Snapped-off pot shaft - UK Vintage Radio Repair and Restoration Discussion Forum

llama · 7th Nov 2021, 7:30 pm

A previous owner had snapped off the volume control, sheer with the outer edge of the threaded bush. The missing parts were - er - missing.

Looking inside I decided exchanging the whole pot would've been a huge task involving many connectors to the rear and heatsinks in the middle with the expected problems of getting good heat transfer but electrically insulated when re-assembling. So I decided to try a fix just from the front.

Using my dremel-oid with its flexible extension drive, I fitted a medium grit rounded grinding stone and gently ground down the 6mm pot shaft until I'd made it reasonably concave. Looking up on the Internet the correct tapping size for an M3 thread, I found my 5/32 drill bit was the closest match I had. It was pretty easy to drill into the pot shaft - it seemed to be some sort of monkey-metal or possibly just aluminium. A set of three M3 taps cost under £2 and made a nice M3 internal thread of sufficient depth to make me feel I could get a good lock-nut action with a suitable bit of M3 studding.

Next I looked through my stock of M3 bits & pieces and found a threaded pillar with M3 inside and an external diameter of 6mm - perfect for the replacement knob. It was about 8mm long. Instead of the M3 studding I'd planned I found an M3 cheese-head screw of just the right length and screwed it really tightly into the threaded pillar, achieving the required lock-nut action for the new knob. I found that a normal M3 nut had just enough clearance around its circumference to be used as the lock-nut for the old pot-shaft. So I loosely threaded it onto the M3 screw and screwed the whole assembly onto the pot-shaft. Not so tight as to strip the thread but tight enough to be sure I'd got the maximum purchase onto my newly tapped thread. I found I'd got a very slim spanner for M3 nuts and that allowed me to lock-nut the M3 nut onto the pot-shaft. A collet-knob from the knob box completed the job.

It remains to be seen whether this assembly will stand the test of time. Gentle use will clearly help in that regard. However, if the assembly works loose I have the option of using superglue on the threads instead of relying on lock-nut action.

Graham

Pamphonica · 7th Nov 2021, 7:48 pm

That sounds like a good fix. I like the idea of starting with a concave end on the shaft. That must make drilling easier.
Good job!
-Jeremy

mark_in_manc · 7th Nov 2021, 11:00 pm

Sounds good. And if it fails, you have room to go up to M4 (and a nice pilot hole to get you started)

G6Tanuki · 8th Nov 2021, 5:55 pm

A cunning fix! In the past I did something similar when fixing a plastic-spindled pot which had been snapped off; I counterbored the remaining bit of the spindle and Araldited a 4BA steel stud into it, then tapped the replacement bit of 6.3mm brass rod 4BA, coated its thread with Araldite and screwed it onto the protruding 4BA stud.

I fuigured that the 4BA stud was probably stronger than the old plastic spindle had been.

7th Nov 2021, 7:30 pm	#1
llama Octode Join Date: Jun 2006 Location: St Osyth, Nr Clacton, Essex, UK. Posts: 1,482	Snapped-off pot shaft A previous owner had snapped off the volume control, sheer with the outer edge of the threaded bush. The missing parts were - er - missing. Looking inside I decided exchanging the whole pot would've been a huge task involving many connectors to the rear and heatsinks in the middle with the expected problems of getting good heat transfer but electrically insulated when re-assembling. So I decided to try a fix just from the front. Using my dremel-oid with its flexible extension drive, I fitted a medium grit rounded grinding stone and gently ground down the 6mm pot shaft until I'd made it reasonably concave. Looking up on the Internet the correct tapping size for an M3 thread, I found my 5/32 drill bit was the closest match I had. It was pretty easy to drill into the pot shaft - it seemed to be some sort of monkey-metal or possibly just aluminium. A set of three M3 taps cost under £2 and made a nice M3 internal thread of sufficient depth to make me feel I could get a good lock-nut action with a suitable bit of M3 studding. Next I looked through my stock of M3 bits & pieces and found a threaded pillar with M3 inside and an external diameter of 6mm - perfect for the replacement knob. It was about 8mm long. Instead of the M3 studding I'd planned I found an M3 cheese-head screw of just the right length and screwed it really tightly into the threaded pillar, achieving the required lock-nut action for the new knob. I found that a normal M3 nut had just enough clearance around its circumference to be used as the lock-nut for the old pot-shaft. So I loosely threaded it onto the M3 screw and screwed the whole assembly onto the pot-shaft. Not so tight as to strip the thread but tight enough to be sure I'd got the maximum purchase onto my newly tapped thread. I found I'd got a very slim spanner for M3 nuts and that allowed me to lock-nut the M3 nut onto the pot-shaft. A collet-knob from the knob box completed the job. It remains to be seen whether this assembly will stand the test of time. Gentle use will clearly help in that regard. However, if the assembly works loose I have the option of using superglue on the threads instead of relying on lock-nut action. Graham __________________ Half my stuff is junk - trouble is, I don't know which half!

7th Nov 2021, 7:48 pm	#2
Pamphonica Nonode Join Date: Mar 2008 Location: Virginia Water, Surrey, UK. Posts: 2,877	Re: Snapped-off pot shaft That sounds like a good fix. I like the idea of starting with a concave end on the shaft. That must make drilling easier. Good job! -Jeremy __________________ Jeremy, G8MLK, BVWTVM Friend, VMARS, BVWS Secretary. www.pamphonic.co.uk www.bttt.org.uk

7th Nov 2021, 11:00 pm	#3
mark_in_manc Octode Join Date: Nov 2011 Location: Manchester, UK. Posts: 1,874	Re: Snapped-off pot shaft Sounds good. And if it fails, you have room to go up to M4 (and a nice pilot hole to get you started) __________________ "The best dBs, come in 3s" - Woody Brown

8th Nov 2021, 5:55 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: Snapped-off pot shaft A cunning fix! In the past I did something similar when fixing a plastic-spindled pot which had been snapped off; I counterbored the remaining bit of the spindle and Araldited a 4BA steel stud into it, then tapped the replacement bit of 6.3mm brass rod 4BA, coated its thread with Araldite and screwed it onto the protruding 4BA stud. I fuigured that the 4BA stud was probably stronger than the old plastic spindle had been. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.