Avo 8 mk 5 - UK Vintage Radio Repair and Restoration Discussion Forum

g0hze snowy · 6th Sep 2017, 9:28 pm

Hi, sorry if this question has been answered in the past but he goes:-
I have a AVO 8 MK5 which I think has a faulty needle damping problem, it swings +/- of the zero stop taking a while to stop, it seems to show the voltage ranges ok, waiting for the 15volt battery so I can test the resistance ranges.
Now as I understand it there are some printed 'R's that do this job, from memory I think they are R9-R11, I have measured them with my DVM and they seem to be within the resistance ranges specified.
Have I perhaps overlooked anything or can someone point me onto the right path. cheers, Snowy

WME_bill · 9th Sep 2017, 6:11 pm

Avo 8.
The meter is damped by the universal shunt permanently in parallel with the meter, wich is tapped at various positions for the current ranges. A very ingenious arrangement, which quite avoids contact resistance trouble on current ranges, but requires a more sensitive meter. Hence the use of a 37.5ua meter shunted to 50 ua.
If any part of the shunt is open circuit, then the meter will indeed swing wildly. All the voltage ranges will read about 33% high. Current and resistance ranges will be incorrect.
Have you carefully checked the voltage ranges. Any range will do. A single mercury cell will give 1.35v reliably over many years.
Then check that the all the shunt resistors are complete and in series across the meter - R9 to R15, of which R9 and R10 are printed on the Mk5.
wme_bill

g0hze snowy · 9th Sep 2017, 7:24 pm

Thanks Bill, I will have another go at it in a couple of days, I am trying to rewire my BMW R100 at the moment. regards, snowy

6th Sep 2017, 9:28 pm	#1
g0hze snowy Hexode Join Date: Oct 2013 Location: Peterborough, Cambridgeshire, UK. Posts: 336	Avo 8 mk 5 Hi, sorry if this question has been answered in the past but he goes:- I have a AVO 8 MK5 which I think has a faulty needle damping problem, it swings +/- of the zero stop taking a while to stop, it seems to show the voltage ranges ok, waiting for the 15volt battery so I can test the resistance ranges. Now as I understand it there are some printed 'R's that do this job, from memory I think they are R9-R11, I have measured them with my DVM and they seem to be within the resistance ranges specified. Have I perhaps overlooked anything or can someone point me onto the right path. cheers, Snowy

9th Sep 2017, 6:11 pm	#2
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Avo 8 mk 5 Avo 8. The meter is damped by the universal shunt permanently in parallel with the meter, wich is tapped at various positions for the current ranges. A very ingenious arrangement, which quite avoids contact resistance trouble on current ranges, but requires a more sensitive meter. Hence the use of a 37.5ua meter shunted to 50 ua. If any part of the shunt is open circuit, then the meter will indeed swing wildly. All the voltage ranges will read about 33% high. Current and resistance ranges will be incorrect. Have you carefully checked the voltage ranges. Any range will do. A single mercury cell will give 1.35v reliably over many years. Then check that the all the shunt resistors are complete and in series across the meter - R9 to R15, of which R9 and R10 are printed on the Mk5. wme_bill

9th Sep 2017, 7:24 pm	#3
g0hze snowy Hexode Join Date: Oct 2013 Location: Peterborough, Cambridgeshire, UK. Posts: 336	Re: Avo 8 mk 5 Thanks Bill, I will have another go at it in a couple of days, I am trying to rewire my BMW R100 at the moment. regards, snowy