The case of the twisting cord.....

[Nanozeugma]

An annoying little problem, probably having a simple answer...
However, I can't seem to reason the solution.

Murphy A46.
Decayed natural rubber grommets and strips need replacing.
Per red arrows on attached image.

In order to achieve this, one needs to disassemble the tuning drive mechanism
and of course release the drive cord. the cord is original, nothing wrong with it.
Undisturbed, the cursor runs flat, parallel with the glass.

The irritating problem.
On reassembly, having refitted the tuning cursor on the cord, it will not run
flat against the glass over the gamut of possible travel.
If you get it flat against the glass at one point, it will twist away from the glass
as it it travels, only constrained by the stub of the cursor when it contacts the glass.

I must be missing something, the answer is probably simple, it's still driving me nuts

[stevehertz]

Is this hypothesis or have you actually done this? Either way, it's imperative that no amount of twist is put into the drive cord during the procedure. If, on re-assembly the problem occurs, then awkward though the procedure may be, you will have to manually (using your finger tips) untwist the cord all the way back to the drive wheel anchorage point, if anything putting a 'positive' twist into the cord to get it to lie flat against the dial scale. One more thing, I have had success cutting grommets through one side, sliding them into place and superglueing the severed faces back together. The superglue will hold rubber fine, the difficult bit is making sure that the join is flush. You've got nothing to lose except a grommet, but it's a whole lot easier than a total dismantlement.

[Nanozeugma]

Yes, I have actually done this.
I have no problem taking these apart and fitting new grommets and rubber strips.
The puzzle is imagining the method employed in assembly when fitting the cord to stop
this, otherwise logic would suggest the same issue would occur.

[stevehertz]

Originally Posted by Nanozeugma

Yes, I have actually done this.
I have no problem taking these apart and fitting new grommets and rubber strips.
The puzzle is imagining the method employed in assembly when fitting the cord to stop
this, otherwise logic would suggest the same issue would occur.

Logic doesn't always prevail in the world of variable mechanics (that cord is not a stiff structure) where the amount of twist it holds can vary with handling, time, age etc. Try my suggestion of manually untwisting the cord. In simple terms it's twisted, it needs untwisting?

[G6Tanuki]

I've never understood the logic in fitting those grommets; if they're soft enough to provide any sort of anti-microphonic effect they are also soft enough to deform and introduce annoying backlash and slop into the tuning-drive.

I happily replace them with brass washers/spacers and tighten the bolts down good and tight.

What cord are you using? It's a characteristic of some cords that the 'twistiness' varies with applied tension! [Kite-flyers know all about this: a line which has been put under a lot of tension will often coil itself in strange ways when the kite comes down and the line is lying on the ground ready to be wound onto a spool].

[Nanozeugma]

Originally Posted by stevehertz

Logic doesn't always prevail in the world of variable mechanics (that cord is not a stiff structure) where the amount of twist it holds can vary with handling, time, age etc. Try my suggestion of manually untwisting the cord. In simple terms it's twisted, it needs untwisting?

I will indeed try your suggestion.
Following your remarks, I surmise that the cord has acquired a twist on being released from the tension it has been under for over eighty years
Thank you for your thoughts.

[Nanozeugma]

Originally Posted by G6Tanuki

I've never understood the logic in fitting those grommets; if they're soft enough to provide any sort of anti-microphonic effect they are also soft enough to deform and introduce annoying backlash and slop into the tuning-drive.

I happily replace them with brass washers/spacers and tighten the bolts down good and tight.

What cord are you using? It's a characteristic of some cords that the 'twistiness' varies with applied tension! [Kite-flyers know all about this: a line which has been put under a lot of tension will often coil itself in strange ways when the kite comes down and the line is lying on the ground ready to be wound onto a spool].

Anti microphony would be my reasoning too.
Not sure about the slop and backlash, haven't noted any even in their decayed state.
I happily refit modern PVC replacements which are firmer than the original natural rubber anyway.
The primary issue with the A46 is that as the grommets dry out and collapse over time, the tuning scale develops a very visible "list to port."
Where this type of grommet is under compression, the bottom surface tends to go to jelly and glue itself to the abutting metalwork.
Where the grommet is fitted to a through hole in the chassis to protect against cableform chafing, both upper and lower faces being in free air, they just go rock hard.

[ms660]

Originally Posted by G6Tanuki

I've never understood the logic in fitting those grommets; if they're soft enough to provide any sort of anti-microphonic effect they are also soft enough to deform and introduce annoying backlash and slop into the tuning-drive.

I happily replace them with brass washers/spacers and tighten the bolts down good and tight.

So far as I know the tuning drive spindle directly engages with the tuning gang drum, being on the same mechanical assembly there's no tuning drive backlash caused by using grommets for mounting the assembly on that I can see.

The idea of using grommets or springs is well proven in reducing the microphonic effect.

Lawrence.