Carbon Pot Track Repair - UK Vintage Radio Repair and Restoration Discussion Forum

brianc · 7th Jun 2010, 10:21 am

Hi all
My restoration of a Marconiphone 706 Radio/TV is presenting me with quite a few problems

and, incidentally, suggesting a few solutions. The set is equipped with 2 dual concentric pots - brightness/contrast and line/frame hold. The replacement of these components is, of course, nigh impossible as both sections have .25" shafts. The vertical hold track (350k) was open circuit and obviously, the track was seriously damaged! On opening it, I discovered that the wiping contact had been displaced and the track was being "excavated" by the spring which should have been holding the wiper in position. The damage was severe with the backing material showing through the resistive carbon layer. Rubbing soft pencil into the marks did some good but it I found it impossible to build up enough graphite to give a good surface. Piano technicians use a product called SUPERSLIDE, which is a suspension of graphite powder in a volatile carrier, for lubricating wood to wood bearing surfaces such as found in piano actions. As another interest of mine is player piano restoration, I had some of this "in stock". I painted the track with a thin coat, waited for it to dry and burnished it to harden the surface. To my delight, I got a track resistance of 500k with what seemed to be a linear law. I painted on another coat and, after burnishing, measured 360k. After rebuilding the pot (of course, the faulty unit was the inner one

) and installing it back into the chassis, it proved to work perfectly with no noticeable noise (erratic settings). As an aside, I contacted a company who offer a pot customisation service to industry about a replacement and the very kind lady in customer service said she would try to help and produced a free sample for me but with 6mm and 3mm shafts which I will keep as a spare but who said that customer service is dead

.

Supersilde is available from Heckshers http://www.uk-piano.org/heckscher/ (no association).
Cheers
Brian

ppppenguin · 7th Jun 2010, 10:52 am

Quote:

Originally Posted by brianc

... as both sections have .25" shafts. ...

This sounds logically impossible for concentric pots.

Congratulations on finding a good restoration method.

Lucien Nunes · 7th Jun 2010, 11:05 am

This is interesting - I had experimented with a couple of modern substances that look similar and do the same job as a lubricant, but found the resulting deposit had much too high a resistance to be useful. One was a US-made product that we use to form a slick, glossy coating on elastomer rollers - this produced such a high resistance that only the Megger would reveal that it was conductive at all.

Original Acheson Dag is still available from Silmid, incidentally:
http://www.silmid.com/Products/Indus...ubricants.aspx
Although for some varieties you have to buy a huge quantity. I wonder whether Superslide is a rebranded Dag product?

Lucien

brianc · 7th Jun 2010, 11:57 am

Quote:

Originally Posted by ppppenguin

Quote:

Originally Posted by brianc

... as both sections have .25" shafts. ...

This sounds logically impossible for concentric pots.

Congratulations on finding a good restoration method.

Quite simple Jeffery - the rear section shaft passes through the front section shaft and is bushed out to .25" for the last .5"
Cheers
Brian

brianc · 7th Jun 2010, 12:04 pm

Quote:

Originally Posted by Lucien Nunes

Original Acheson Dag is still available from Silmid, incidentally:
http://www.silmid.com/Products/Indus...ubricants.aspx
Although for some varieties you have to buy a huge quantity. I wonder whether Superslide is a rebranded Dag product?

Lucien

Some of these (the volatile ones) seem to be similar to the Heckshers product and could work.
Cheers
Brian

chipp1968 · 7th Jun 2010, 1:37 pm

I have these pots on my HMV 907 and the Brightness contrast combo is the trouble area ,the rear one which i couldn't spray .I could open the front one in situ , but it will need sorting out as it seams to directly affect the best area of tuning in the signal properly. So this solution is very good news .I had a bad vol pot as well , but after repeated applications of spray it improved greatly .I dont know how it is now as the radio went silent some months ago so i havant got that problem sorted out .It will be good too soon as its such a good sounding radio.

brianc · 7th Jun 2010, 11:20 pm

It's a bit of a problem getting at the front section of the pot! The rear section shaft is thin enough to pass through the front section shaft and is then made up to .25" using a bush which is splined onto the thinner shaft. This bush must be removed by using a pin-punch to drive the thin shaft back out of it. It is vital to use a pin-punch of the correct diameter. Once the bush is off, the rear section can be removed and the front section accessed. On reassembly, great care must be taken to press the smaller shaft back into the bush. When I do it again, I will ream the bush out to be a close fit and use superglue to fix it. In this way, no stress is placed on the fragile bits.
Cheers
Brian

I've just noticed that Chipp1968 refers to the section that is easily accessible as the front section - I have referred to it as the rear section. Maybe we should call it the section furthest from the threaded bush!

7th Jun 2010, 10:21 am	#1
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Carbon Pot Track Repair Hi all My restoration of a Marconiphone 706 Radio/TV is presenting me with quite a few problems and, incidentally, suggesting a few solutions. The set is equipped with 2 dual concentric pots - brightness/contrast and line/frame hold. The replacement of these components is, of course, nigh impossible as both sections have .25" shafts. The vertical hold track (350k) was open circuit and obviously, the track was seriously damaged! On opening it, I discovered that the wiping contact had been displaced and the track was being "excavated" by the spring which should have been holding the wiper in position. The damage was severe with the backing material showing through the resistive carbon layer. Rubbing soft pencil into the marks did some good but it I found it impossible to build up enough graphite to give a good surface. Piano technicians use a product called SUPERSLIDE, which is a suspension of graphite powder in a volatile carrier, for lubricating wood to wood bearing surfaces such as found in piano actions. As another interest of mine is player piano restoration, I had some of this "in stock". I painted the track with a thin coat, waited for it to dry and burnished it to harden the surface. To my delight, I got a track resistance of 500k with what seemed to be a linear law. I painted on another coat and, after burnishing, measured 360k. After rebuilding the pot (of course, the faulty unit was the inner one ) and installing it back into the chassis, it proved to work perfectly with no noticeable noise (erratic settings). As an aside, I contacted a company who offer a pot customisation service to industry about a replacement and the very kind lady in customer service said she would try to help and produced a free sample for me but with 6mm and 3mm shafts which I will keep as a spare but who said that customer service is dead . Supersilde is available from Heckshers http://www.uk-piano.org/heckscher/ (no association). Cheers Brian

7th Jun 2010, 11:05 am	#3
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Carbon Pot Track Repair This is interesting - I had experimented with a couple of modern substances that look similar and do the same job as a lubricant, but found the resulting deposit had much too high a resistance to be useful. One was a US-made product that we use to form a slick, glossy coating on elastomer rollers - this produced such a high resistance that only the Megger would reveal that it was conductive at all. Original Acheson Dag is still available from Silmid, incidentally: http://www.silmid.com/Products/Indus...ubricants.aspx Although for some varieties you have to buy a huge quantity. I wonder whether Superslide is a rebranded Dag product? Lucien

7th Jun 2010, 1:37 pm	#6
chipp1968 Rest in Peace Join Date: Jul 2006 Location: Somerset, UK. Posts: 2,356	Re: Carbon Pot Track Repair I have these pots on my HMV 907 and the Brightness contrast combo is the trouble area ,the rear one which i couldn't spray .I could open the front one in situ , but it will need sorting out as it seams to directly affect the best area of tuning in the signal properly. So this solution is very good news .I had a bad vol pot as well , but after repeated applications of spray it improved greatly .I dont know how it is now as the radio went silent some months ago so i havant got that problem sorted out .It will be good too soon as its such a good sounding radio.

7th Jun 2010, 11:20 pm	#7
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: Carbon Pot Track Repair It's a bit of a problem getting at the front section of the pot! The rear section shaft is thin enough to pass through the front section shaft and is then made up to .25" using a bush which is splined onto the thinner shaft. This bush must be removed by using a pin-punch to drive the thin shaft back out of it. It is vital to use a pin-punch of the correct diameter. Once the bush is off, the rear section can be removed and the front section accessed. On reassembly, great care must be taken to press the smaller shaft back into the bush. When I do it again, I will ream the bush out to be a close fit and use superglue to fix it. In this way, no stress is placed on the fragile bits. Cheers Brian I've just noticed that Chipp1968 refers to the section that is easily accessible as the front section - I have referred to it as the rear section. Maybe we should call it the section furthest from the threaded bush!