Thanks! Saved me a walk downstairs for my hard copy!

The STC I have on the bench is not the same but very similar in many respects.

Did you try C220 and C222? If not, tack a 100μF 350v across each of them to check.

VDRs rarely fail but it is possible, or a wrong one may have been fitted. Try temporarily disconnecting VDR 2. Turn the height down first and bring it up again carefully in case it was the problem. If it makes a big difference when out of circuit, a 22nF in series with 10k makes a suitable snubber to replace it. It's needed to protect the 6BM8 and its socket from high retrace voltages.

The extra resistor you mentioned would have been added to try to correct the fault by biassing the valve further down the curve.

That NOS Mullard 6BM8 must have cost you a motza! The 6BM8 is an audiophool valve. The much cheaper Czech and Chinese ones work but P10 will need to be wound up a little.

Thanks Ian
I replaced both C220 and C222. C220 is now 100μF 450V and C222 is again 8μF at 450V
I’ll check VDR2 tomorrow. The NOS 6BM8 was part of a bunch of valves given to me by a friend downsizing .
I’ll report back tomorrow
Cheers
Scott

I had no idea the 6BM8 was in any way rare. I have so many (used) that they are all thrown in a box with the 6BX6s, 6BL8s, 1S2A and other common TV valves which will probably never get used.

I know they were also used for audio, but really, they are (or were) a very common valve.

Hi Ian,
Well I removed VDR2, no real change. The height set to minimum bring top and bottom in as you would expect. I could adjust the Lin and top Lin for an even but compressed picture. Increasing the height takes the bottom of the picture down but the top moves but not as much or as evenly as the bottom - as the VDR was out of circuit I didn’t increase the height too much but enough to get the results described above. So it still appears to be an issue with the top of the picture.
Another thing I noticed is the picture blooms a bit and focus is a little ordinary at high brightness, also, when the volume is turned up and down rapidly the pic blooms with that too. It could be a supply issue too

Any ideas?

Cheers
Scott

Lots of ideas, but…… very hard when chassis is not right in front of you.
I know some other experienced people are assisting, but one thing I noticed, you mentioned a resistor was placed parallel to the correct value on the cathode, vertical output.
Well may have been done when the not correct output transformer was replaced, and a technicians attempt to get the linearity right.
Makes sense to me.
As mentioned before, I think you have numerous problems.(Faulty Bits).
JJ

The only potentially defective part is the vertical output transformer. All other components have been either tested and confirmed ok and anything defective replaced.
Cheers

Still lots of possibilities.
Yoke wrong for tube or the other way around.
Blooming and other regulation problems do not indicate faulty vertical output transformer.
Weak 6BM8, possibly but I’m sure you have tried several.
These sets are old, and the history of repairs may not be what you want to hear.
I remember a saying, “as long as it moves and talks it’s ok”.
And also remember faults as you have described, but do not want set to be removed for workshop investigation, because they may miss “days of our lives” or “the young and breastless”.
JJ

I wonder if the wrong secondary winding was used, i.e swapped around?

What is the DC resistance of each winding, with the yoke unplugged?

Thanks
Do you mean on the vertical output transformer?
Scott

Hi Scot, that would be the yoke.
The windings halves are symmetrical so should have equal inductance and resistance each side.
Be nice if you could measure BOTH values, the suspicion would be shorted turns on one coil.
That would give you the symptoms you have.

With every thing else covered the only next thing to do is substitute the output transformer.
Crap, we are back where we started!!!

Thought, the secondary and plate primary are symmetrical but I note the third winding, the brown yellow and orange wires.
The yellow wire tap is offset not centre tapped. The suspicion is the brown and orange are swapped or wrong colour coded?
Possibly an ohms test will show which end is which in relation to the tap.

Fred.

Thanks Fred
I measured the Yoke the other day. With the Vertical output to it unplugged each coil measured 2.4 ohms (across the 150ohm resistors on the yoke). So that would seem to be ok.
Cheers
Scott

No, yoke is OK. I can see that from the video.

What I meant was to check the DC resistance of all 3 windings on the transformer. Maybe the yoke drive secondary and the feedback secondary have been swapped, resulting in a mismatch from the 6BM8 to the yoke. Think of it as trying to drive a car uphill in too high a gear.

It might also be possible to connect the feedback winding in series with the plate winding and derive the feedback by more conventional means.

Cool.
I’ll check it out when I’m back in the workshop next week.
I’ll also email you a pic of the transformer then as well.
Cheers
Scott

Hi Scot, well it looks like the focus is on the output tranny.
The original may have failed and a tech has put on the latest at the time substitute.
So as Ian points out the winding useage is under suspicion.
the tech may have just replaced colour by colour, it worked, and closed the job card off.
While it works, the winding spec, the turns, inductance etc etc may not be optium and it may never work correctly.
Agree with Ian, to look very closely at which winding is doing what.
I would expect the plate primary to have have a higher DC resistance, the yoke secondary a lot lower. (just like an audio tranny).
The feedback winding I dont know but should be obvious as it is tapped.
Fred.

Hi Scot, well it looks like the focus is on the output tranny.
The original may have failed and a tech has put on the latest at the time substitute.
So as Ian points out the winding useage is under suspicion.
the tech may have just replaced colour by colour, it worked, and closed the job card off.
While it works, the winding spec, the turns, inductance etc etc may not be optium and it may never work correctly.
Agree with Ian, to look very closely at which winding is doing what.
I would expect the plate primary to have have a higher DC resistance, the yoke secondary a lot lower. (just like an audio tranny).
The feedback winding I dont know but should be obvious as it is tapped.
Fred.