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Old 2nd Jun 2022, 11:32 am   #21
wd40addict
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Default Re: AVO 7 voltage reading low.

Unlike the 8 the 7 does use the 1.5V battery on the AC ranges. It's used to bias the rectifier, note the 0.2M resistor in circuit. It doesn't make a huge difference to the readings, but full accuracy requires its presence.
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Old 2nd Jun 2022, 12:15 pm   #22
The Philpott
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Default Re: AVO 7 voltage reading low.

Yep, this meter should have that AC bias feature (introduced during WW2 as my 1939 meter doesn't have it.)

If going for a full repair, note the 1.5v cell powers the low and medium resistance ranges, and the high resistance range requires 9v. (Easiest way of doing this is grafting a PP3 into the battery compartment)

Suggest getting DC volts working then go from there...the main cutout is a possible source of bad contact but in this case i think it's unlikely.

Dave
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Old 3rd Jun 2022, 11:18 am   #23
Alchad1
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Default Re: AVO 7 voltage reading low.

Morning,

Update, baby steps but slight progress. Yesterday could get a reading on DC but not AC, tried a few drops of contact cleaner on the various selector contacts and exercised them quite a bit. End result was no reading on either DC or AC! After much head scratching I discovered that if I pressed in on the reset button the needle would move and show a reading.

This morning I investigated further and to cut a long story short I removed the shaft from the reset button and gave all the contacts a very good clean up with one of the other halfs' emery boards. Reassembled and the meter is now reading DC successfully but still not AC.

So any suggestions as to why this could be?

Thanks

Alchad
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Old 3rd Jun 2022, 12:12 pm   #24
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Default Re: AVO 7 voltage reading low.

The 'reset' cutout has (graphite/carbon?) flat contacts that open after the small (pointed) silver/gold contacts have opened. This is to avoid the precious metal being eroded, ie it directs any arc flash to the carbon contacts should the cutout operate in use. This opening sequence should be checked to see that all is correct. (Easy to remedy if it's not)

You don't want anything harsher than 400 wet & dry paper on the carbon contacts, (unless they've been cooked) and you don't want anything harsher than alcohol on the metal contacts (again, unless they've been eroded...like mine had.) Keep the nail board for fettling revell/airfix models if i were you!

Lack of AC readings: Are ALL of the leaf switches actually opening and closing properly? Double check this before considering whether the rectifier is damaged. The rectifier is at the bottom right of your first photo, incidentally.

Dave
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Old 3rd Jun 2022, 2:00 pm   #25
Alchad1
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Default Re: AVO 7 voltage reading low.

If you mean the leaf switches on the selector switches - they seem to be opening and closing properly although can't tell if the contacts are dirty or oxidised. The photo below is an close up of the bottom right of the Avo, so I assume the rectifier is the round black object? The top is heavily oxidised, does this indicate damage?

Thanks again for the time.

Alan C
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Old 3rd Jun 2022, 3:44 pm   #26
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Default Re: AVO 7 voltage reading low.

Yes that's the rectifier...and the last two i saw that were corroded on the outside were actually faulty, so this could be significant. The rectifier's usually marked with ~ ~ + - so no need to refer to circuit diagram. How's your soldering? I think 4 Germanium diodes would do the job (someone advise?) but they're heat sensitive so need clamping with something to wick the heat away. Alternatively you could swap out the old copper oxide rectifier for a second hand one.

Dave
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Old 3rd Jun 2022, 5:14 pm   #27
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Default Re: AVO 7 voltage reading low.

Quote:
Originally Posted by The Philpott View Post
How's your soldering? .

Dave
Not bad on larger things, not too brilliant on electrical components, but having come across this thread ....

https://www.vintage-radio.net/forum/...=160057&page=2

I might have one last attempt at fixing and have just bought some BAT49 diodes on Ebay, however I may need some guidance from Superscope as to what is meant by "re-pack" with the diodes - I'm assuming they need to be soldered?? . I'll have a go at removing the old rectifier tomorrow.

Thanks again,

Alchad
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Old 3rd Jun 2022, 5:46 pm   #28
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Default Re: AVO 7 voltage reading low.

Yes, re-packing or re-stuffing just infers gutting an old component shell and inserting modern component/s, mainly to preserve the old appearance. In this case it would make it easier to install and tidy as well, bearing in mind your rectifier is tucked away in quite a tight little corner.Note that these rectifiers do come in slightly different sizes, yet another thing sent to try and catch you out. Yours, being old, is probably a large one...good.

Yes, Ian (superscope) would have soldered the diodes in.

A classic mistake (which i did once) is soldering in diodes the wrong way round!

Dave
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Old 3rd Jun 2022, 7:38 pm   #29
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Default Re: AVO 7 voltage reading low.

Alchad,

It would seem that your meter has at least two problems;

1. The cut-out contacts, which as Dave has explained, are duplicated. There are two small points which are a metal or alloy which is intended to have a low resitance contact but are not intended to break a large fault current. These should only be cleaned with nothing more abrasive than paper moistened with isopropyl alcohol or switch cleaner. The large disc contacts are a carbon compound in meters of this age - metal in later models.

When the cut-out is tripped, the small contacts should break first so that the larger discs break the fault current. I don't know if there is an "official" method of cleaning the large contacts, but Dave's advice on using fine, or very fine, wet-or-dry paper seems a good policy to me.

The cut-out is likely to need some cleaning and adjustment and this has been described several times within the forum.

2. The rectifier incorporates four copper-oxide rectifier diodes which have been obsolete for a very long time. The forward voltage of a copper oxide diode is around 0.3 volts. The BAT49 diodes which you have bought are Schottky diodes with a Vf of around 1.0V so may not be suitable. Germanium diodes, such as OA91/95 do work well as a replacement for copper oxide.

To keep the appearance original you can take the trouble to fit germanium diodes within the original rectifier housing but an assembly as in the atatchment will be acceptable functionally. These glass diodes need careful handling both to avoid mechanical stresses, which can crack the glass, and to avoid heat damage while soldering.

PMM
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Old 4th Jun 2022, 2:53 pm   #30
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Default Re: AVO 7 voltage reading low.

PMM,

Thank you for the reply.

I think I've sorted the cut-out problem.

The rectifier on the other hand....I removed it and took it apart as carefully as I could but one copper oxide disc fell out and I'm puzzled that of the three pairs of disc which were intact, two have the copper disc on top of the copper oxide one, but one has the oxide on top of the copper. From what I read on another post I thought they all should be copper on top of oxide. Also, assuming I sort out the disc orientation question, internally all seemed pretty much in good condition, apart from the springs which were slightly oxidised.

Thank you also for the picture of a substitute rectifier. Sad to admit that would probably take me out of my comfort zone as far as soldering is concerned, plus nor would I know which wire should go where.

Alan C
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Old 4th Jun 2022, 4:48 pm   #31
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Default Re: AVO 7 voltage reading low.

I think the easiest way to understand what's going on is to look at a Four Diode
Bridge Rectifier Circuit. That should make things clearer.

Treat each Copper Oxide/Copper Disc Pair as a single Diode.


Ian
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Old 4th Jun 2022, 5:36 pm   #32
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Default Re: AVO 7 voltage reading low.

- I suppose at this point...the amount of time you spend on it hinges on what your intentions are for the meter.

There's no way i would get a meter to working order with the sole intention of selling it; it just isn't worth the candle. Fundamentally there's two types of buyers- the steampunkers i alluded to at the start, and the collector/user type. The latter generally isn't fazed by having to carry out these sorts of repairs- they enjoy it.

Dave
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Old 6th Jun 2022, 2:05 pm   #33
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Default Re: AVO 7 voltage reading low.

Afternoon, final update, I've decided to give the meter away - free to a good home, see

https://www.vintage-radio.net/forum/...10#post1475910

Basically, as explained in the post, I was trying to temporarily connect up the rectifier I'd taken apart and put back together to see if it worked, but one of the very small terminals connected to a coil came loose (see photo). As I said in the other post I suspect it may have been loose before which may have been the reason it wasn't working on AC, who knows, but repairing that is way outside of my abilities!

If any of you guys who have been kind enough to help me would like it, you would have first call.

Thanks for your help and time.

Alan C
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Old 15th Jun 2022, 9:56 pm   #34
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Default Re: AVO 7 voltage reading low.

I'll continue on this thread as Alan has sent me this meter!

The cutout: The gold/silver spikes still weren't quite closing properly and about half a volt was being lost through the large arc protection contacts. Having bent down one 'leg' DCV function was restored on the 10v range, predictably dead-on accurate @ 10v, 7.5v, 5v and 2.5v. The 1000v winding is o/c but non-function of the 400v and 100v ranges turned out to be o/c trimmer windings rather than o/c of the main windings (why are these trimmers found to be o/c so often? It's as if the short windings were wound under too much tension)

The low and medium resistance ranges are now working, haven't tried high range.

The anomalous silver (hammer effect) painted case is aluminium, and most likely from a Mk2 as it mentions the PF sockets. The debris is from corrosion of the aluminium so i'll strip the insulation out and clean it. There's an interesting maintenance sticker on the side, i will photograph it.

The main leaf switch stack had an issue with lack of 'throw' for the upper two contacts- the pushrod seems too short so i glued a 20 thou shim to the leaf above to give more movement. Fiddly- this leaf stack is well defended from attack from all directions!

I've rebuilt the rectifier and tackled a detached wire and missing terminal post on the transformer, but something's up with the AC readings and only the barest movement of the needle occurs. More head-scratching required..I am leaning towards blaming the transformer but...there could be a dry joint somewhere.. Current isn't reaching the rectifier.

The signal to call it an evening came when a large mosquito landed on the 'divide by 2' button! Thanks to Alan, it's a very interesting meter, different in details to my older one.

Dave
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Old 17th Jun 2022, 5:23 pm   #35
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Default Re: AVO 7 voltage reading low.

Service sticker- (Farmer? Farnell?)

I have a spare Mk2 transformer which came with a bundle of bits, so removed the suspect one for better access and comparison. Sure enough, the long thin winding that feeds the rectifier is o/c- it should be around 204 ohms according to the spare tx. As the coloured cable ties show the terminal locations are different, but as the insulation is likely to be more stable on a later unit i will adapt the wiring to suit.. I have some PTFE insulated multicore which is just stiff enough for the job- the only way to make all the solder joints without further dismantling is to make the leads too long and loosely coiled, so they can coil up back into the meter as the tx is lowered in. Not the correct way to do it, but 'as little dismantling as possible' is fine by me.

There's a horizontal brass rod with threaded ends holding the tx stable, which makes it only just possible to remove it without removing the bottom resistance board.

There's one of those mysterious SAI diodes across the rectifier, which doesn't appear to do anything (one end cap was loose anyway so i removed it.) Former member Spch discussed this at length years ago as it didn't seem logical to him how it was wired in..

Also pictured are two bobbins for the 1v and 10v ranges- i see that these are replacements as the ghosts of the original slab windings can be seen behind them.

To have produced such compact designs in the days before CAD must have needed great vision. If all dimensions of these meters were 25% greater they would be a delight to work on- but potentially too bulky to have been properly competitive in the marketplace!

Dave
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Old 17th Jun 2022, 10:44 pm   #36
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Default Re: AVO 7 voltage reading low.

TX connected up, fed meter with 14vac on the 100Vac range. Turned attention to the three tiny wires at the other end of the transformer....fortunately got it correct first time- blue wrapped one to the leaf switch tag, fawn wrapped ones to ~ and ~ on the rectifier.

Needle deflection seen from my viewpoint at the rear of the meter, of approximately the correct amount. Phew! To be continued.

Dave
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Old 18th Jun 2022, 8:13 pm   #37
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Default Re: AVO 7 voltage reading low.

1- There are only 3 wires at this end of the transformer, the 2 (fabric insulated) loose wires have no obvious destination (they were both connected to one terminal on the old transformer, and at least one of them relates to trickle biasing the rectifier to improve low voltage AC accuracy....perhaps the Mk2 did not have this biasing facility..)

2- A bit of a green PTFE tangle but functional and not impinging on anything else. (Yes, i did have the workshop door open..)

3a- An unusual o/c winding- the legend reads
'PROV PAT 5124-1943 and others 298.000 PLUS'
It related to the 1000v ranges and dropped a max. of 600v. (300kohms)
There is a a stiff spring down the middle which also forms the leg at the bottom. One end of the winding joins to the spring/leg and the other end joins to the band at the top.

The 150kohm shunt is very similar, and is functional-but wasn't connected!

3b- SAI diode (damaged)

3c- An odd sliding rheostat variable resistor which was fitted as the reference for the medium ohms range.. It was set at maximum (500ohm) and it made sense to chop it out and fit component resistors to a value of 438ohm. It's now possible to toggle between low and med ranges and maintain zeroing.

Confession- I had no idea until i dug into this meter that the ACV ranges relied on elements of the transformer being functional. This is why i suspected the rectifier, which in all probability was OK!

All the ranges (apart from capacitance and 1000v which i haven't yet played with) have now been found to be working, and surprisingly accurate. The movement is rather nose heavy which might explain why the needle doesn't always zero too precisely. Easily sorted, then i'll exercise the movement by feeding a slow pulse through for a few minutes. Loosening a jewel 1/8th of a turn is a last resort, probably won't be needed in this case, as the needle seems to move normally otherwise.

Battery compartment covers- note the distance between the mounting holes for bakelite/metal covers is subtly different, but it's enough to stop them being interchangeable.

Dave
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Old 18th Jun 2022, 8:18 pm   #38
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Default Re: AVO 7 voltage reading low.

Nice one Dave,

That was a bit more work than we all expected wasn't it?

Another Avo saved to fight another Day.


Ian
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Old 18th Jun 2022, 10:17 pm   #39
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Default Re: AVO 7 voltage reading low.

Dave,

This meter is lucky to have found its way to you.

Congratulations on your effort and perseverence.

PMM
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Old 19th Jun 2022, 12:33 am   #40
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Default Re: AVO 7 voltage reading low.

Thanks all.
Curiously i bought a small batch of dismembered Model 7 bits off ebay years before i even had a Model 7- and i can't recall why. Ultimately the leaf switches replaced the ruined ones in the 1939 meter, and the transformer now replaces the damaged one in this later meter.
I've also found a large light blue 0.3% 300k welwyn resistor that was part of the same batch..

Dave
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