What is a "Metrosil"?? - UK Vintage Radio Repair and Restoration Discussion Forum

Sean Williams · 8th Oct 2005, 9:13 pm

OK, Both the fergie and the cossor have these - what on earth are they? some sort of fast rectifier? some form of voltage dependent resistor?

Answers on a posting..........

Cheers
Sean

mickjjo · 8th Oct 2005, 9:29 pm

Yes, It is a voltage dependent resistor, an early form of semiconductor, There are some useful tips on substitution on this site:-

http://freespace.virgin.net/andy.valve/techa.htm

Regards, Mick.

.

dazzlevision · 9th Oct 2005, 8:02 am

Hello,

Unfortunately, the Metrosil referred to in the previous post was not used for EHT regulation but as a source of an A1 supply, so the substitution circuit is not relevant to this application.

The Metrosil (originally maufactured by Metropolitan-Vickers Ltd, I believe) used for EHT regulation in 1950s mono sets was a long grey rod, around 1cm in diameter, with silvering at both ends, to allow soldered connections. Ekco were also fond of using these - inside a nylon insulating tube.

The same sort of thing was used in Pye 691/693/697 hybrid 625 only colour sets as the focus control - only it was a Mullard type. The Mullard type may still be available from scrap Pye (or GEC/Sobell dual standard CTV) hybrid CTV set. A much shorter version was used in many colour sets in the focus control circuit, using a tap-off from the EHT tripler.

Remember thought, the EHT in a large screen CTV is 25kV but only around 15kV in the mono sets we are talking about, so you may have to shorten the effective length of the "colour" Metrosil accordingly.

What usually happens is the Metrosil material crumbles - particulalrly in the ferguson sets, where a metal clamping ring is fitted - towards the low voltage end of the device.

The set will work without its Metrosil but EHT regulation will be impaired.

Regards,

Dazzlevision

Sean Williams · 9th Oct 2005, 8:14 am

Hi All,

thanks for that - the metrosil in the two applications I have here so far is indeed to supply tha A1 feed to the CRT - the Cossor one seemed to be a little intermittent, but it might have been a dry joint, because touching wood since resoldering the legs we have a healthy supply!

The substitution circuit looks good though!

Cheers
Sean

8th Oct 2005, 9:13 pm	#1
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	What is a "Metrosil"?? OK, Both the fergie and the cossor have these - what on earth are they? some sort of fast rectifier? some form of voltage dependent resistor? Answers on a posting.......... Cheers Sean __________________ Engineers make things work and have spare bits when finished

8th Oct 2005, 9:29 pm	#2
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: What is a "Metrosil"?? Yes, It is a voltage dependent resistor, an early form of semiconductor, There are some useful tips on substitution on this site:- http://freespace.virgin.net/andy.valve/techa.htm Regards, Mick. .

9th Oct 2005, 8:02 am	#3
dazzlevision Dekatron Join Date: Sep 2003 Location: Near Swindon, North Wiltshire, UK. Posts: 3,613	Re: What is a "Metrosil"?? Hello, Unfortunately, the Metrosil referred to in the previous post was not used for EHT regulation but as a source of an A1 supply, so the substitution circuit is not relevant to this application. The Metrosil (originally maufactured by Metropolitan-Vickers Ltd, I believe) used for EHT regulation in 1950s mono sets was a long grey rod, around 1cm in diameter, with silvering at both ends, to allow soldered connections. Ekco were also fond of using these - inside a nylon insulating tube. The same sort of thing was used in Pye 691/693/697 hybrid 625 only colour sets as the focus control - only it was a Mullard type. The Mullard type may still be available from scrap Pye (or GEC/Sobell dual standard CTV) hybrid CTV set. A much shorter version was used in many colour sets in the focus control circuit, using a tap-off from the EHT tripler. Remember thought, the EHT in a large screen CTV is 25kV but only around 15kV in the mono sets we are talking about, so you may have to shorten the effective length of the "colour" Metrosil accordingly. What usually happens is the Metrosil material crumbles - particulalrly in the ferguson sets, where a metal clamping ring is fitted - towards the low voltage end of the device. The set will work without its Metrosil but EHT regulation will be impaired. Regards, Dazzlevision

9th Oct 2005, 8:14 am	#4
Sean Williams Dekatron Join Date: Feb 2005 Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW Posts: 3,517	Re: What is a "Metrosil"?? Hi All, thanks for that - the metrosil in the two applications I have here so far is indeed to supply tha A1 feed to the CRT - the Cossor one seemed to be a little intermittent, but it might have been a dry joint, because touching wood since resoldering the legs we have a healthy supply! The substitution circuit looks good though! Cheers Sean __________________ Engineers make things work and have spare bits when finished