120 and 180 line viewing

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Re: 120 and 180 line viewing

Postby Steve Anderson » Sat Jul 26, 2014 12:02 am

Steve, yep, I see what you mean. You appear to get non-composite video and either separate line OR frame syncs in the mechanical modes. And it would appear to be a square-wave rather than pulse. That could be simply solved with a monostable, but without separate frame AND line you're kinda stuck.

There could be fudges around this, but I think that I would resign myself to a no-go situation.

Steve A.
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Re: 120 and 180 line viewing

Postby Panrock » Sat Jul 26, 2014 3:20 am

Not a good day trying to deal with the two problems.

The RF herringbone is of too fine a grain to be visible on the lowest definition standards but is annoying on the higher ones. It's not coming from the vision strip. Damping the video output doesn't affect it. I'm unable to safely prod around the tube cathode 'brightness' end of things with a decoupling test capacitor, in case it's getting in there, since this is at minus EHT. My feeling now is that this oscillation could be caused by the VCR517 tube itself. It is, after all, a sort of amplifying valve.

The flyback lines I can probably live with, given a signal with a decent frame blanking period (like on a 405-line signal) and when the brightness control is not too far advanced. I fear they are just a feature of this circuit, They seem to be emphasised with a 120 and 180-line signal. It would be nice to (safely) incorporate frame flyback suppression to remove them. Probably best not done at the tube though. Ideas?

Steve O
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Re: 120 and 180 line viewing

Postby Panrock » Sat Jul 26, 2014 7:04 am

Finally for today, here are three warts-'n-all pictures of the results I'm currently getting, showing the problems.

Only the 240-line standard is 4:3. The converter (correctly) chops the pictures on 180 and 120 to 6:5.

Steve O
Attachments
120.jpg
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180.jpg
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240.jpg
240.jpg (38.86 KiB) Viewed 7472 times
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Re: 120 and 180 line viewing

Postby Steve Anderson » Sat Jul 26, 2014 12:58 pm

Hmmm...if 'twere me I would be tempted to return C43 back to its original value and increase the values of VR5 and R49 - but that's a bit of a fiddle-arse operation. My thinking is that the impedance of V17 is not low enough to discharge the increased C43 in a short enough period of time.

Nominally the combined resistance of VR5 and R49 is 3M2 (50% pot rotation), so double this to 6M4, subtract half of VR5's value = 5M4, say 5M6 for R49. Be prepared to try values either side (4M7/6M8). I was concerned about grid-conduction/current, but while sync pulses are present that shouldn't happen. If no sync pulses the ramp could be non-linear (or even non-existant), but it doesn't matter then.

Also the 405 standard has 15 lines of field blanking, about 1.5ms...not sure what the 120/180/240 blanking periods are.

As for the herringbone pattern, try breaking the junction of C61 and R34, bring up the brightness to a mid-grey and see if said pattern has gone. It's interesting to note that it appears to be worse at the bottom of the screen, which implies something timebase-related. Maybe even PSU ripple induced, but then I would expect it to slowly drift up/down the screen.

Steve A.

I wish people would put component values actually on the circuit diagrams, it's a pain having to flick back-and-forth to a components list.

I also can't find any data on the EHT transformer beyond the 2-0-2V winding for the rectifier heater, no secondary EHT winding volts/current appears to be specified...unless I've simply missed it.

Steve, can you post "Revised sync separator from Nov '53 issue" here - that may help...unless you can link to it at your website. And any other changes/errata you may have...beyond your own which I already have.
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Re: 120 and 180 line viewing

Postby Panrock » Sat Jul 26, 2014 8:41 pm

Steve A, thanks for the comments, which I shall now work through. Agree about the way the component values are listed separately is cumbersome and annoying. I shall try to find that old PT magazine. In any case, I monkeyed about with the sync separator of old, and it is probably best to attempt to trace the schematic for you.

There's also an issue with hum on the scans. More likely than not this is due to imperfection in the magnetic screening from the Mu-Metal shield, which is absolutely critical.

Steve O

PS. PT mag long lost. I have decided to provide you with a complete timebase and s/separator schematic showing the actual current setup and with all component values marked - both original and now. It's the least you can expect... :wink: Watch this space.
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Re: 120 and 180 line viewing

Postby Panrock » Sun Jul 27, 2014 2:56 am

Panrock wrote:PS. PT mag long lost. I have decided to provide you with a complete timebase and s/separator schematic showing the actual current setup and with all component values marked ...

Here is the schematic of the rig as it is now, set up for the lower standards, including component values and incorporating all the various changes. Some component numbers have been included, for convenience, should you wish to refer to that list of reasons why they were changed. This chart should be accurate, but derives partly from my tracing of the circuit. No, I don't understand how it can possibly work either! :lol:

Steve A, I also have other versions of this diagram with the actual changes colour coded, should you require them.

Cheers (and thanks!)

Steve O
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Re: 120 and 180 line viewing

Postby Panrock » Sun Jul 27, 2014 4:23 am

Steve Anderson wrote:As for the herringbone pattern, try breaking the junction of C61 and R34, bring up the brightness to a mid-grey and see if said pattern has gone.

Just done this. The interference remains. The tendency for it to be related to scan position would figure with my hypothesis it's tube-related, given that this contains electrostatic elements. Or maybe one of the EHT smoothing capacitors is doing a 'funny' with its ESR at a certain frequency?

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Re: 120 and 180 line viewing

Postby Steve Anderson » Sun Jul 27, 2014 12:50 pm

Without being able to 'get in there' with a scope-probe myself it's always difficult to diagnose these sort of things at a distance.

Now I have not heard of a CRT going 'active' before, but it is a glorified triode so perhaps it is possible. Keeping C61 disconnected, try a 100nF cap to ground (400V) from the junction of R58/59/61/62. If that does nothing similar caps from the wipers of VR7 & VR8 to ground.

Next, if the above does nothing, with C61 still disconnected, temporarily place a 100nF between 1 & 2 of the CRT (grid/cathode). What I'm trying to determine is is the pattern modulating the CRT grid circuit or is it rubbish modulating the sweep waveforms (mainly horizontal) which will lead to a similar result. i.e. I'm still pointing my finger at the timebases....but I have been wrong in the past and surely will be again in the future.

You'll note that of all the sub-systems in this TV the timebases have had the most changes/modifications by PT and yourself.

If only I were in Sedgeberrow now, between the two of us I'm sure we could have this fixed in time for a celebratory Sunday pub-lunch! The attached looks ideal and local!

Steve A.
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Re: 120 and 180 line viewing

Postby Panrock » Mon Jul 28, 2014 5:49 am

Hi Steve A

Steve Anderson wrote:Now I have not heard of a CRT going 'active' before, but it is a glorified triode so perhaps it is possible. Keeping C61 disconnected, try a 100nF cap to ground (400V) from the junction of R58/59/61/62. If that does nothing similar caps from the wipers of VR7 & VR8 to ground.

You mean R57, 58, 61, 62? Makes no difference.

Steve Anderson wrote:Next, if the above does nothing, with C61 still disconnected, temporarily place a 100nF between 1 & 2 of the CRT (grid/cathode).

Again, no difference. So it's the tube. Or some local interference to which the electron beam is responsive. Or possibly (more unlikely) layout in the remainder of the tube feeder wiring.

I already have my 'brightest' tube installed, but it wouldn't be hard to unplug it and try another.

Steve Anderson wrote:If only I were in Sedgeberrow now, between the two of us I'm sure we could have this fixed in time for a celebratory Sunday pub-lunch! The attached looks ideal and local!

Sounds like a commisatory pub lunch would be in order for the moment.:wink: But if you ever visit, you'll be welcome! Preferably before I stash my Mirror Screw away - next to the huge Grosvenor and its camera - in the cramped store room, where it will in effect, be 'put beyond use'.

Any thoughts about the 'revised' sync separator circuit I've posted? This is the one currently in use. Otherwise I guess the next thing is to try re-balancing the frame timing R's and C as you suggested.

Steve O
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Re: 120 and 180 line viewing

Postby Steve Anderson » Mon Jul 28, 2014 4:30 pm

Hi Steve, I didn't study your revised version that well, I was in a bit of a rush that day. Now I see what you've altered around the vertical sync separator/integrater. But I think there's an error in the revision. I can't see how V14 gets its anode volts unless via the screen circuit. It may well be correct but it's not a configuration I have seen before. But I'm no expert on valve/tube sync separators.

I did mean R57, 58, 61, 62, oops...but it was worth a try. Likewise the CRT grid/cathode bypass.

If you feel in the mood do try another CRT. Do you have any spare SP61's? Or you could swap a few around...and see if there's any change.

I'm still thinking it's the timebase(s) that are at fault here.

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Re: 120 and 180 line viewing

Postby Panrock » Mon Jul 28, 2014 6:08 pm

Steve Anderson wrote:Hi Steve, I didn't study your revised version that well, I was in a bit of a rush that day.

Talking of 'rush', you already know how much I appreciate your input. Please only have a look this when you have time and the fancy takes. :) In any case, this morning the working week has kicked in and the Argus will now have to be relegated to the floor to make room on the bench. Henceforth I'll probably only be doing this for 'relaxation' in the evenings.

Steve Anderson wrote:Now I see what you've altered around the vertical sync separator/integrater. But I think there's an error in the revision. I can't see how V14 gets its anode volts unless via the screen circuit. It may well be correct but it's not a configuration I have seen before. But I'm no expert on valve/tube sync separators.

I was equally surprised when I saw it, thus the cryptic comment in a previous post. So I checked it again. I vaguely remember having my eyebrows raised when I saw it in the 'Nov 53 issue' too - but I followed instructions...

Steve Anderson wrote:If you feel in the mood do try another CRT. Do you have any spare SP61's? Or you could swap a few around...and see if there's any change.

Will try another CRT.

I have one spare SP61, which I sub'd for V18 after the following. I had noticed touching V18 caused an effect on the picture (which it shouldn't). Touching this valve seemed to add hum to the frame scan, causing vertical shift and scan distortion. The RF patterning remained. Changing the valve made no difference, so I checked the connections underneath that connect the valve electrodes and screen to chassis. They were all present and correct. A mystery.

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Re: 120 and 180 line viewing

Postby Steve Anderson » Mon Jul 28, 2014 7:34 pm

Hmm, OK, they know better than I in these matters, I have no way of confirming or otherwise that this is valid, let's assume it is. By implication the first version they published wasn't quite up to the task - or something - and needed 'refining'. The values in the version posted above on Saturday are the PT values or your own?

I would tackle the flyback 'foldover' first then this herringbone issue, but it's your Argus...of the two I'm thinking (hoping) the 'foldover' is going to be the easiest to fix as long as the frame blanking of 120/180/240-lines isn't too short. (in terms of absolute time not number of lines).

I guess you mean the external metallisation of V18/SP61's, is it possible to check that this stuff is grounded, it may have some resistance of its own and therefore may not read a dead short to chassis (or cathode as the case may be, most are grounded cathodes anyway). A few hundred Ohms maybe, less being better. Compare it with the others. (I'm assuming you did a visual inspection, if you did check continuity, my apologies).

I'll await the results of the CRT swap-out, not quite as quick as a standard tube/valve change but hopefully not too much of a pain.

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Re: 120 and 180 line viewing

Postby Panrock » Mon Jul 28, 2014 8:01 pm

Thanks Steve A. I'll reply to your points properly in due course. Meanwhile, over on the VRAT Forum, the guys there are looking out for us a "Nov '53 issue" to check my weird circuit against...

Steve O

PS. The circuit Jon's come up with over there from Nov '53 does look very similar to mine, except his is more logically laid out and some of the values are different.

The values in current.gif are those actually being used right now.
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Re: 120 and 180 line viewing

Postby Panrock » Mon Jul 28, 2014 9:20 pm

Here's my current circuit with actual component values used, re-drawn, for greater clarity, in the same style as Jon's. Hope it's helpful.

Steve O
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Re: 120 and 180 line viewing

Postby Steve Anderson » Mon Jul 28, 2014 9:49 pm

*With head in hands* Here we go...first, the two versions you've posted in this tread are different, not just in layout, but electrically. They are both different to the original PT version (which is expected)...and the scrap view on the VRAT forum is different again to all. Or am I all confuzzled again?

Now the one that looks the most sane to me is 'redrawn_current.gif', your previous post. The one on VRAT looks OK too.

However, from what has gone before you don't appear to have a sync separator problem, just a flyback one. It's taking too long to get back to the top of the screen, the triggering seems OK. So I suggest leave the separator alone and try the time-constant mod I suggested earlier, VR5,R49 & C43. i.e. leave VR5 as is, return C43 back to original and increase R49 to 5M6, possibly 4M7 or 6M8.

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