AVO 8 question.

[sparkymike]

On an Avo 8 I am renovating, I noticed that the transformer fixings were missing. A small strip of Tufnel is still under the transformer but the nut and washer was missing from the inner side and on the coil board side a spacer is missing. Should this spacer be non conductive, or could it be metal?
I managed to replace the shake-proof washer and 8BA nut on the inner side with great difficulty, as access is limited, but patience won the day.
Mike.

[pmmunro]

Mike,

Are these the fixings you are concerned about (shown ringed in red)? The nuts are screwed onto the piece of brass threaded rod which passes right through the transformer laminations.

The boards are simple held in place by a phosphor bronze wavy washer and a brass nut.

PMM

[sparkymike]

See photo. The lower domed bakelite piece is missing and the hexagon piece is fixed direct to the front plate, which leave a gap where the transformer foot should bolt to it. Is that bakelite domed piece part of the moulding or is it removable without losing the fixing screw ?
Mike.

[Superscope]

Hi Mike,

I thought that, that Domed piece was integral to the front panel, but if you say
yours is missing then it must be a separate piece.
Can't imagine anybody would have cut or filed it away!

I don't currently have a Model 8 Scrap meter broken down to Panel level, but
if I get some time later I will remove the Transformer from a scrap meter for a
closer look.

If it is a separate piece, you are welcome to it for the price of postage.
If it is molded to the front Panel, I guess that means Avo tried different
methods at different times.

I assume the Photo you posted is not the Meter in question.




Ian

[The Philpott]

The dome is almost certainly part of the front facia moulding. I recall one of mine was fractured and got reinforced/built up with araldite.

(crossed with Ian's reply)

Just looked at the carcase of an early Model 8 and the boss is integral with the front facia, however it is cylindrical rather than domed- it may be that this was seen as a weak point and became domed later on.

Dave

[pmmunro]

Mike,

I've just checked a Model 9 Mk II front panel and the domed spaced is definitely a part of the front panel moulding on this one.

I don't recall ever seeing a separate piece used as a spacer but you should be able to tell if it was a separate piece on your meter. It could have been broken off but if that is what happened, it would have left a rough surface. Even if that had been smoothed over, some marks would be visible as the rest of the paenl in the area is very smooth, almost shiny. It is quite possible that a separate domed spacer could have been used in some stage of production but in that case I would expect the intergral 6 BA stud at the lower end of the haxagonal spacer to be longer than the standard which is 1/4 inch long. Otherwise there would be only 1/16 of an inch to hold into the front panel.

It should be possible to make a replacement spacer; is 3/16" high and tapped 6 BA. It might be necessary to drill the panel a bit deeper and tap the new depth. It might also be possible to glue a tapped spacer to the front panel but for adequate strength it would be better to tap into the front panel. This would then need a longer stud on the hexagonal spacer, possibly by converting it to a through spacer and using a length of 6BA screwed rod.

PMM

[sparkymike]

I will have a closer look to see if the domed piece was broken off or not.
Mike.

[sparkymike]

Had another look and the dome piece broke off flush and dead flat. Now have a spacer in place.
Thanks for various help posts.
Mike.

[sparkymike]

I tested the meter on DC and all ranges are ok, but when testing on AC,
on 1000v range, mains voltage is around 280/300 and on 250 range needle is off the scale. Could this be a certain resistor gone low value, or one of the coils short circuited ?
Mike.

[micheal]

Could be the Shunt resistor across the bridge rectifier gone high 670ohms.

Mike

[sparkymike]

Hi Mike,
where is that resistor located? There is one behind the meter movement
but that is a 6.3K and impossible (nearly) to measure without removing the movement.

[sparkymike]

Found it, I was looking for a resistor component, but it is a very small coil.
It measure 630ohms.
Mike.

[micheal]

It’s also worth checking the leaf switch contacts are clean and operating correctly.

Mike

[sparkymike]

If switch contacts were soiled, surely that would give a lower than correct value on the scale ? ie. more resistance.
I did wonder if the hair springs were catching up and causing the problem but they both look fine.
Mike.

[micheal]

Coil spring trouble would affect both dc and ac.
If it’s easy to do disconnect one end of the shunt resistor across bridge as you may have been measuring other thing in circuit see if you get closer to the 670 ohms

[pmmunro]

Originally Posted by sparkymike

If switch contacts were soiled, surely that would give a lower than correct value on the scale ? ie. more resistance.
I did wonder if the hair springs were catching up and causing the problem but they both look fine.
Mike.

Leaf spring contacts can give problems.They are best cleaned with a strip of paper soaked in a claner such as isopropyl alchol. Spray cleabers are to be avoided as the mist has a way of coalescing on the turns of the hairspring of the movement. This can cause the turns to stick together, affecting the linearity of the pointer deflection.

Careful washing with a small artists paint brush and isopropyl alchol with a piece of tissue to collect the washings should help.

The large contacts f the range switches can be cleaned with the same solvent on a cotton bud head and a suitable bent rod or hook replacing the original stem. These contacts are not lubricated.

The rectifier is made up of 4 copper oxide diodes fitted inside the "pot" to which the 670 ohm resistor is attached. These diodes can be replaced by germanium ones, such as OA91 or OA 95.

PMM

[sparkymike]

The rectifier does not look like the original type and some previous owner has been playing around in that same area with obvious tell take clues such as burnt insulation in a few places. I probably have a donor Avo of same type so will change that rectifier out anyway.
Mike.

[sparkymike]

I connected up my variac to the meter and got following results on AC switch settings.
8 volts 10 volts meter reading
20 '' 25 '' '' ''
81 '' 100 '' '' ''
200 '' 250 '' '' ''
239 '' 350 '' '' ''

I have another model 8 identical to the above and that is giving same readings
error.
Mike.

[pmmunro]

Mike,

The rectifier shwn in your photograph seems to have been a type which AVO used in later meters or in repairs. You may find that the square block contains four germanium diodes.

If I understand correctly. you have two Model 8 Avometers showing the same, apparently wrong, readings on alternating voltage. What are you comparing them against?

PMM

[sparkymike]

Measuring mains voltage and variac output with digital meter.(radio shack)
I can double check with a Fluke meter if needed.

Mike.