AVO 8 mk7 Ohms Fault - UK Vintage Radio Repair and Restoration Discussion Forum

chriswood1900 · 28th Mar 2016, 11:37 am

Can anyone assist with a problem with an AVO8 mk7 which works fine on all ranges except Ohms x1, The ohms range will adjust to full scale but produce wildly inaccurate readings. I have traced out the circuit for a mk6 which I understand is much the same and the only component that would appear to affect this is R1 the 18-Ohm resistor shunt. When I substitute and 18 ohm resistor between the neg terminal and the + 1.5 battery terminal all returns to normal, I would be grateful if someone could confirm my diagnose, my problem is I can't find where the resistor is on the circuits boards try as I may to trace the circuit I seem to get lost, has anyone got the component layout for a Mk7? When I have found it I intend to bridge it with a high stab selected metal film resistor will this be OK?
kind regards
Chris

Phil G4SPZ · 28th Mar 2016, 2:39 pm

I'm not familiar with the Mk 7 but I reckon you've cracked the problem, Chris. That appears to be a "printed resistor" which must have gone o/c, so your proposed solution seems the best practical approach.

chriswood1900 · 28th Mar 2016, 4:10 pm

Thanks Phil but I'm struggling to find the location of the O/C resistor.
Chris

Phil G4SPZ · 28th Mar 2016, 4:20 pm

Yes, but to an extent it doesn't matter where it is; if you can wire the new resistor between the correct end of the 'zero ohms /100' pot and the cut-out, it will do the same job.

Hopefully someone with better knowledge of the mark 7 will be able to assist further.

neonlamp · 28th Mar 2016, 4:53 pm

What a coincidence- just had my Mk7 in pieces and could have photographed the resistor, but had reassembled it before I saw your request. What you are looking for is a printed, rectangular, ceramic unit approx 10x25mm right next to the ohmsx1 pot at the outer edge of the PCB holding the three pots.
Hope you can find it.

Cheers

chriswood1900 · 28th Mar 2016, 7:49 pm

Thanks to Phil and neonlamp it is now fixed. Once I knew where to look I discovered a dry joint caused by the lead being cut to short where circled. Extending the lead and cleaning and re-soldering and all is working. Whilst at it I looked around for other joints and found the black wire on the 10 amp fuse dry as well. For future ref whilst my lash up with an 18ohm resistor proved the fault it was not a fix as I had bypassed several switches in the process.
Regards Chris

Phil G4SPZ · 28th Mar 2016, 8:53 pm

Well done, Chris! Glad you found the fault.

The Avo Model 8 Marks 5, 6 and 7 represented a radical redesign of the traditional hand-built Avometer after Thorn acquired Avo. Apparently the aim was not only to cut costs but to improve reliability. Early Avometers were reputed to suffer from dodgy soldering, and although the soldering was often not pretty, I have to say I've never found any dry joints in the many earlier models I've worked on. Clearly, your experience suggests that dry joints had not been entirely eradicated even towards the end of Avometer production!

I have a Mark 5 in my collection, but as it works perfectly I've resisted the temptation to open it up just for a look.

neonlamp · 28th Mar 2016, 10:28 pm

Good to hear you found the problem. I suspect someone has "repaired" your Avo at some point- doubt Avo/Megger would have left a lead short and if anything I've seen great dollops of solder on most of the connections I've seen in Avo 8's.

Phil- I don't like opening up the tin either, but sometimes things need sorting!! Today's problem was a sticky needle (among other issues) which turned out to be a small iron/steel filing/splinter.
Now I need to fit new ohms pots the next service call- getting noisy. Nearly all Avo's over 10 years seem to suffer noisy pots (which will soon be all of them).

Cheers

trsomian · 31st Mar 2016, 4:15 pm

A fellow I knew at a previous job had worked at a place where the entire kit of spare parts for AVO model 8 were kept in the stores. Apparently, bit by bit he got all the parts and assembled himself a meter! Maybe others did the same, and you have got one of those.

chriswood1900 · 1st Apr 2016, 3:58 pm

It is unlikely mine came from any dodgy source! it is ex MOD and does not look like it had ever been opened so not got at by a squady, I think it was just one which had an error in assembly.
Chris

28th Mar 2016, 11:37 am	#1
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,486	AVO 8 mk7 Ohms Fault Can anyone assist with a problem with an AVO8 mk7 which works fine on all ranges except Ohms x1, The ohms range will adjust to full scale but produce wildly inaccurate readings. I have traced out the circuit for a mk6 which I understand is much the same and the only component that would appear to affect this is R1 the 18-Ohm resistor shunt. When I substitute and 18 ohm resistor between the neg terminal and the + 1.5 battery terminal all returns to normal, I would be grateful if someone could confirm my diagnose, my problem is I can't find where the resistor is on the circuits boards try as I may to trace the circuit I seem to get lost, has anyone got the component layout for a Mk7? When I have found it I intend to bridge it with a high stab selected metal film resistor will this be OK? kind regards Chris Attached Thumbnails

28th Mar 2016, 2:39 pm	#2
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: AVO 8 mk7 Ohms Fault I'm not familiar with the Mk 7 but I reckon you've cracked the problem, Chris. That appears to be a "printed resistor" which must have gone o/c, so your proposed solution seems the best practical approach. __________________ Phil Optimist [n]: One who is not in possession of the full facts

28th Mar 2016, 4:10 pm	#3
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,486	Re: AVO 8 mk7 Ohms Fault Thanks Phil but I'm struggling to find the location of the O/C resistor. Chris

28th Mar 2016, 4:20 pm	#4
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: AVO 8 mk7 Ohms Fault Yes, but to an extent it doesn't matter where it is; if you can wire the new resistor between the correct end of the 'zero ohms /100' pot and the cut-out, it will do the same job. Hopefully someone with better knowledge of the mark 7 will be able to assist further. __________________ Phil Optimist [n]: One who is not in possession of the full facts

28th Mar 2016, 4:53 pm	#5
neonlamp Retired Dormant Member Join Date: Sep 2010 Location: Tavistock, Devon, UK. Posts: 84	Re: AVO 8 mk7 Ohms Fault What a coincidence- just had my Mk7 in pieces and could have photographed the resistor, but had reassembled it before I saw your request. What you are looking for is a printed, rectangular, ceramic unit approx 10x25mm right next to the ohmsx1 pot at the outer edge of the PCB holding the three pots. Hope you can find it. Cheers

28th Mar 2016, 7:49 pm	#6
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,486	Re: AVO 8 mk7 Ohms Fault Thanks to Phil and neonlamp it is now fixed. Once I knew where to look I discovered a dry joint caused by the lead being cut to short where circled. Extending the lead and cleaning and re-soldering and all is working. Whilst at it I looked around for other joints and found the black wire on the 10 amp fuse dry as well. For future ref whilst my lash up with an 18ohm resistor proved the fault it was not a fix as I had bypassed several switches in the process. Regards Chris Attached Thumbnails

28th Mar 2016, 8:53 pm	#7
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: AVO 8 mk7 Ohms Fault Well done, Chris! Glad you found the fault. The Avo Model 8 Marks 5, 6 and 7 represented a radical redesign of the traditional hand-built Avometer after Thorn acquired Avo. Apparently the aim was not only to cut costs but to improve reliability. Early Avometers were reputed to suffer from dodgy soldering, and although the soldering was often not pretty, I have to say I've never found any dry joints in the many earlier models I've worked on. Clearly, your experience suggests that dry joints had not been entirely eradicated even towards the end of Avometer production! I have a Mark 5 in my collection, but as it works perfectly I've resisted the temptation to open it up just for a look. __________________ Phil Optimist [n]: One who is not in possession of the full facts

28th Mar 2016, 10:28 pm	#8
neonlamp Retired Dormant Member Join Date: Sep 2010 Location: Tavistock, Devon, UK. Posts: 84	Re: AVO 8 mk7 Ohms Fault Good to hear you found the problem. I suspect someone has "repaired" your Avo at some point- doubt Avo/Megger would have left a lead short and if anything I've seen great dollops of solder on most of the connections I've seen in Avo 8's. Phil- I don't like opening up the tin either, but sometimes things need sorting!! Today's problem was a sticky needle (among other issues) which turned out to be a small iron/steel filing/splinter. Now I need to fit new ohms pots the next service call- getting noisy. Nearly all Avo's over 10 years seem to suffer noisy pots (which will soon be all of them). Cheers

31st Mar 2016, 4:15 pm	#9
trsomian Hexode Join Date: Oct 2010 Location: Box End, Beds. UK. Posts: 271	Re: AVO 8 mk7 Ohms Fault A fellow I knew at a previous job had worked at a place where the entire kit of spare parts for AVO model 8 were kept in the stores. Apparently, bit by bit he got all the parts and assembled himself a meter! Maybe others did the same, and you have got one of those.

1st Apr 2016, 3:58 pm	#10
chriswood1900 Octode Join Date: Jan 2007 Location: Dorridge, West Midlands, UK. Posts: 1,486	Re: AVO 8 mk7 Ohms Fault It is unlikely mine came from any dodgy source! it is ex MOD and does not look like it had ever been opened so not got at by a squady, I think it was just one which had an error in assembly. Chris