I believe that there is a tendency to overlook the fact that, Test equipment, TV and Radio, all use basically the same resistors & caps as the old radio's of the era.

What that translates to is that they fail the same way. I have replaced caps in an AVO 7X, Peak 200H output range, Several Signal Generators and even a Heathkit GDO. The OS-1 Heathkit Oscilloscope was so bad and unstable that I wiped the board & replaced every component on it: That sorted it

There are resistors of the late fifties that are are about as stable as a bucket of weeping gelignite and its nothing unusual like the Heathkit, to dump the lot. It should be policy to not just replace power supply electrolytic caps but to replace all electrolytic caps, as not only do they dry out & loose capacity, the others do the same.

Testing can reveal horror stories. If you use the electronic device LCR-T4 (or similar) which is a very handy little widget (got one). Do make sure that the cap (any cap) is discharged fully. I have been reliably informed that not doing that, will kill it.

I have a policy now where if a NOS cap has been sitting a while, or I don't know its history. It is put on the reformer & gradually run up to as close as I can get to working volts. The bad ones rarely make it to the top.

There are ways of using the reformer for testing in circuit.

Well gents here’s the update.
All caps now changed - only a couple of resistors were out of spec. The rest were within 10% so they were left.
End result is the vertical stage is still a little odd with it vertically impossible to get the top linearity right. The picture is a little squashed at the top while the rest is ok. I’m not sure what could be changed component wise to give it more adjustment on the top linearity.
The Horizontal Output and HT was rectified by replacing the electrolytic caps along with the 6CM5, 6EA8 and the 1B3GT. Picture is no longer jittery either. There’s still a bit of blooming with the brightness and contrast but nothing serious.
I’ll be interested in any thoughts on the top linearity.
Cheers
Scott

Whilst I treated TV's like the plague & tried to avoid them, that did not always work. As a result I have not avoided acquiring a couple of TV servicing books and someone put out a TV quick fix guide that gave clues to what valve may be implicated, when thins started to go awry.

May be some info there if I can tear myself away from other jobs?

Marc

Hi Scott, the point now is to find if the non linear screen sweep is a function of the parts that determine the wave shape, or, is the output stage running non linear.

thoughts.

question: can you overscan the vertical sweep by adjusting the "height"?
thats another way of saying does the output valve have plenty in hand.
If the height control is flat out anyway, we have a problem.
usual reason, tired valve or, grid cap leaking causing overbias!

question: is the bias voltage 17v correct????

question: is the EHT set too high?
If you have heaps of CRT brilliance, the EHT may be able to set down in value a bit.
the "width" core in the EHT box should do that.
the horizontal may be overscanned a bit anyway
That reduces the amount of drive needed to sweep the screen vertically.
Then you can back off the height, reset the "top lin" and "lin" pots and get a better screen sweep.

question: are the vertical wave shapes as shown on the circuit?
harder to determine as you have work on a hot chassis with extender leads!
if the wave shape from the oscillator is not that funny peak and ramp shape then the sweep on the screen will never be linear.

the wave shape is that funny shape because the tube has coil sweep not plate sweep.
the coils are non linear from inductance hence a perfectly linear sweep drive would not produce a linear sweep and picture from the coils.

Hmmmmm.
Fred.

Hi Ian and Fred
Thanks, if you like, I can email you the schematic for the TV, I’m not overly experienced with TV though an fine with valve technology. It would also assist me in identifying if the set has the various aspects you mentioned above. We can then look at specific components.
Cheers
Scott

Hi Fred,

Thanks for you reply. Ok, to answer your questions:

Yes you can adjust the vertical height to over scan, though it has far more effect to the bottom of the picture rather than the top.
Difficult to check the 17v as the set can’t be operated when the chassis is in service position. Due to the design, the chassis has to be slid all the way to the rear of the cabinet to fold it down, all leads (inc CRT) are then too short.

CRT brilliance I wouldn’t say is ‘heaps’ but is reasonable, contrast is a little weak though.

The wave shape forms are hard to test for the above reason re the chassis access.

Cheers
Scott

Does the vertical tend to "squish" about 1/4 of the way down the raster from the top as you increase the height?

Or is it folding over at the very top?

I actually have one of these chassis on the bench right now! It was full of leaky UCC paper caps. And inexplicably both the 4th IF 6BX6 and the RF amp 6CW7 were down to air, with no obvious cause.....

Don't forget to ditch the two paper caps on the mains input, where the cord is terminated. An initial attempt at power-up on the dim bulb instantly tripped the safety switch!

Thanks Ian,
Yes, increasing the vertical height causes the top to fold over itself.
I’ll email you a video of it so you can see for yourself. This is about the best I could get the picture though you can see it’s brighter in the top part of the screen, shape is not great and in the scene with credits you can see the compression at the top.
Hope that’s ok and makes sense.
Cheers
Scott

Hi Scott, Ian is the man for this one with hands on experience!

The output valve is definitely either running out of linear swing, or, being fed an incorrect signal.
The trick is to find out which.
The fold over at the top indicates "clipping" like an audio amp overdriven.
The valve is supposed to have plenty of swing to spare and so does not approach saturation or cut off.

This situation you have is awkward, it would be nice to substitute a new valve and a new output tranny and a new yoke one at a time and see what happens!

You have to assume all these parts are ok at this point and the problem is either lack of HT or a waveshaping part out of spec.
As you tests and replacements indicate all these points are good and the observations indicate the valve is being operated non linear just to sweep the screen face, this is getting to the point where you would like to make up extender leads and work on a hot chassis with a CRO.

Lots of assumptions of good parts here!
I would hate the cause being one un replaced part in the circuit.
I would also hate it if the problem was the thermistor or the resistors on the yoke!

Fred.

Hi Fred,
The 6BM8 is new so not the valve, transformer is not original so not sure about that, though it does provide height and linearity but just issues at the top of the screen.
As for the YOKE those 150ohm resistors and the thyresistors, are they on the actual yoke and accessible?
I’m
Hoping to hear from Ian too as I emailed him
Cheers
Scott

Hi Scott the thermistor and resistors should be soldered in the yoke assembly.
As Ian says the vertical stage is not far off an audio amplifier with the LOPT just like an audio tranny and matching a fairly low impedance load (the yoke) to the valve. If the transformer or the yoke are not quite the match for some reason then the 6BM8 struggles.
I have no data at all on the parts just the circuit diagram in my JR field service manual.

I would be tempted to find the thermistor and temporary clip lead it out to see what difference that makes.
If the thermistor has moved to high ohms from age that will spoil the matching.
I do not know what the thing should measure no value on my circuit, although it has to be low in the single ohms at room temp.
Ian will know.

My circuit shows a VDR wired across the transformer primary, another unknown value item that could scupper the stage if faulty low ohms. I would expect that one to have open circuit at normal voltage and fire off to clip voltage surges over the rated value (?v)
One could try disconnecting one end to see what happens.

Every part is suspect until you test or replace all of them.

Fred.

The last comment of Fred's, does hold water. I am continuously pointing out, that as you transfer the waxed paper & worn out electrolytic caps to the bin, you test every resistor and any Mica or ceramic cap that ends up with one end free in the exercise. That saves a huge amount of frustration & rework.

That will, I have found, with radio's in practice over decades; that substantially increases the probability into the 90% range, of it firing up, with perhaps a few minor issues, or none.

Marc

Thanks Fred and Marc
Fred, I had a look at those 2 150ohm resistors in the yoke. They measure very low around 2.4 ohm but I still suspect that’s because they are in the circuit- I could clip them out and re measure them. The themister has me stumped as I have no idea what it’s supposed to be rated therefore testing could be an issue.

I’m hoping Ian may have added to this discussion at some point
Given his vast experience with these things.

Scott

I wrote an email but then the NBN went down! Back up again now.

Main thing is the cathode and screen bypasses.

What brand is the 6BM8? I've found that new replicas can be a little wanting when used in a vertical stage. Pye T21 case in point. You need to reduce the cathode resistor a bit.

From the video, and assuming you have replaced the electros mentioned, it could be that the transformer is a bad match to the yoke. Forget about the 150 ohm resistors by the way.

I have an Astor parts chassis that I think contains a suitable donor transformer. 110 degree, 6BM8 driving a series connected yoke like yours. You'll need to change the circuit though because it has no feedback winding.

Let me know about the electros, the cathode resistor and B+ dropper values.

Hi Ian
Welcome back!
Got to love the NBN…
Thanks for your reply, the Astor transformer sounds like a good solution.
I’m not all that experienced with TVs so my knowledge is a little limited. The 6BM8 is NOS Mullard.
As for the cathode & screen bypasses, I’ll need your guidance on what actual components I need to check. I can then identify them in the schematic and check them in the TV. I looked up an example of a cathode bypass cap and comparing it to the schematic, I think you’re referring to C218 with is .068μF and the associated electro is C221 100μF. Both these have been replaced with the correct values as per the schematic (.068 & 100μF ) the resistor R232 has been replaced with the correct 330ohm as per the spec. Prior replacement, there was another resistor in parallel with the 330 bringing the value to around 240 ohm.
Hope this makes sense.
Have you got the schematic? If not, here’s a link to it.

https://www.kevinchant.com/uploads/7/1/0/8/7108231/stc_t236_envoy.pdf

Cheers
Scott