Another Bush TV12. - UK Vintage Radio Repair and Restoration Discussion Forum

FERNSEH · 3rd Feb 2020, 6:40 pm

My "Birmingham" Bush TV12 had a no sync fault. Tried another EF50 sync separator valve but no improvement. The 'scope confirmed that the full video vaveform was present at the grid pin of the sync valve but closer observation showed that the sync tips weren't clamped to chassis potential.
As a quick test a silicon diode was connected between the grid of the EF50 and chassis, sure enough the sync tips are now at chassis potential. Turned out the control grid contact on the valveholder is broken. A replacement has been fitted and now the set displays a steady picture, but there are still a number of other faults to sort out.
The first picture shows the high quality ceramic B9G valveholder. Seems a pity to use it for such a mundane task as in the sync separator.

DFWB.

high_vacuum_house · 3rd Feb 2020, 9:12 pm

I am wandering if the unusually high quality valve holder and the EF50 were war surplus Components that were to hand rather than producing new valveholders from scratch.

Christopher Capener

mark pirate · 4th Feb 2020, 8:07 am

Those paxolin valve holders can be troublesome, not had any problems with them on my TV12, but replaced a few on an LV20 due to corrosion.

I do like the spring clips fitted on Pye sets, they really get a firm hold on those EF50's

Pye obviously learned a few things when using EF50's in radar during the war.
I think the pin length was a design flaw on an otherwise excellent valve.

Mark

Jac · 4th Feb 2020, 10:20 am

Very expert fault finding David!

Jac

FERNSEH · 5th Feb 2020, 11:22 am

Hello Jac,
the TV12B has two other faults to correct. As the attachment shows the vertical linearity could be better and the vision alignment needs attending to. Sound on vision and over emphasis of the vision HF component.

I'm considering replacing the MW22-14 CRT with the ion trap MW22-16. The original tube has evidence of ion burn and as this set has almost 7KV EHT available I'm certain there will be a worthwhile improvement in picture quality by fitting the later tube.

DFWB.

FERNSEH · 5th Feb 2020, 5:01 pm

Circuit diagram of the frame timebase. V3 (EF50) functions as a Miller transiton oscillator.

From r-type.org: http://www.r-type.org/articles/art-135.htm

The foldover might be caused by the changes in the values of the components associated with C2. Flyback time too long.

DFWB.

3rd Feb 2020, 6:40 pm	#1
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Another Bush TV12. My "Birmingham" Bush TV12 had a no sync fault. Tried another EF50 sync separator valve but no improvement. The 'scope confirmed that the full video vaveform was present at the grid pin of the sync valve but closer observation showed that the sync tips weren't clamped to chassis potential. As a quick test a silicon diode was connected between the grid of the EF50 and chassis, sure enough the sync tips are now at chassis potential. Turned out the control grid contact on the valveholder is broken. A replacement has been fitted and now the set displays a steady picture, but there are still a number of other faults to sort out. The first picture shows the high quality ceramic B9G valveholder. Seems a pity to use it for such a mundane task as in the sync separator. DFWB. Attached Thumbnails

3rd Feb 2020, 9:12 pm	#2
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,935	Re: Another Bush TV12. I am wandering if the unusually high quality valve holder and the EF50 were war surplus Components that were to hand rather than producing new valveholders from scratch. Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

4th Feb 2020, 8:07 am	#3
mark pirate Dekatron Join Date: Mar 2008 Location: Worthing, West Sussex, UK Posts: 5,185	Re: Another Bush TV12. Those paxolin valve holders can be troublesome, not had any problems with them on my TV12, but replaced a few on an LV20 due to corrosion. I do like the spring clips fitted on Pye sets, they really get a firm hold on those EF50's Pye obviously learned a few things when using EF50's in radar during the war. I think the pin length was a design flaw on an otherwise excellent valve. Mark

4th Feb 2020, 10:20 am	#4
Jac Heptode Join Date: Jun 2006 Location: Eindhoven, Netherlands. Posts: 642	Re: Another Bush TV12. Very expert fault finding David! Jac

5th Feb 2020, 11:22 am	#5
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Another Bush TV12. Hello Jac, the TV12B has two other faults to correct. As the attachment shows the vertical linearity could be better and the vision alignment needs attending to. Sound on vision and over emphasis of the vision HF component. I'm considering replacing the MW22-14 CRT with the ion trap MW22-16. The original tube has evidence of ion burn and as this set has almost 7KV EHT available I'm certain there will be a worthwhile improvement in picture quality by fitting the later tube. DFWB. Attached Thumbnails

5th Feb 2020, 5:01 pm	#6
FERNSEH Dekatron Join Date: Mar 2005 Location: Gateshead, Tyne and Wear, UK. Posts: 7,444	Re: Another Bush TV12. Circuit diagram of the frame timebase. V3 (EF50) functions as a Miller transiton oscillator. From r-type.org: http://www.r-type.org/articles/art-135.htm The foldover might be caused by the changes in the values of the components associated with C2. Flyback time too long. DFWB. Attached Thumbnails