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Old 21st Oct 2020, 4:47 pm   #1
Nanozeugma
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Default AVO Test Bridge.

Found on a Facebook Marketplace advertisement, described as a "measuring box," located on the South Coast, this looked in very reasonable cosmetic condition.
So I took a chance.
I had arranged to collect it, but a nasty tummy bug put paid to that and the owner kindly offered to post it.
Hermes - oh dear - luckily it arrived unscathed.
Some concerted elbow grease and Bakelite polish removed years of dirt, paint splashes and surface scratches.
Internally the unit is built like a brick outhouse, the fabric sleeving used was in good order.
The Westinghouse rectifier was tested and found fit.
Five TCC wax capacitors were tested and found to be as leaky as one might expect. One tubular electrolytic had dried out and had zero capacitance.
The only capacitor still sound was a 1uF 250v unit in a grey steel can, there were two waxies in parallel across the 1uF which do not appear on the circuit diagram - presumably to pad the value.
All resistors were within tolerance and left alone. The three precision resistors are actually coils of resistance wire on miniature bobbins.
Needing only defective capacitors replaced and a range switch cleaned, the unit works perfectly and is surprisingly accurate.
One thing worth bearing in mind with the leakage test facility is that approximately 120 vdc is applied across the capacitor under test, there is no facility to control the applied voltage. Not suitable for low working voltage capacitors.
Also, the test capacitor is left charged on disconnection, discharge before touching the legs.
As you can see from the image, the capacitor under test - one originally fitted inside the unit and extremely leaky - proved that the leakage indicator neon is working fine.
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Old 21st Oct 2020, 5:13 pm   #2
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Default Re: AVO Test Bridge.

Very nice find!

Not so sure about poaching goodies from our patch though

Peter
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Old 21st Oct 2020, 10:07 pm   #3
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Default Re: AVO Test Bridge.

Does this have a grey twin flex? I had to replace the one on mine as the plastic was cracking badly.Quite a job as the switch cannot be got at without removing the transformer. The 1mfd cap also was u/s. All OK now.Les
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Old 22nd Oct 2020, 7:50 am   #4
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Default Re: AVO Test Bridge.

Quote:
Originally Posted by TowerRadio View Post
Does this have a grey twin flex? I had to replace the one on mine as the plastic was cracking badly.Quite a job as the switch cannot be got at without removing the transformer. The 1mfd cap also was u/s. All OK now.Les
The mains cable on mine is gold coloured PVC twin flex which follows the old British mains colours - red and black. It is sound, so I left it alone. As you remark, replacing it would be a challenge. If a mains earth is considered desirable, one may be connected by the knurled nut on the case provided for that purpose. I was pleased that the 1uf grey steel can capacitor was still reading perfectly o.k. as I had nothing remotely physically similar to replace it with.
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Old 22nd Oct 2020, 9:55 am   #5
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Default Re: AVO Test Bridge.

A job well done.These are nice bits of kit,mine is my father's and I remember him showing me the HT present when checking leakage about 60 years ago. I wonder at want point all the wax caps started to go bad (TCC in mine).Les
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Old 22nd Oct 2020, 12:41 pm   #6
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Default Re: AVO Test Bridge.

Quote:
Originally Posted by TowerRadio View Post
I wonder at what point all the wax caps started to go bad
No idea. Obviously when these wax coated capacitors - and similar - were manufactured, likely no-one conceived that they would be in service in (in the case of this instrument) at least 73 years.
Hunts (A.H. Hunt & Co. originally of Garratt Lane, London SW18) had a reputation for excellent quality control at the time. The name has since (it would seem) have become synonymous with leaky capacitors, probably because they were found in so many appliances. It's a bit unfair on Hunts because if they were rubbish, manufacturers would have quickly stopped buying them. TCC, Dubilier, Sprague etc are just as leaky after the elapse of decades.
The waxies in mine were also TCC.
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Old 22nd Oct 2020, 2:32 pm   #7
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Default Re: AVO Test Bridge.

All caps in the main fail ,not relevant who made them but because of old age.
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Old 23rd Oct 2020, 2:03 pm   #8
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Default Re: AVO Test Bridge.

I would be interested to know what the HT voltage is in this device. Or what it should be.

My AVO test bridge had a faulty 'stick' rectifier and so I replaced it with a diode and series resistor.

The schematic does not mention the HT voltage.

To be on the safe side I used a high value series resistor.
The HT was quite low but the test bridge works.

Selenium rectifiers start to degrade from the day they are manufactured.

It would be useful to have a standard replacement circuit for this type of rectifier.
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Old 23rd Oct 2020, 4:36 pm   #9
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Default Re: AVO Test Bridge.

Nice work. I use mine on a regular basis, having recapped it. I did not comprehensively re-calibrate it since I'm not a purist, but simply tested a range of brand new caps to verify that the test results were reasonably accurate. It is very handy for low capacitance components for which my cheap Chinese component tester is a waste of time. Cheers, Jerry
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Old 24th Oct 2020, 2:36 pm   #10
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Default Re: AVO Test Bridge.

Quote:
Originally Posted by Silicon View Post
I would be interested to know what the HT voltage is in this device. Or what it should be.

My AVO test bridge had a faulty 'stick' rectifier and so I replaced it with a diode and series resistor.

The schematic does not mention the HT voltage.

To be on the safe side I used a high value series resistor.
The HT was quite low but the test bridge works.

Selenium rectifiers start to degrade from the day they are manufactured.

It would be useful to have a standard replacement circuit for this type of rectifier.
The HT across the reservoir electrolytic (C7 4uF) read 268 volts after fitting a good quality modern axial 4.7uF 450v cap - with the R6 on L (leakage) - no test component attached.
With R6 on C/R and adjusted to get closest to balance on the internal meter the HT was 252 volts across C7 as above.
Externally, with R6 on L (no component under test) the voltage across the terminals was 124 volts.
Readings taken with a modern DVM.

The Westinghouse stick rectifier in mine was good and remains in situ. The parts list describes it as a Westinghouse W40 but I have been unable to find specs for it, the only guidance I have seen on testing one is that if it reads significantly differently in one direction versus the other it's probably ok.
I assume it is selenium.

Hope that helps a bit.
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Old 24th Oct 2020, 5:39 pm   #11
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Default Re: AVO Test Bridge.

I have an AVO all wave oscillator with a similar power supply using a Se stick rectifier. The output amplitude stability of this oscillator depends to some extent on a rather soggy HT supply, caused by the forward resistance of the original rectifier.

After replacing the filter capacitors, I wired a 1N4007 in series with the stick to relieve it of voltage stress and to avoid consequential damage in the event of its failure. Wired in this manner, the HT regulation (and the performance of the oscillator) remains as original.

Leon.
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Old 1st Nov 2020, 11:30 am   #12
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Default Re: AVO Test Bridge.

I have recently been given an AVO Bridge Test set, it looks to be visually in good condition. The meter is fitted into a purpose made wooden box with divisions, for the thick two leads which have HOLDTITE COLLTT of LONDON on the ends, substancial crock clips.it is lined out with felt On the box lid it is marked with the following.
BRIDGES, TEST
AVO, N0.1 MK1
EQUIPMENT
WY2438
Then on the inside of the lid the following
CASES
TESTEQUIPMENT
N0.5.WY2176.
the box is faded RAF blue.
I haven't powered it up, but will have a good look around for those caps before doing so. I'm curious about it former home life, and the purpose made wooden case, any clues. Ted
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Old 1st Nov 2020, 4:21 pm   #13
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Default Re: AVO Test Bridge.

If you haven't seen it,Ted, the Restoration Manual is available in this thread: https://www.vintage-radio.net/forum/...d.php?p=519396
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Old 1st Nov 2020, 5:02 pm   #14
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Default Re: AVO Test Bridge.

Thank you for pointing out the restoration manual, I now have a copy. Also, found out the meter was army REME, there is a small label on the back of the meter, and it says it was calibrated by REME 1965. I was REME in 1953 we had AVO 7's available to us for repair jobs. Ted
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