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Old 1st Apr 2024, 5:12 pm   #1
WaveyDipole
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Default AVO 8 MkII AC voltage readings low

In November 2023 I acquired an AVO 8 MkII, serial 82194-C-1158 in November 2023. It already has a mention in post #2708 on the AVO Survey thread, so I guess there is no need to log it again:

https://www.vintage-radio.net/forum/...=4343&page=136

First a bit of history:

Unfortunately its case was badly damaged with hefty cracks around the battery compartment. I will mention at this point, that it didn't come from Pamphonica, but another member. The battery compartment itself and the thick Bakelite cover were intact for most part, with the latter having slight damage near one of the screw holes. The rest of the case was in need of some serious TLC. Fortunately it came with all of the Bakelite splinters that had broken off so after dismantling and removing the battery compartment I was able, piece by piece, to re-build the damaged case. Most of the pieces were bonded together with superglue, but some re-enforcement was added on the inside of one corner using Milliput, which was also used to fill a small gap. In the meantime, the meter unit had been sat on the shelf over the winter, having only had a quick DC volts check to ensure that the movement was still working, had good deflection and was not sticking. Fortunately, it was found to be good on all counts.

Now to bring things up-to-date:

Since the case repair is now complete, I decided to have a closer look at the meter. Readings for all DC voltage and current ranges are reasonably accurate, if fractionally on the high side sometimes. It zeroes on all resistance ranges (with a bit of persuasion on the x1 range) and resistance readings on a sample of resistors look good as well.

However, it does have a problem with higher AC voltage readings. This is similar to the problem I encountered with an AVO 7 discussed here:

https://www.vintage-radio.net/forum/....php?p=1570697

However, the fault here is a bit different and does not affect all ranges. Up to and including the 100V range it measures fine, but on the 250V and 1000V ranges it measures very low. For example, mains reads only around 160V, but correctly on the DMM. I checked the resistors in the chain, particularly the 99, 150 and 750k and all seem OK. Thinking back to the previous thread, I also substituted the copper-oxide rectifier, which is a known problem, with Germanium diodes. However, the problem persists. I also disconnected and checked diode SA1. It seems to have a quite high reverse voltage of around 1.3V, but other than that, it reads correctly like a diode and even with it disconnected, the readings on the meter remain just as low. I also checked and cleaned with Servisol, the right-hand rotary control contacts for those ranges. Still no change. Wiggling the switch a little also has no effect.

I am following the circuit diagram here:
https://www.vintage-radio.net/forum/...=4343&page=136

I am open to suggestions?

Last edited by WaveyDipole; 1st Apr 2024 at 5:19 pm.
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Old 1st Apr 2024, 11:21 pm   #2
The Philpott
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Default Re: AVO 8 MkII AC voltage readings low

My notes on a faulty Model 8/III read:-
'AC volts 53% low on 2,500v, 1,000v & 250v ranges, others OK.
Upper leaf switch contact on DC selector bent to make better contact'

It could be significant that I had the same odd results; not all AC ranges were wrong.

I'm not clear whether you've cleaned just the radial sliding contacts, or the leaf switch stack contacts...I believe the latter may be the problem..and may also need tweaking after any blackness has been gently removed. On mine i had to bend one of the leaves- bodgery perhaps but NOS leaf switch stacks, you'll understand, are unobtainium!

These particular bad contacts can often be agitated/persuaded with a long cocktail stick or similar while the meter is reading AC.. the pointer (you hope) suddenly leaps upward.

I'll admit I haven't referred back to your link to the Model 7 repair, yet.

Dave
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Old 1st Apr 2024, 11:54 pm   #3
WaveyDipole
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Default Re: AVO 8 MkII AC voltage readings low

Dave, thanks. Hopefully tomorrow I will have a look at the upper leaf switch contact as you suggest.
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Old 2nd Apr 2024, 8:55 am   #4
pmmunro
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Default Re: AVO 8 MkII AC voltage readings low

WD, As you probably know, cleaning the switch contacts with a spray risks getting droplets on the turns of the hairsprings if the movement is on the front panel. Use fairly thick paper soaked in switch cleaner instead.

PMM
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Old 2nd Apr 2024, 10:29 am   #5
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Default Re: AVO 8 MkII AC voltage readings low

PMM, thanks for the reminder. Indeed I wouldn't want to contaminate the hairsprings!
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Old 2nd Apr 2024, 5:35 pm   #6
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Default Re: AVO 8 MkII AC voltage readings low

Had a look at the contacts on both sides but can't see anything that looks weak or loose obvious. They all seem to be making good contact. I took some photos of the wiring as I will probably have to remove the switch assembly. I did however notice, almost by accident, that the 250V range gives the correct reading if I turn it half a notch towards the 100V mark. That does seem to suggest something going on around the AC range knob or leaf switches, but at this point that's rather speculative.
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Old 5th Apr 2024, 11:30 am   #7
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Default Re: AVO 8 MkII AC voltage readings low

I think I have solved the problem. I removed the contact assembly on the AC side yesterday, cleaned up the contacts and put it back, but this made no difference. There was clearly something going on with the AC rotary control however as a wiggle to halfway between the 50V an 100V setting made it reading come right. The 100V setting was also getting a little temperamental. I figured it was something to do with the rotary control but doing continuity checks on both the rotary and leaf switches didn't reveal anything. I then noticed the screw ar the top of the rotary control. I initially tightened it by a quarter turn but that made the 100V read low all the time. So I turned it back and slackened it by half a turn and it now works OK. There seems very little difference in the operation of the control (it doesn't feel tighter or looser) but it now reads correctly on both 100V, 250V and 1000V ranges. I am not quite sure what this adjustment has done, but it seems to have fixed those ranges.

I next checked the 2500V range. Following this thread:

https://www.vintage-radio.net/forum/...48#post1422748

I used the lower right Red connector and the top right Black connector. On feeding it 400V the pointer barely moves. The DC switch is set to AC and the AC switch to 1000V. I know that the rectifier is OK, so the next step is probably to locate switch FG and check it.

Last edited by WaveyDipole; 5th Apr 2024 at 11:49 am.
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Old 5th Apr 2024, 1:40 pm   #8
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Default Re: AVO 8 MkII AC voltage readings low

WD,

Your main problem seems to have been the, not uncommon, loosening of the face cam locking screw. The sole function of this screw is to keep the cam in place and it seems that it has come just loose enough to allow the cam to rotate away from its correct position. This will have had the effect of shifting the correct rotational position at which the leaf spring contacts made contact, or released.

Knowing that this quite often happens, I probably should have thought to suggest this problem to you but at least it's good to know you found and correctedc the problem.

PMM

Last edited by pmmunro; 5th Apr 2024 at 1:41 pm. Reason: Pressed "send" too soon
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Old 5th Apr 2024, 3:32 pm   #9
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Default Re: AVO 8 MkII AC voltage readings low

Same here Peter! It skipped my mind totally.

WD- The cam timing has to be very precise when the screw is tightened, but it's not an onerous job as such.

Dave
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Old 5th Apr 2024, 7:12 pm   #10
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Default Re: AVO 8 MkII AC voltage readings low

The 2.5kV range does work! Please forgive me for a "senior moment". I had not put the meter back in the case, forgetting that the top sockets were not connected directly but via spring contacts and resistors on the back of the battery compartment. Once back in the case, the 2.5kV range it worked as expected at least up to the 400V source I had available.

I now get correct readings on all ranges. I have a couple of small tasks to do on it before the repair is complete. The red socket cover has a crack in it and was loose. It have just glued this on. The meter needs a more thorough clean and batteries. I saw someone selling a 15V adapter on eBay and might get one of those. It would look neater than my home made effort. Also the screws on the battery compartment are too short and barely grip. It needs slightly longer ones. Other than that, work is about complete.

The first photo clearly shows the amount of damage to the case that has been repaired. The second one with the battery cover does not look as bad, but plenty of cracks are still in evidence. The meter itself is in pretty good shape.
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Old 5th Apr 2024, 7:41 pm   #11
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Default Re: AVO 8 MkII AC voltage readings low

...I think...you may have an earlier battery compartment cover, teamed up with (shorter) screws that are an ideal length for the later type aluminium cover.

There's an element of doubt in my mind though, as there's a small difference in the distance between the mounting screw holes for earlier and later types....of the order of 1.5mm! I think this was accidental in origin, it's too small an anomaly for there to be a good reason for it.

If your meter has the early type dimensions I may be able to dig out an undamaged early type battery cover (but if you want me to look measure the distance between the screw holes on the meter, precisely in millimetres, as the early and late type aren't properly interchangeable)

Dave
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Old 5th Apr 2024, 10:31 pm   #12
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Default Re: AVO 8 MkII AC voltage readings low

Spookily enough I noticed this post on the same day as my AVO read low volts on AC! In my case it was the leaf contact assembly which operates when the function is set to AC.
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Old 6th Apr 2024, 11:31 am   #13
WaveyDipole
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Default Re: AVO 8 MkII AC voltage readings low

Quote:
Originally Posted by The Philpott View Post
...I think...you may have an earlier battery compartment cover, teamed up with (shorter) screws that are an ideal length for the later type aluminium cover.

There's an element of doubt in my mind though, as there's a small difference in the distance between the mounting screw holes for earlier and later types....of the order of 1.5mm! I think this was accidental in origin, it's too small an anomaly for there to be a good reason for it.

If your meter has the early type dimensions I may be able to dig out an undamaged early type battery cover (but if you want me to look measure the distance between the screw holes on the meter, precisely in millimetres, as the early and late type aren't properly interchangeable)

Dave
I had wondered whether that was the case. I had not seen Bakelite covers on these before. I was also curious as to what the slots and clip were for?

The distance between the centres of the screw holes measures 134mm, or 5.5in as it would have been when these were made.
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Old 6th Apr 2024, 8:02 pm   #14
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Default Re: AVO 8 MkII AC voltage readings low

Yes, the slots and clip are for the probes originally supplied- in the meter manual for some versions there's a line drawing of the meter c/w accessories which shows the probes. Most of the early probes have long since worn out or been lost or discarded.

I'll clarify that measurement...as i recall the aluminium covers come in BOTH sizes but the bakelite covers are all one size (Just to confuse things).

Dave
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Old 6th Apr 2024, 8:44 pm   #15
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Default Re: AVO 8 MkII AC voltage readings low

Post No.44 in this thread refers:-

https://www.vintage-radio.net/forum/...=191602&page=3

Glad i found my original measurements, memory can't always be relied on!

My Avo 8 MkIII measures 133.5mm (perhaps the MkII was different then..)
The bakelite covers i have knocking around are 134mm.

Dave
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Old 17th Apr 2024, 10:33 pm   #16
WaveyDipole
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Default Re: AVO 8 MkII AC voltage readings low

I found some screws that were 4mm longer and these are just right. All that remains is to sort the 15v battery.
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Old 2nd May 2024, 4:40 pm   #17
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Default Re: AVO 8 MkII AC voltage readings low

Battery now sorted. Everything works as it should.
Thank you for the comments.
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