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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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25th Feb 2018, 11:27 am | #1 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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. Last edited by John M0GLN; 25th Feb 2018 at 11:39 am. |
25th Feb 2018, 12:57 pm | #2 |
Heptode
Join Date: Jun 2010
Location: Zurich, Switzerland
Posts: 538
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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 |
25th Feb 2018, 2:03 pm | #3 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |
25th Feb 2018, 4:45 pm | #4 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 482
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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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26th Feb 2018, 3:34 pm | #5 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |
26th Feb 2018, 3:50 pm | #6 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 482
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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.
Last edited by See_Mos; 26th Feb 2018 at 4:03 pm. |
3rd Mar 2018, 5:17 pm | #7 |
Octode
Join Date: Jul 2004
Location: Dundee, UK.
Posts: 1,797
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Re: Toasted Avo 8 Mk V shunt
John,
Would you like a replacement shunt board which has been salvaged from a scrap meter? PMM. |
4th Mar 2018, 10:44 am | #8 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |
5th Mar 2018, 12:26 pm | #9 |
Hexode
Join Date: Nov 2003
Location: Ossett, West Yorkshire, UK.
Posts: 482
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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 |
5th Mar 2018, 1:43 pm | #10 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |
18th Mar 2018, 11:10 am | #11 | |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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Re: Toasted Avo 8 Mk V shunt
Quote:
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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18th Mar 2018, 7:01 pm | #12 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
Posts: 4,081
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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. |
19th Mar 2018, 8:51 pm | #13 |
Octode
Join Date: Jan 2004
Location: Southport, Merseyside, UK.
Posts: 1,156
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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 |