AVO Mk-II Valve Tester restoration.

[marcvos80]

Hey guys,

I'm Marc and new to the forum.
Today I acquired an old AVO Mk-I (I think it is while there is no type tag anymore) which I'm planning to restore into working condition.

The tester looks heavily used and rough, and repainted sometime long ago..

Hopefully I can get it back to work!
The mains cord was cut off (I guess with a good reason), so I have to check what's wrong with the tester and see if the mains transformer is still functioning.
I'll start from there..

The first thing a found is that a previous owner has been working on it (I guess without good results).
The old mains cord was soldered directly to the PT, bridging the on/off switch and SET~ switch.

Does someone have a picture of the correct mains wiring around the voltage selector and fuse and the wiring on the PT tabs?

Attached some pictures of the tester.

I'll keep you guys posted.

[marcvos80]

Ah my bad!
The mains wasn't bridged, but soldered to some unused tabs on the PT.

Filament power comes up, some lights are burning.

Next thing: check all secondary voltages and diodes.

[Kevin Hoyland]

Morning Marc.
Just seen your post i can send you Photos of all the inside of the Tester if it will Help. Have you looked up Avo mk 1 and Avo mk2 valve Tester on this forum you will see over the years a lots of post and photo have been posted.
Good look with the tester and i hope the Meter is in working order. Test all the Resistor Replace all the capacitors one at a time and when put the voltage on bring it up slow.

Kevin.

[marcvos80]

Hi Kevin,

If you can send me some good inside photographs would be really nice, thank you!

I haven’t worked on the tester since my last post due to some other tube amp related stuff coming my way, but I’ll start soon!

Also, is there a place to source a replacement front panel?
My tester looks heavily used or like the tester sat in direct sun light all its life.
For the restoration I want the front panel to look as neat as if it came out of the factory this morning. Any thoughts on that?

Thanks a lot in advance!

Marc

[marcvos80]

Today I've started to check all the parts and replace when out of spec.
All the wire wound resistors are within spec, so I only need to replace some of the old carbon film which are out of spec.

When tracing the schematic I found out that my tester is in fact a Mk2 tester, without the typical Mk2 front handles and document drawer.
Is has 19 tubes sockets, the red-green gauge, 120 Ohm resistors at the B9 socket and an additional 75 Ohm resistor at the selector thumb wheel.

I've chosen to only replace the out of spec parts and additionally all rectifiers (yes, also the EA50) with BYW96E's as suggested in the Radio Bygones article.

Parts will arrive soon, so I will keep you posted when the replacement parts are in place!

Attached some pictures of the tester on the bench.

Best,
Marc

[marcvos80]

Checking out the meter today.

I've read that the meter should draw 440uA at FSD, so I set up a small test setup to see what is happening with the meter.

I'm a bit puzzled while at 440uA the meter hits exactly the red ~ mark (see attached photo), but I guess it should be FSD @ 440uA, right?

At FSD the meter reads 505uA so I have to compensate the meter with an extra VR at the back of the meter, right?

Just double checking with you guys if my steps are correct.
Btw, I noticed that the meter draws 7uA when it's at zero.. Is that correct?

Thanks!

[marcvos80]

Oops.. forgot to attach the photo's..

[ms660]

460uA FSD in the EMER manual that I'm looking at.

Lawrence.

[retailer]

It looks as if your meter has lost sensitivity - if this is the case then you do have a few options the cheapest being to fit a meter amplifier, not real difficult, many forum members have done this, a forum search will bring up numerous examples. Other options are to either have the meter serviced and remagnetised or source a replacement meter from somewhere.

[marcvos80]

Today I reassembled the tester to see if changing the components would at least bring me an "alive" tester. And yes! it did!
I even "tested" a tube (not taking the measured values as true) but it looks like all functionality is there.

Next step is to start calibration, but before I can start that I have to fix the sensitivity issue on the movement.

I found some posts on the forum regarding correction of the magnet by glueing extra neodymium magnets to the meter.
That's my next step to check out and see if that fixes the meter movement and gets it back to 460uA FSD.

I hope to come back again with good news soon!
Any cues or hints are welcome!

[Dekatron]

In checking a few different AVO and EMER documents that I have copies of the FSD should be 440uA (just as Euan writes in his article) and the resistance is somewhere between 110-120 Ohm in the meters that Euan and I have checked - with the shunt RV7 @2.5KOhm you should then be able to adjust the reading to get correct measurements.

Now, I don't remember all discussions with Euan and Garry Tempest, who wrote a similar restoration article but in the Autumn 2000 edition of The Radiophile, but I think that the meter with the shunt in place should work as a 500uA meter with a 100 Ohm total resistance, 60uA flowing through the RV7 shunt and the rest, 440uA, through the meter.

I also remember that Euan said that this meter was harder to adjust with external magnets as its design made it harder to fit the magnets in good places. A meter amplifier like the one that Gerry wrote about might be a simpler way to go as you won't have to fiddle with the brittle movement then.

[ms660]

The meter FSD I quoted was from a copy of EMER document Y812 Part 2 and Y814, were those meters ever used or was it an error on their part?

Lawrence.

[Radio Wrangler]

As it's a mark-II I'd better update the thread title.

David

[Dekatron]

It isn't that much of a difference in using a 460ua or a 440ua meter as long as the internal resistance is low enough so you can calibrate it to become the 500uA / 100 Ohm meter with a 50mV drop. With 460ua you will have to have some 108.7 Ohm internal resistance to reach 50mV and the shunt RV7 then needs to be 1250 Ohm.

I've never seen a meter that has had 460uA with low enough internal resistance, every meter I have come across, and heard of from other owners, has had an internal resistance above 112 Ohm.

Perhaps AVO changed the specifications over time or it could be one of their famous "errors".

[marcvos80]

Hi guys,
I'm a little bit lost in the discussion here.
I've done my measurements again and come to a 510uA @ 114,8 Ohm FSD on the meter.

The RV7 shunt resistor is not going to help me I guess because it will only reduce the current through the meter.
So I think that adding magnets to the meter, or build a meter amplifier should be my best option?
Which option would you suggest is best in this case?

[marcvos80]

Originally Posted by Radio Wrangler

As it's a mark-II I'd better update the thread title.

David

Thanks!

[Dekatron]

As I understand it from what Euan told me it was very hard to fit Neodymium magnets to the movement so I believe that a meter amplifier is an easier way to go.

[marcvos80]

I found that there was a magnetic shunt on the movement which I fiddled around a bit.
It did change the FSD, but not as much as I hoped / needed.
The best setting I found was a reduction of 25uA to 483uA FSD.
Still not good enough!
Before building an external meter amplifier I will try to see if I can correct the meter by adding some extra magnets. If that doesn't work, the meter amplifier is my next step.

[Radio Wrangler]

As time goes by and the charge of the original magnet falls further, you'll need to keep playing with added magnets. With a meter amplifier, you can have a simple pot to adjust the meter sensitivity back to where it should be. I reckon the meter amplifier approach is a longer-term fix.

David

[marcvos80]

Thanks for the addition David.
I've searched the forum for some hours now, but couldn't find extra information about the meter amplifier.
Does anyone have a link to a thread which describes the build of the meter amplifier?