CRM 121 Focus Problem - UK Vintage Radio Repair and Restoration Discussion Forum

Niechcial,Steve · 22nd Oct 2019, 9:16 pm

This tube is fitted to a Decca 101 using a permanent magnet focus assembly. The focus is user adjustable by a knob on the back of the set.
The problem is that from cold, the focus needs progressively adjusting every few minutes until after an hour it is finally stable. Starting again from cold means either winding the focus magnet back again or waiting the hour until the focus comes in. My first thought was that the EHT was going up or down, but using an EHT meter showed that was not the case. The emission on the tube is not bad and the brightness and contrast are stable.
I'm wondering whether I might do some good by stripping off a bit of cathode with the CRT rejuvenator or whether this is some other gun fault. All suggestions welcomed!

Freya · 22nd Oct 2019, 9:25 pm

My thought would be the tube is on the soft side, not much help. What's the getter looking like.

FERNSEH · 22nd Oct 2019, 9:49 pm

Hi Steve,
The Decca 101 has permanent magnet focussing, is that correct?
TV chassis is similar to the Regentone Big 12.

DFWB.

Sideband · 22nd Oct 2019, 10:20 pm

These Mazda tubes have been dubbed 'airbags'....they go soft over time. When you say the tube is 'not bad' what does that mean? If the emission is not optimal (I doubt it is with a 60 year old CRT) then you may well get focus errors if the cathode is 'lazy' and takes time get to full emission. It will also be caused by molecules of gas (air) that is impairing the vacuum.

Rejuvenation may well help but it has to be accepted that it may only be a temporary cure.

Niechcial,Steve · 22nd Oct 2019, 11:09 pm

Thanks guys. Yes it is permanent magnet focussing. The getter is nice and silver and there is no sign of a blue glow but I guess could still be a bit soft.

Heatercathodeshort · 25th Oct 2019, 6:23 pm

Typical Mazda CRM121 symptoms Steve. The chassis in the Decca is the popular Plessey that can be used with many tube types. It has 2v, 4v and 6.3v heater tappings on the mains transformer marked in black. The focus unit has a wide adjustment.

We have a similar receiver, a Regentone 'BIG12' at the museum that has the infamous CRM121. The emission is good with a nice getter but the picture is poor. Critical focus and just generally lacking in contrast and brightness. When time permits I will probably fit a Mullard MW31-18/74 or maybe something more exotic!

The HT line is 250v, just about enough for the A1 but it can be easily derived direct from the mains with a silicon diode and a bit of smoothing. Quite a rare model. Regards, John.

Niechcial,Steve · 26th Oct 2019, 1:51 pm

Thanks John. I've got an MW and I will probably go that route. If more A1 would b benefit there is always the possibility of a simple voltage doubler off the mains!

FERNSEH · 26th Oct 2019, 4:28 pm

HKS wrote: "The HT line is 250v, just about enough for the A1 but it can be easily derived direct from the mains with a silicon diode and a bit of smoothing. Quite a rare model. Regards, John."
Mullard recommend that the first anode should be >150volts above the cathode voltage. I believe that in Regentone Big 12 sets which are fitted with a Mullard MW31-16 CRT the cathode voltage is reduced by a potential divider.

DFWB.

Niechcial,Steve · 5th Nov 2019, 9:38 pm

Here are a couple of pics of the Decca DR101. It was the first old telly I bought- back in 1985 before the days of converters so I used to run it from a vcr with 405 tapes through a home built modulator. The cabinet is in lovely condition and when I got it it had the Decca turret tuner conversion fitted. I took that out, but I may put it back. The tube actually seems to be improving a bit with use. The band across the test card is of course a camera artefact.

FERNSEH · 6th Nov 2019, 1:50 am

From the 1957 Mullard Maintenance Manual.
Obtaining the first anode voltage for tetrode CRTs.

DFWB.

Heatercathodeshort · 6th Nov 2019, 11:36 am

That tube looks very good from the picture.John.

22nd Oct 2019, 9:16 pm	#1
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	CRM 121 Focus Problem This tube is fitted to a Decca 101 using a permanent magnet focus assembly. The focus is user adjustable by a knob on the back of the set. The problem is that from cold, the focus needs progressively adjusting every few minutes until after an hour it is finally stable. Starting again from cold means either winding the focus magnet back again or waiting the hour until the focus comes in. My first thought was that the EHT was going up or down, but using an EHT meter showed that was not the case. The emission on the tube is not bad and the brightness and contrast are stable. I'm wondering whether I might do some good by stripping off a bit of cathode with the CRT rejuvenator or whether this is some other gun fault. All suggestions welcomed!

22nd Oct 2019, 9:25 pm	#2
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,990	Re: CRM 121 Focus Problem My thought would be the tube is on the soft side, not much help. What's the getter looking like. __________________ Stephen _________"It`s only an old telly" ___

22nd Oct 2019, 9:49 pm	#3
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: CRM 121 Focus Problem Hi Steve, The Decca 101 has permanent magnet focussing, is that correct? TV chassis is similar to the Regentone Big 12. DFWB.

22nd Oct 2019, 10:20 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,577	Re: CRM 121 Focus Problem These Mazda tubes have been dubbed 'airbags'....they go soft over time. When you say the tube is 'not bad' what does that mean? If the emission is not optimal (I doubt it is with a 60 year old CRT) then you may well get focus errors if the cathode is 'lazy' and takes time get to full emission. It will also be caused by molecules of gas (air) that is impairing the vacuum. Rejuvenation may well help but it has to be accepted that it may only be a temporary cure. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

22nd Oct 2019, 11:09 pm	#5
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: CRM 121 Focus Problem Thanks guys. Yes it is permanent magnet focussing. The getter is nice and silver and there is no sign of a blue glow but I guess could still be a bit soft.

25th Oct 2019, 6:23 pm	#6
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: CRM 121 Focus Problem Typical Mazda CRM121 symptoms Steve. The chassis in the Decca is the popular Plessey that can be used with many tube types. It has 2v, 4v and 6.3v heater tappings on the mains transformer marked in black. The focus unit has a wide adjustment. We have a similar receiver, a Regentone 'BIG12' at the museum that has the infamous CRM121. The emission is good with a nice getter but the picture is poor. Critical focus and just generally lacking in contrast and brightness. When time permits I will probably fit a Mullard MW31-18/74 or maybe something more exotic! The HT line is 250v, just about enough for the A1 but it can be easily derived direct from the mains with a silicon diode and a bit of smoothing. Quite a rare model. Regards, John.

26th Oct 2019, 1:51 pm	#7
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: CRM 121 Focus Problem Thanks John. I've got an MW and I will probably go that route. If more A1 would b benefit there is always the possibility of a simple voltage doubler off the mains!

26th Oct 2019, 4:28 pm	#8
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: CRM 121 Focus Problem HKS wrote: "The HT line is 250v, just about enough for the A1 but it can be easily derived direct from the mains with a silicon diode and a bit of smoothing. Quite a rare model. Regards, John." Mullard recommend that the first anode should be >150volts above the cathode voltage. I believe that in Regentone Big 12 sets which are fitted with a Mullard MW31-16 CRT the cathode voltage is reduced by a potential divider. DFWB.

5th Nov 2019, 9:38 pm	#9
Niechcial,Steve Hexode Join Date: Jan 2007 Location: Beckenham, London, UK. Posts: 373	Re: CRM 121 Focus Problem Here are a couple of pics of the Decca DR101. It was the first old telly I bought- back in 1985 before the days of converters so I used to run it from a vcr with 405 tapes through a home built modulator. The cabinet is in lovely condition and when I got it it had the Decca turret tuner conversion fitted. I took that out, but I may put it back. The tube actually seems to be improving a bit with use. The band across the test card is of course a camera artefact. Attached Thumbnails

6th Nov 2019, 1:50 am	#10
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: CRM 121 Focus Problem From the 1957 Mullard Maintenance Manual. Obtaining the first anode voltage for tetrode CRTs. DFWB. Attached Thumbnails

6th Nov 2019, 11:36 am	#11
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: CRM 121 Focus Problem That tube looks very good from the picture.John.