Cermet (?) pot replacement? - UK Vintage Radio Repair and Restoration Discussion Forum

GW4FRX · 5th Mar 2019, 7:52 pm

One of the pots on the board in the pic has an intermittent and very noisy track which won't respond to any amount of propanol, Servisol or anything else. I remember being able to buy these from the likes of RS many years ago but haven't seen any for a long time, and trawling the internet this afternoon has got me precisely nowhere.

Anyone suggest where I can find a 100K lin in this style? The overall diameter is about 15mm and the depth about 6mm.

VT FUSE · 5th Mar 2019, 8:19 pm

I have had problems with that style in the past and after removal changed them for open style Egen carbon track type. If you want enclosed track types then Piher types are a good replacement,I do not think the style you show is Cermet? from memory, once removed from board, the cause of the poor contact is often caused by tarnish on the wiper and its associated slip ring like arrangement,this can be cleaned with fine metal polish and the track with IPA etc,I think the pot is held together using a tiny C clip or suchlike.

McMurdo · 5th Mar 2019, 8:59 pm

Haven't seen any for a long time, the pins on those are close enough to accept a modern replacement, usually quite thick pins. I've had a few over the years where, when you unsolder them, the base part which forms the track falls into 2 pieces. Are they 300mW?

MrBungle · 5th Mar 2019, 9:29 pm

Ugh those things are cancerous. My telequipment stuff was full of them. Usually replace them with cheap enclosed ones https://www.bitsbox.co.uk/index.php?...roducts_id=713

If you need more power bourns 3386P are good but expensive.

Diabolical Artificer · 6th Mar 2019, 7:21 am

Don't think they're Cermet, just bog standard carbon. I have some used ones in my spares draw out of as Mr B says a Tele scope. Could see if there's a 100k one if any use.

Andy.

GW4FRX · 6th Mar 2019, 3:43 pm

Many thanks for the replies, all very useful and illuminating. It's difficult to know whether to keep the pots in service for the sake of originality knowing that they're likely to be unreliable or change them for something modern which won't quite look the part. Such seems to be the conflict between conservation and restoration.

In the meantime I'll happily reimburse postage or make a suitable contribution to a charity of choice if anyone happens to have a 100K lin languishing in the junk box looking for a good home.

And no, this sort doesn't seem to be held together by a circlip or whatnot and it's rather difficult to see how it was assembled in the first place. Beastly thing!

MrBungle · 6th Mar 2019, 4:07 pm

Did some digging in the junk box, err cupboard, and found one solitary one left on a spare TQ board I have lying around which was not 100k and sacrified it for the cause. Here's how it is constructed:

Click image for larger version

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m0cemdave · 6th Mar 2019, 9:50 pm

That looks like a moulded track type, probably made by Plessey. I think I have a datasheet somewhere, will have a rummage.

m0cemdave · 6th Mar 2019, 10:35 pm

From an early 1970's Farnell catalogue:

www.spamabyss.net/radio/MPpots.jpg

Note that these could be moulded track carbon, wirewound, or cermet.

Also various different pinouts - in addition to those on the page, I have seen some with a large threaded centre pin.

GW4FRX · 6th Mar 2019, 10:43 pm

Quote:

Originally Posted by m0cemdave

From an early 1970's Farnell catalogue:www.spamabyss.net/radio/MPpots.jpg Note that these could be moulded track carbon, wirewound, or cermet. Also various different pinouts - in addition to those on the page, I have seen some with a large threaded centre pin.

That's a very useful find -- many thanks. They look exactly like the thing on the Radford board although I'm not quite sure what flavour of pot is is since there are no markings on it anywhere.

Argus25 · 7th Mar 2019, 4:37 am

I recognize these large format presets, or very very similar, with the same pinouts/geometry, they are still available from RS components, go there and search 186-205 a 47k version or 186-211 the 100k version (that one is on back order it says), but they are still current parts. They range from 100R to 1MR. They were Tyco 16mm PC910 Series.They are 1W rated.

Have a look at page 2 of this article where I used one in an automotive voltage regulator:

http://www.worldphaco.com/uploads/HI...CS_ARCHIVE.pdf

Diabolical Artificer · 7th Mar 2019, 6:38 am

Had a quick look, no 100k so far, but do have a small bag of 10T 100k LIN with a footprint which is near, EG the two back legs line up but the wiper leg would need bodging. These are also of less wattage, I'd say 1/4w. Will send you one if you drop us your address.

As to reliability of the originals, the ones I have appear to be fine. I used a few on projects like PSU's with no issue's.

Andy.

GW4FRX · 7th Mar 2019, 9:51 am

Quote:

Originally Posted by Argus25

I recognize these large format presets, or very very similar, with the same pinouts/geometry, they are still available from RS components, go there and search 186-205 a 47k version or 186-211 the 100k version (that one is on back order it says), but they are still current parts. They range from 100R to 1MR. They were Tyco 16mm PC910 Series.They are 1W rated.

Well, bless my soul. Very many thanks for that -- I combed RS and Farnell the other day but didn't think to search on 'trimmer resistors'. One of those looks as though it'll fill the bill nicely.

Case solved!

GW4FRX · 7th Mar 2019, 9:54 am

Quote:

Originally Posted by Diabolical Artificer

Had a quick look, no 100k so far, but do have a small bag of 10T 100k LIN with a footprint which is near, EG the two back legs line up but the wiper leg would need bodging. These are also of less wattage, I'd say 1/4w. Will send you one if you drop us your address.

Andy -- that's greatly appreciated but it seems from the above that a modern more-or-less equivalent is available from RS so I'll go with that. But thank you very much indeed for the offer.

5th Mar 2019, 7:52 pm	#1
GW4FRX Pentode Join Date: Aug 2005 Location: Shrewsbury, Shropshire, UK. Posts: 149	Cermet (?) pot replacement? One of the pots on the board in the pic has an intermittent and very noisy track which won't respond to any amount of propanol, Servisol or anything else. I remember being able to buy these from the likes of RS many years ago but haven't seen any for a long time, and trawling the internet this afternoon has got me precisely nowhere. Anyone suggest where I can find a 100K lin in this style? The overall diameter is about 15mm and the depth about 6mm. Attached Thumbnails

5th Mar 2019, 8:19 pm	#2
VT FUSE Hexode Join Date: Aug 2010 Location: Malvern, Worcestershire, UK. Posts: 347	Re: Cermet (?) pot replacement? I have had problems with that style in the past and after removal changed them for open style Egen carbon track type. If you want enclosed track types then Piher types are a good replacement,I do not think the style you show is Cermet? from memory, once removed from board, the cause of the poor contact is often caused by tarnish on the wiper and its associated slip ring like arrangement,this can be cleaned with fine metal polish and the track with IPA etc,I think the pot is held together using a tiny C clip or suchlike.

5th Mar 2019, 8:59 pm	#3
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,274	Re: Cermet (?) pot replacement? Haven't seen any for a long time, the pins on those are close enough to accept a modern replacement, usually quite thick pins. I've had a few over the years where, when you unsolder them, the base part which forms the track falls into 2 pieces. Are they 300mW? __________________ Kevin

5th Mar 2019, 9:29 pm	#4
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Cermet (?) pot replacement? Ugh those things are cancerous. My telequipment stuff was full of them. Usually replace them with cheap enclosed ones https://www.bitsbox.co.uk/index.php?...roducts_id=713 If you need more power bourns 3386P are good but expensive.

6th Mar 2019, 7:21 am	#5
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,669	Re: Cermet (?) pot replacement? Don't think they're Cermet, just bog standard carbon. I have some used ones in my spares draw out of as Mr B says a Tele scope. Could see if there's a 100k one if any use. Andy. __________________ Curiosity hasn't killed this cat...so far.

6th Mar 2019, 3:43 pm	#6
GW4FRX Pentode Join Date: Aug 2005 Location: Shrewsbury, Shropshire, UK. Posts: 149	Re: Cermet (?) pot replacement? Many thanks for the replies, all very useful and illuminating. It's difficult to know whether to keep the pots in service for the sake of originality knowing that they're likely to be unreliable or change them for something modern which won't quite look the part. Such seems to be the conflict between conservation and restoration. In the meantime I'll happily reimburse postage or make a suitable contribution to a charity of choice if anyone happens to have a 100K lin languishing in the junk box looking for a good home. And no, this sort doesn't seem to be held together by a circlip or whatnot and it's rather difficult to see how it was assembled in the first place. Beastly thing!

6th Mar 2019, 4:07 pm	#7
MrBungle Dekatron Join Date: Jun 2016 Location: London, UK. Posts: 3,687	Re: Cermet (?) pot replacement? Did some digging in the junk box, err cupboard, and found one solitary one left on a spare TQ board I have lying around which was not 100k and sacrified it for the cause. Here's how it is constructed:

6th Mar 2019, 9:50 pm	#8
m0cemdave Octode Join Date: Feb 2009 Location: Bletchley, Buckinghamshire, UK. Posts: 1,223	Re: Cermet (?) pot replacement? That looks like a moulded track type, probably made by Plessey. I think I have a datasheet somewhere, will have a rummage.

6th Mar 2019, 10:35 pm	#9
m0cemdave Octode Join Date: Feb 2009 Location: Bletchley, Buckinghamshire, UK. Posts: 1,223	Re: Cermet (?) pot replacement? From an early 1970's Farnell catalogue: www.spamabyss.net/radio/MPpots.jpg Note that these could be moulded track carbon, wirewound, or cermet. Also various different pinouts - in addition to those on the page, I have seen some with a large threaded centre pin.

7th Mar 2019, 4:37 am	#11
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Cermet (?) pot replacement? I recognize these large format presets, or very very similar, with the same pinouts/geometry, they are still available from RS components, go there and search 186-205 a 47k version or 186-211 the 100k version (that one is on back order it says), but they are still current parts. They range from 100R to 1MR. They were Tyco 16mm PC910 Series.They are 1W rated. Have a look at page 2 of this article where I used one in an automotive voltage regulator: http://www.worldphaco.com/uploads/HI...CS_ARCHIVE.pdf

7th Mar 2019, 6:38 am	#12
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,669	Re: Cermet (?) pot replacement? Had a quick look, no 100k so far, but do have a small bag of 10T 100k LIN with a footprint which is near, EG the two back legs line up but the wiper leg would need bodging. These are also of less wattage, I'd say 1/4w. Will send you one if you drop us your address. As to reliability of the originals, the ones I have appear to be fine. I used a few on projects like PSU's with no issue's. Andy. __________________ Curiosity hasn't killed this cat...so far.