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Old 29th Sep 2019, 9:47 am   #81
Argus25
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Default Re: Bush TV22 with TC184 B3 converter.

Quote:
Originally Posted by peter_scott View Post
I have just changed R52 (on the Trader Sheet) from 100k to 10k and it looks like that is all that is required to obtain interlacing. If anyone is wondering why alternate lines are dim this simply the persistence of the phosphor and the camera shutter not being open long enough to average out the light from the two scans. In the first photo the rightmost image of the group of three is the TV22 with 10k. I've added a close-up of the 10k photo. Then another of the 100k.

Peter
It's a wonder the Engineers at Bush didn't notice this at the time. However, I suspect that on a brand new set where all of the capacitors in the set were in perfect condition, and H pulses on the power supply rails were at a minimum, it just might have been a marginal condition and sometimes not had the line pairing.

So probably in their factory prototype sets they never spotted the problem.

Later, even if they did realise the 100k value was way too high, they were not exactly going to call all the sets back in and draw attention to the defect, which the average customer would never have spotted on a screen this size.

Then it gets noticed again nearly 60 years later.

It very much reminds me of an issue that remained hidden in Atari's Pong arcade game for over 40 years. It produced a spooky effect at times in game play and it was caused by the PCB tracks not matching the designer's schematic. It got called "The Ghost in the Machine bug" (an I.Robot reference) and was overlooked, even by the designer, at the time.
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Old 29th Sep 2019, 12:06 pm   #82
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Default Re: Bush TV22 with TC184 B3 converter.

An interesting one. I have just checked one of my TV22s. The interlace could be described as reasonable for a 9" tube but reducing the value of R11 [Bush manual] to around 18k has improved it dramatically.
It is very difficult to photograph using one of today's digital cameras with their high 'shutter' speeds but I took these pictures about an hour ago with the reduced resistor value and the results are interesting.

Going through the Bush data for the TUG26, the 16" metal cone version of the TV22, they had obviously had a few queries about poor interlace. Their cure was to try a number of ECL80 valves until one was found that suited the circuit. They also suggested careful adjustment of the vertical hold control.
I find this most odd with a company like Bush. Surely it is not that difficult to design a decent sync separator? It's only a few components.
As I think David mentioned the best interlace I have encountered is with the Ekco circuit used for over ten years from 1951. Interlace is 100% over the locking range of the frame hold control with very solid line and frame lock. I have no doubt that large numbers of Ekco models survived their entire life without adjustment to the controls. Regards, John.
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Old 29th Sep 2019, 4:04 pm   #83
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Default Re: Bush TV22 with TC184 B3 converter.

Had a fiddle with the TV32 and made a small improvement (not quite right yet) to the alignment but also applied Hugo's frame sync time constant reduction to this set. I reduced R1 270k to 120K. As was, the frame had no interlace and was jittering. With the reduction the jitter has gone and it has interlace.

Photos are before and after on the right.

Peter
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Old 30th Sep 2019, 3:24 pm   #84
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Default Re: Bush TV22 with TC184 B3 converter.

Before anyone questions it, I misread the diagram.

Peter
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Old 30th Sep 2019, 11:07 pm   #85
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Default Re: Bush TV22 with TC184 B3 converter.

As I noted I would expect the V sync system in the TMB272 would be fine with a short time constant high amplitude pulse. There is a balance though, and the sync injection level can be too high and if it is the noise in the sync can cause a reset or restart effect on the V osc and you end up with a sudden black blanking bar appearing in the middle of the screen somewhere and rolling upwards after that to re-lock. Also the filtering has to be enough to smooth out the edges of the H pulses on the V sync pulse. The Bush TV22 was simply miles off.
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Old 30th Sep 2019, 11:49 pm   #86
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Default Re: Bush TV22 with TC184 B3 converter.

....But it's not the first time I have seen this, both my pre WW2 sets, the KTE-5 and the Meissner had exactly the same issue with an integrator with 10 times the suitable time constant. On those though, because of the 5 inch screen the interlace was less of a problem that the poor vertical lock. It was also odd in that at the time, in 1939, the correct time constant for the vertical sync separator's integrator had already been outlined by RCA and was 50uS, at least for the American composite sync train, and that value also seems close to correct, or at least in the ball park, for the 405 line signal's sync.
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Old 1st Oct 2019, 12:49 pm   #87
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Default Re: Bush TV22 with TC184 B3 converter.

The attachment shows how the grid bias for the output valve is obtained. R7 and C5 smooth negative going waveform across C4. Resistors R5 and R6 serve to apply the correct grid bias for V2B.

DFWB.
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Old 1st Oct 2019, 6:10 pm   #88
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Default Re: Bush TV22 with TC184 B3 converter.

Ironically I've been struggling to achieve interlace on the TV32 and also to remove jitter from the frame and whilst a faster time constant is desirable for the former my problem has been the presence of line pulses on the frame syncs. I guess a two stage filter would improve things but simply upping the size of filter cap in this single RC sorts both the jitter and lack of interlace.

Peter
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Old 1st Oct 2019, 9:39 pm   #89
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Peter,
to perform the correct integration function of the broad pulses the time constant should be slightly greater than the duration of a broad pulse which for 405 line TV is 40microseconds. So it follows the TC should be no greater than, say 60uS.

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Old 1st Oct 2019, 10:13 pm   #90
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Default Re: Bush TV22 with TC184 B3 converter.

The attachment shows the integrated frame sync pulse train. The 405 line system didn't have the luxury of equalising pulses.

DFWB.
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Old 1st Oct 2019, 10:23 pm   #91
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Default Re: Bush TV22 with TC184 B3 converter.

Thanks David,

The circuit has 10k and 0.001uF so 10uS. I changed it to 50uS for the photo in my last post. Actually I've probably gone over the top because the feed isn't direct from the sync sep anode. The load is split 39k from the rail and 27k to the anode so my 10k is probably more like 26k so it started life with a 26uS time constant. I should have just doubled the capacitor size.

Peter

p.s. Just reduced my capacitor to a doubling in value and it does work fine.

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Old 2nd Oct 2019, 10:10 am   #92
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Default Re: Bush TV22 with TC184 B3 converter.

An alternative to the usual resistor and capacitor frame pulse integrator is to employ an inductor. The attachment shows the sync separator circuit employed in the Masteradio T852. The 0.5H inductor will present a high impedance to the line sync pulses and will serve as the anode load. Note the frame sync coupling to the blocking oscillator. The 0.002uF capacitor and 27Kohm resistor form a differentiator and flyback will be initiated by the positive going frame pulse.

DFWB.
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Old 2nd Oct 2019, 11:29 am   #93
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Default Re: Bush TV22 with TC184 B3 converter.

Thanks David, I'll bear the inductor instead of a second RC in mind and the differentiator although I do have good interlacing but there is something noisy somewhere that twitches momentary losses of interlace. I replaced the frame speed pot that was obviously faulty and am slightly suspicious of the height and frame form pots but I haven't probed around enough yet.

Another thing is that the set is very slow to warm up and starts with excess height. I'll need to be brave and check the EHT.

Peter
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Old 2nd Oct 2019, 12:44 pm   #94
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Peter,
The Masteradio frame sync separator is similar to the circuit used in the pre-war GEC BT9121.
There is a frame sync circuit in one of the 1950s Practical Television magazines which fully exploits the properties of the broad frame sync pulse train.
Without the circuit in front of me I can only guess that a resonant circuit is tuned to 20,250cps which allows only the frame sync pulses to pass through.
The waveforms show the frame sync pulses in almost total isolation from the line sync.
The only disadvantage of this circuit, if it is one, is that the frame sync separator will not operate on a simple single broad pulse.

DFWB.
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Old 2nd Oct 2019, 4:11 pm   #95
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Default Re: Bush TV22 with TC184 B3 converter.

Hi David,

My interlace was poor with the 0.001uF filter but with 0.002uF it's pretty clean and solid but drops out occasionally. I think the occasional frame twitch is due to a noisy height pot and this might be a factor in the interlace drop-outs.

The frame sync at C16 0.001uF on the circuit looks stable to me. See below.

VR9 in the top left is described as an interlace adjustment but it doesn't alter much over the course of its range except when taken to its lowest resistance at which point the frame frequency takes a jump. I think it just adjusts the amplitude of the sync pulse and presumably the effective threshold point.

Peter
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Old 2nd Oct 2019, 10:08 pm   #96
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Peter,
The negative going pulse is the waveform that's always present at the anode of the blocking oscillator irrespective of there being a sync pulse or not.
The blocking oscillator requires a positive sync pulse at the valve grid to initiate the flyback. A negative sync pulse can be applied to the anode and the transformer will act as an inverter so that the valve grid receives the positive sync pulse.

DFWB.
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Old 2nd Oct 2019, 11:28 pm   #97
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Default Re: Bush TV22 with TC184 B3 converter.

The problem really is that good interlace cannot be guaranteed unless the vertical sync is correctly filtered off the main sync pulse train, this is especially important when the signal has equalising pulses.

These are the facts: one stage of RC filtering , even with the correct time constant of around about 50uS is only just adequate. Two stages of filtering using the formula I mentioned earlier in this thread is much better, but three stages is perfect.

The reason is that with only one stage of filtering, by the time the serrations from the H & equalising pulses are properly filtered the vertical pulse is low in amplitude with a slow rising edge, asking for interlace troubles an poorer vertical look. A three stage filter does not have these issues and is a much more effective BPF for the task, enabling a fast rise V sync, free of serrations too.

RCA settled on a standard 3 stage filter, it had three 10k resistor's and three capacitors (I could post the values if anyone is interested). This "standard network ultimately appeared in many monochrome valve TV's and later was built into hybrid modules used in many Japanese TV's and VCR's.

The thing that is odd, despite in the USA, most set makers using this (on RCA's advice ) and most television tech training manuals teaching it as standard practice, British manufacturers paid little if any attention to it and most just stuck with one stage filters (and occasionally two stage filters were used which were better). Also, there appears to have been a complete absence of the knowledge of the the fact that poor interlace results from a poorly designed vertical sync filter, none of my British text books refer to it, or mention it as the primary cause of trouble and it appears that nobody on the forum knew about it either and had invoked all other kinds of causes for faulty interlace, when the topic of poor interlace came up. So it might have not been standard practice to teach this to TV techs in the UK.

So many old vintage UK TV sets can be improved for perfect vertical pulse separation and perfect interlace, just by modifying them to a simple three stage filter.

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Old 2nd Oct 2019, 11:34 pm   #98
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Default Re: Bush TV22 with TC184 B3 converter.

Hi David,

I'm confused! I think the scope trace is a filtered version of the familiar waveform that I've pasted on the picture below but with the picture information clipped off.

This is the only signal coming from the sync separator and it connects to the anode of the blocking oscillator not the grid.

Peter
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Old 3rd Oct 2019, 12:23 am   #99
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Default Re: Bush TV22 with TC184 B3 converter.

The penny's dropped. I understand the confusion. I should have disconnected the blocking osc. before displaying the sync waveform as currently it shows a mix of the oscillator waveform and the sync.

Peter
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Old 3rd Oct 2019, 12:30 am   #100
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Default Re: Bush TV22 with TC184 B3 converter.

....ps, the only way to assess the quality of the separated V sync is to disable the vertical oscillator (don't forget to extinguish the CRT beam current).

In any case, you can easily convince yourself that RCA was correct, if you sit down at a bench with just a TV pattern generator, a scope and a breadboard. Use the composite sync output from the generator and filter off the V sync with just one stage of RC filtering, looking with the scope. You will not get the result shown in post #71 of a properly filtered sync, it will be more like the result in post 90, with serrations, and the problem here is the the filtered pulse has the same amplitude on its rising edges at two different times, begging for interlace troubles. If you increase the capacitor value to eliminate these, the rising edge has a slow rate of change and the amplitude is attenuated and then, in use, the V osc is closer to threshold at the point of triggering and is more prone to be triggered into the next cycle by interfering H pulses.

Trying then adding more RC filter stages and it becomes obvious that this is how the pulse should be separated to attain a smooth fast rising edge free of serrations as shown on the image in post #71.

I could also add, if you have a TV and you find that the position of the vertical hold control affects the interlace, this is the inevitable result of a poorly separated V sync, with serrations on its rising edged and the start of the scan can be triggered at different times and is completely cured by a three stage filter, which ideally the set should have had in the first place, or a bare minimum two stage filter, of the correct time constant.

Last edited by Argus25; 3rd Oct 2019 at 12:49 am. Reason: Typo
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