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Old 16th Sep 2019, 8:59 pm   #61
rambo1152
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Default Re: Yet Another Bush TV22

Quote:
Originally Posted by Panrock View Post
The video is actually very soothing to watch!
I've seen hum portrayed as accurate as that on virtual CRT monitors in animated gaming footage. Considering the people involved probably have yet to celebrate their 20th birthday, you've got to be impressed with them.
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Old 16th Sep 2019, 9:52 pm   #62
peter_scott
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Default Re: Yet Another Bush TV22

Quote:
Originally Posted by Alistair D View Post
Could the heater isolation transformer be causing a stray field?
Al
I think you may be onto something there Alistair.

Quote:
Originally Posted by peter_scott View Post
Looking at waveforms wrt ground I can't see any movement in the frame output drive or anode but if I look on the secondary of the output transformer then (wrt ground) the wiggles are clear to see.
Peter
My attention has moved to the scan coils and Alistair's comment about the HKS transformer. I have just unbolted it and by rotating it by 90 degrees I can make a little change to the frame scan. I think I may need to consider mounting the transformer somewhere else.

Alistair, you are doubly a hero. Firstly for rescuing me with the isolating transformer when I found myself with an HKS CRT and secondly for correctly identifying my silly positioning of it on the chassis causing myself and many folks here and VRat hours of puzzlement. I am very grateful to everyone but especially to you.

Very many thanks,

Peter

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Old 16th Sep 2019, 11:09 pm   #63
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Default Re: Yet Another Bush TV22

A convenient place (but a wrong place) for an HKS isolation transformer.
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Old 17th Sep 2019, 12:03 am   #64
Alistair D
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Default Re: Yet Another Bush TV22

Glad you got to the bottom of the problem.

Al
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Old 17th Sep 2019, 12:14 am   #65
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Default Re: Yet Another Bush TV22

I hadn't realized you had added a transformer nearby. Sometimes the radiated fields can be quite high, depending on the primary magnetization current and extend far from the transformer body. One trick if you are forced to put it close to sensitive circuitry or other transformers is to put a copper flux band on it and rotate it for minimal interference.

I once had a transistor set where there was interference from the stray magnetic field of the vertical deflection yoke, to a driver transformer core in the audio circuit.

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Old 17th Sep 2019, 1:01 am   #66
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Default Re: Yet Another Bush TV22

Glad you solved it Peter.
I was supprised it had an effect from so far away. I do notice though that if you look at your photo, the transformer is roughly in line with the scan coils, so maybe it is less a case of the beam being directly influenced, but more a case of the core of the scan coils being influenced by the transformer (presumably this would enhance the effect).

Cheers.
Andy
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Old 17th Sep 2019, 7:52 am   #67
peter_scott
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Default Re: Yet Another Bush TV22

Yes Andy, I'm pretty sure it was the scan coils coupling with the transformer because I could see the distortion on the frame output.

When I fitted it several years ago stray fields didn't occur to me. Last night I tried various orientations in the same location but none worked. This morning I have found that I can locate it under the main deck mounted on the wooden side member below the LOPT and I can't see any frame wiggles. This time I have checked that the mains is not in sync with my video source.

Thanks to all,

Peter

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Old 17th Sep 2019, 11:24 am   #68
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Default Re: Yet Another Bush TV22

Still got rubbish line linearity and uneven focus but I think I'll leave it that.

Peter
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Old 17th Sep 2019, 2:24 pm   #69
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Default Re: Yet Another Bush TV22

One interesting thing with this fault that virtually guaranteed that the ripple seen was not due to ripple voltage on the HT supply voltage; is that the width of each scanning line was unaffected. The scan width is proportional to the HT voltage (more so than the height which changes a lot less with changes in HT) so if you have enough ripple on the HT rail you will see the width of the scanning lines modulated and see the hum signal down the sides of the scanning raster.
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Old 17th Sep 2019, 3:08 pm   #70
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Default Re: Yet Another Bush TV22

Hi Peter,
It's noticed that the picture is compressed about the centre of the screen, the point where the transition from reclaimed energy from the flyback and conduction of the line output valve takes place. Have you adjusted the line drive trimmer? Also, it might be a good idea to 'scope the line drive on the PL38 grid.

DFWB.
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Old 17th Sep 2019, 5:06 pm   #71
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Default Re: Yet Another Bush TV22

Hi David,

I've had a fiddle with the line drive and it looks a bit better. I've added the line drive waveform. Junction of R31 & 32.

Peter
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Old 17th Sep 2019, 6:34 pm   #72
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Default Re: Yet Another Bush TV22

Hi Peter,

Just a little diversion, a while back you mentioned problems setting up the picture, centreing it and getting the ion trap set right, could it be the ion trap magnet isn’t as strong as it used to be? I had one once in a set, I forget which one it was now, where no matter what I did the picture was cut off in one corner, and a bit darker to one side, I also had to have it close to the base of the crt for it to work, I re-magnetised it with a big magnet and was then able to set the ion trap further up the neck, and cured the cut off corner too.

Regards
Lloyd
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Old 17th Sep 2019, 7:12 pm   #73
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Default Re: Yet Another Bush TV22

Thanks Lloyd,

I just tried another one that I had lying around and it may still need to be stronger but I think it has given me some improvement especially in overall focus.

You guys are great!

Peter
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Old 17th Sep 2019, 10:28 pm   #74
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Default Re: Yet Another Bush TV22

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Originally Posted by peter_scott View Post
Thanks Lloyd,

I just tried another one that I had lying around and it may still need to be stronger but I think it has given me some improvement especially in overall focus.

You guys are great!

Peter
Picture attached, out of focus and too much contrast.

When I finished my TV22 and optimized the H drive I did notice a zone of abnormal linearity where the damped current is fading away and the PL38 plate current is increasing. It caused a zone of flatness the circle perimeter as the rate of increase of yoke current with time reduced (or the beam deflection velocity slowed just a little in that zone) Picture attached to show where the defective area was between the green lines, giving it a slightly egg shape effect. Also just to the right of the right green line, seen by the rectangles, the scan is a little compressed there too. Not too severe, ok perhaps for a domestic TV but it wouldn't have cut it for a studio video monitor. Interesting that it is different from yours so some of the circuit constants mustn't be exactly the same.
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Old 17th Sep 2019, 11:17 pm   #75
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Default Re: Yet Another Bush TV22

I think I have some way to go (if I ever get there) before I get to the linearity of your screen shot. Since taking my previous screen shot I checked the value of the anode load of the line oscillator triode and found it to be 920k instead of 820k and got a slight improvement but it's still far from linear.

I also need to look at the frame linearity that currently defies adjustment despite a good range in the controls.

Peter
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Old 17th Sep 2019, 11:34 pm   #76
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Default Re: Yet Another Bush TV22

Looking at your raster, just in the zone of the first box(drawn by the white lines) on the left side about 10% into the line scan, it looks as though the linearity is crowded a little and the beam brightness increases there. I think there is an issue with the energy recovery by the damper, possibly the valve has low emission or the boost capacitor high ESR. Also it would be worth replacing that 2uF electro with another capacitor too if not done yet.

I don't have a standard scope recording yet of the H drive at the grid of the PL38, especially to check for example if the point at which the PL38 gets driven into grid current (and the drive wave is clipped) matches yours.

When I am able I am going to collect all the waveform data from this stage using my wide bandwidth 50kV capable probe, so as to document exactly what each waveform looks like on every circuit point, but I have not got around to it yet.
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Old 17th Sep 2019, 11:38 pm   #77
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Default Re: Yet Another Bush TV22

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Not too severe, ok perhaps for a domestic TV but it wouldn't have cut it for a studio video monitor.
Whilst I do love a nice Bush TV22, they have their limits. Have a look at the picture on any domestic PYE set however and you will see perfect linearity.

Cheers
Andy
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Old 17th Sep 2019, 11:58 pm   #78
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Default Re: Yet Another Bush TV22

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possibly the valve has low emission or the boost capacitor high ESR. Also it would be worth replacing that 2uF electro with another capacitor too if not done yet.
My boost cap is new and swapping my PZ30 appears to make no difference. I think I remember buying one of my PZ30s new and it certainly hasn't clocked many hours. I did rewind my line linearity transformer back in 2015 and perhaps I've adversely altered its characteristics.

Peter
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Old 18th Sep 2019, 1:05 am   #79
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Default Re: Yet Another Bush TV22

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Have a look at the picture on any domestic PYE set however and you will see perfect linearity.
Cheers
Andy
Andy,

I get pretty excited by good scan linearity. Have a look at the H & V scan linearity on this 14" monochrome monitor, scroll to page 20 of this article:

http://worldphaco.com/uploads/The_19...o_monitor..pdf

They managed this with three basic things, one was the outstanding design of the deflection yoke (made by a specialist company Syntronic Instruments) photo of it on page 19, it has correction magnets built into it. Then if that was not enough, they used both current & voltage feedback around the vertical deflection amplifier to help the linearity there.

(This is the monitor that uses Tektronix's mind blowingly clever gated negative feedback black level clamp. This is one of the few sets that also uses a UJT as the vert oscillator, but I did find one other, the VDU in the IBM5155 computer uses two transistors wired as a PUT to do it, I also get enthusiastic by oddball designs)
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Old 18th Sep 2019, 12:09 pm   #80
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Default Re: Yet Another Bush TV22

Still not quite there on the line scan but got improvement with a different combination of linearity transformer and line drive. The frame linearity was just leaky caps that I hadn't replaced. Picture centring is still on the margin but maybe if I can increase my ion trap magnetism I can get the improvement that Lloyd suggests.

Peter
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