Repairing pots - UK Vintage Radio Repair and Restoration Discussion Forum

Mike Phelan · 2nd Dec 2006, 6:30 pm

The variable resistor type, that is.

I have seen quite a few posts on here about needing to replace various post, and often these are difficult to find now. Of course, a dose of switch cleaner will have been tried and failed to make any difference.

Many of the earlier ones can be repaired – here’s how:
There are a few causes of the problem – the most likely ones are the slider contact being tarnished and the rivets holding this item or the track having poor contact.
Actual wear of the track itself is the least likely cause.

Dismantle the pot, carefully bending the tabs out of the way. Usually the spindle and everything else will come out, as in this one from a sixties Thorn radio, or there will be a circlip on the spindle.
On-off switches are often rivetted on and the rivets can be drilled out, replacing them with nuts and bolts.
Larger rivets can be drilled and tapped 10 or 12BA and the heads put on the inside

The one I used as an example was the volume control and DP switch from a Ferguson 3152, of which a bit more anon in Success Stories.
It was that bad that, even with switch cleaner, just a slight touch on the knob produced more crackling that a hog roast.
[IMG]http://static.***********/113/312079544_8888c9a0bd.jpg[/IMG]

When I dismantled it, it was plain to see that the ring on the slider was very badly tarnished indeed – there was a black film of silver sulphide.
I had to rub the sliver vigorously with Brasso to clean it.

Shown here is how I punch all the rivetted connections; seen here is something called a clockmaker’s staking set, but that is not a requirement; a small centre punch is just as good.

[IMG]http://static.***********/108/312079548_65466d8f1d.jpg[/IMG]
It is worth taking care of the lubrication as well; Vaseline on the contact parts, normal grease on the spindle.
The four studs on the switch were also polished; these have a sliding contact.

Some switches can weld the contacts together; if they cannot be separated, unfortunately then the switch is scrap.
Quite often it is possible to swap parts on pots where they are of the same make, but where you cannot find one with a switch, or the spindle is too short, or whatever.

Worth having a go!

paulsherwin · 2nd Dec 2006, 8:08 pm

Has anybody had any success dismantling the pots found in many 50s Philips sets? These have a metal front and body which seems to have been pressed from a single sheet of metal. The track and wiper lugs emerge from a round paxolin sheet which is pressed into the rear of the metal body and retained by a lip. I can't see any way of dismantling these without destroying the part.

Paul

Radio_Dave · 2nd Dec 2006, 9:39 pm

Quote:

Originally Posted by Mike Phelan

Worth having a go!

Definitely! If a good spray with lubricated switch cleaner doesn't do the trick I always dismantle, clean and lubricate as you have suggested and in the, vast, majority of cases the pot works properly again

Even if the track has worn through I have sometimes been able to put a 's' shaped kink into the wiper to move its contact onto new carbon.

David

peter_scott · 3rd Dec 2006, 12:21 am

Mike,

That's very interesting. I had always just assumed track wear was the problem. Dave's idea of, what I assume as, a change of wiper radius is also interesting.

Thanks,

Peter.

stephanie · 3rd Dec 2006, 6:44 am

Quote:

Originally Posted by paulsherwin

Has anybody had any success dismantling the pots found in many 50s Philips sets? These have a metal front and body which seems to have been pressed from a single sheet of metal. The track and wiper lugs emerge from a round paxolin sheet which is pressed into the rear of the metal body and retained by a lip. I can't see any way of dismantling these without destroying the part.

Paul

I dismantled one (because of a crack in the track) and replaced the track with one from a scrap set.

There's a litle retaining ring in a notch at the point where the shaft comes out of the front bushing. It's fiddly...you need something like a couple of dental picks to pop it out. Make sure you don't lose it!!!!

skodajag · 3rd Dec 2006, 9:32 pm

Has anyone successfully dismantled the pots which Philips and Mullard used on their pre-war monoknob sets? They don't have a conventional spindle but rather a lever emerging from the side which is spring loaded (on the outside of the pot) and operated by one of the dreaded bowden cables. The cable pulls the lever in one direction and the spring returns it in the other.Those on my set have stiffened up to the point that the return spring isn't strong enough to return them. Switch cleaner didn't free them up but, strangely, heating gently with a hair dryer frees them perfectly. But when they cool down they become stiff again. I think dismantling is the next step but those I've spoken to so far suggest it might be possible to get them apart, but not back together again (which of course takes us all back to our childhood). Any sugestions? (Paul: I take it that your request here for help with later philips pots means that this request is legit on this thread?)
Jonathan

paulsherwin · 3rd Dec 2006, 9:44 pm

Quote:

Originally Posted by skodajag

(Paul: I take it that your request here for help with later philips pots means that this request is legit on this thread?)

It is as far as I'm concerned - this is a general discussion of techniques for dismantling and repairing pots, not a request for help with a specific radio

Paul

Mike Phelan · 4th Dec 2006, 9:05 am

David
Your idea of moving the wiper to an unworn bit of track is a very good idea. I actually did it on the one above but missed mentioning it!

Tony
Thank you. I never managed to think in straight lines all the time; there tend to be a few ninety-degree bends in there ...

Jonathan
Sounds like the lubricant on the cables has gummed up? Probably need to dismantle them as far as you can and wash in IPA or petrol and re-lube.

dave walsh · 4th Dec 2006, 9:44 am

I've printed of your repair details for ref Mike but also simply because both the work and the illustrations are of such high quality. Haven't got over the aerial repair yet. You should do an illlustrated book on micro engineering techniques. Thanks for your very clear mod comment Paul.
Dave W

GMB · 4th Dec 2006, 2:02 pm

This may only apply to wirewound pots, I haven't tried carbon yet.....

I made a discovery in this area just a few days ago. Faced with some totally scratchy wirewound pots in a bridge - so bad as to be almost unusable - I was about to embark on a major repair job. But before gutting these highly non-standard multi-stacked pots I tested out this idea: maybe the scratchiness is only transient and it is the transients that are most of the irritation, not the actual dead spots themselves. These were linear pots and calculation revealed that you can connect surprisingly low resistors from slider to each track end without messing to the linearity. So I tried it and the results were magic! With some (albeit wrong) continuity maintained at all times the effect of the dreadful scratchiness was almost completely eliminated.

Aerodyne · 5th Dec 2006, 3:32 pm

It might be worth mentioning (see, 90 degrees, Mike) that another cause of noise on pots may not be the fault of the pot itself but - dependent greatly on the circuit configuration - due to DC getting to the track due to a leaky coupling cap. The cure is self-evident!
-Tony

Mike Phelan · 5th Dec 2006, 3:38 pm

Absolutely, Tony. Or that poor circuit design shoves DC on the pot and it needs a capacitor putting in.

Mike Phelan · 28th May 2007, 3:01 pm

I have just resuscitated the thread after repairing a volume control from a Bush TR130, which was very noisy and the switch was sticking as well (more in Success Stories when it is all finished).

The switch responded to switch cleaner so fortunately I did not have to drill the rivets out, and cleaning the track and ring were as before, but there is an important point to watch; there are two carbon brushes that will fall out and get lost when you take the spindle out, if you are not aware of this - pictures shown.

2nd Dec 2006, 6:30 pm	#1
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Repairing pots The variable resistor type, that is. I have seen quite a few posts on here about needing to replace various post, and often these are difficult to find now. Of course, a dose of switch cleaner will have been tried and failed to make any difference. Many of the earlier ones can be repaired – here’s how: There are a few causes of the problem – the most likely ones are the slider contact being tarnished and the rivets holding this item or the track having poor contact. Actual wear of the track itself is the least likely cause. Dismantle the pot, carefully bending the tabs out of the way. Usually the spindle and everything else will come out, as in this one from a sixties Thorn radio, or there will be a circlip on the spindle. On-off switches are often rivetted on and the rivets can be drilled out, replacing them with nuts and bolts. Larger rivets can be drilled and tapped 10 or 12BA and the heads put on the inside The one I used as an example was the volume control and DP switch from a Ferguson 3152, of which a bit more anon in Success Stories. It was that bad that, even with switch cleaner, just a slight touch on the knob produced more crackling that a hog roast. [IMG]http://static.*********/113/312079544_8888c9a0bd.jpg[/IMG] When I dismantled it, it was plain to see that the ring on the slider was very badly tarnished indeed – there was a black film of silver sulphide. I had to rub the sliver vigorously with Brasso to clean it. Shown here is how I punch all the rivetted connections; seen here is something called a clockmaker’s staking set, but that is not a requirement; a small centre punch is just as good. [IMG]http://static.*********/108/312079548_65466d8f1d.jpg[/IMG] It is worth taking care of the lubrication as well; Vaseline on the contact parts, normal grease on the spindle. The four studs on the switch were also polished; these have a sliding contact. Some switches can weld the contacts together; if they cannot be separated, unfortunately then the switch is scrap. Quite often it is possible to swap parts on pots where they are of the same make, but where you cannot find one with a switch, or the spindle is too short, or whatever. Worth having a go! __________________ Mike.

2nd Dec 2006, 8:08 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,970	Re: Repairing pots Has anybody had any success dismantling the pots found in many 50s Philips sets? These have a metal front and body which seems to have been pressed from a single sheet of metal. The track and wiper lugs emerge from a round paxolin sheet which is pressed into the rear of the metal body and retained by a lip. I can't see any way of dismantling these without destroying the part. Paul

3rd Dec 2006, 12:21 am	#4
peter_scott Dekatron Join Date: May 2003 Location: Edinburgh, UK. Posts: 3,274	Re: Repairing pots Mike, That's very interesting. I had always just assumed track wear was the problem. Dave's idea of, what I assume as, a change of wiper radius is also interesting. Thanks, Peter.

3rd Dec 2006, 9:32 pm	#6
skodajag Pentode Join Date: Mar 2003 Location: Heathfield, East Sussex, UK, Posts: 227	Re: Repairing pots Has anyone successfully dismantled the pots which Philips and Mullard used on their pre-war monoknob sets? They don't have a conventional spindle but rather a lever emerging from the side which is spring loaded (on the outside of the pot) and operated by one of the dreaded bowden cables. The cable pulls the lever in one direction and the spring returns it in the other.Those on my set have stiffened up to the point that the return spring isn't strong enough to return them. Switch cleaner didn't free them up but, strangely, heating gently with a hair dryer frees them perfectly. But when they cool down they become stiff again. I think dismantling is the next step but those I've spoken to so far suggest it might be possible to get them apart, but not back together again (which of course takes us all back to our childhood). Any sugestions? (Paul: I take it that your request here for help with later philips pots means that this request is legit on this thread?) Jonathan

4th Dec 2006, 9:05 am	#8
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Repairing pots David Your idea of moving the wiper to an unworn bit of track is a very good idea. I actually did it on the one above but missed mentioning it! Tony Thank you. I never managed to think in straight lines all the time; there tend to be a few ninety-degree bends in there ... Jonathan Sounds like the lubricant on the cables has gummed up? Probably need to dismantle them as far as you can and wash in IPA or petrol and re-lube. __________________ Mike.

4th Dec 2006, 9:44 am	#9
dave walsh Dekatron Join Date: Jan 2006 Location: Ramsbottom (Nr Bury) Lancs or Bexhill (Nr Hastings) Sussex. Posts: 5,817	Re: Repairing pots I've printed of your repair details for ref Mike but also simply because both the work and the illustrations are of such high quality. Haven't got over the aerial repair yet. You should do an illlustrated book on micro engineering techniques. Thanks for your very clear mod comment Paul. Dave W

4th Dec 2006, 2:02 pm	#10
GMB Dekatron Join Date: Aug 2003 Location: near Reading (and sometimes Torquay) Posts: 3,100	Re: Repairing pots This may only apply to wirewound pots, I haven't tried carbon yet..... I made a discovery in this area just a few days ago. Faced with some totally scratchy wirewound pots in a bridge - so bad as to be almost unusable - I was about to embark on a major repair job. But before gutting these highly non-standard multi-stacked pots I tested out this idea: maybe the scratchiness is only transient and it is the transients that are most of the irritation, not the actual dead spots themselves. These were linear pots and calculation revealed that you can connect surprisingly low resistors from slider to each track end without messing to the linearity. So I tried it and the results were magic! With some (albeit wrong) continuity maintained at all times the effect of the dreadful scratchiness was almost completely eliminated.

5th Dec 2006, 3:32 pm	#11
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Repairing pots It might be worth mentioning (see, 90 degrees, Mike) that another cause of noise on pots may not be the fault of the pot itself but - dependent greatly on the circuit configuration - due to DC getting to the track due to a leaky coupling cap. The cure is self-evident! -Tony

5th Dec 2006, 3:38 pm	#12
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Repairing pots Absolutely, Tony. Or that poor circuit design shoves DC on the pot and it needs a capacitor putting in. __________________ Mike. Last edited by Darren-UK; 9th Jan 2008 at 11:33 pm. Reason: Removed keep-thread-open request due to archiving.

28th May 2007, 3:01 pm	#13
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Repairing pots I have just resuscitated the thread after repairing a volume control from a Bush TR130, which was very noisy and the switch was sticking as well (more in Success Stories when it is all finished). The switch responded to switch cleaner so fortunately I did not have to drill the rivets out, and cleaning the track and ring were as before, but there is an important point to watch; there are two carbon brushes that will fall out and get lost when you take the spindle out, if you are not aware of this - pictures shown. Attached Thumbnails __________________ Mike.