To stuff, or not to stuff? - UK Vintage Radio Repair and Restoration Discussion Forum

SurreyNick · 21st Sep 2014, 2:52 pm

To stuff, or not to stuff? That is the Question.

To put it bluntly, I’m in a quandary.

Pictured is a vintage Telsen Mica Capacitor, from c.1930s, deconstructed. The set I am building employs six of these little baseboard mount caps of varying values. Some of those I have still have the values impressed in the pitch within which the capacitor is embedded, others don’t. The pitch itself, in all cases, has become crystalline and very brittle and so will need replacing. The question in my mind though is, at the same time as renewing the pitch, should I replace the capacitor for a more modern equivalent?

I know these Mica capacitors have a reputation for being very reliable, and I know how to test if they can still hold a charge but, in all probability, they have sat unused for fifty-plus years and I’m worried they might fail under use, or become leaky. Furthermore, for those where the values have been lost, is there a way to establish the value? My test gear is limited to an AVO8 and a DMM. And with my limited gear, is there a way to establish their efficacy (for example through some sort of soak test)?

With so many questions the easiest thing to do would be just to put in new caps. But I have deliberately sought out vintage components and, call it nostalgia or sentimentality, part of me wants to use these components in original form wherever possible. BUT once built and connected together I don’t want to have to deconstruct the set to replace a part that has gone faulty because it was likely to, and it could and should have been upgraded.

So there we have it. To stuff, or not to stuff? That is the Question.

I don’t have sufficient experience to judge, and hope those with more knowledge and experience can share their thoughts on these questions.

If the recommendation is to replace, what type of cap would you recommend? The ceramic disc type as pictured in the second photo, or some other type? The caps will be embedded, probably in Milliput epoxy putty, and the highest value will be 0.1uF.

Thanks

Nick

21st Sep 2014, 4:15 pm

As these where mostly used for coupling/decouping a ceramic would do fine, don't use epoxy it is hard and can break components when it sets and shrinks a bit. Try and find some pitch, you could scrounge a bit from a road resurfacing crew, take a tin with you.

Radio Wrangler · 21st Sep 2014, 6:03 pm

Black, non-acetic silicone rubber would also look right and be easily removed if ever necessary.

David

PJL · 21st Sep 2014, 6:38 pm

These mica caps generally don't go wrong but its worth testing for capacitance (does your DMM measure capacitance) and leakage (put it in series with a 10K resistor and measure the current when put across the HT).

They are hand made and tolerance will have been very wide from new, +100% is not unusual. Its unlikely you have a 0.1uF, they are typically 300pF (marked .0003) or a few 1000pF (marked .001).

I am just stuffing one which measured no capacitance, had a loose bolt and someone had speared the wax probably with a plan to see if they could take it apart.

Diabolical Artificer · 21st Sep 2014, 6:49 pm

Nick its up to you and depends on lots of variables. Like rarity of set, value and above all your working practices. Some folk change a few caps to get a set working,others strip a set back to the chassis and re-build. Most of us fall in somewhere in-between.

I know from your previous posts that you've been thinking about this some time. Forgive me but maybe you need to focus on the whole set, rather than one cap.

Just my penny's worth, Andy,

SurreyNick · 21st Sep 2014, 7:45 pm

It's a fair point Andy, and yes, I have been thinking about this for some time.

I am in fact working on two projects: A full restoration of a 1927 Cossor Melody Maker, which I intend to rebuild as original and authentic as possible; and this other project, which is a homebrew.

Its a three-valve TRF and I am clear upon what I am aiming for with this set. It's a juxtaposition of original 1920s/1930s vintage components mounted on the surface a contemporary shallow cabinet, within which all the ugly stuff such as the wiring, tuning drive etc. is hidden. As to value... irrelevant, as it's a gift for my wife. As to rarity... of components, high, otherwise irrelevant. Priority?... reliability and efficacy, as there is no point in gifting it to the wife if it breaks down or doesn't work well.

With this in mind I am happy to upgrade the caps. I've had to fabricate some components (such as the coil) and had to repair others too (such as the HF Choke), so it's not essential if these caps are 'untouched'. If they were likely to be good and reliable I would have left them alone, but if they are likely to vary in tolerance as much as stated then I'll put new caps in. I wouldn't do this on the restoration of the Melody Maker though. With that restoration I will keep looking until I find useable components.

My thanks to the thread contributors. I will stuff, using the materials suggested.

Nick

21st Sep 2014, 2:52 pm	#1
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	To stuff, or not to stuff? To stuff, or not to stuff? That is the Question. To put it bluntly, I’m in a quandary. Pictured is a vintage Telsen Mica Capacitor, from c.1930s, deconstructed. The set I am building employs six of these little baseboard mount caps of varying values. Some of those I have still have the values impressed in the pitch within which the capacitor is embedded, others don’t. The pitch itself, in all cases, has become crystalline and very brittle and so will need replacing. The question in my mind though is, at the same time as renewing the pitch, should I replace the capacitor for a more modern equivalent? I know these Mica capacitors have a reputation for being very reliable, and I know how to test if they can still hold a charge but, in all probability, they have sat unused for fifty-plus years and I’m worried they might fail under use, or become leaky. Furthermore, for those where the values have been lost, is there a way to establish the value? My test gear is limited to an AVO8 and a DMM. And with my limited gear, is there a way to establish their efficacy (for example through some sort of soak test)? With so many questions the easiest thing to do would be just to put in new caps. But I have deliberately sought out vintage components and, call it nostalgia or sentimentality, part of me wants to use these components in original form wherever possible. BUT once built and connected together I don’t want to have to deconstruct the set to replace a part that has gone faulty because it was likely to, and it could and should have been upgraded. So there we have it. To stuff, or not to stuff? That is the Question. I don’t have sufficient experience to judge, and hope those with more knowledge and experience can share their thoughts on these questions. If the recommendation is to replace, what type of cap would you recommend? The ceramic disc type as pictured in the second photo, or some other type? The caps will be embedded, probably in Milliput epoxy putty, and the highest value will be 0.1uF. Thanks Nick Attached Thumbnails

21st Sep 2014, 4:15 pm	#2
Guest Guest Posts: n/a	Re: To stuff, or not to stuff? As these where mostly used for coupling/decouping a ceramic would do fine, don't use epoxy it is hard and can break components when it sets and shrinks a bit. Try and find some pitch, you could scrounge a bit from a road resurfacing crew, take a tin with you. Last edited by Guest; 21st Sep 2014 at 4:16 pm. Reason: spelling (not an exhaustive search)

21st Sep 2014, 6:03 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,877	Re: To stuff, or not to stuff? Black, non-acetic silicone rubber would also look right and be easily removed if ever necessary. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

21st Sep 2014, 6:38 pm	#4
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: To stuff, or not to stuff? These mica caps generally don't go wrong but its worth testing for capacitance (does your DMM measure capacitance) and leakage (put it in series with a 10K resistor and measure the current when put across the HT). They are hand made and tolerance will have been very wide from new, +100% is not unusual. Its unlikely you have a 0.1uF, they are typically 300pF (marked .0003) or a few 1000pF (marked .001). I am just stuffing one which measured no capacitance, had a loose bolt and someone had speared the wax probably with a plan to see if they could take it apart.

21st Sep 2014, 6:49 pm	#5
Diabolical Artificer Dekatron Join Date: Jul 2009 Location: Sleaford, Lincs. UK. Posts: 7,662	Re: To stuff, or not to stuff? Nick its up to you and depends on lots of variables. Like rarity of set, value and above all your working practices. Some folk change a few caps to get a set working,others strip a set back to the chassis and re-build. Most of us fall in somewhere in-between. I know from your previous posts that you've been thinking about this some time. Forgive me but maybe you need to focus on the whole set, rather than one cap. Just my penny's worth, Andy, __________________ Curiosity hasn't killed this cat...so far.

21st Sep 2014, 7:45 pm	#6
SurreyNick Heptode Join Date: Apr 2014 Location: Wigton, Cumbria, UK. Posts: 728	Re: To stuff, or not to stuff? It's a fair point Andy, and yes, I have been thinking about this for some time. I am in fact working on two projects: A full restoration of a 1927 Cossor Melody Maker, which I intend to rebuild as original and authentic as possible; and this other project, which is a homebrew. Its a three-valve TRF and I am clear upon what I am aiming for with this set. It's a juxtaposition of original 1920s/1930s vintage components mounted on the surface a contemporary shallow cabinet, within which all the ugly stuff such as the wiring, tuning drive etc. is hidden. As to value... irrelevant, as it's a gift for my wife. As to rarity... of components, high, otherwise irrelevant. Priority?... reliability and efficacy, as there is no point in gifting it to the wife if it breaks down or doesn't work well. With this in mind I am happy to upgrade the caps. I've had to fabricate some components (such as the coil) and had to repair others too (such as the HF Choke), so it's not essential if these caps are 'untouched'. If they were likely to be good and reliable I would have left them alone, but if they are likely to vary in tolerance as much as stated then I'll put new caps in. I wouldn't do this on the restoration of the Melody Maker though. With that restoration I will keep looking until I find useable components. My thanks to the thread contributors. I will stuff, using the materials suggested. Nick