AVO 8 mk3 repair question - UK Vintage Radio Repair and Restoration Discussion Forum

G4YVM David · 17th Sep 2016, 2:20 pm

I have been given an avo 8 mk3 which has a burnt out range. i'd like to know whether its possible to repair.

The range is (sorry for the description), looking at the rear of the unit as it comes out of the case, second from left. The right hand windings are the big chunky ones, the far left is a band of fine wire...my burnt one is second from left. It alsso has (or had) a 68ohm resistor across it...thats gone bang too. The wire loom is totally fried.

I can replace the resistor easily enough, but what am i aiming for with the wire windings?

Thanks
D

camallison · 17th Sep 2016, 4:13 pm

Do you have the service manual for it? That should tell you the value. Earlier this year, there was a discussion on this.

Colin

[Edit - the best description I found was http://www.petervis.com/avo-meters/a...vo-8-mk-3.html. ]

camallison · 17th Sep 2016, 4:26 pm

I also have a scan of the user manual which gives component values as well as the circuit diagram. If you wish a copy, PM me with your email address.

Colin

G4YVM David · 17th Sep 2016, 6:09 pm

PM sent, thanks
D

G4YVM David · 17th Sep 2016, 6:10 pm

Thats the item..a shunt coil. Im sure I have suitable wire in my wire box.

D

The Philpott · 17th Sep 2016, 6:14 pm

I have a feeling this is one of the low voltage AC ranges, probably the 2.5 volt one. If so, the transformer is part of the circuit. That the piggy back replacement resistor has immolated itself leads me to believe you may have a short deep within the transformer.

Replacing the transformer is an option but the LV AC ranges are thrown out of kilter as a result, and then need tweaking..?

While you are in there it is well worthwhile checking the switch cams are in phase with the legends on the front of the AVO, and also that the leaf switches are meeting and parting when they should. (the leaf switch open/close protocol is shown on the standard service document.)

camallison · 17th Sep 2016, 6:33 pm

Quote:

Originally Posted by G4YVM David

PM sent, thanks

Nothing received.

Colin

G4YVM David · 17th Sep 2016, 9:02 pm

Thanks. Ill check the resitance of the transformer.

If thats gone too its a scrapper...i only paid a fiver ��

D

Herald1360 · 18th Sep 2016, 10:42 am

I suspect that the busted whole you have there is worth less than the sum of its remaining good parts......

The Philpott · 18th Sep 2016, 5:58 pm

For reference- Both my mod8/mk3 meters, when interrogated from the front terminals with a sensitive DMM, resist to the extent of 245ohm when on 2.5vAC setting.

So presumably this is the gross figure including the trimmer winding. 68ohm for the aftermarket trimmer resistor you have is probably a standard value off the shelf, most likely somewhere in ballpark of 62 to 64 is what the factory will have wound onto the board.

I did look at an early mod 8 which i use for spares- and the winding you mention was burnt out on that as well!

17th Sep 2016, 2:20 pm	#1
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	AVO 8 mk3 repair question I have been given an avo 8 mk3 which has a burnt out range. i'd like to know whether its possible to repair. The range is (sorry for the description), looking at the rear of the unit as it comes out of the case, second from left. The right hand windings are the big chunky ones, the far left is a band of fine wire...my burnt one is second from left. It alsso has (or had) a 68ohm resistor across it...thats gone bang too. The wire loom is totally fried. I can replace the resistor easily enough, but what am i aiming for with the wire windings? Thanks D

17th Sep 2016, 4:13 pm	#2
camallison Heptode Join Date: Feb 2013 Location: Yarm, North Yorkshire, UK. Posts: 535	Re: AVO 8 mk3 repair question Do you have the service manual for it? That should tell you the value. Earlier this year, there was a discussion on this. Colin [Edit - the best description I found was http://www.petervis.com/avo-meters/a...vo-8-mk-3.html. ] Last edited by camallison; 17th Sep 2016 at 4:19 pm.

17th Sep 2016, 4:26 pm	#3
camallison Heptode Join Date: Feb 2013 Location: Yarm, North Yorkshire, UK. Posts: 535	Re: AVO 8 mk3 repair question I also have a scan of the user manual which gives component values as well as the circuit diagram. If you wish a copy, PM me with your email address. Colin

17th Sep 2016, 6:09 pm	#4
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Re: AVO 8 mk3 repair question PM sent, thanks D

17th Sep 2016, 6:10 pm	#5
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Re: AVO 8 mk3 repair question Thats the item..a shunt coil. Im sure I have suitable wire in my wire box. D

17th Sep 2016, 6:14 pm	#6
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,107	Re: AVO 8 mk3 repair question I have a feeling this is one of the low voltage AC ranges, probably the 2.5 volt one. If so, the transformer is part of the circuit. That the piggy back replacement resistor has immolated itself leads me to believe you may have a short deep within the transformer. Replacing the transformer is an option but the LV AC ranges are thrown out of kilter as a result, and then need tweaking..? While you are in there it is well worthwhile checking the switch cams are in phase with the legends on the front of the AVO, and also that the leaf switches are meeting and parting when they should. (the leaf switch open/close protocol is shown on the standard service document.)

17th Sep 2016, 9:02 pm	#8
G4YVM David Heptode Join Date: May 2014 Location: Salisbury, Wiltshire, UK. Posts: 998	Re: AVO 8 mk3 repair question Thanks. Ill check the resitance of the transformer. If thats gone too its a scrapper...i only paid a fiver �� D

18th Sep 2016, 10:42 am	#9
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: AVO 8 mk3 repair question I suspect that the busted whole you have there is worth less than the sum of its remaining good parts...... __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

18th Sep 2016, 5:58 pm	#10
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,107	Re: AVO 8 mk3 repair question For reference- Both my mod8/mk3 meters, when interrogated from the front terminals with a sensitive DMM, resist to the extent of 245ohm when on 2.5vAC setting. So presumably this is the gross figure including the trimmer winding. 68ohm for the aftermarket trimmer resistor you have is probably a standard value off the shelf, most likely somewhere in ballpark of 62 to 64 is what the factory will have wound onto the board. I did look at an early mod 8 which i use for spares- and the winding you mention was burnt out on that as well!