns of the hairsprings stuck together, entirely altering their characteristics. The only effective cure is to dismantle the movement and wash the moving coil assembly in isopropyl alcohol - not to be undertaken lightly unless you have the infromation, resources and experience. In the service manuals Avo advised against using flux-cored solder on hairsprings as the flux could creep and cause adhesion of the turns. However, in the interests of getting a good joint, I believe it may be reasonable to use a modern low-residue flux solder and carefully wash off as much as possible, taking care that it is washed free and not inwards. Does anyone have detailed knowledge of the tendency for modern fluxes to cause such problems? On the subject of time-on-test I don't really have enough experience of the two-panel tester to say exactly what is reasonable, but valves do take a long time to fully stabilise and it would be essential for accurate work on a VCM that they should be fully stabilised. I would be surprised if Avo had designed the two-panel tester in such a way that smaller valves could not be tested over a reasonably long period but I must take the advice of others here. To decide it would be necessary to know what the limiting factor would be when using the two-panel tester.

16th Mar 2005, 11:39 pm	#26
Ken Hexode Join Date: Oct 2003 Location: Near Glasgow, UK. Posts: 309	Re: AVO two panel valve tester Hi pmmunro Thanks for the advice on using isopropyl alcohol to clean the wire wound pots. I noticed when I had the tester apart that even a small air movement was enough to cause the needle to move. This has also put me of using a spray. Even if I sprayed away from the movement I think the blast could be deflected by other parts towards the needle. Hi Neil F Thanks for the information on the adaptors. My panel does have the 'divide by seven transformer' and so I have just one 'free' space. I don't have anything I can use as a nine pin 'plug' but now I know there is nothing very clever going on and the connections are just pin to pin I'll make something up. I'll look out for the EF80s and will let you know how they test. I will send three more for you to test if thats still OK. Regards, Ken.