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Old 3rd Oct 2019, 12:49 am   #101
FERNSEH
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Peter,
Re the TV32 frame timebase circuit. The frame sync coupling capacitor C14 will have on the left side plate the negative going flyback pulse from the oscillator valve anode.
The oscillator valve is only conductive during the flyback period, the rest of the duty cycle the valve is cut-off. C5 is discharged during the flyback and will charge during the forward scanning stroke.
The frame sync separator circuit employing an inductor to filter out the line sync pulses can be found on pages 307 and 308 of the March 1960 Practical Television magazine. The circuit employs two 6AM6 pentodes.

DFWB.
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Old 3rd Oct 2019, 7:57 am   #102
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Default Re: Bush TV22 with TC184 B3 converter.

If the frame oscillator free runs then there's no problem for the CRT in simply disconnecting the sync signal.

Peter
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Old 3rd Oct 2019, 12:09 pm   #103
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Default Re: Bush TV22 with TC184 B3 converter.

With the standard circuit I get poor/no interlace and first photo shows the sync signal disconnected from the oscillator in the top trace and connected in the bottom one.

With the filter capacitor changed from 0.001 to 0.002uF I get good interlace and the waveforms in the second photo.

Peter
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Old 3rd Oct 2019, 2:49 pm   #104
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Default Re: Bush TV22 with TC184 B3 converter.

Whilst the set can give good interlace, periodically it can lose it. It also appears to depend on the line hold setting!
The effects where it loses interlace are not video aliasing and I'm not making any adjustments throughout the video. A noisy component somewhere?

https://youtu.be/2R-vwATrK9s

Peter
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Old 3rd Oct 2019, 6:08 pm   #105
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Default Re: Bush TV22 with TC184 B3 converter.

"A noisy component somewhere?"

Hi Peter,
I often think the insulation properties of the component group boards is suspect. My TV22 is on the bench and later today I'll hook up the oscilloscope to various points in the sync separator and frame timebase. Let's see how they compare with your set. The 100K to 10K resistor mod hasn't been done yet.

DFWB.
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Old 3rd Oct 2019, 10:32 pm   #106
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Default Re: Bush TV22 with TC184 B3 converter.

Attachments show the integrated frame sync pulse across C13 and at the cathode of the Westector diode.

Second picture shows the frame sync pulse after setting the 'scope to delayed triggering.

DFWB.
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Old 3rd Oct 2019, 10:53 pm   #107
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Default Re: Bush TV22 with TC184 B3 converter.

Quote:
Originally Posted by peter_scott View Post
Whilst the set can give good interlace, periodically it can lose it. It also appears to depend on the line hold setting!
The effects where it loses interlace are not video aliasing and I'm not making any adjustments throughout the video. A noisy component somewhere?

https://youtu.be/2R-vwATrK9s

Peter
It's not surprising there is poor interlace and that it could be affected by the H hold control. After all there is no buffer stage on the sync step outputs for the V and H channels. A large H osc component is coupled onto the sync sep's load by the H oscillator.

As you can see from your scope recordings the V sync has been inadequately filtered and there are still excessive H serrations. As I noted on the previous posts, the filter/integrator has to be able to remove these serrations without compromising the fast rise if the leading edge of the pulse. It's poor design that is all and easily fixed.

Have a look again at the separated vert sync pulse in post #71, this is how it should look...do you see any serrations on it or the baseline voltage leading to it ?...they are there if you look closely but below a level that can trigger the V oscillator to start at the wrong time.

The problem stems from the fact that at the very point the grid voltage on the V osc has increased to about to bring the valve into conduction to start the next field, it only takes a very small voltage to trigger it and if it's not the rising edge of the V sync, it will lose interlace.
Of course this does include noise voltages (but that is not obvious on your scope trace it doesn't look noisy) and if there was an issue like leakage on the group boards, or poor power supply filtering to the V osc, that is not going to help.

It appears to me it is just the V sync filtering itself that has not been properly done. Just replace the network with RCA's standard 3 stage filter and it will likely fix it, attain a pulse as smooth with the fast rise of that shown in post#71.

Like I also mentioned before, sit down at a bench with a generator and composite sync signal & scope and build a few RC filters yourself with one, two and three stages of filtering to convince yourself it's a good idea.
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Old 3rd Oct 2019, 11:23 pm   #108
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Default Re: Bush TV22 with TC184 B3 converter.

More waveforms. Scope probe connected to the sync coupling capacitor C10 (0.01uF).
With a 405 line signal inputted and the frame hold locked the first attachment shows the waveform present at either end of C10. The second picture is with the scope set to delayed triggering.
The third picture shows the waveform at C10 with the frame oscillator stopped.
The sawtooth waveform is in fact the sync pulse, goes to show the excessive time constants in the frame sync circuits.
With no signal input the scope trace flat lines.

DFWB.
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Old 3rd Oct 2019, 11:43 pm   #109
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Default Re: Bush TV22 with TC184 B3 converter.

Yes, this is the problem with the TV22, gross over integration of the pulse. Though you might find as you decrease the resistor to 10k, the serrations appear in the sync pulse. I had estimated about 22k would be a reasonable compromise in post #70.

The problem is that if there are residual H serrations in the V sync signal , due to the fact that the signal is AC coupled into the V osc, the serrations are seen as sync pulses, so as soon as the V oscillator gets into a condition where it's sensitive to triggering for the next field a serrations triggers it and not the leading edge of the V sync pulse.

So both over integration and under integration can result in poor interlace.

In transistor sets the circuitry is often less problematic, the DC axis of the signal is often more easily maintained and slice levels are easy to set up, so often, interlace issues relating to aberrant triggering of the V osc are far less common.

Try replacing the diode with say a 15k resistor and change the 100k to 15k, and re-record the frame sync pulse, I would say it would probably be pretty good as it would be a reasonable two stage filter then.

Last edited by Argus25; 3rd Oct 2019 at 11:57 pm.
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Old 4th Oct 2019, 9:59 am   #110
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Default Re: Bush TV22 with TC184 B3 converter.

I think I've been too timid with the filtering in post #103. Below is the Hugo's RCA sync pulse with the two stage integrator that generated it. As before, I have shown the sync pulse without connection to the oscillator in the top trace and with it connected in the lower one. I needed to slow the scope timebase down by a factor of 10 to reproduce the RCA scope image.

And yes, with this heavily slugged sync pulse the interlace looks good. Unfortunately it doesn't stop my set from drifting out of interlace. I am beginning to wonder if this might be another case of mains interference. I've got 600mV of ripple on the HT.

Peter
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Old 4th Oct 2019, 11:04 am   #111
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Default Re: Bush TV22 with TC184 B3 converter.

I think I can confirm the the 600mV HT ripple is distorting the signal from the sync separator and that's causing the interlace drift.

Peter
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Old 4th Oct 2019, 11:57 am   #112
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Default Re: Bush TV22 with TC184 B3 converter.

That separated pulse looks very good to me.

Yes it is important that the V osc is protected from interference coming in on its HT line. Often there is just a series resistor and a 0.1uF filter cap, which was intended to kill off H pulses. If there is 50Hz ripple getting in, put a 4.7uF electrolytic cap, or similar, in parallel with the 0.1uF cap should cure it.
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Old 4th Oct 2019, 12:26 pm   #113
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Default Re: Bush TV22 with TC184 B3 converter.

The frame timebase circuit of the TV32 series does not employ an interlace diode and considering this chassis was also used in 17" models good interlace would be of prime importance. Integration components C16 and R12 have a time constant of only 10microseconds which is of course too short but it's possible that further integration takes place in the oscillator.
So the frame sync circuit of the TV22 will be rewired to the specification of the TV32. I will report back later how the results go.

DFWB.
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Old 4th Oct 2019, 1:10 pm   #114
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Default Re: Bush TV22 with TC184 B3 converter.

Hi David,
I will be interested to see how you get on. My 17" T36 uses the TV32 circuit, it is really bad at interlacing and the frame lock is poor. Maybe something else is wrong with it...

Cheers
Andy
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Old 4th Oct 2019, 1:12 pm   #115
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Andy, I've rewired the frame sync circuit in the TV22 to the much simpler arrangement used in the TV32.
Absolutely hopeless! Interlace is only possible at a critical setting of the frame hold control. Also, the frame hold is so hard that adjustment of the control alters the picture height.
So it's time to take a look at my TV32 and see what the picture looks like on that set.

DFWB.
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Old 4th Oct 2019, 1:18 pm   #116
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Default Re: Bush TV22 with TC184 B3 converter.

Hi all,
I've just modified my pet TV22. Yes, I did have it on the dining table.
I managed to find an ok old 10K 1/2W resistor, though the bands have been a bit scratched off (in the middle of picture 2, going across the edge of the tag board near the rear of the CRT bowl).
The result is good interlace and much much better frame lock, not that it was bad in the first place.
Prizes for guessing the TV show in picture 5.

Cheers
Andy
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Old 4th Oct 2019, 1:59 pm   #117
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Default Re: Bush TV22 with TC184 B3 converter.

Quote:
Originally Posted by FERNSEH View Post
Hi Andy, I've rewired the frame sync circuit in the TV22 to the much simpler arrangement used in the TV32.
Absolutely hopeless!
Hi David,

It is much better with the RCA filter 10k/0.005 + 10k/0.005 in post #110.

This has a time constant of 150uS so rather longer than we had been thinking.

I'd be interested to know if the TV22 also wafts in and out of interlace with the HT ripple like my TV32.

Peter
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Old 4th Oct 2019, 5:14 pm   #118
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Default Re: Bush TV22 with TC184 B3 converter.

Quote:
Originally Posted by FERNSEH View Post
Hi Andy, I've rewired the frame sync circuit in the TV22 to the much simpler arrangement used in the TV32.
Absolutely hopeless! Interlace is only possible at a critical setting of the frame hold control. Also, the frame hold is so hard that adjustment of the control alters the picture height.
As noted the reason for this is H serrations on the V sync pulse and the V osc is being triggered by those.

That TV32 circuit looks awkward in that there appears to be little decoupling of the V oscillator's power supply. So I would guess in that set, even after the V pulse filtering is corrected, unless the main power supply electros are in perfect order, there might still be some interlace issues. Perhaps this is one reason why this problem might have got worse as the set ages.

I don't think the addition of the diode in the TV22 is at all helpful and it's better replaced with a resistor to help make a 2 stage filter of the correct time constant. And at least in the TV22 it's easier to minimise the effects of power supply ripple by adding a capacitor across the existing 0.1uF.

One way to get the filter perfect in the TV22, is to replace the diode with one gang of a dual gang 50k pot and replace the 100k resistor with the other gang of the pot. Disable the oscillator and adjust the pot until the H serrations on the filtered pulse just disappear. Then, measure the pot and use resistors that are that value or the next standard value higher. Then at least you will have a good separated V sync pulse. Then if still issues, put a 4.7uF electro across the 0.1uF filter cap in the anode circuit of the V osc. After this, you will have done all you can.

Last edited by Argus25; 4th Oct 2019 at 5:29 pm.
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Old 4th Oct 2019, 8:14 pm   #119
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Default Re: Bush TV22 with TC184 B3 converter.

Finally got a chance to take a look at the TV32. The set takes ages to fully warm up and it was more than four minutes before anything appeared on the screen.
The TV32 still displays a bright picture although the MW36-44 CRT is showing signs of age. The tube is not the original.
The big surprise is that the interlace is satisfactory. Good interlace can be achieved irrespective of frame hold setting, no critical position.

DFWB.
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Old 4th Oct 2019, 10:34 pm   #120
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Default Re: Bush TV22 with TC184 B3 converter.

Hi David,

Mine is the same on warm-up but not on interlace. Could you possibly measure the ripple on your HT for me?
Do you find your interlace control does much?

Peter

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