Avometer 8 Mk 5 - UK Vintage Radio Repair and Restoration Discussion Forum

sparkymike · 25th Jul 2014, 11:49 am

I have an Avo meter 8 mk 5 here that needs repair. I would like to repair it if at all possible, as it was the one I used at work for many years, until some bright client blew it up. I claimed the remains and it has been in my loft for several years until now.
Both of the flimsy printed circuit "ribbon" cables need replacing for a start. Is it possible to do away with these and hard wire it as per earlier meters ? Are these point to point or do some tracks cross over as per computer cables ?
The movement is free, but does not seem to have any damping. In other words, the needle takes a long while to become stationary, if the case is rocked. Is this electrical damping and do the batteries need to be in place for this to take effect ? One other point, the printed instructions on the rear of the case refer to a 15 volt battery, but next to where that fits is a label reading FC No. 041603 8V, or is that just the serial number ? Thinking on, I have never heard of an 8 volt battery so the later is probably correct.
Perhaps this meter is past it, but a few sentimental reasons for getting it up and running again.
Mike.

pmmunro · 25th Jul 2014, 12:53 pm

Mike,

The printed film wiring can be replaced with conventional wiring. You will have to connect the individual wires to the stubs over which the film wiring was fitted, after it is removed. The wires can then be loomed neatly and the other ends made off.

Model 8s are designed so that the chain of shunt resistors are permanently connected across the movement shunting one quarter of the current around the movement. I.e. 37.5 micro amps for the movement and 12.5 in the shunt to make up the full 50 micro amps fsd. This contributes greatly to the damping and lack of damping usually indicates that something in the shunt chain is disconnected.

The number you quite is the serial number; the battery used for the high resistance range is the same as for other Model 8 meters. These can still be bought but the price induces many users to devise their own substitute as you will see in other threads.

PMM

sparkymike · 25th Jul 2014, 5:36 pm

I need to know the layout of the small film cable which goes from main board points 4/5/6/7/8/9 to points next to the transformer ,as this is missing.A picture of the larger film cable is in in the manual, but not the small one.
Has anyone got one on the bench they could photo please. Mike.

Vicboduk · 25th Jul 2014, 7:10 pm

Mike,

You are in luck! I hope these help.

Vic

sparkymike · 25th Jul 2014, 9:26 pm

Thanks Vic, Just what I need to check things out and get it right. (but maybe not original !!) Some rainbow ribbon cable should do the job.
The main film cable has had three repairs so far, so I will have to check that out for continity. There is one component that is damaged and is a small thin white sqare about 7mm. wide. Any ideas what it was ? It is on the top of the small pcb next to the transformer. Hopefully this was not an AVO only component. !! There are three other similar ones on the same panel.
Mike.

sparkymike · 28th Jul 2014, 9:32 am

further to my last post, it looks like the component I am trying to identify is a ceramic printed resistor. I need to know the value . It is the top one on the small square printed circuit board.(top when meter is inverted on the bench).
Mike

Vicboduk · 28th Jul 2014, 2:52 pm

Hi Mike,

I think the resistor you are referring to is R6. This will affect the 10Vac range.
By coincidence that is the problem with the meter I’m working on. Its value is dependent on which model you have, earlier versions it will be 7kΩ, later versions 800Ω.
In both versions this resistor is trimmed for calibration.
My meter is of the later generation and the fault was found to be R32 8.2kΩ o/c which is in series with R6. In my case R6 had been trimmed to 1.104kΩ.

Vic

sparkymike · 28th Jul 2014, 3:06 pm

Hi Vic,
Thanks. Yes that's the little blighter !! Where did you find the information for that component ? I could not see it on the circuit in the manual. There is a component list of sorts in the manual, but a lot of parts are missed out.
My meter is serial no.is FC 041603 8V. Does that make it early or late production.? I suppose that I can just bridge that out with a similar resistor of like value value, until I have found the main fault.
Mike.

Vicboduk · 29th Jul 2014, 1:41 pm

Hi Mike,

It seems you are working from the user's manual. Use your favourite search engine and look for the AVO 8 Mk5 service manual. This contains some extra information.
As for the version you have I think "pmmunro" may be able to give you a more informed response.
After the 8V on the serial number there should be a date code this will help narrow down things a little more.

You could temporarily use a 10kΩ potentiometer, adjust for calibration, and measure the value then substitute for close tolerance fixed resistors.

Vic

sparkymike · 29th Jul 2014, 2:40 pm

Hi Vic,
Thanks for advice. I was thinking along the same lines, maybe use a small preset pot. I would like to remove the large pcb to get a closer look at the film cable where it is soldered to the rotary switches. Do the two switch rotors lift off with the board ,or am I missing something here ?
I believe the Avo manual I downloaded is the service manual, but will check to see if there is a more comprehensive one.
Mike.

sparkymike · 29th Jul 2014, 5:34 pm

I have progressed a little further forward I think, I had to replace three of the tracks in the main ribbon film. Two were os and the third looked as if it was about to fail. By removing the three 2BA screws on the main pcb it is possible to raise it a little and get a better view of the tracks and switch contacts. The switch rotators lift with the board, so no need to touch them.
While I have been doing this I noticed a small crack near the top edge of the second largest pcb, so I will have to remove that to get a better look see. I am due this Thurday for a cataract operation, so all being well and fingers crossed, I will be able to see round corners afterwards !!
So all the above would not have helped in the meter operation. This one has had a tough life, but may yet work again.
As a matter of interest, is anyone out there making new ribbon films for the AVO ?
Mike.

sparkymike · 30th Jul 2014, 4:38 pm

I have been working my way round the circuit and all seems fine so far. I had wondered if the cut-out was faulty, but a check from the main neg terminal to the printed circuit shunt is ok. There is a test point shown on the drawing for the movement. Can this be located without any dismantling?
Regarding if this meter is early or late, the three pots are all 22K, so this points to a later model according to the circuit I have.

sparkymike · 30th Jul 2014, 5:32 pm

Now found the movement tails. Red and black soldered to edge of main board marked on underside of board 2 and G. Manual quotes T. Am I on wrong point there?
I am reading 8.7K across those two points. Is that correct, or is moveemnt faulty?
Mike.

sparkymike · 2nd Aug 2014, 9:36 pm

I urgently need a clear side view photo of the underside of the main circuit board showing the two wipers. Can anyone post one please.
Mike.

sparkymike · 2nd Aug 2014, 10:01 pm

Well think I have found another problem among the many. See thumbnail. Am I correct in what I am thinking here ?
Mike.

pmmunro · 2nd Aug 2014, 10:32 pm

Mike,

I assume you're thinking that there should be a contact channel on each arm of the lower wiper, other wise why are there two when there is only one arm on the top wiper?

It's a logical question, but there should only be the one contact channel which bridges between the inner tracks and the outer. There is also a one-segment offset between the top wiper and the bottom one on both switches. Viewed from the back, the lower wiper is one segment clockwise ahead of the top one.

PMM.

sparkymike · 3rd Aug 2014, 8:06 am

Hi PMM,
Yes you got my drift exactly. Strange AVO did not use a similar arm as the outer one. Perhaps original intention was to fit a felt pad in the spare lower slots to clean the tracks thus balancing the arm and keeping the pressure even ?
A further reason that started me off on this blind alley was that there is a fine piece of wire stuck in one of the empty slots and I had assumed that was the remains of a spring for the contact plate. I will have to remove this with tweezers.
So I am back to the actual movement of which I would like to know resistance of the coil, as per a previous posting. I can then hopefully proceed. If the AVO is fixable I would like to do it ,but it is rapidly getting to the stage where I will put it to one side for future thought . I have a certain amount of patience, but that is rapidly depleting !!
Mike.

cantata · 19th Jan 2015, 1:48 am

Hi Mike

You may find this service manual of use, if you haven't got it already!

Michael

25th Jul 2014, 11:49 am	#1
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Avometer 8 Mk 5 I have an Avo meter 8 mk 5 here that needs repair. I would like to repair it if at all possible, as it was the one I used at work for many years, until some bright client blew it up. I claimed the remains and it has been in my loft for several years until now. Both of the flimsy printed circuit "ribbon" cables need replacing for a start. Is it possible to do away with these and hard wire it as per earlier meters ? Are these point to point or do some tracks cross over as per computer cables ? The movement is free, but does not seem to have any damping. In other words, the needle takes a long while to become stationary, if the case is rocked. Is this electrical damping and do the batteries need to be in place for this to take effect ? One other point, the printed instructions on the rear of the case refer to a 15 volt battery, but next to where that fits is a label reading FC No. 041603 8V, or is that just the serial number ? Thinking on, I have never heard of an 8 volt battery so the later is probably correct. Perhaps this meter is past it, but a few sentimental reasons for getting it up and running again. Mike.

25th Jul 2014, 12:53 pm	#2
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avometer 8 Mk 5 Mike, The printed film wiring can be replaced with conventional wiring. You will have to connect the individual wires to the stubs over which the film wiring was fitted, after it is removed. The wires can then be loomed neatly and the other ends made off. Model 8s are designed so that the chain of shunt resistors are permanently connected across the movement shunting one quarter of the current around the movement. I.e. 37.5 micro amps for the movement and 12.5 in the shunt to make up the full 50 micro amps fsd. This contributes greatly to the damping and lack of damping usually indicates that something in the shunt chain is disconnected. The number you quite is the serial number; the battery used for the high resistance range is the same as for other Model 8 meters. These can still be bought but the price induces many users to devise their own substitute as you will see in other threads. PMM

25th Jul 2014, 5:36 pm	#3
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 I need to know the layout of the small film cable which goes from main board points 4/5/6/7/8/9 to points next to the transformer ,as this is missing.A picture of the larger film cable is in in the manual, but not the small one. Has anyone got one on the bench they could photo please. Mike.

25th Jul 2014, 7:10 pm	#4
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Avometer 8 Mk 5 Mike, You are in luck! I hope these help. Vic Attached Thumbnails __________________ It has to be here... I've just put it down!

25th Jul 2014, 9:26 pm	#5
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 Thanks Vic, Just what I need to check things out and get it right. (but maybe not original !!) Some rainbow ribbon cable should do the job. The main film cable has had three repairs so far, so I will have to check that out for continity. There is one component that is damaged and is a small thin white sqare about 7mm. wide. Any ideas what it was ? It is on the top of the small pcb next to the transformer. Hopefully this was not an AVO only component. !! There are three other similar ones on the same panel. Mike.

28th Jul 2014, 9:32 am	#6
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 further to my last post, it looks like the component I am trying to identify is a ceramic printed resistor. I need to know the value . It is the top one on the small square printed circuit board.(top when meter is inverted on the bench). Mike

28th Jul 2014, 2:52 pm	#7
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Avometer 8 Mk 5 Hi Mike, I think the resistor you are referring to is R6. This will affect the 10Vac range. By coincidence that is the problem with the meter I’m working on. Its value is dependent on which model you have, earlier versions it will be 7kΩ, later versions 800Ω. In both versions this resistor is trimmed for calibration. My meter is of the later generation and the fault was found to be R32 8.2kΩ o/c which is in series with R6. In my case R6 had been trimmed to 1.104kΩ. Vic Attached Thumbnails __________________ It has to be here... I've just put it down! Last edited by Vicboduk; 28th Jul 2014 at 3:11 pm.

28th Jul 2014, 3:06 pm	#8
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 Hi Vic, Thanks. Yes that's the little blighter !! Where did you find the information for that component ? I could not see it on the circuit in the manual. There is a component list of sorts in the manual, but a lot of parts are missed out. My meter is serial no.is FC 041603 8V. Does that make it early or late production.? I suppose that I can just bridge that out with a similar resistor of like value value, until I have found the main fault. Mike.

29th Jul 2014, 1:41 pm	#9
Vicboduk Hexode Join Date: Aug 2007 Location: S.W. London, UK. Posts: 416	Re: Avometer 8 Mk 5 Hi Mike, It seems you are working from the user's manual. Use your favourite search engine and look for the AVO 8 Mk5 service manual. This contains some extra information. As for the version you have I think "pmmunro" may be able to give you a more informed response. After the 8V on the serial number there should be a date code this will help narrow down things a little more. You could temporarily use a 10kΩ potentiometer, adjust for calibration, and measure the value then substitute for close tolerance fixed resistors. Vic __________________ It has to be here... I've just put it down! Last edited by Vicboduk; 29th Jul 2014 at 1:50 pm.

29th Jul 2014, 2:40 pm	#10
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 Hi Vic, Thanks for advice. I was thinking along the same lines, maybe use a small preset pot. I would like to remove the large pcb to get a closer look at the film cable where it is soldered to the rotary switches. Do the two switch rotors lift off with the board ,or am I missing something here ? I believe the Avo manual I downloaded is the service manual, but will check to see if there is a more comprehensive one. Mike.

29th Jul 2014, 5:34 pm	#11
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 I have progressed a little further forward I think, I had to replace three of the tracks in the main ribbon film. Two were os and the third looked as if it was about to fail. By removing the three 2BA screws on the main pcb it is possible to raise it a little and get a better view of the tracks and switch contacts. The switch rotators lift with the board, so no need to touch them. While I have been doing this I noticed a small crack near the top edge of the second largest pcb, so I will have to remove that to get a better look see. I am due this Thurday for a cataract operation, so all being well and fingers crossed, I will be able to see round corners afterwards !! So all the above would not have helped in the meter operation. This one has had a tough life, but may yet work again. As a matter of interest, is anyone out there making new ribbon films for the AVO ? Mike.

30th Jul 2014, 4:38 pm	#12
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 I have been working my way round the circuit and all seems fine so far. I had wondered if the cut-out was faulty, but a check from the main neg terminal to the printed circuit shunt is ok. There is a test point shown on the drawing for the movement. Can this be located without any dismantling? Regarding if this meter is early or late, the three pots are all 22K, so this points to a later model according to the circuit I have.

30th Jul 2014, 5:32 pm	#13
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 Now found the movement tails. Red and black soldered to edge of main board marked on underside of board 2 and G. Manual quotes T. Am I on wrong point there? I am reading 8.7K across those two points. Is that correct, or is moveemnt faulty? Mike.

2nd Aug 2014, 9:36 pm	#14
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 I urgently need a clear side view photo of the underside of the main circuit board showing the two wipers. Can anyone post one please. Mike.

2nd Aug 2014, 10:01 pm	#15
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 Well think I have found another problem among the many. See thumbnail. Am I correct in what I am thinking here ? Mike. Attached Thumbnails

2nd Aug 2014, 10:32 pm	#16
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,813	Re: Avometer 8 Mk 5 Mike, I assume you're thinking that there should be a contact channel on each arm of the lower wiper, other wise why are there two when there is only one arm on the top wiper? It's a logical question, but there should only be the one contact channel which bridges between the inner tracks and the outer. There is also a one-segment offset between the top wiper and the bottom one on both switches. Viewed from the back, the lower wiper is one segment clockwise ahead of the top one. PMM.

3rd Aug 2014, 8:06 am	#17
sparkymike Octode Join Date: Dec 2006 Location: Colchester, Essex, UK. Posts: 1,076	Re: Avometer 8 Mk 5 Hi PMM, Yes you got my drift exactly. Strange AVO did not use a similar arm as the outer one. Perhaps original intention was to fit a felt pad in the spare lower slots to clean the tracks thus balancing the arm and keeping the pressure even ? A further reason that started me off on this blind alley was that there is a fine piece of wire stuck in one of the empty slots and I had assumed that was the remains of a spring for the contact plate. I will have to remove this with tweezers. So I am back to the actual movement of which I would like to know resistance of the coil, as per a previous posting. I can then hopefully proceed. If the AVO is fixable I would like to do it ,but it is rapidly getting to the stage where I will put it to one side for future thought . I have a certain amount of patience, but that is rapidly depleting !! Mike.