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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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Old 27th Sep 2022, 3:04 pm   #1
Vintage Engr
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Default Taut band meter repair- Ferrograph RTS2

This could be any taut -band meter, but I'm including the model number of the item of test gear that I've taken it out of.

I understand that this is a fairly well known fault on the RTS2, as if one does not take care with switch-settings, the needle can make some fairly erratic & violent excursions!
So, the coil is o.k. it's a 100uA movement. However the taut-band has broken in the middle, so there's no chance of re-soldering at one end, which can sometimes be achieved.
My question is, does anyone know what the band is made from, it's obviously non-ferrous, and is flat & springy.

It looks as if I could repair it, given that I could find the correct material for the band. I guess I only need about 50mm, that would allow for holding it at each end while I solder it to the tension springs.
Any ideas gratefully received,

Thanks,
David.
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Old 28th Sep 2022, 12:02 am   #2
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Default Re: Taut band meter repair- Ferrograph RTS2

I have a Hewlett Packard taught band movement that had some corrosion on the central pole piece that impeded the coil, I initially thought I could take it apart to clean off the corrosion but once I had a good look at the construction I decided against it, the type and cross section of the suspension wire will affect the amount of tension it provides - the wrong type of material or wrong size will throw the fsd of the meter out. However as the movement is no longer functional it is worth a try - you've nothing to lose, possibly the torsion wire from an anniversary (400 day) clock, I once repaired one, the wire used in these is flat, you just have to find the right cross section.
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Old 28th Sep 2022, 9:23 am   #3
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Default Re: Taut band meter repair- Ferrograph RTS2

Thanks Retailer,

I was looking at clock hairsprings yesterday, but they are mainly listed by the tension force, rather than physical size. I shall however measure the size & then have another look. It is a flat cross-section, as would be expected.
Other than that, I have seen a 1mA version of the meter, so if all else fails, I might fit that with an op-amp.

David
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Old 29th Sep 2022, 11:34 am   #4
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Default Re: Taut band meter repair- Ferrograph RTS2

This is an update on where I am with this project now. Having more or less given up finding a replacement Sifam meter of the right physical size, I have now found a 1mA FSD version, & have decided to use an op-amp driver, (similar to the Avo valve-tester mods).
This has already been done by others, using a circuit from an old P.W. article.

What I will do, is to add some meter protection. Whilst repairing this Ferrograph RTS-2 I discovered that with the original circuit & the 100uA meter, it was possible, during normal operation, to overload the meter by around 100-200% which is why so many of these units end up with damaged meters. I have had my original unit for around 30 years, & was always worried by the erratic meter excursions. I was originally going to make one test set out of 2, but will now try & get both going, meter availability permitting of course.

David.
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Old 30th Sep 2022, 12:10 pm   #5
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Default Re: Taut band meter repair- Ferrograph RTS2

You might try a 400 day clock suspension spring. They are listed on ebay by thickness ranging from 0.046mm to 0.152mm. Looks like a ticklish job though.

You see these clocks from time to time a bootfairs or charity shops - you might get lucky.

The thickness is important - the torsion varies by the 4th power of thickness.

Last edited by Cobaltblue; 30th Sep 2022 at 12:28 pm. Reason: Don't use Euphemisms Rule C1
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Old 30th Sep 2022, 12:14 pm   #6
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Default Re: Taut band meter repair- Ferrograph RTS2

Thanks,
I shall obviously keep the damaged meter, & if I can will aim to repair it.

David.
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Old 30th Sep 2022, 2:35 pm   #7
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Default Re: Taut band meter repair- Ferrograph RTS2

Can you give full details of the meter, and a picture of the scale?
You might get lucky - somebody might have one lying around in a jumk box.
Is it a Sifam 42A and if so, what's the DC resistance?
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Old 30th Sep 2022, 6:37 pm   #8
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Default Re: Taut band meter repair- Ferrograph RTS2

Although they do not advertise it Sifam also offer a repair service for their meters.

I found this out when I contacted them on the off chance they could supply me with a bezel for one of the meters on my Roband PSU. Not only did they still have stock of the part they sent one FOC.

It would be worth contacting them.

http://www.sifam.com/default.asp

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Old 1st Oct 2022, 12:28 pm   #9
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Default Re: Taut band meter repair- Ferrograph RTS2

Quote:
Originally Posted by m0cemdave View Post
Can you give full details of the meter, and a picture of the scale?
You might get lucky - somebody might have one lying around in a junk box.
Is it a Sifam 42A and if so, what's the DC resistance?
Thanks,

Yes it is a Sifam 42A, and was one of Sifam's 'Clarity' range, no longer manufactured. The FSD is 100uA, and the D.C. res. is 1020 ohms.

The scale is not a problem, as I now have one good original scale, which I can copy (I have two RTS-2 test sets requiring a meter & one of them the scale has faded into obscurity).
I hopefully have a used 1mA version of the meter arriving next week, so I shall experiment with the op-amp driver for that.
If that proves o.k., I will probably look for another 1mA unit, as it should be a little more robust than the original 100uA type.

Someone in the USA with the same test set, did actually get Sifam to make a one-off meter using one of their current (ouch!) range. However, it was very expensive, & needed a 3D printed bit of plastic to make it properly fit the aperture on the Ferrograph test set. At least using a s/hand meter means that it fits perfectly.
I hadn't thought of asking Sifam for a repair, but I guess that would also be expensive.

Hopefully I can get both units working again.

David
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Old 2nd Oct 2022, 7:33 pm   #10
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Default Re: Taut band meter repair- Ferrograph RTS2

I think I may have a used one of those stashed in a drawer. Unfortunately I'm away from home for the next 2 weeks, but will dig it out on my return and test it.

Type 42A, taut band, 100uA - and something like 1100 ohms DCR. With audio voltmeter and dBm scales if I remember right.

If it turns out to be what you need, it will be yours for the postage cost.
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Old 2nd Oct 2022, 11:57 pm   #11
G8GUH Gerry
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Default Re: Taut band meter repair- Ferrograph RTS2

Hi David,

I repaired one of these meters by transplanting a different meter movement into it. Details are in an article here - pages 5-7.

Good luck!

Gerry
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Old 4th Oct 2022, 1:46 pm   #12
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Default Re: Taut band meter repair- Ferrograph RTS2

Quote:
Originally Posted by m0cemdave View Post
I think I may have a used one of those stashed in a drawer. Unfortunately I'm away from home for the next 2 weeks, but will dig it out on my return and test it.

Type 42A, taut band, 100uA - and something like 1100 ohms DCR. With audio voltmeter and dBm scales if I remember right.

If it turns out to be what you need, it will be yours for the postage cost.
That's the one!
Yes please if you still have it.
Thanks,

David.
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Old 4th Oct 2022, 2:08 pm   #13
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Default Re: Taut band meter repair- Ferrograph RTS2

Quote:
Originally Posted by G8GUH Gerry View Post
Hi David,

I repaired one of these meters by transplanting a different meter movement into it. Details are in an article here - pages 5-7.

Good luck!

Gerry
Thanks Gerry,

What a great article, & also some superb restoration work. I notice that your replacement meter is a standard D'Arsonval moving coil type rather than the original taut band. I also now have two RTS-2's to hopefully bring back to original specification.
Did you put in any extra meter protection, (I've not totally read your article yet), as it's very easy to overload the meter, simply by inadvertently either having the osc. level too high & going to 'read osc.', or even just turning the mains on tends to send the meter hurtling way past its intended FSD.

My original RTS-2 was also an early one that I've owned for almost 40 years, & has served me well. That too has the captive mains lead & voltage selector on the rear. The newer one that I have is much later, incorporating all the mods up to the end of production. I have one Auxiliary test Unit that came with my older test set. I'm hopefully going to spruce that up a bit as well, the speaker has gone off-centre and its a relatively high impedance one. I don't know why as the Plessey audio I.C. can drive a lower impedance.

I'll post later on when I've completed them.
Thanks again, much appreciated,

David.
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Old 5th Oct 2022, 4:28 am   #14
G8GUH Gerry
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Default Re: Taut band meter repair- Ferrograph RTS2

I don't think I installed any meter protection, but its probably a good idea to do so - as you noted, the meter can have some 'wild' movements and often pegs at FSD. The original meter from the unit I refurbished had been removed and installed in a second (later model) RTS-2 unit. I retained that unit and sold on the refurbished one and the matching ATU-1 unit to a friend who is more into tape recorders than I am. The ATU-1 unit was in good condition and worked well - but I had no real use for it.
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Old 5th Oct 2022, 11:51 am   #15
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Default Re: Taut band meter repair- Ferrograph RTS2

Most of my working life was involved with broadcast & industrial video & audio equipment, so the balanced audio in/out on the Auxiliary unit is useful to me. Most of my audio kit at home is balanced audio.

I'm spoilt really as I also own a Lindos audio analyser.

However sometimes the analogue 'feel' of the RTS-2 can be more user-friendly, plus as the Lindos has an up-to-date calibration, I can use that to check the RTS-2.
Regarding the meter ballistics on the RTS-2, yes it be very violent, the same happens every tine the unit is powered off, it actually audible as the needle hits the end of travel. I will have to do something about it, - I don't want two more wrecked meters!

David.
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