Help with AVO 8 fault

[G3VKM_Roger]

Hi All,

I have a Avo 8 multimeter inherited from a deceased friend about ten years ago. I haven't used it for two or three years but I last used it for some HV measurments on a transmitter and all was normal.

I now find that with a good D cell fitted but no 15volt battery I get normal operation on Ohms and X100 but no V, I AC or DC readings. I made a temporary hook-up to an improvised 15V battery but no joy. Fuse is OK, movement OK and zeros fine on resistance.

I've had a look back at previous threads on Avo faults but there are quite a few to choose from, so are there any Model 8 gurus who can offer advice?

Cheers

Roger

[Sinewave]

These meters do not need a battery for any voltage or current readings, so you can eliminate any battery issues.

Do you feel happy opening it up? We could talk you through tracing the circuit if you wanted. Check some of the contacts.

Which mark of 8 is it?

[The Philpott]

Hi Roger, I am not a guru but i have kicked 3 Model 8's (Mk III) and 2 Model 9's (Mk II) into a reliable state.

The circuit diagram for the Model 8 Mk II or III is to be seen everywhere (there is one on this website, and also probably one on richardsradios, if you don't have the original paper working instructions to refer to)

I am assuming you have a Mk I, II or III, and if so:

The circuit diagram gives you the leaf switch position protocols, and errant clearances (or blackening of the silver) in these switches is a common problem...This is most likely what has happened to your meter. It is sometimes necessary to tweak (ie bend!) one or more of the leaves to return the switch stack to life. (If AVO kept in a box for many years with rubber test leads in a warm environment sulphur can blacken the contacts, for example)

There is also a rotor (with cam lobes) on the back of both rotary selectors, these rotors have occasionally been known to come loose, which then puts the operating cams out of phase with the leaf switches. Doesn't usually happen unless there has been arcing, causing the sliding contacts to jam.

(I assume you mean with a D cell you get Ohms and Ohms divided by 100 to function?)

Please bear in mind i know nothing about Model 8 Mk IV and onwards- and some or all of the above may not apply to them!

Dave P

[G3VKM_Roger]

Thanks to you both.

The meter reads only on Ohms and Ohms x100 and there are no DC or AC readings for any current or voltage range, including the HV terminals.

I have a downloaded AVO Servicing Manual (in PDF, 23 pages) but am unsure if it is the correct one.

The meter is a model 8 Mark 5 and is labelled:

P/N 5170-211
S/N 13995
8V/5/74

I'd be quite happy to open it up, and would be grateful for any assistance with that and repairs. A dirty or bent contact sounds the most likely problem.

Cheers

Roger

[Sinewave]

The mark 5 is relatively straight forward in terms of switching ranges, so it may be a case of one of the contacts not making. When you open it up you'll see the ac and dc range contacts, see if you can test for continuity on the switch points on the drawing below.

https://www.petervis.com/avo-meters/...t-diagram.html

[G3VKM_Roger]

OK, many thanks. I will try and get the meter opened this weekend and report back.

Cheers

Roger

[G3VKM_Roger]

I spent a little time on my Avo 8 last night and saw from the prise marks around the case seal that it had been opened before, possibly by the previous owner.

A brief visual check didn't show any obvious damage or wiring faults but there seems to be an oily substance all over the rear of the switch assembly, I'm wondering if it might be the residue from some sort of swiitch cleaner used to clear a contact fault at some time.

I will try to clean this substance off and check for it's presence on the switch contacts. I have only Servisol switch cleaner here, is that suitable?

Cheers

Roger

[The Philpott]

Someone may have inadvisedly loaded it up with wd40 or 3 in 1 oil..

Servisol in my old yellow/white/black can is simply alcohol (Servisol Aero-Klene 50), so would hopefully act as a solvent for greasy deposits. (Obviously any fluids should be kept away from the meter movement.) I recall there is- or was- another version of Servisol that leaves an oil film behind. (red can..?)

[G3VKM_Roger]

Yes, WD40 sprang to mind with me, although there's no smell that WD40 has. My Servisol is No 10 red can and I do have a Maplin "servicing spray" which the labels claims is a degreaser and suitable for video recorder heads, probably passed its sell-by date!

Cheers

Roger

[G3VKM_Roger]

Update: Looks like my Model 8 is back in business.

I cleaned off all the greasy deposit on the switch boards and then found that downward pressure gently applied to the "DC" knob caused the meter to read again momentarily. I found that there was a small amount of vertical play in the spindle of the switch. Holding the spindle where it comes through the board I pressed down on the DC control knob and felt a very slight movement, after which the meter worked OK.

I could see that the similar shaft end on the AC switch has two circlips fitted but there are no slots that I can see on the DC switch. Hopefully the shaft won't loosen again!

Next step will be to check operation on all ranges and then build a 15V battery from button cells.

Oh, my meter is missing the little latch on the battery compartment, so if anyone has a scrap meter for spares I'd be happy to buy a battery door.

Cheers

Roger

[Sinewave]

Glad you got it working.

Until recently I was sure CPC sold the 15v batteries but I can't find them there anymore. Once my current stock is exhausted, I'll make an adaptor to plug it into a bench power supply for bench use.

[avocollector]

I use a neat workaround of 5 x 2032 3v disc shaped batteries (they fit if you angle them) and a small spring to contact the other end plus a bit of foam to keep it in place. Seems to work quite well in the 8 Mk V although other models require different adaptations.

[avocollector]

Oops forgot to say I take the spring from used ballpoint pens and clip down to size - you could also use an M5 nut in place of the spring apparently.

[G3VKM_Roger]

Originally Posted by avocollector

I use a neat workaround of 5 x 2032 3v disc shaped batteries (they fit if you angle them) and a small spring to contact the other end plus a bit of foam to keep it in place. Seems to work quite well in the 8 Mk V although other models require different adaptations.

Thanks for the suggestion. I have a couple of unwanted four packs of 2016 disc cells so I will see if I can improvise a way to fit them, heatshrinking five of them together should be OK if I use minimum heat and then wire tails to the original battery connectors.

Cheers

Roger

[Refugee]

The 2032 is slightly thicker and with 5 and a 15mm pipe stop end they are the same length as the original. They then fit with a bit of tape and even the original label so that the existing springs hold them together.

[G3VKM_Roger]

Originally Posted by Refugee

The 2032 is slightly thicker and with 5 and a 15mm pipe stop end they are the same length as the original. They then fit with a bit of tape and even the original label so that the existing springs hold them together.

That's a neat solution and I may have the fittings to hand, if so will give it a try.

All the best,

Roger

[The Philpott]

I think i might try that next time rather than (ten off) Ag10/LR54 stuffed inside a genuine battery case. Post #15 only has seven opportunities for bad connections, re-stuffing a BLR 121 complete with original end caps has thirteen!

Added to this the prospect of electrolyte leakage can be virtually ignored, high capacity, and anything up to 10yrs shelf life. There is an occasional oddity with small lithiums that after lengthy storage they can appear dead, and a medium current needs to be drawn to break through an insulating layer that has formed inside.

If there is verdigris on the avo's arrow shaped contact, this could over time migrate over onto the brass stop-end and turn it black. (more prevalent in damp atmospheres)

Dave