AVO Test Bridge

[Superscope]

Does anybody know what Items or accessories were supplied in the
Military Avo Test Bridge Set?

I know there was a later version which looked quite different, but I'm
particularly interested in this one, which I believe is the first Series.

I assume there were a Set of Leads included at least.

There are some curious circular cut outs in the internal construction
of the Box which obviously housed something.

Does anybody in the Forum have a complete set , or photos of the
items?

I haven't managed to track down any Military Documents for it yet, if any still exist.
The Bridge in this Set is dated to 1944 and looks to be virtually unused.

It is housed in the exact same size and style Case used for the Avo Meter Model 48A
of the same period.



Ian

[TowerRadio]

Padded hole-spare valve,other holes,external caps to extend range?Les

[Superscope]

Thanks Les,



That makes sence.

Do you happen to know what Values the external Caps might have been?

Just need to see now if anybody has any Photos of them.
Would be interesting to see how they were constructed.





Ian

[Superscope]

I am still interested if anybody has any Photos or information
on the Reference Capacitors which allegedly were part of the
Military Avo Test Bridge Box set.

There is surprisingly little information on these Test Bridges,
other than the Restoration Manual, and Working Instructions,
both of which refer to the Later Variant using the 6B8G Valve.

Three different Valves have been used.
Early Bridges (1939-1941 est) seem to have used the Mullard EBF2
Between about 1942-1945 ( est) Bridges seem to have used the Marconi L63
Later Bridges (1946-1947 est) seem to have used the Marconi 6B8G

The EBF2 and 6B8G are both Double Diode Pentodes, so apart from
the slightly different Pin assignments, would these be interchangable?
This is just out of interest. My Bridge doesn't use either of these.

The majority of these Bridges seem to have used the L63, which is a Triode.

I haven't found a Schematic for the L63 version, so I'm not sure how the
circuit was changed to accomodate a Triode instead of a Pentode.
Although I will compare my Bridge with the later Circuit when I get Time.

What would be the reason for changing from the original Pentode to a Triode
and then back again ?

I'm guessing most if not all were supplied to the Military.

Almost certainly launched in 1939 around August/September.
Latest example known about seems to be from 1947.
If anybody has a later one, it would be good to know.


Ian

[Superscope]

I was asked recently about the Avo Cases used for the Test Bridge.

The Box Set Case of course is in Post #1
The only Photo I have seen of what is probably the regular Bridge only
Case is attached.

There are of course, no guarantees that this is original to the
Bridge, but it seems likely it is.

I have not yet identified the M^S Military marking on the Case,
so if anybody knows which area of the Military it belongs, it would
be good to know ?



Ian

[Superscope]

Just discovered this Test Bridge in a Completed Auction.
Not Ebay surprisingly!
There was no date, so I don't know when this finished.

So it appears the Avo Test Bridge was also supplied in
the Standard Style Avo Varnished Pine Box.

Maybe the RAF just painted these Boxes themselves in their
own Grey/Blue Colour.

Interestingly, all the accessories, as usual, are missing, so I am
no closer in my photographic search for what was in those Boxes!



Ian



Ian

[wd40addict]

I used to have one of these, the padded compartment contained a spare L63. There were no other accessories with my example, not even any leads.

[HamishBoxer]

I have just purchased one of these last Thursday but it came without any box. Still, I am delighted with it I add at what I paid. Not off any auction sites . PS Must check on valve type.

[HamishBoxer]

6B8 is fitted in mine.

[Superscope]

Any chance we can have the Serial Number ?

I'm guessing it will likely be a 1946 or 1947 model.

The 6B8G version seems to be late in the production.

But now I said that, your Bridge will probably prove me wrong!

I've not had time to try my one out yet.
I have resisted the temptation to just plug it in.

I will work through the Restoration Guide at some point.



Ian

[Superscope]

Can anybody shine a light on what exactly this component is?

It looks and behaves like a Diode, but seems to exhibit a 2.9 Volt
drop whilst forward biased.

It doesn't appear in the Circuit Diagram, and is placed across the
Moving Coil Movement, so certainly looks like it is for some kind of
reverse Current protection.

If it's just a regular Diode, why this Heavy duty contruction ?
or is this just because of it's age?


Ian

[TowerRadio]

Its a Westector,copper oxide type of vintage rectifier in a small(for the time) package.Maybe an addition. Les.

[Superscope]

Thanks Les,

Does 2.9 Volts sound reasonable?

I didn't think copper oxide had that higher Volt drop ?

It's original to the Bridge, not an addition.

Ian

[HamishBoxer]

Hi Ian, not found that serial number yet. Bit like my Avo8 , missing I think.

[TowerRadio]

I have a booklet somewhere by Westinghouse documenting these rectifiers,must try and find it.You can always hide an OA81 and discretely unhook the old rectifier.Les

[ThePillenwerfer]

There were two versions of the AVO Test Bridge. The earlier one has an L63 Triode and a Westector and the later one a 6B8G Double-Diode-Pentode. The only circuit diagram that seems to exist is of the 6B8G version.

See page 8 of this: https://www.vintage-radio.net/forum/...1&d=1466164593

[Superscope]

Just been reading another Thread about the Westectors
https://www.vintage-radio.net/forum/...d.php?t=114179

Using a regular Diode checker, probably doesn't tell you much about
the health of the Westector.
Interesting to read that the construction inside is the same as the original
Avo meter Copper Oxide Bridge Rectifiers.
Seems they can be repaired, as indeed I have done with the Avometer ones.

As I mentioned a while back, I have found Three different Versions of this Bridge.
The very early ones seem to have used the EBF2.

I am in the process of Drawing up a Schematic for the L63 Version, hence
whilst going through the Device I had some componant Questions.
There are quite a few Value Changes to the Resistors related to the different
Valve.

Will post it up here when I'm finished.


Also,

Does anybody know Peter Wotton?
Is he a Forum Member?

I would like to update the Avo Test Bridge Restoration Manual
with Peters permission.
Otherwise, I have to write a whole New Version which would be
duplicated effort.



Ian

[Superscope]

Happy New Year to everybody.


Nobody seems to know Peter Wotton, so here are my Two Pennies worth
in trying to update the Avo Test Bridge Restoration Manual Peter Wrote.

Pretty much re-written from scratch to avoid copyright issues, but I
have kept the same basic format and some of Peter's original wording
may have crept in.
Not always easy to re-write a perfectly good sentence!

Special thanks to "ThePillenwerfer" for proof reading the final and offering
a few suggestions.

It took considerably longer to do than planned, but hopefully, it will be of
interest to somebody.

The pdf is over 5Mb so exceeds the Forum limits, so I have split it into
Two Files.

Download both into the same directory and run the Zip File with a suitable
UnZip application.
The UnZip application should recombine them both into a single pdf File.
WinRAR was used to create the Archive, but other Apps are available!

Feedback and corrections welcome.



Ian

[Superscope]

and Part Two of the Zip archive.

The Forum software, doesn't seem to like the .Z01 extention, so the
.Zip needs to be removed from this File before extraction is attempted.






Ian

thanks very much, very interesting, lots of photos, drawings, a really good job, thanks!

It is more than complete!!!