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.

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

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.
:beer:
Mark

Re: Another Bush TV12.
 

Very expert fault finding David!

Jac

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.

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.