Spotted on ebay US, Muirhead 10k Voltage Dividing Resistance

[Gyro]

One for the metrologists. I'm a bit surprised that this is still on ebay US after a few months (no seller connection)...

http://www.ebay.co.uk/itm/Muirhead-Voltage-Dividing-Resistance-Decade-Rotary-Resistor-Block-D-801-D-/400932744345?hash=item5d59742c99:g:G1sAAMXQWzNShq8s

It's a strange non-KVD divider (it uses twice as many resistors!). it might possibly be of interest as part of a mini metrology lab.

Here are the resistor porn photos of mine:

https://www.eevblog.com/forum/testgear/kelvin-varley-reistor-porn/msg773878/#msg773770

YMMV of course.

[woodchips]

Can't read the part number in the photo, but the earlier A201D voltage dividing resistors are, according to the Muirhead catalogue, of the Rayleigh type so the total resistance is kept constant.

Stated accuracy is +/-0.1% +/-(0.002 ohm x number of dials), price in 1951 was £53.

Sure I have a five decade one somewhere, can't see it at the moment.

[Gyro]

Hi woodchips,

It is / was a D-801-D. 4 Decade 10k divider, 250V RMS max. Yes, the total input resistance is kept constant.

I'm wondering if Rayleigh divider is the term I've been looking for all these months. Muirhead made Kelvin Varley ones too. Not sure why the difference given how many extra resistors were involved in the D-801-D. Accuracy-wise mine seems to manage 0.01-0.02%, down at my measurement limits. Edit: End resistance comes out at 1ppm (11uV for 10V in).

Any chance that you have a catalogue page for this one? 

Chris

P.S. 5 Decade sounds nice, I remember you saying you had some 'treasures' 

[Dave]

Stated accuracy is +/-0.1% +/-(0.002 ohm x number of dials), price in 1951 was £53.

Just to put it into perspective:
Accounting for the inflation over the decades, those 53 GBP would be equivalent to today's 1638 GBP, so not exactly a toy for the average hobbyist.

Calculated here.

[timb]

So, I put in an offer and got it. Will post some photos once it comes in. Looks like a neat little device.

[Gyro]

@Dave: Yes I know. Expensive laboratory stuff that was built to last and which don't degrade significantly is a real bargain these days. I have some laboratory balances that fall into that category too.

@timb: Congratulations, I was hoping that someone on here would get it!  It really needed to go to someone in the US, shipping to Europe was horrendous. Hopefully yours will be as pretty inside as mine, and smell as nice too... Although potentiometers are far less subject to abuse than resistance boxes and it appears to have come out of a standards lab (it looks to have been owned by EMI in the UK at one stage too!). You potentially have the makings of a mV resolution precision power supply there with a 10V reference and some buffering.
Edit: It should clean up nicely by the way (as long as you don't scratch the paint getting the stickers off). All the front panel lettering is engrave and fill so you can brighten it up by filling with a white permanent marker and quick wipe with suitable solvent. I think the black paint is cellulose.

[lowimpedance]

One for the metrologists. I'm a bit surprised that this is still on ebay US after a few months (no seller connection)...

http://www.ebay.co.uk/itm/Muirhead-Voltage-Dividing-Resistance-Decade-Rotary-Resistor-Block-D-801-D-/400932744345?hash=item5d59742c99:g:G1sAAMXQWzNShq8s

It's a strange non-KVD divider (it uses twice as many resistors!). it might possibly be of interest as part of a mini metrology lab.

Here are the resistor porn photos of mine:

https://www.eevblog.com/forum/testgear/kelvin-varley-reistor-porn/msg773878/#msg773770

YMMV of course.

Since I was scanning info from an old Muirhead catalogue I scanned the pages for the D-801 and some other related stuff for yours and timb information.

I have the A-201,just love the all that bakerlite and timber  .

[lowimpedance]

Some more info !.

[Gyro]

@ lowimpedance, Thanks very much, I've been looking for that catalogue page for ages! (In fact I've just deleted a reply in the metrology section where I was asking if you had this very thing  ). You're a gentleman Sir. 

Yes I do admire the real teak case on yours!

[woodchips]

If a thread goes off the first page then gets forgotten, as I had forgotten about this.

But, why if th efile is described as a pdf does it say it is a binary file when I try to download?

Having a clearout and found a few of these ratio boxes, anyone interested? A couple are useful, but more than that is dead space.

[Gyro]

Hi Woodchips, I'm interested, which ones do you have?

[Jeff_Birt]

http://www.ebay.com/itm/Muirhead-Voltage-Dividing-Resistance-Decade-Rotary-Resistor-Block-D-801-D-/401179129765?hash=item5d6823b7a5:g:G1sAAMXQWzNShq8s

Still seems to be on eBay, same photos and all. Wonder if the previous sale fell through or he had more than one?

[Jeff_Birt]

I took a chance and made an offer, and it was accepted. Maybe I bought a pig?

[Gyro]

Hopefully you haven't  . I think timb bought the other one, so hopefully he can give some indication by now, they both look to have come from the same source. Even if it needs some cleaning up on the outside, the inside is hopefully pristine.

[Jeff_Birt]

Well, I bout 1/4th of a pig

The 1K resistor section is burnt up. The switch is fine and the resistance of each resistor section is still surprisingly close to 1K, they are closer to 1% than 0.1%. If you appreciate fine craftsmanship you will appreciate the build quality. The switches are silky smooth, very, very nice.

I could not find any information about exactly what type of wire they used, I only saw a reference to 'resistance wire' in the documentation I could find, perhaps nichrome. One part of me wants to try and wrap the resistor sections again. Where on earth do you buy enameled nichrome wire? (OK, I did get some hits when Googling.) Another part of me says carefully remove the original resistor sets and solder in some Vishay thin film 0.1%. Thoughts?

Since the unit was sold as used, "OK" condition I'm not sure if I should ask the seller for some amount of reimbursement for it being damaged. If it had been marked for parts or as is I would not feel inclined to try and get a partial refund.

[Vgkid]

The wire used is manganin, or its variants. My rule of thumb on ebay, is to always assume it is broken, unless shown otherwise.

[Gyro]

Oh, that's a shame Jeff.    It's sad when something that well crafted has suffered some abuse. Being a potentiometer, it is much less likely than a straight resistance box. Are ALL 18(?) of the resistors in the 1k section burned? That sounds a bit unlkely - if there is still at least one good resistor then you should be able to measure it to accurately size the replacements. Yes, the resistors are likely to be Manganin.

Just a thought... are you certain that the 1k section is actually burned and not just a different varnish colour to the other decades?  As I say, it would be pretty unlikely for all the resistors in one section to burn as they are never all in circuit at the same time.    Pictures???

Have you tried testing it in voltage divider mode? it's the voltage division ratio that is nore important than the absolute resistance values. EDIT: Don't go snipping anything out until you've checked this!

As you say the craftsmanship is superb, you'd never be able to buy switches like that these days, even for starting from scratch.

[Jeff_Birt]

Sorry, I thought I had attached photos. There are four separate ceramic cards the resistors are wound on. The cards are in pairs with two being linked in one corner. One set of cards is much worse than the other. Due to the charred nature of the worse side I'm reluctant to put a voltage across but perhaps with the current limit on the power supply set very low I will try it. Using my HP 3478A (just calibrated) I measured the resistance across the input to output terminals (leaving common rail disconnected). Each resistor in each stage can easily be tested in this manner and all except the 1K stage were well within tolerance.

As for repairing the device, one would certainly not want to start snipping. It would be a case of unsoldering the links to the offending switch and removing the switch. In a similar manner each lead from the cards would be de-soldered and carefully pull loose from the switch. Well, in thinking about it now it might be better to remove the lower corner link between the card pairs first. That way you could remove the cards one at a time from the switch.

I have a micrometer to measure the wire diameter and can estimate the amount of wire needed by counting the wraps on a less damaged section. That could then be compared to the stated resistance per inch(mm) of the new wire. I'm thinking if I do rewrap the resistors I'll have to take my meter back to work and recalibrate it in the 4-wire mode and make a set of 4-wire test leads. Although, It may not make much of a difference at 1K Ohm.

[Gyro]

Hmm yes, somebody's had a really good go at cooking that!  It looks as if someone put a lot of volts on it with a shorted output (or overloaded it in 'resistance box' mode) with the other decades set to zero.

I'm just wondering, did the 1k resistors drift high or low (probably low I guess). There's just the off-chance that if high, you could trim them down to correct value by adding high value resistors in parallel without too much degredation. It would obviously depend what input voltage you're planning to use it on (max spec is 250V) but if the input terminals still read 10k then I'd be tempted to start at 1V and then, cautiously, 10V, giving a 1mV resolution divider.

Your repair plan looks good, as it is a Rayleigh divider, the 1k resistors need to exactly match the total resistance of the next decade (in combination with the successive decades). That is hopefully the same value as the un-burned resistor (the last one looks ok).

Yes, I would still go for 4-wire measurement at 1k.

It's sad but I think still well worth the effort of restoration. I would send your photos to the seller as it probably doesn't count as 'OK condition'.

[Jeff_Birt]

Yes, the resistance measurements came out a bit low, along the lines of 996 Ohms which is closer to 0.4%. The stability of the resistance is of course completely unpredictable now. I even measured the resistance at the resistor leads to the switch to rule out a switch contact problem.

By coincidence I happen to receive a Vishay Y00071K00000V0L, 0.005% 1K resistor this week. That should provide an excellent reference for testing my rewound resistors. Now to find the wire...

Oh, one last questions if you will. I suspect the coating on the cards is varnish, is that correct?

[Gyro]

Hi Jeff,

Yes, the coating on the cards is varnish, probably too old to be polyurethane (shellac?), but it ought to dissolve off fairly easilly with alcohol.

You're right that the stability will have become unpredictable, it's a shame they dropped lower though. One lucky thing though, if you look at the next decade down you will see that the resistors are each made up of two counter wound windings in parallel to minimise inductance - I don't think they extended this winding method up to the 1k resistors (higher resistance swamps the inductance).

Good luck with the rewind.

Chris

[woodchips]

Forgot about this thread, again.

So far I have found five of the four decade Muirhead D801-D divider boxes.

One works fine except that 0.9 reads 1.0.
One works fine except the 0.001 decade, seized solid (this is a common problem, all could do with some lubricant)
One is open circuit, this is in a steel, not wood, case
One doesn't work but can't remember why
One has the 0.1 shaft freely rotating

If anyone wants one then I don't have the cheek to charge, just the cost of postage, weight 3kg so not cheap.

[Gyro]

Hi Woodchips,

It sounds as if you and Jeff could get into a decade section exchange scheme. Even the burned sections would be useable as very high quality 2 pole 10 way (actually 11 physically) switches. I'm just trying to think how easy they would be to use as the basis of a KVD, it might involve a bit more cross wiring than ideal maybe. I know the KVD ones normally have staggered contacts.

Already having a perfect D-801-D, I'm stuggling with myself to justify another, I am really curious about what other similar treasures (Muirhead or other) that you might have in your pile though!?

Chris

[Jeff_Birt]

I don't think I would have the heart to scavenge on for parts unless it was not feasible to repair. I ordered some .1% Vishay resistors with a similar TC to repair mine while I try to rewind the resistors. Whilst looking for the correct wire to use I came upon a paper about the Manginin wire wound resistors that served as the NIST standard from 1940 to 1990. The fella who developed it fretted over the change in the wires structure as it was wound so after winding it was heat treated to normalize the wire. If anyone is interested I will post the PDF. NIST uses a different method as the primary reference now, but it is only used to calibrate the wire wound resistors.

[woodchips]

No, one is enough, but very useful as a standard voltage divider.

I have listed other items for sale here, and Tek 7000 scopes on Tekscopes, but the world has moved on. A flickery green trace isn't the writing of the 2010's, if not colour digital then it has no value.

I bet the average age of the people who read this thread is over 50! I am 62.

[Vgkid]

I bet the average age of the people who read this thread is over 50! I am 62.

I'm 26...
Me and timB will keep the age down.

[Gyro]

Well I'm going to push it up again... You've got a couple of years on me woodchips. 

I already have my own green trace thanks. I wouldn't mind finding a Muirhead KVD though!