Help needed with AVO Model 9 Mk. 2 AC current range

[BlownFuse]

Hello everybody, this is my second post and I’m looking for help, guidance & advice. I hope someone can shed-light on the problem.
I very recently acquired a new AVO Model 9 Mk.2 (Ebay).
Serial Number 7597-1166 (not currently listed in survey)
Manufactured: Vauxhall Bridge Road on back panel.
The AVO is in what I would call remarkable cosmetic condition, it looks almost new. Much of the Bakelite retains its original glossy appearance and with few slight scratches and the odd minor scuff. Clearly has not had a hard-life.

Problems
Inside, I noticed strange scorch-marks on the current transformer. [photograph]
Then, I noticed a problem with the 10Amp AC range, I haven’t tested the lower AC current ranges - yet.

Test-circuit
8.6 V AC (50Hz) output from transformer, fed via 4R7 10W resistor through A DMM (Brymen BM869s) set to 10A range and the Mod.9Mk.2 and two other AVOs for comparison (all AVOs set to their 10A AC range), all connected in series.

While passing 1.6 Amps through the meters, there was zero deflection on Mod.9Mk.2 10 A AC range.
Observation
When switched from 10A to 1A AC, the Mod.9Mk.2 responded by reaching FSD much more rapidly than either a Model 8 Mk. II, or a Test Set No. 1, both of which in comparison, respond quite leisurely on their 1A AC ranges. There’s something like a five fold difference in the meters’ rapidity of deflection response to a 60% overload.


Resistance measurements of AC current ranges*
Meter: Mod.9Mk.2
10 A 0.04Ω
5A – (range not present on this model)
1A 31.4Ω**
0.1A 31.35Ω**
0.01A 31.27Ω**

For comparison
Resistance measurements of AC current ranges*
Meter: Test Set No. 1
10 A 0.04Ω***
5A 0.04Ω*** / ****
1A 0.07Ω***
0.1A 0.077Ω***

* Resistance measurements performed with Brymen BM869s DMM
** Suspect measurement.
*** Similar results obtained from my Model 8 Mk. II.
**** Model 8 Mk. II has no 5A AC range, it has a 2.5A AC range instead.

The Model 8 Mk.I/II service manual suggests looking at the DC voltages. They are a little-low on the Mod.9Mk.2. Is this significant?
The DC voltage tests (All wired in parallel)
BM869s AVO Range TestSetNo.1 Mod.9Mk.2 Mod.8Mk.II
1.57V 3V/2.5V 1.57V 1.52V 1.57V
9.24V 10V/10V 9.2V 9.04V 9.2V
12.04V 30V/25V 12.1V 11.75V 12.1V

I presume the current transformer is NBG.
Does anyone know whether the transformer the same as in model 8s?
I looked at the Mod.9Mk.2 user manual and there’s no part number listed, that I noticed.
Does anyone have access to the service manual for the Mod.9Mk.2?


Help!

[The Philpott]

It will take me a while to assimilate your query (cos i am tired!) but first thoughts are that (i think) the highest rated current windings on the transformer are on the outside. I had a dry joint which disabled the 10A AC range on a model 8/III, and it was easily accessible. I noted that the 1A range behaved normally though (unlike your scenario.)

The abnormally high reading on the 1A range suggests to me that the windings have shorted together, either through overload/breakdown of the insulation, or heat breaking through from an overload situation on the 10A winding.

Dave P

[The Philpott]

Additional- Something tells me that a transformer transplant can throw the (two lowest) AC volts ranges out of calibration (On a Model 8 it can, at least) - but you might choose to live with this.

[BlownFuse]

Dave P,
Thanks for your thoughtful input. In-fact it triggered a review of what I'd already stated and new lines of research. And new research reveals new discoveries,

First of all, I started looking at transformer windings.
While a number of the circuit diagrams state the number of turns in the various transformer windings, none so far do in the versions of the manual for the Model 9 Mark 2 that I have found.
I have a paper manual version from AVO Dover, and a downloaded green version from Vauxhall Bridge Road, neither indicate the number of turns per winding.

Does anyone know a source?

AVO Transformers
Model & Mark ; Windings
8 (Mk. I): 3750; 750; 375; 75; 45; 3; 2;1/2 -
8 Mk. II: 3750; 750; 375; 75; 45; 3; 2;1/2 -
8 Mk. III: 3750; 750; 375; 75; 45; 3; 2;1/2 -
8 Mk. 4: 5000; 4500; 450; 45; 5; ½ -
8 Mk. 5: 5000; 4500; 450; 45; 5; ½ -
8 Mk. 6: 5000; 4500; 450; 45; 5; ½ -

Observation June 9th 2017
Hysteresis effect noticed in resistance measurements on Model 9 Mark 2 AC current ranges.
Could something be loose? I suspect the AC range selector switch [switch SC?] (Note to self: the one seen on the right).
Can anyone shed light on this?
Might this have a bearing on the non-functioning of the AVO’s AC current ranges?

Performed With Brymen BM869 positive to AVO positive etc.
AC Range CW ACW
10mA 110.68kΩ 31.33kΩ
100mA 110.5kΩ 31.49kΩ
1A 110.41kΩ 31.47kΩ
10A 0.07Ω 0.07kΩ

Performed With Brymen BM869 positive to AVO negative etc.
AC Range CW ACW
10mA 113.85kΩ 33.507kΩ
100mA 113.82kΩ 33.54kΩ
1A 113.77kΩ 33.56kΩ
10A 0.06Ω 0.05kΩ

[pmmunro]

Blownfuse,

The Spares List for the Model 9 Mk II is attached. This gives the turns ratios for the transformer. The second attachment is the circuit diagram for the Model 8 MkI and II from the service manual.

You will see that unfortunately the turns ratios for the two transformer types are entirely different.

As Dave/The Philpott states, the high current turns are on the outside of the winding. If only the outer turns are damaged, it should not be too difficult to rewind the turns affected. The biggest difficulty would be in extracting the laminations from the former without distorting the iron, but if only the outer turns are damaged, it may not be necessary to remove the laminations.

The third attachment is a drawing of the Model 8 Mk I/II current transformer. These are auto transformers, so an open circuit in any part of the winding will disconnect the whole transformer. The transformer is shunting the input to the rectifier and so causes a considerable damping effect on the movement. This would account for the rapid pointer response; I would also expect the pointer to swing much more freely than normal and take a long time to settle when displaced.

Before going too much further, have you checked the DC ranges? These are considerably simpler than the AC ranges and the meter cannot indicate correctly unless the basic movement circuit is responding as it should. If the DC ranges are intact, you should be able to restore the meter to full functionality unless the damage on the AC side is very extensive.

PMM

[The Philpott]

Interesting that you mention the range selector switches. There is actually a recommended procedure where a fault in the sprung leaf contacts is suspected; you grab a selector switch at the 12o'clock and 6o'clock positions and manipulate it back and forth in the panel to try and change the suspect reading that you are getting from the needle. This has the effect of forcing a worn or maladjusted pair of contacts to manifest themselves. (Not so easy to do vigorously on a model 8 or 9 as the switch knobs are tapered of course)

It is only possible to assess that the spike contacts inside are 'dancing' correctly if you slide white paper behind them to improve the view. Ideally one would replace them but they are no longer available and i have always got away with carefully bending a leaf to re-establish correct operation.

Yours might be perfectly ok, but it's certainly a common failure mode to look out for if you notice other anomalies unrelated to the scorched transformer