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Old 28th Aug 2019, 10:17 am   #1
Ian - G4JQT
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Default Bush TV22 "cog-wheeling"

I've not used my TV22 for about a year, but it is now displaying an intermittent instability making the test card circle look like a cog wheel; i.e. all the vertical lines are slightly jittery.

Adjusting brightness or horizontal hold controls seems to have an effect, sometimes clearing it from the top or bottom half of the screen.

It appears to be heat related as it usually settles down and disappears after about 15 minutes. Doesn't seem to be a dirty valve holder as I've cleaned them all. I have a feeling it's a known 'feature' but can't find reference on the forum. Any ideas?

Incidentally, this is the TV22 MKII, and the 2.5 MHz frequency gratings are clearly visible (although not in the photo; the picture also needs centreing and tweaking), but on my two MKI TV24s the 2.5MHz is only just visible. Is the MKII IF strip using EF80s (plus a few other changes) known to have a better video band width? Or is it an alignment issue?

Thanks.

Ian
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Old 28th Aug 2019, 10:41 am   #2
Lloyd 1985
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Default Re: Bush TV22 "cog-wheeling"

Is that test card from an aurora with the 1k tone playing? Just wondering if it could be a bit of sound on vision that clears up after a while? If it’s present on all other signal sources, or without the 1k tone then that rules that one out!

Regards
Lloyd
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Old 28th Aug 2019, 10:46 am   #3
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Default Re: Bush TV22 "cog-wheeling"

Could be sync/video caps,to EHT "Brushing",bad earthing on crt etc.Not Alignment fault.
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Old 28th Aug 2019, 10:48 am   #4
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Default Re: Bush TV22 "cog-wheeling"

Thanks Lloyd, but no it's not sound of vision. I've got the same effect on an Aurora and a Domino. Also confirmed by removing the sound phonos.
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Old 28th Aug 2019, 10:51 am   #5
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Default Re: Bush TV22 "cog-wheeling"

Quote:
Originally Posted by HamishBoxer View Post
Could be sync/video caps,to EHT "Brushing",bad earthing on crt etc.Not Alignment fault.
Thanks Hamish.

The alignment query was separate. I noticed the video bandwidth seemed slightly better on the MKII than the MKI. I guess that's quite possibly one of the improvements Bush made with the MKII.
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Old 28th Aug 2019, 11:10 am   #6
Peter.N.
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Default Re: Bush TV22 "cog-wheeling"

'Cogging' usually displaces larger sections of the picture, it doesn't look like a sync fault, more like an
EHT arc as Hamish says.

Peter
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Old 28th Aug 2019, 11:30 am   #7
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Default Re: Bush TV22 "cog-wheeling"

Hi Ian, I think your terminology is incorrect - I would describe that as jitter.

Cog wheeling is a specific type of sync fault where the video content gets into the sync path and is the reason for the black & white castellated border to the test card as the fault causes the line timebase to follow the leading edge of the video causing entire groups of lines to be displaced according to whether the video starts off with a black or white castellation.

In particular, the circle of the test card turns into an impressive cogwheel, hence the name.

I'm sure I've seen many pictures illustrating this effect but can't find one at the moment.

Anyway, back to the fault in hand. The heat business is unusual - heat related faults usually appear when the set gets hot, not the other way round! I wonder if a freezer spray might be useful here? Wait for the fault to clear, then spray individual components in the sync separator and line oscillator areas to see if you can induce the fault.

I assume you've already checked that all the resistors in these areas are within tolerance?
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Old 28th Aug 2019, 11:56 am   #8
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Default Re: Bush TV22 "cog-wheeling"

Thanks Terry. Yes, I think my terminology was wrong. Jitter is probably a better description.

This set is the complete rebuild I wrote up in the BVWS Bulletin, Autumn and Winter 2007, so all caps and resistors are new, although of course there could be a fault now, or a fault in something that I didn't replace - a wound component for example. It worked unbeautifully for years.

It doesn't seem to be a dry joint as I've poked and knocked about all over the inside of the chassis components and wiring.

I can't see or hear any EHT arching, but following suggestions here I need to have a much closer look in the dark. But to me it looks more like some instability/oscillation as it's not random.

Anyway, thanks all for the suggestions so far...

Ian
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Old 28th Aug 2019, 1:54 pm   #9
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Default Re: Bush TV22 "cog-wheeling"

You would not normally hear EHT,if it was bad crt earthing.May be try a meter lead from CRT aqua dag to chassis.

As a quick check and possible elimination.

The fact it has been unused a while makes me think along the lines I have stated.
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Old 28th Aug 2019, 1:56 pm   #10
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Default Re: Bush TV22 "cog-wheeling"

Another thing, Ian, it's impossible to tell from the photo but is the jitter occurring at field rate? Not, of course, that I can think of a logical cause of interlaced jitter!

Components which were new 12 years ago won't be new today! True, you can say they haven't suffered the intensive use that they would have done when the set was new.

But how many time have we replaced 1M and higher values in much younger sets because they have drastically changed in value, yet the the power they have dissipating is not much more than if they'd be left in the original packet they came in?

Of course, if any were NOS for authenticity, they could be many times older.
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Old 28th Aug 2019, 5:10 pm   #11
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Default Re: Bush TV22 "cog-wheeling"

Check C25 the 16uF electrolytic. This decouples the Line oscillator and the IF chassis.

Keith
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Old 10th Sep 2019, 6:42 pm   #12
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Default Re: Bush TV22 "cog-wheeling"

Still looking at this. I think I'll describe it as "line jitter", as it's more random than my OP photo shows. Maybe a tell-tail sign is that it only happens at a particular brightness. That can be set by turning brightness control up and contrast down, or vice verca; when at a particular brightness the line instability/jitter starts.

I've briefly substituted V4, V5, V6 and V6 (EF80) on RF chassis to no avail. Looking at various waveforms with a scope around V3, V4 and V6 and in the RF sync circuits nothing seems amiss, although maybe a random micro-second variation wouldn't be obvious anyway.

There are no odd noises coming from anywhere either.

Getting stuck now as I don't really know enough to investigate much further.

Keith - I checked C25 16uF by bridging it an no difference. No randomly varying voltages across it on a scope either.
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Old 10th Sep 2019, 7:29 pm   #13
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Default Re: Bush TV22 "cog-wheeling"

Ian, you still haven't said whether this could be field related or not. When it occurs, is it every other line which is displaced, even if the effect runs up or down the screen. If alternate lines are displaced, it must be field related and that might point to the areas needing a more critical inspection.
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Old 10th Sep 2019, 7:51 pm   #14
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Default Re: Bush TV22 "cog-wheeling"

Hi Ian,
Try swapping the two ECL80 valves. It might be an unwanted effect of the frame blocking oscillator. I once had a Ferguson 941T that generated UHF interferance which was solved by changing the blocking oscillator valve.

Cheers
Andy
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Old 10th Sep 2019, 8:44 pm   #15
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Default Re: Bush TV22 "cog-wheeling"

If it is brightness related this possibly points to an EHT fault.
Check that the CRT dag coating is earthed to chassis as a poor connection here will give the fault symptoms you describe. It is also possible that the EY51 has seen better days.
John
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Old 10th Sep 2019, 8:54 pm   #16
Ian - G4JQT
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Default Re: Bush TV22 "cog-wheeling"

I don't think I can post a video on the forum, but I've put a short one showing the effects on the UK Vintage Radio fb page.


Unhelpfully it's turned the image by 90 degrees, but anyway I think it shows what happens when the brightness is turned up. I might be wrong, but I don't think it's related to the EHT.

At its worst - but most stable - it does appear to be alternate lines that are displaced.

Last edited by Ian - G4JQT; 10th Sep 2019 at 9:00 pm.
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Old 10th Sep 2019, 9:42 pm   #17
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Default Re: Bush TV22 "cog-wheeling"

In the RF deck there is a 50mfd capacitor which decouples the cathodes of the RF amplifier V1 and the vision and sound IF amplifier V3. It is designated as C4 in decks with EF91 valves and C5 in the EF80 version.
Might be worthwhile checking this component.

DFWB.

Last edited by FERNSEH; 10th Sep 2019 at 9:48 pm.
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Old 10th Sep 2019, 10:05 pm   #18
Ian - G4JQT
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Default Re: Bush TV22 "cog-wheeling"

Well I'll be... ! Thank you Jonathan Eastwood from the UKVR fb forum.

I did look in the dark on Jonathan's suggestion and saw reflected inside the LOPT metal shield a tiny, faint blue speck. A wire was barely making contact with its solder tag and was arcing at the bottom of the LOPT former. And although adjusting the brightness didn't make any visible difference to the spark, it obviously did!

It must have been like that for a very long time because once I fixed it, the EHT came up and some picture and ion-trap adjustments were needed.

Thanks Jonathan, but also thanks to everyone who made suggestions.

Regards,

Ian

Last edited by Ian - G4JQT; 10th Sep 2019 at 10:14 pm.
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