Avo mk2 valve tester help required

[oceanobsession]

Hi allan , ive just pulled the trigger on these ,


http://www.ebay.co.uk/itm/252060185981?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT



 http://www.ebay.de/itm/261772914242?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT


mainly due to shaft size it gives me more room to put a thread in the end off the one and join them with something like this

http://www.ebay.de/itm/121794271178?_trksid=p2060353.m1438.l2648&ssPageName=STRK%3AMEBIDX%3AIT  I'm going to mount them with 2mm flat plate cut and shaped like the original pot , mount them with lock nut in the middle then space with drilled out tube as original ,
http://www.ebay.de/itm/251397292147?_trk...1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT
some measurements on this one me phil.

[AlanBeckett]

Phil,

They look good, and you'll have a chance to bugger one of each before you get it right
'twas ever thus.

Alan

[Robert Darwent]

The black tester on the left in your photographs is a VCM Mk.I not a Mk.II - the main visual difference is the lack of chrome handles on either side of the front panel. The two models are essentially the same but note - not completely identical, there are subtle circuit differences.

Regards

[oceanobsession]

The black tester on the left in your photographs is a VCM Mk.I not a Mk.II - the main visual difference is the lack of chrome handles on either side of the front panel. The two models are essentially the same but note - not completely identical, there are subtle circuit differences.

Regards

Hi I did notice that it had three transformers inside and the other pair have two , I did struggle a bit with the manual but there is a quick ref card in the lid off the one that is great to follow , what does the switch do which is the one like the on and off switch 1x and 8x I think it said ,
cheers phil. 

[oceanobsession]

Just a quick update , ive now had all the pots arrive from Russia , but they may not do the job , basically there is a separate transformer that
supplies 25v to these potentiometers and then feeds the other side of the main vu metre , once the current is equal the gauge reads zero , so
you would not want any voltage leaving the pot when in the off position , otherwise it would mess with the ma reading , also the pot needs to
supply the voltage slowly and allow all 25v if its needed , I did notice a foot to lift the sweeper clear of the track when off on the old pot , so this   would also have to be checked , there is one original pot that is working fine , also one avo mk2 that seems to be better than the others , on
one of the testers the neg grid pot is out of tolerance , you can adjust it for half way but its out at 1v and 10v , main meters in both mk2 testers
seem to read low , £100 to get them recalibrated and cleaned also re magnetized  , worth doing if everything else is ok , I keep reading and trying to understand the workings of these things , cheers me phil. 

[AlanBeckett]

If it really is meant to become O/C at the end of its travel how about a Microswitch and a simple Cam on the Shaft?

Alan

[oceanobsession]

Ive had another look at the russion pot and it does lift the sweeper off its track at both ends . you can put a piece of heat shrink in sutch a way that the sweeper rests on it breaking the circuit . apparently you can get away with only one pot but they say don't go to the pub first before testing any valves , the original pot 500ohm is lin and the 1400 ohm is log , so I suppose its a case of playing , cheers alan  me phil.     

[AlanBeckett]

Don't know about the single Pot approach. If they could have got away with one they probably would have. Not only that, they're not simply in Series or Parallel.
You'll have to suck it and see.

Alan

[Radio Fixer]

Don't know how I failed to spot this thread but many years ago, about 12 probably, I restored a Mk1 and a Mk2 and wrote how to fix them articles. A web search on AVO Valve Testers should have found me as they went literally all over the world. Maybe some help in understanding and other faults. If you want the articles I can e-mail them to you. Others in Radio Bygones, years later did restorations and drew a more complete circuit diagram if you can get hold of it.

Set Zero pot

"This pot is tricky because of the two controls in tandem. I plotted the characteristics and the front voltage setting control is linear whereas the rear has the characteristic logarithmic long tail. It is this long tail that gives the fine control to easily set zero. I looked at two ways of replacing this."

I wrote up two ideas about getting around this pot being faulty and tried the second one which worked fine but is a little difficult to implement (the first didn't need trying as it was the 10 turn pot idea).

Gary

[oceanobsession]

Hi gary , anything you have would be helpful , I did look at the 10 turn pots these look very good ( 361420867276) ebay , but if you do leave it
in the on position and forget it takes longer to wind it off  more chance of damage to the meter , the other thing that worries me is when in the
off position would there still be any current affecting the ma reading , looks like the old pots lift the wiper from the track to make sure , apparently the mk1 don't test rec valves but ive none to try , the one mk2 I have has a neg grid pot that seems to go out off range you can adjust the pot for 5 volts but when you test it at 1 volt and 10 volts it a way bit out , I'm thinking this should just be the pot , and the second mk 2 is in better condition with just a sticky mu meter £100 to have it rebuilt , ive ordered an analogue mm to check further , the mk1 has three transformers and seems to be put together differently cheers for your help my email is     phillydivon@gmail.com
 
thanks again phil.