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Old 26th Sep 2019, 7:06 pm   #61
Keith
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Default Re: Bush TV22 with TC184 B3 converter.

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Originally Posted by FERNSEH View Post
The interlace is not very good in my set, the problem being that there is line pairing irrespective of the frame hold setting.
My TV22 also doesn't seem to interlace. Is this a common problem with the design or is there a problem with the way I generate the half line pulses I wonder?
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Old 26th Sep 2019, 7:37 pm   #62
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Keith,
Both the Aurora SCF405 and Hedghog converters can if desired insert equalising pulses into the frame sync pulse train. I've found with the TV22 the interlace is actually worse when the equalising pulses are present.

DFWB
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Old 26th Sep 2019, 7:42 pm   #63
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Keith,

If you are generating your signals in the same way as me then I suspect the TV22 is the problem here.

Both photos below are from the same source but the image on the right is from my TV22.

Peter
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Old 26th Sep 2019, 10:00 pm   #64
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Default Re: Bush TV22 with TC184 B3 converter.

Hi again Peter,


I'm using the monostable circuit from http://www.nostalgiatech.co.uk/SynchRed%20(Large).jpg.


I've tried minor adjustments to the timing but to no avail. I'd be interested in any pointers to problems in the receiver itself.
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Old 26th Sep 2019, 11:28 pm   #65
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Keith,

Yes, same source as me.

Peter
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Old 27th Sep 2019, 7:19 am   #66
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Default Re: Bush TV22 with TC184 B3 converter.

There are good reasons why the interlace in the TV22 is poor. Most of the explanations for poor interlace overlook the obvious. Mechanisms such as the V osc triggering on H pulses etc are invoked.

But the primary cause, of the poor interlace in the TV22 (actually like many pre-ww2 sets) is two things, that both happen together, namely over integration of the sync pulses (it loweres the level) and too low a level of sync pulse injection to the V osc.

If you look at the TV22 after the unnecessary diode they have there, you will notice the time constant of the vertical integrator components are outrageously long, with values such as 100K and 0.01uF when really the values should be more like 0.005uF and 10k, so it out by a factor of 20, yes that bad. This is why there is poor interlace.

An ideal time constant for a vertical integrator (to extract the V sync but without serrations from H sync, yest still producing a vertical separated pulse with a fast rise) in a two stage filter has a time constant of 50uS. So typically the filter should be a series 10k resistor and 0.005uF to ground then another series 10K and 0.005uF to ground and Bob is your Uncle.

PS , for a two section filter, the time constant is R1(C1 + C2) + R2C2.

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Old 27th Sep 2019, 10:04 am   #67
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Default Re: Bush TV22 with TC184 B3 converter.

Thanks Hugo,
I shall take a look in that area and report back in due course.
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Old 27th Sep 2019, 10:18 am   #68
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Default Re: Bush TV22 with TC184 B3 converter.

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Originally Posted by Argus25 View Post
If you look at the TV22 after the unnecessary diode they have there
Interesting. I was actually going to suggest that replacing MR1 was the first thing to try...

Cheers
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Old 27th Sep 2019, 3:33 pm   #69
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Default Re: Bush TV22 with TC184 B3 converter.

For discussion later, the circuit of the sync separator and frame pulse integrator and clipper.

DFWB.
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Old 27th Sep 2019, 11:33 pm   #70
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Default Re: Bush TV22 with TC184 B3 converter.

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Originally Posted by FERNSEH View Post
For discussion later, the circuit of the sync separator and frame pulse integrator and clipper.
The filter capacitors were not as large as I remembered but the resistor is too large. I would recommend as a trial make the 100k about 22k or thereabouts by tacking a resistor across it and after that if no improvement, try replacing the diode with a 22k resistor.

If that doesn't work, disable the vertical oscillator and look at the separated V sync pulse on the scope to see if has been suitably separated and filtered.
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Old 28th Sep 2019, 12:31 am   #71
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Default Re: Bush TV22 with TC184 B3 converter.

... I've attached a picture of what a properly separated V sync should approximately look like (with or without equalizing pulses) at the grid of a vertical osc stage with the osc disabled. In circuits where it gets injected into the plate load area it simply gets transformed by the blocking osc transformer to become a + going pulse at the grid.

When the level is too low and especially if it is over integrated, at the moment the grid voltage is rising, about to bring the vert osc valve into conduction, it is getting to the point where a very tiny grid voltage transient (such as interference from H pulse on the supply rail, or from nearby wiring) trigger the vert osc into the next cycle instead, then the half line delay is lost and line pairing occurs.

With a better decent amplitude and properly separated V sync, the V osc is triggered definitely by that timing, at a time where its grid voltage is lower (more negative) and therefore the osc stage less sensitive to interfering H pulses that could trigger it instead.

So the bottom line is if the V sync pulses are too weak and over integrated with a slow rising edge, more likely line pairing will occur.

Also with regard to the TV22 circuit, normally the diode is not required and if you look at their circuit, there is no DC return path on the diode's anode, the only possible path is the diode leakage itself which is ill controlled or leakage in faulty aging capacitors.

Also, with this circuit it is very important that the filter capacitor C7 0.1uF is in perfect order, or H pulses will modulate the primary of the blocking osc transformer and make the line pairing worse.
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Old 28th Sep 2019, 1:37 am   #72
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Default Re: Bush TV22 with TC184 B3 converter.

Hi all,
just for reference I have attatched part of the circuit of the Ekco TMB272 which is very similar, but I notice here the diode is properly biased.

Of course the TMB272 is a much nicer set (dives for cover) - no clowning around with this design

Cheers
Andy
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Old 28th Sep 2019, 3:26 am   #73
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Default Re: Bush TV22 with TC184 B3 converter.

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Originally Posted by beery View Post
just for reference I have attatched part of the circuit of the Ekco TMB272 which is very similar, but I notice here the diode is properly biased.
Interesting, only one stage of integration (that is still ok but in theory the H serrations are better filtered off with a two stage filter) and a very short time constant of about 24uS (RCA recommended 50uS). This will result in a very fast rising high amplitude V sync pulse though. Probably one advantage of mixing in the syncs in the plate circuit, the transformer itself has LPF properties that help reduce its response at line frequencies versus vertical rate, so probably you can get away with the shorter time constant in the integrator components, than in the case where the sync is introduced directly to the grid.

(The time constant is at least 200uS, or more in the TV22, could be longer too, depending on the behavior of the diode).

Does the TMB272 have any line pairing/ interlace issues ? I would expect that it would not have any issue, unlike the TV22.

PS : I've always liked the look of the TMB272, but I was not able to get one sent here. However, some years ago there was a fellow in the UK who sold me a CRT from one and that is what I have in my TV22. Because it did not have an external aquadag, I added an EHT filter capacitor inside the EHT cage (you can see it in the photo I posted previously post #24) and made an acrylic insulator top to go over it to prevent any corona discharges.

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Old 28th Sep 2019, 10:38 am   #74
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Default Re: Bush TV22 with TC184 B3 converter.

Except for the omission of the frame sync coupling capacitor C96 the Ekco T330 uses a similar frame sync circuit.
I can assure you that the T330 displays a perfectly interlaced picture.

DFWB
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Old 28th Sep 2019, 10:44 am   #75
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Default Re: Bush TV22 with TC184 B3 converter.

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Originally Posted by FERNSEH View Post
Except for the omission of the frame sync coupling capacitor C96 the Ekco T330 uses a similar frame sync circuit.
I can assure you that the T330 displays a perfectly interlaced picture.

DFWB
So this circuit has an even shorter time constant than the TMB272, only 14uS. That is 14 times less than the TV22. Of note though, there is generally no need for the diode, it was never included in American circuits. I am doubtful of the advantage it confers, though there may be one. Its pretty obvious though that the TV22 has an excessively over integrated V sync pulse.
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Old 28th Sep 2019, 11:27 am   #76
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Default Re: Bush TV22 with TC184 B3 converter.

Ekco describe the diode as a frame pulse shaper, but the main purpose of the diode is to remove any line sync pulses. More complex frame sync circuits were employed in UK made TVs such the two diode circuit shown in the first attachment. This circuit was used in many Pye TV receivers.
The second attachment shows the very simple frame sync coupling employed in many Ferguson receivers. Instead of the usual integrator the frame pulse is supplied to the oscillator by a series capacitor which functions as a differentiator. As simple as the circuit is the interlace is generally good.

DFWB.
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Old 28th Sep 2019, 12:20 pm   #77
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Default Re: Bush TV22 with TC184 B3 converter.

Quote:
Originally Posted by FERNSEH View Post
Ekco describe the diode as a frame pulse shaper, but the main purpose of the diode is to remove any line sync pulses. More complex frame sync circuits were employed in UK made TVs such the two diode circuit shown in the first attachment. This circuit was used in many Pye TV receivers.
The second attachment shows the very simple frame sync coupling employed in many Ferguson receivers. Instead of the usual integrator the frame pulse is supplied to the oscillator by a series capacitor which functions as a differentiator. As simple as the circuit is the interlace is generally good.

DFWB.
I don't think that circuit with the two diodes is very smart at all. Also considering the forward bias applied, it would be difficult even with large signals to get the diode out of conduction. The diodes just gate the signal through and benefits from any reverse blocking function seem less plausible.

In fact the circuit in the TV22 would be a fine example that proves this point, where in fact rectification effects are not possible due to the absence of a specific DC reference on the diode's anode.

There is absolutely no need for a diode to be included in a vertical sync separator circuit, and there never has been.

The science behind the separation of the vertical sync pulse from the composite sync train, is well established and no rectification effects are either necessary, or helpful. The problem is one of a well designed LPF with a suitable time constant, the ideal is a three stage filter, more stages than that have no utility value.

When I see this sort of thing it makes me wonder whether the motivation for putting these diodes in the circuit was something else altogether. You often see some unusual circuit configurations where the manufacturers were trying to avoid patent infringements, so they come up with a circuit configuration that deliberately doesn't resemble another set maker's design, but the added components do nothing useful. You might wonder why Stromberg carburetors have a rubber diaphragm around the piston, and SU's do not, its the same reason.

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Old 28th Sep 2019, 3:42 pm   #78
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Default Re: Bush TV22 with TC184 B3 converter.

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
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Old 28th Sep 2019, 4:26 pm   #79
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Default Re: Bush TV22 with TC184 B3 converter.

Hi Peter,
Following up Hugo's advice it is clear that the value of R52 (trader sheet, R11 Bush SM) is too large. Referencing to the circuit diagram on post 69 it can be assumed that integrating capacitors C11 and C13 are in parallel and along with the anode load resistor R16 (15K) the time constant will be 45microseconds, just slightly longer than the broad pulses. Although the presence of the other anode load resistor R15 (39K) will reduce the TC of C11,C13 and R16 somewhat.
It might be worthwhile looking at the circuit diagrams of later Bush TVs. Models TV22A and TV24C employ a different timebase circuit using a cross coupled frame multivibrator.

DFWB.
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Old 28th Sep 2019, 5:46 pm   #80
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Default Re: Bush TV22 with TC184 B3 converter.

Hi David,

Yes, the multivib sets have a substantially lower time constant.

Peter
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