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Old 25th Feb 2018, 11:27 am   #1
John M0GLN
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Default Toasted Avo 8 Mk V shunt

The current thread regarding an Avo 8 reading high made to have another look at my Mk V with a hope of getting it going again, it's also reading high, 11.1V instead of 9.2V measuring a PP3 battery, and as you can see from the attached photo someone, honestly not me, has in the past overloaded it and nearly burnt a shunt out, it measures 0.8Ω, I've looked at the circuit and I can find 3 printed shunts, 0.04Ω, 0.36Ω and 3.6Ω, so it must be either the first or second, I can't find any reference to the shunt numbers to identify them in the manual but as it's the physically biggest shunt I assume it is the 0.04Ω, can anyone confirm that for me please? There of course might be other problems as well but I'll start with this one which is definitely damaged.

Thanks

John as well

Edit, I've perhaps wrongly assumed that it is a shunt, I'm not sure and I've just measured the Fluke and that measures 0.4Ω with its leads shorted out so the toasted copper strip might be 0.4Ω as well, I don't have any specific low resistance measuring equipment, any advice welcomed.
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Old 25th Feb 2018, 12:57 pm   #2
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Default Re: Toasted Avo 8 Mk V shunt

Hello John,
I assume, you have a (bench) PSU: supply these "shunt" from it as Current generator with i.e. 0,1 or even 1,0A, than measure with an DMM what is an Voltage drop over that? Try it please more times and in both polarities, and calculate its average value...
rgds, Karl
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Old 25th Feb 2018, 2:03 pm   #3
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Default Re: Toasted Avo 8 Mk V shunt

Thanks Karl, I'll have to rig up something like that if I need to make a new shunt, however looking at it more closely I don't think it is a shunt, in the photo you can see some thin printed tracks to other parts of the circuit, just below and to the right of where the yellow wire is connected there is a track and at the left side of the burnt track there are 6 sets of 5 tiny tracks, it's just possible to see them and I've no idea what they are, but looking at the schematic the shunts have no other connections other than at their ends.
Perhaps because it was a similar fault to the one on the other Avo 8 thread that I jumped to the wrong conclusion, it's difficult to tell what is what,

John
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Old 25th Feb 2018, 4:45 pm   #4
See_Mos
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Default Re: Toasted Avo 8 Mk V shunt

It is the shunt for the higher current ranges. The thick one being for the 10 amp. The material is not copper but a temperature stable material, possibly nichrome. Calibration was done by cutting the bridges.
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Old 26th Feb 2018, 3:34 pm   #5
John M0GLN
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Default Re: Toasted Avo 8 Mk V shunt

So it is a shunt, I tried a little experiment and shorted it out with a piece of copper wire and it made no difference to the reading so I had a closer look, which I should have done in the first place, and it's been got got at, you can see in the not very good photo, on the left there are 4 wire ends, blue, black, yellow and red which should go somewhere but don't, and on the right hand side 2 blacks and 2 reds soldered together in a very un-Avo like manner.
Bit of a shame as from the outside it looks almost brand new.

John
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Old 26th Feb 2018, 3:50 pm   #6
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Default Re: Toasted Avo 8 Mk V shunt

Those flexi-board interconnects are terrible and crack very easily and cannot be unsoldered without causing damage. I came across one that had been completely rewired.
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Old 3rd Mar 2018, 5:17 pm   #7
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Default Re: Toasted Avo 8 Mk V shunt

John,

Would you like a replacement shunt board which has been salvaged from a scrap meter?

PMM.
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Old 4th Mar 2018, 10:44 am   #8
John M0GLN
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Default Re: Toasted Avo 8 Mk V shunt

Thank you very much for your offer, I would like one to get it working again, but do you know what is missing from the cut ends of the 4 wires shown in the attached picture and perhaps why have the red & black wires to the RH side have been cut and re-soldered? I did bypass the burnt shunt with a piece of copper wire and it made no difference to the reading error so it's almost certain that there are further faults with it, I can certainly replace the shunt board with the one you have but unless I can resolve the other issues it might be to no avail and I've wasted a good board.

I've sent you a PM.

John
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Old 5th Mar 2018, 12:26 pm   #9
See_Mos
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Default Re: Toasted Avo 8 Mk V shunt

Battery wires to the rear casing. On mine Blue/green 15V neg, Orange 15v pos, Black fuse, red 1.5v pos.

and another picture of the flexi strip for the shunt
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Old 5th Mar 2018, 1:43 pm   #10
John M0GLN
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Default Re: Toasted Avo 8 Mk V shunt

Thanks for that info', it's one less thing to concern me as I don't think I'd want to be using the resistances ranges.

John
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Old 18th Mar 2018, 11:10 am   #11
John M0GLN
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Default Re: Toasted Avo 8 Mk V shunt

Quote:
Originally Posted by pmmunro View Post
Would you like a replacement shunt board which has been salvaged from a scrap meter?
I received the shunt board that pmmunro very kindly sent me and swapped it over for the burnt one, I tested the Avo on the volts range with a 9V battery and it still read about 2V too high, this didn't really surprise me as the burnt shunt board is for the current ranges and needed replacing to use these, from the manual too high a reading is down to the swamp resistor, now I don't know what it looks like or where it is, but as it's in series with the movement I disconnected the wire from the movement and wired in a small variable resistor which I adjusted to get the meter reading correctly, measuring this after correcting the readings gave a resistance of 1676Ω which I'll make up from a couple of fixed resistors. I'm a little puzzled as to why the swamp resistor is so far out, the total resistance of the movement + swamp should be 2667Ω and I've had to add 1676Ω, I can see a shunt being burnt out from a current overload but the swamp only has to carry 37.5μA.
I've just tested it on DC up to 400V, AC up to 300V and DC up to 2A against a Fluke 73 and any error is not much more than the Avo's needle width.
So many thanks to pmmunro and all who helped me to repair the meter.

John
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Old 18th Mar 2018, 7:01 pm   #12
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Default Re: Toasted Avo 8 Mk V shunt

I can't imagine that the swamp resistance on this meter is a wound bobbin (like earlier ones) but if it were i would put the discrepancy down to shorted turns within the winding.

It's feasible that someone has put an incorrect component in. I have seen some very odd values patched into second-hand meters as a result of people not double checking their maths.
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Old 19th Mar 2018, 8:51 pm   #13
John M0GLN
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Default Re: Toasted Avo 8 Mk V shunt

I think I've found the reason why I can't locate the swamp resistor, someone in the past must have removed it, the resistors that I added to make the meter read correctly I inserted between a solder tag on the meter movement and the red lead that came from it, I've just traced this red lead and also a black lead that was soldered to the meter chassis and these are the leads that are shown just to right of centre in the attached photo in post #8, these leads have been crudely re-joined together and go to the switch pcb, I've re-soldered the red lead back on the the meter movement solder tag and put my resistors between the other end of the lead and the pcb. I needed 1676Ω which I've made up from a 1K5 and a 180R and it works fine.
I can't think why both the red and black leads have been tampered with as the circuit shows the swamp in the +side of the movement.

John
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