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Old 14th Jun 2019, 1:06 pm   #1
ColinTheAmpMan1
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Default Muirhead Decade Resistance Box A-25-K

I have just purchased this beautifully-made decade box. The switches have to be seen to be believed. It has four decades: tenths, units, tens and hundreds. There are ten resistors for every decade, which are the same value for each decade and therefore additive in series. The problem that I have is with the "hundreds" decade, where two of the resistances are faulty.

These resistances are mounted as two banks of five each. The faulty resistors are at least on the same bank. The third resistance measures 200 ohms and the fifth one is o/c. Does anyone have any experience of fixing such a fault or know how I might go about a repair? Getting to the faulty bank doesn't look easy without removing it and that doesn't look particularly simple.

I realise that I can purchase 0.1% resistors from Mouser, for instance, but since the banks are comprised of five wire-wound resistances each, I would have to buy five 100 ohm, 0.1%, 1W resistors and have the job of removing the whole bank and replacing it with individual resistors.

I also realise that I could get this professionally repaired (https://www.calibrate.co.uk/manufacturer/muirhead/ for instance), but at what cost? Probably more than I paid for it originally.

My ideal situation (I think) would be to source a good, original and serviceable bank of five 100 ohm resistors and do a swap. Of course, this would still involve removal of the faulty bank, but at least some originality would be preserved.

TIA, Colin
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Old 14th Jun 2019, 5:24 pm   #2
TonyDuell
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Default Re: Muirhead Decade Resistance Box A-25-K

It's no help in fixing it, but I think I know why one resistor reads twice what it should be. It's common for these resistors to be wound as 2 windings in opposite directions to reduce their self-inductance. The 2 windings are electrically connected in parallel, so if one goes open-circuit you get the double-value resistor you've seen.
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Old 15th Jun 2019, 10:38 am   #3
woodchips
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Default Re: Muirhead Decade Resistance Box A-25-K

According to the Muirhead catalogue the tenths switch uses the bifilar loop winding, the ohms, ten ohms and hundred ohms switches use the Ayrton-Perry winding.

The hundreds decade has an accuracy of +/-0.1%, maximum current 75mA, inductance varying from 0.2 to 8 uH from step 1to 10.

Must say that I normally chuck these away if I get any, no one wants them, far too large and no 4mm plug terminations. Don't think I have any kicking around at the moment.
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Old 15th Jun 2019, 1:45 pm   #4
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Default Re: Muirhead Decade Resistance Box A-25-K

I have the Cropico version, looks the same but with grey knobs. It's a prized possession, they were jaw-droppingly expensive new and a modern unit from Time Electronics starts at 500 and goes up depending on the range required. Fix it!
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Old 15th Jun 2019, 3:06 pm   #5
Ed_Dinning
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Default Re: Muirhead Decade Resistance Box A-25-K

Hi Colin, depending on the resistors that are dead, I probably let you have some resistance wire to wind your own, or replacement 1% resistors that can be selected for the exact value needed.
These are useful devices, but only at DC or low audio frequencies.

Ed
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Old 15th Jun 2019, 6:25 pm   #6
Craig Sawyers
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Default Re: Muirhead Decade Resistance Box A-25-K

Thank heavens that some here treasure these electrical works of art, and don't like woodchips "chuck these away".

Maybe not in the same class as AVO valve testers, but when valves were not in vogue vast numbers of these went into landfill, being simply "chucked away". I personally know someone who did that with ten VCM163's. At the time no one wanted them. They were far too large.

My friend has been kicking himself because now they are worth at least a grand each.

Craig
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Old 15th Jun 2019, 6:40 pm   #7
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Default Re: Muirhead Decade Resistance Box A-25-K

I have a couple of resistance boxes -- and I use them. I can't remember all the details, one is an old wooden thing, 4 decades, which claims to be able to handle considerable current. I've used that for testing telephone circuits with extra resistance in various places (e.g. Planset extenal extensions with resistance in the 'earth').

Another is a more modern 6 decade box that I paid the very high price of 1.00 at a museum sale....

And I have a little resistance box made by Time Electronics that has a single dial with settings in degrees celsius. It has the same resistance between its termainals as a Pt-100 resistance thermometer sensor at that temperature and is used for testing and calibrating the electronics of such thermometers (as I have an HP data logger with that function, the resistance box is useful to me).

One thing I have learnt over the years is just about anything, however obscure, is wanted by somebody somewhere. I also hate throwing out anything if I feel that somebody can make use of it, especially if it is impossible to find in general. So I have boxes of obscure parts (and worse) looking for the right person...
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Old 15th Jun 2019, 8:07 pm   #8
John_BS
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Default Re: Muirhead Decade Resistance Box A-25-K

I recently used one to mimic the fuel-tank transducer in a Mercedes truck.

Not this one, though (0.01% to 50kHz)
John
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Old 17th Jun 2019, 11:38 am   #9
woodchips
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Default Re: Muirhead Decade Resistance Box A-25-K

I chuck them away because no one buys them. I did actually go and do a search, and found five, plus probably another 8 or so in the remote stores. Who wants them? Postage will be about 15 each.

I also found nine capacitance boxes, mix of air, mica, Pacitor, decade etc types. These came in two trailer loads of items bought in a sale many years ago, total cost near 2,000 plus the driving. Along with power supplies, wattmeters, AC bridges, many more C and L boxes etc etc, and so far sold nothing of it. These were bought to sell so can't offer them on the forum.

Most of this stuff came from Cranfield or RM college of Science, did a clear out, or close down.

You will see nice mahogany boxes of old test gear in antique shops for 50 or something, no one buys them. No one knows what they are. An old car is in everyones experience, test equipment, or old computers like my Burroughs aren't.

Sad, but true, if you want one then buy it, will go in the skip when you die.
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Old 17th Jun 2019, 3:36 pm   #10
ColinTheAmpMan1
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Default Re: Muirhead Decade Resistance Box A-25-K

I thought that it might be useful to show some pictures of the resistance banks. They are non-inductive Ayrton-Perry wound.

The pictures show:
1) The entire "Hundreds" switch with the faulty bank on the right and showing the three red paint spots that I think identify this as a bank of 100 ohm resistors.
2) The inside of the good bank.
3) The bad bank of five 100 ohm resistors.
It is very difficult to see any damage or evidence of overheating.

Quote:
Originally Posted by Ed_Dinning View Post
Hi Colin, depending on the resistors that are dead, I probably let you have some resistance wire to wind your own, or replacement 1% resistors that can be selected for the exact value needed.
These are useful devices, but only at DC or low audio frequencies.

Ed
Hi Ed,
It's kind of you to make this offer, but as you can see from the photos that I've now posted, the resistor banks would have to be removed anyway. Also, I have no experience of winding resistors, especially Ayrton-Perry. Finally, the resistors are 0.1% tolerance and my only ohm-meter is a Fluke 175, which I don't think will be too good at that accuracy. I do have a Marconi Universal Bridge TF 2700, which I have modified with 0.1% resistors, but again, I'm not at all sure how I might use it to wind 0.1% resistors.

Maybe someone will come up with an appropriate Muirhead 5 x 100 ohm bank?

Colin.
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Old 17th Jun 2019, 5:19 pm   #11
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Default Re: Muirhead Decade Resistance Box A-25-K

Incidentally Ed, if decade boxes are only really useful at DC or low audio frequencies, why do the best ones use non-inductive resistors? Isn't that adding unnecessary expense?
Colin.
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Old 17th Jun 2019, 7:01 pm   #12
Ed_Dinning
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Default Re: Muirhead Decade Resistance Box A-25-K

Hi Colin, no, it is removing inductance that is there at all frequencies and affect your readings.
If you know the approx gauge of wire and required resistance it is easy to wind a slighly higher value then trim to value using a bridge circuit based on the good sections of your R box.

Ed
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Old 17th Jun 2019, 7:35 pm   #13
Craig Sawyers
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Default Re: Muirhead Decade Resistance Box A-25-K

If you get the bank out, you could replace each 200 ohm resistor with five 1k resistors in parallel.

For example Farnell 1083251 is a 0.1% resistor with a tempco of 15ppm/C and 250mW power rating. Since resistors in general are better than the stated tolerance, and assuming that they are randomly distributed across the tolerance band you would end up with a 1.25W precision resistor of 200 ohms.

If you can get in there, you could just add two set of resistors in parallel with the dead sections.

Craig
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Old 17th Jun 2019, 8:07 pm   #14
emeritus
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Default Re: Muirhead Decade Resistance Box A-25-K

If my memories of the "Post Office Box" in school Physics lessons are correct, non-inductive resistors would be highly desirable where the decade box forms one arm of a Wheatstone bridge and a sensitive galvanometer is used to detect the point of balance.

I have a fairly modern 5 decade resistance box myself, salvaged from Marconi Marine a couple of decades ago, but haven't used it - yet!
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Old 18th Jun 2019, 10:27 am   #15
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Default Re: Muirhead Decade Resistance Box A-25-K

Quote:
Originally Posted by Craig Sawyers View Post
If you get the bank out, you could replace each 200 ohm resistor with five 1k resistors in parallel.

For example Farnell 1083251 is a 0.1% resistor with a tempco of 15ppm/C and 250mW power rating. Since resistors in general are better than the stated tolerance, and assuming that they are randomly distributed across the tolerance band you would end up with a 1.25W precision resistor of 200 ohms.

If you can get in there, you could just add two set of resistors in parallel with the dead sections.

Craig
I am pretty certain that I can de-solder and remove the faulty bank (just six solder joints on the switch). Incidentally, it is one of the hundreds of ohms bank (the 200-ohm resistors at 0.1% 1W that I am also looking for are for a different decade box). I don't think that there is enough room for five resistors in parallel - perhaps I ought to post another photo with a reference to make it clearer. The whole bank is 6cm wide by 4cm tall, making each resistance section 1cm by 4cm.

The other difficulty is that while one of the windings has gone completely o/c, the other one only has half of its Ayrton-Perry winding blown and so is showing 200 ohms. This will presumably also not be non-inductive with one half o/c.

Colin.
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Old 18th Jun 2019, 10:37 am   #16
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Default Re: Muirhead Decade Resistance Box A-25-K

Quote:
Originally Posted by Ed_Dinning View Post
Hi Colin, no, it is removing inductance that is there at all frequencies and affect your readings.
If you know the approx gauge of wire and required resistance it is easy to wind a slighly higher value then trim to value using a bridge circuit based on the good sections of your R box.

Ed
I am possibly being really dense here, but I still don't understand. I would have thought that a few micro-henries in parallel with a hundred ohms would only have some effect at higher frequencies, but I have to admit that I haven't done the arithmetic.

I am not confident that I could make a winding like these. The Ayrton-Perry winding doesn't look easy to me, as I have never wound an ordinary, straightforward resistor even.

Just a thought, if I can't get some other solution, could you replace the two faulty sections for me if I posted the faulty bank to you? I would expect to pay you for your time, expertise and wire, of course.

Colin.
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Old 19th Jun 2019, 8:42 am   #17
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Default Re: Muirhead Decade Resistance Box A-25-K

Quote:
Originally Posted by ColinTheAmpMan1 View Post

Maybe someone will come up with an appropriate Muirhead 5 x 100 ohm bank?

Colin.
Somewhere in my thousands of parts I have some Muirhead WW resistors. Not in banks of 5 but separate sections. Look similar to a single item in your banks. Can't remember what values they were but were at least 0.1% or better. Don't know how they were wound so will have to find them and see. They were in sealed packets.

Will message you when I find them.
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Old 19th Jun 2019, 4:08 pm   #18
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Default Re: Muirhead Decade Resistance Box A-25-K

Hi Colin.

Have found the resistors - have 2 x 100ohm as per picture. Opened one to check the size - 45x 13mm plus the wires. I haven't looked at the spec so maybe you will be able to determine if these might be suitable.
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Old 20th Jun 2019, 11:32 am   #19
ColinTheAmpMan1
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Default Re: Muirhead Decade Resistance Box A-25-K

Quote:
Originally Posted by vidjoman View Post
Hi Colin.

Have found the resistors - have 2 x 100ohm as per picture. Opened one to check the size - 45x 13mm plus the wires. I haven't looked at the spec so maybe you will be able to determine if these might be suitable.
Attachment 185372
Hi,

I have noodled around on the web, but I can't find any further info about your Muirhead resistors. Do you think that they are Ayrton-Perry wound? I suspect that they might well do the job. How much do you want for the pair, plus P+P?

Colin.
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Old 21st Jun 2019, 9:16 am   #20
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Default Re: Muirhead Decade Resistance Box A-25-K

Colin. Have sent you a PM.
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