What's the purpose of the screening can on the VHF tuner's valve - UK Vintage Radio Repair and Restoration Discussion Forum

Nickthedentist · 26th Mar 2018, 8:35 pm

Hello,

Sorry for a daft Q, but what's the purpose of the screening(?) can on the valve (ECC85 etc). in the VHF tuner of many 1950s radios?

I have just restored some Cossors; in two of them, it was missing, and didn't seem to make the blindest bit of difference. If anything, their valves stayed cooler and there was less tuning drift.

Many thanks,

Nick.

Herald1360 · 26th Mar 2018, 9:12 pm

May reduce LO radiation? Not perhaps a problem with only 88-100MHz coverage (except for PMR/police etc who used the 100-108MHz band), but the LO when a set's tuned to radio 2/3/4 will be inband on sets going to 108MHz.

Tractionist · 26th Mar 2018, 9:33 pm

Thanks Chris [and Nick for asking] ..... I've often wondered that too ......

toshiba tony · 26th Mar 2018, 9:46 pm

Also on some sets they even screened IF stages, personally I thought this was a bit of over use of shielding, but things were made much better then.

mark2collection · 26th Mar 2018, 9:59 pm

I guess they act much like the conductive coating on valves from the 30's/40's.

If the can has a spring inside, would this also reduce microphony?

I have an American tuner from the 1950's I'm itching to fix/write about & it's smothered in valves with screening cans! Removing them does make a difference, if you place you hand near the front-end with the cans removed, the set drifts off station ...

Mark

Radio Wrangler · 26th Mar 2018, 10:29 pm

Quote:

Originally Posted by Herald1360

May reduce LO radiation? Not perhaps a problem with only 88-100MHz coverage (except for PMR/police etc who used the 100-108MHz band), but the LO when a set's tuned to radio 2/3/4 will be inband on sets going to 108MHz.

Upsetting people in the top part of the broadcast band is not as serious as if you tune up the band and the LO goes over 108MHz. Those frequencies belong to instrument landing systems and are continuously monitored for interferers.

For many years the upper part of the broadcast band was kept empty of high power broadcasts so aviation radios stood a chance. Now you've got pop music on 107.9 and an ILS localiser on 108.1. Far too close for any meaningful filtering, the receiver front ends have to work with 30mW of interferer just off channel.

Receiver LO radiation can go in the channel.

David

26th Mar 2018, 8:35 pm	#1
Nickthedentist Dekatron Join Date: Jan 2005 Location: Oxford, UK. Posts: 17,858	What's the purpose of the screening can on the VHF tuner's valve Hello, Sorry for a daft Q, but what's the purpose of the screening(?) can on the valve (ECC85 etc). in the VHF tuner of many 1950s radios? I have just restored some Cossors; in two of them, it was missing, and didn't seem to make the blindest bit of difference. If anything, their valves stayed cooler and there was less tuning drift. Many thanks, Nick.

26th Mar 2018, 9:12 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: What's the purpose of the screening can on the VHF tuner's valve May reduce LO radiation? Not perhaps a problem with only 88-100MHz coverage (except for PMR/police etc who used the 100-108MHz band), but the LO when a set's tuned to radio 2/3/4 will be inband on sets going to 108MHz. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

26th Mar 2018, 9:33 pm	#3
Tractionist Heptode Join Date: Apr 2016 Location: Cheltenham, Gloucestershire, UK. Posts: 875	Re: What's the purpose of the screening can on the VHF tuner's valve Thanks Chris [and Nick for asking] ..... I've often wondered that too ...... __________________ Red to red, black to black. Throw the switch and stand well back!

26th Mar 2018, 9:46 pm	#4
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 978	Re: What's the purpose of the screening can on the VHF tuner's valve Also on some sets they even screened IF stages, personally I thought this was a bit of over use of shielding, but things were made much better then.

26th Mar 2018, 9:59 pm	#5
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 471	Re: What's the purpose of the screening can on the VHF tuner's valve I guess they act much like the conductive coating on valves from the 30's/40's. If the can has a spring inside, would this also reduce microphony? I have an American tuner from the 1950's I'm itching to fix/write about & it's smothered in valves with screening cans! Removing them does make a difference, if you place you hand near the front-end with the cans removed, the set drifts off station ... Mark