120 and 180 line viewing

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Re: 120 and 180 line viewing

Postby Panrock » Mon Jul 28, 2014 10:12 pm

Steve Anderson wrote:*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?

Think you may be - can't blame you though. My own two circuits should be electrically identical. I've just checked again. One likely point of confusion could be a 22K on the earlier diagram that (from its position) looks like an anode load resistor but actually is part of the frame pulse network! Thus the dire need for a re-draw...

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

These are very similar. The only differences to the sync separator: a grid leak resistor I added to V14 and some changed component values.

Steve Anderson wrote: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.


Will do! :)

Steve O
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Re: 120 and 180 line viewing

Postby Steve Anderson » Mon Jul 28, 2014 10:39 pm

OK, I'll await results from both impending tests.

The two circuits posted here are different...have a close look around V14, in the first version there's a resistor, 22k from screen to anode, the screen has a cap, 10u, to ground...that's a clue.

I know what it's like when you get 'too close' to a drawing, it happens to us all!..

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Re: 120 and 180 line viewing

Postby Panrock » Mon Jul 28, 2014 10:50 pm

Steve Anderson wrote:OK, I'll await results from both impending tests.

The two circuits posted here are different...have a close look around V14, in the first version there's a resistor, 22k from screen to anode,

Yes, so is there in the second version.

Steve Anderson wrote: the screen has a cap, 10u, to ground...that's a clue.

And again, that cap is also in the second version, moved up to where its anode rail decoupling function is clearer...

Anyway I've now added a comparison diagram, with the various components colour coded. I still reckon they are the same.

Steve O
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Re: 120 and 180 line viewing

Postby Panrock » Tue Jul 29, 2014 4:52 am

Steve Anderson wrote: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.

Success! :D

I don't have a good stock of high value resistors but was able to lay my hands on a pair of 10 megs to put in parallel. The result.. 120-lines and 240-lines are now well-nigh perfect, with just a tiny bit of it remaining on 180-lines (previously by far the worst). Maybe 180-lines has the shortest frame blanking?

I consider this problem now solved (thanks!). Returning to the old value of C43, there is far more height available too and the timebase seems generally more efficient.

Yes, I did check the continuity-to-screen of V18 - it's fine. However I have just discovered this valve's cathode tag connection was disconnected! It must have been getting to its cathode via internal connections. Honest guv'nor... :oops: it was hidden behind another wire.

I append the latest circuit below, now working well.

Next... the RF (seems better....not that bad really) and the hum on video and scans (only really objectionable on the 24Hz standards). I strongly suspect this is not down to insufficient smoothing. And if it were on the EHT smoothing there would be hum on focus too - there isn't. It could be a Mu-Metal issue... or pickup on the high impedance scan feeds. Nevertheless the next thing is to try bridging various HT rails with a fat C.

I have updated the Argus web page with the latest diagrams.

Steve O
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Re: 120 and 180 line viewing

Postby Steve Anderson » Tue Jul 29, 2014 1:08 pm

Steve Anderson wrote:I know what it's like when you get 'too close' to a drawing, it happens to us all!..Steve A.


...and it just happened to me! Probably if I had drawn it out by hand I would have seen that they are one and the same. Sorry to waste time, my fault.

Panrock wrote: I have just discovered this valve's cathode tag connection was disconnected! It must have been getting to its cathode via internal connections. Honest guv'nor...


Odd. There isn't much information around r.e. the SP61, a Mazda datasheet and one or two base diagrams. None of which show any internal connection between cathode, g3 or the metallisation. And when each of them is brought out on a separate pin you assume they're not internally connected. Indeed the Mazda datasheet mentions using g3 within an AGC circuit. Whatever - it's fixed it appears.

I'm starting to wonder if V18 was actually doing anything? You did mention incresed height, perhaps the CRT was being driven only via one vertical plate...as I said before..."Whatever - it's fixed it appears."

Has this also got rid of the herringbone pattern?

In addition to hanging big C's around the place it may be worth swinging the CRT out to the left-side of the chassis (viewed from front) to see if there's any change in screen disturbance/ripple. This puts that big transformer visible in the photo right behing the CRT, usually the best place for it. Hopefully the leads to the CRT will allow this. This may help identify if it's supply ripple or magnetic. Though I'm not sure where the other transformer(s) is/are located.

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Re: 120 and 180 line viewing

Postby Panrock » Tue Jul 29, 2014 6:52 pm

Steve Anderson wrote:Odd. There isn't much information around r.e. the SP61, a Mazda datasheet and one or two base diagrams. None of which show any internal connection between cathode, g3 or the metallisation. And when each of them is brought out on a separate pin you assume they're not internally connected. Indeed the Mazda datasheet mentions using g3 within an AGC circuit.

Err yes, I did rather jump to conclusions there...

Steve Anderson wrote:I'm starting to wonder if V18 was actually doing anything? You did mention increased height, perhaps the CRT was being driven only via one vertical plate..."

I don't think so. The increased height was contemporaneous (fancy word) with changing the capacitor and resistor.

Steve Anderson wrote:Has this also got rid of the herringbone pattern?

No, but it seems less noticeable; well nigh invisible on the lower definition standards.

Steve Anderson wrote:In addition to hanging big C's around the place it may be worth swinging the CRT out to the left-side of the chassis (viewed from front) to see if there's any change in screen disturbance/ripple. This puts that big transformer visible in the photo right behing the CRT, usually the best place for it. Hopefully the leads to the CRT will allow this. This may help identify if it's supply ripple or magnetic.

A good tip. Will do.

Steve Anderson wrote:Though I'm not sure where the other transformer(s) is/are located.

The EHT and timebase heater transformers are below-decks toward the rear. Come to think of it, reversing the phase of one or more of their mains feeds might have some field cancellation effect. But I'll try the tube 'swing-out' test first to establish if this effect is a factor.

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Re: 120 and 180 line viewing

Postby Steve Anderson » Tue Jul 29, 2014 10:30 pm

Now...once you've got you ripple/magnetics sorted here's what I would have a go at next. Now this is me, if once you've got rid of/reduced ripple/magnetic factors and you're happy - call it a day.

However, my thinking is that as these SP61's are top-cap grid tubes there's some 3" (OK it's small) of exposed unscreeded lead above the chassis. In each case (V16 & V18) have quite high grid-circuit impedances, 2M or so DC, and at the frequencies they are dealing with the caps are quite reactive (C53/52 & C49/50). To save the cost of two resistors in each timebase output the reactance of the caps acts as two components. These two tubes (V16/18) are configured ideally as anode followers with a gain of -1. It's the tube equivalent of a unity-gain inverting op-amp.

Dropping the value of R46/55 to 1M and increasing C53/52 & C49/50 to double their nominal values may, a) quench any marginal instabilty, b) reduce and pick-up in the grid leads by a factor of two. In the case of the horizontal tube, V16 it's input capacitance is about 11pF, add in socket and wiring capacitance and it's starting to look similar to a phase-shift oscillator.

Anyway, enough. It's just what I might have a look at if I had the time and inclination. I'm not asking you to do so.

Steve A.

Please excuse my use of 'tubes' rather than 'valves'...even though British I habitually call them tubes. But then again, perhaps I should call it a toob....
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Re: 120 and 180 line viewing

Postby Panrock » Wed Jul 30, 2014 5:26 am

Steve A, your 'scheme' interests me for two reasons.

Firstly, if it quenches any instability, this could have a bearing on the herringbone.

Secondly, by reducing the grid impedance, it could reduce hum.

I think that first I shall see if there are 'traditional' causes for the above, namely:

    Herringbone caused by the CRT (try substituting).
    Hum caused by magnetic induction (try swinging out the CRT).

... then if these methods don't bear any fruit, move on to trying your latest suggestions.

Dropping the value of R46/55 to 1M and increasing C53/52 & C49/50 to double their nominal values...

Why stop at a factor of two though? How far could we go in multiplying down and up these R's & C's? Imagine...super-low hum and super-high stability! :wink:

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Re: 120 and 180 line viewing

Postby Steve Anderson » Wed Jul 30, 2014 5:12 pm

Panrock wrote:Firstly, if it quenches any instability, this could have a bearing on the herringbone.
Secondly, by reducing the grid impedance, it could reduce hum.

Why stop at a factor of two though? How far could we go in multiplying down and up these R's & C's? Steve O


Well, you do have to consider the loading on the previous stage which will reduce gain. I believe you rotated the CRT such that the usual Y plates are used here for the horizontal deflection as it seemed you couldn't get quite enough width. The Y plates are always nearest the gun assembly which provides the greatest deflection sensitivity and lowest parasitic capacitance therefore easing the gain/bandwidth trade-off in 'scope vertical amplifiers. The designers of the Argus were trying to squeeze every last drop of gain they could out of those SP61s.

What I suggested yesterday was just that, a suggestion. I have no idea if it will clean up matters but it can't harm except for the potential loss of gain. If you are going to try this, start with the horizontal.

First though I would sort out the hum...I doubt that hum is being induced in the grid circuit. It's either ripple or magnetic...or a bit of both...good luck.

Steve A.

An afterthought...I recall somewhere you mentioned that the main HT supply was derived via half-wave rectification...correct me if I'm wrong. Anyway, continuing...if it were full-wave it should be possible to differentiate whether it's 50Hz modulation (magnetic) or 100Hz (ripple).
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Re: 120 and 180 line viewing

Postby Panrock » Thu Jul 31, 2014 9:26 am

About 2/3 of the hum disappeared when the tube was 'swung out'. The remaining hum looked like 100Hz.

I spent a good few hours this evening getting all the Hammerite off the mu-metal shields and their mating surfaces (a tedious job), so the whole thing could be electrically earthed to the metalwork. Then disaster!

I tried all three of my 'good' tubes: two VCR517's and (shown) my CV2810. All displayed this same twisted, defocussed, badly locked, oscillating, mess on the screen. Is what I've done a "no no"? Could the earthed shields be acting as a distorting electrode?

Or perhaps there is a faulty connection to one of the tube pins? The only good thing is there now seems to be less hum.

Back to the drawing board. :twisted:

Steve O

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Re: 120 and 180 line viewing

Postby Steve Anderson » Thu Jul 31, 2014 11:49 am

Steve, any chance that during scrubbing off the Hammerite that the mu-metal has gotten magnetised? Dropping it, banging it (hammering) may do this.

If so the only answer I can think of is an industrial-sized degausser. Radio and recording studios always had these in the past, but now the use of tape as a recoding media has vanished, so have the degaussers. Hospitals also have them to degauss surgical instruments - I know that seems odd but there it is.

I have heard of guys discharging large capacitors through a few turns of wire can also do this...time for a Google...

Steve A.

Also try without the mu-metal screen, tolerate the hum for a moment, but does everything else return to some semblance of normality? Whilst running bring the mu-metal close to the CRT. If there's a large influence it probably means it has been magnetised somehow.

Grab an old CRT monitor/TV which if any good had a deguassing coil around the faceplate. 'Modify' it to suit the mu-metal. Don't omit the PTC thermistor which controls the current.
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Re: 120 and 180 line viewing

Postby Panrock » Thu Jul 31, 2014 6:22 pm

Steve, yes the mu-metal was given a right juddering and scraping during the process.. a great idea. Thanks!

I actually have an old Degausser, which I made up from a load of colour set degaussing loops in series and phase, all across a low voltage transformer.

I'll give it a whirl.

Steve O

P.S. Then I noticed something else. Long ago, I see I 'repaired' pin 10 contact inside the tube base. I remember now - the original contact had been broken. This new contact had got pushed back. I pulled it forward again. This connects to part of the focussing assembly in the tube. I also thoroughly degaussed everything and.. switched on.

Normal service has now been resumed! :D

P.P.S. So I have found my old CV2810 is fine after all. I am now getting a much brighter blueish 'black and white' picture. It's funny how quickly one gets used to the 25Hz flicker...

The RFI has gone, whether due to changing the tube or properly electrically screening it. The only remaining obvious problem is there is still a little hum.
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Re: 120 and 180 line viewing

Postby Steve Anderson » Thu Jul 31, 2014 8:41 pm

Good news. I guess what is left is probably ripple, and to eliminate what fraction may be magnetic is simply going to be too much of an effort to deal with.

So once the ripple is dealt with your "One capacitor modification" is concluded.

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Re: 120 and 180 line viewing

Postby Panrock » Thu Jul 31, 2014 8:51 pm

Steve Anderson wrote:So once the ripple is dealt with your "One capacitor modification" is concluded.

HAHAHA. It's been a burden for you too (sorry).

Not finished yet. The Argus will now be inverted on the bench once again, while I look into the ripple (on a scope). The flyback on 180-lines (only) could be tighter too...

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Re: 120 and 180 line viewing

Postby Steve Anderson » Thu Jul 31, 2014 9:01 pm

The vertical flyback timing may be just marginal, if you really wish to persue this try reducing C43 and increasing the R49/VR5 combination pro-rata. Though I wouldn't go any more than half/twice the current values. Say 330nF for C43. (8M or so for R49).

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