AVO Test Bridge

[WaveyDipole]

I am still some way from recovered, although I am getting up and about a little now. Spend a few minutes looking the Bridge this morning bearing in mind recent comments. I bypassed the selenium diode and installed a silicone diode in its place. Replaced the 100k with a 120k 2W to allow some voltage drop and replaced the smoothing capacitor. Used a 10μF as that is all I had to hand at a suitable voltage rating (400V) apart from some 1μF. The DC voltage is at about the same as it was before the substitution. However, unfortunately all previous problems persist including deflection on the meter on the Voltmeter setting, some AC voltage at the rectifier output and incorrect readings on the C1 range.

I also managed a quick test using croc clip leads to bypass the presets connected to R6 so as to re-instate the original configuration but no difference there either.

Superscope, you make an interesting point about R12. Its resistance reads 230Ω, so a little short of the 250Ω indicated on the diagram. I will have to check whether I have something at around the 20Ω or maybe hook the resistance decade box and experiment to see whether it makes a difference.

[WaveyDipole]

I set the resistance decade box to 250Ω and attached it in place of the resistor. I then powered up and observed the movement pointer. The resistance was varied 50Ω in either direction (between 200Ω and 300Ω) but no change to pointer position. The original resistor has now been re-instated. Will have to have a think where to go from here.

[WaveyDipole]

Could anyone tell me what type of caps these are please?

I have looked online but have found no useful info. The black ones on the right look like encapsulated paper, but are very close to value (20nF) when measured. Paper types would have been well adrift and these do seem rather small for paper given their voltage rating. Could they be a film type?

What about the metal can one on the left? This one is significantly adrift of its value (59nF vs 50nF).

I am not sure of the tolerance of either as its not stated so I presume it can't be very critical?

[WaveyDipole]

Just had a bit of a breakthrough!

I discovered that the component earth and external earth terminals were reversed. The component earth was wired to R350 (bridge point A) and the external earth directly to the 50V secondary (bridge point D). The standards common rail was also wired incorrectly to bridge point D instead of bridge point A. Curiously, judging by its length and forming, that connecting wire looks original. The R5 tag is closer so one might have expected the wire to be shorter and bent at right angles at the R5 tag rather than the transformer connecting tag about half an inch further along. Error at the factory?

Having corrected the wiring in line wit the diagram, I can now read capacitors on the C1 range correctly. A 100pF capacitor reads exactly 1 as expected, which wen multiplied by 100pF gives a reading of exactly 100pF. A 25pF capacitor read approximately 22.5pF. I re-checked the other ranges and they seem to correctly as well. In addition, that 220nF capacitor which I had trouble reading previously, can be read correctly on both ranges C2 and C3.

The voltmeter problem still persists, so still one problem to sort.

[dazzlevision]

Originally Posted by WaveyDipole

Could anyone tell me what type of caps these are please?
What about the metal can one on the left? This one is significantly adrift of its value (59nF vs 50nF).

The one on the left has the Erie Electronics logo, but that capacitor looks like it was originally made by A H Hunt, whom Erie acquired in the late 1960s.

According to my 1973 Erie catalogue, they are WF 197 series Duolectric metal cased metallised lacquered paper and Polyester film interleaved capacitors.
Standard tolerance is 20% (OM), but 10% (OK) to special order.

Their list number for this capacitance and voltage was AF502.

[Superscope]

None of those Capacitors look original though!


Excellent News on the Earth reversal breakthrough.

When I started reading the post, I started to worry in case
my Schematic was in error! (Maybe it is? Please let me know)
I will have a look inside my Bridge later and check to see if
the wiring is done in a similar fashion.

Did this have any effect on the Voltmeter Fault ?
Looking at the schematic it looks like it probably did.


Ian

[WaveyDipole]

Ian,

No, your schematic seems to be quite correct. When I discovered this wiring error, before changing anything, I also checked against the original schematic which I found online in a couple of places including here:

https://frank.pocnet.net/instruments...Testbridge.pdf

Although this schematic does not use the earth symbols on the terminals and does not mark the L and V positions on potentiometer R6, it is quite clear which terminal goes to which side of the 50V winding. The External terminal connects to R5, the component one direct to transformer winding. I double and triple checked the wiring against both diagrams before reporting in case I got muddled up as well.

Although correcting the wiring solves the C1 range and other measurement problem that I had, it does not solve the Voltmeter fault. This Voltmeter one is rather curious. I disconnected C6 (50nF) from the switch and Bridge Point C (both terminated on S2 terminal B) and voltages dropped to negligible. Each connection individually (S2 and Bridge Point 2) seems to add some deflection and their combined effect is cumulative. Position 'L' also deflects considerably.

[WaveyDipole]

The two caps have now been proactively replaced. I was able to use the 20nF polystyrene cap that was fitted in error to replace one of them. The other was replaced by a new 47nF film cap.

I have been considering whether to remove the two preset type resistors when I came across this even stranger thread:

https://www.vintage-radio.net/forum/....php?p=1300216

Still, other than this and my example, no other pictures that can be found online seem to show any resistors connected to the two earth terminals. If no one can suggest a specific reason why they might be there, I think I might remove them, if only because they do not seem to be particularly robust and may present a point of failure or increased resistance when the contact is less than optimal.

The two-core cable has now been replaced with a 3-core one and chassis earth is connected to mains earth. There is actually a copper leaf contact fixed to the transformer core which males contact with the case. I hadn't noticed this earlier since it was oxidised. It has now been cleaned up and pulled outward a little to ensure good contact with the case. Interestingly, connecting the mains earth to the earth chassis point has reduced the amount of deflection in voltmeter mode from around 1V to around 0.5V.

There is still something of an issue with the C1 mode. Although it shows exactly 1 (x 100pF) for a 100pF capacitor, it shows around 800pF for a 1nF capacitor so is more than 20% out at the top end of the scale. There is some indication in the manual that the bridge may be less accurate on this range, but I was not sure whether to expect it to be quite that far out though. Still, was are talking about an instrument from 1947! The other ranges (capacitance and resistance) seem quite accurate.

[paulsherwin]

Thread reopened by request.

[Superscope]

Many thanks Paul.


It appears Avo did indeed add adjustable resistors to each side of the main
Dial Potentiometer R6 towards the end of production. Namely 1947/48.
These should be 100 Ohms each (or there about).

I now know of Three examples, that have the same style Wire-wound Bar resistors
as linked too earlier.

Adjustment was made by soldering one connection along the winding at the
required point.
Meaning that one end was anchored onto the Earth Terminal and the other end was
left hanging in mid air.

WaveyDipoles Bridge is slightly different, so it might be that this was just an early
version of the mod, or possibly an owner or service agent adding the Mod themselves.

The purpose seems to have been to make the calibration of R6a/R6b easier and more
accurate to achieve, and one assumes after WWII the use of a couple of extra components
wasn't a problem.
Previous to these added resistors, Dial Calibration would of required tedious Dial
and Potentiometer adjustment.

The small isolated break in the the R6 winding at the V end was as surmised previously,
to allow the Wiper to go down fully to the Valve Voltmeter ground.
With the Dial in the V position, the Wiper sits on an isolated spot and acts like Switch,
isolating and connecting the Ext ground Terminal directly with the VVM ground without
the added resistor sitting in the way.


It was all a bit Heath Robinson for sure, but I guess it did the Job.


Ian

[Superscope]

I think that there is also a mistake in the Schematic Diagram.

I don't think it effects the Circuit functionality, but it might lead
to confusion when trying to trace the circuit.

Maybe somebody else could also check it, but unless I'm
going completely mad (always possible of course), the Bridge
current limit resistor R5 is NOT in the Standards Common rail (Ext Earth).

On the Two Bridges I have here, R5 appears to be in the (X earth) terminal Rail.
Basically, on the L end of R6 not the V end.

If true, the mistake is on Avo's original Schematics and everybody has copied it
blindly, myself included.

I'm working on an updated Service Manual which should be ready shortly.


Ian

[WaveyDipole]

Well I had another look at these "potentiometer" devices in my unit. They are rather unusual.

They are wire-wound and the winding seems to be made around a thick piece of enamelled copper wire which is wrapped around the round former and the ends twisted together to form a terminal. Only one end of the winding is connected to this terminal. The other is tucked into holes in the former. The "leg", which was soldered to a terminal in the Test Bridge is the wiper and is not movable. Its the round former that rotates.

Both devices were set at their minimum setting so the approximately 0.7Ω-1.2Ω reading I reported was at almost the minimum point. When turned to maximum resistance this measures somewere between 5Ω and 10Ω. It is very difficult to get an exact reading because it is continuously fluctuating. However, it is nowhere near the 100Ω reported by Superscope. The construction is very much "Heath Robinson".

Apologies that the photos are a bit low resolution. They were done using the USB Microscope which is supposed to be "Full HD" but is giving me only 640x480 resolution for some reason.

Regarding the position of R5, in my notes I see that I have recorded the same observation and assuming that this was a wiring error, rewired my unit to match the circuit. This did improve but not adequately resolve the accuarcy of the C1 scale reading. I will be very interested in the outcome of this question. If necessary I will return the wiring back to its original state, including those potentiometers that were removed as it now looks like they might have been part of an original factory modification.

[Chrispy57]

I hope that it may be of some interest to learn that the Avo Test Bridge production did actually continue until at least May of 1948, according to the serial number of the nice example that I acquired recently. Proof can be seen in the penultimate photo in the link supplied.

Cheers
Chris

https://www.ebay.co.uk/itm/Vintage-A...p2047675.l2557

[WaveyDipole]

Thank you. That does seem to be a nicely preserved example complete with a two pin plug!

[Superscope]

Hi Chris,

That certainly looks like a very nice example.
It should be the 6B8G Valve inside.

Have you had a peek inside yet?
If not, do you plan too?

The burning question is whether it has the Bar Resistance Windings
added to either side of R6 (The Dial potentiometer) as per the photo
on Post # 50

Thanks for the number, I have added it to the Avo Multimeter Survey.
It is a very late one, and could indeed be the last year of manufacture, only
time will tell.
Yours is now the second from 1948 in the survey and strangely enough, both are
from May.
No others have been seen from 1948 or later.



Ian

[Chrispy57]

Hi Ian - glad the info was of interest. I will keep a note of your questions for when I do get around to checking it out - I have a couple of projects on the bench at the moment and don't want to jump in half-hearted. I wondered if this was a "Civilian" offering since some of the other photos I have seen show the WD logo and arrow on the front panel?

How is your blood pressure at the moment? Googling can be rather unhealthy - I stumbled across this ill-fated example, attached!

Cheers
Chris

[Superscope]

Hi Chris,

Blood Pressure is fine, but only because I spotted that poor old Test Bridge
some time back:

https://www.vintage-radio.net/forum/...postcount=2845

At the risk of being Shot at by other Forum members, it's not the worst attempt to
Steampunk an Avo I've seen.


Ian

[Chrispy57]

Hi Ian - and I think they claim that it sold for over £125, hmm mine only cost £17.65 ..... I wonder .....

Cheers
Chris

[Chrispy57]

Hi Ian,
I'm pleased to be able to report that I have now turned my attention to the Avo Test Bridge, having finished the previous project occupying bench space, and can answer your questions.

"The burning question is whether it has the Bar Resistance Windings
added to either side of R6" - YES it does have these two resistors in situ, each measuring about 100 Ohms on a quick DVM check.

The valve is the 6B8G type, as you had anticipated.

I can see the large clips for a selenium rectifier, but this has a fairly substantial silicon diode fitted, labelled F8.

Some wax capacitors are drippingly waxy, but I haven't done any checks at this stage.

It came apart very easily, with nice clean screws all present.

I've added a few photos for your perusal.

Cheers
Chris

[Superscope]

Originally Posted by Superscope

I'm working on an updated Service Manual which should be ready shortly.


Ian

Better late than never!

Can't imagine anybody was waiting for it, but since I did update it, I might as well
make it available.
This time I have simply split it into Two pdf Files rather than go through the Win rar route.
This is due to the file limit size on the Forum.

Hope this useful


Ian