Commodore 1084s monitor - smoking scan coil

[Keith]

This monitor hasn't been used for a few months but kept in a dry place. On switch on there was nothing on the screen and a slight intermittent high frequency noise. There was a slight burning smell. With the case removed a thin wisp of smoke could be seen coming from the scan coil area. No other signs of damage.

PSU was running but with low voltage outputs. Collector of horizontal output transistor had flyback pulses but only about 200V. DC current into output stage from 125V rail (down to about 60V) was 0.5A. No CRT heater visible.

With the horizontal scan coil disconnected, PSU voltages came back up to nominal, collector pulses now ~600V. DC current about 150mA and heater glowing. No smoke seen. On switch off a defocused vertical band was briefly visible on the screen. Checked various HV capacitors and diodes around horizontal o/p stage - all OK.

Any advice would be appreciated (I can post part schematic tomorrow if required).

[McMurdo]

smoking scan coils (with loaded lop stage) always reminds me of a disease of orion or similar yokes where the rubber wedges decompose and liberate some conductive, corrosive substance into the copper which eats it away slowly.

[1955APREN]

Hi
I have not any experiece of monitors but this type of fault as been seen on 14ins tv the rubber wedges that go betwen the scan coils and glass flare of the crt seem to pick up condenation this runns down into the scan coils making a short.
If you can remove scan coils from crt you should be able to see short. what make and number is the crt. hope this might help.
regards Derrick

[1955APREN]

Sorry Mcmurdo beat me to it as I was typingl

[Keith]

Thanks for the replies. I must admit I feared that it must be a problem in the yoke itself but couldn't think what could have caused it. I guess it's going to have to come off but even assuming I can clear the short, what are my chances of getting it back on and still having acceptable convergence and purity?

[Welsh Anorak]

A problem might arise if you have to remove a portion of the wire to make the repair. This isn't too much of a problem with your average TV but a monitor will show up any small imperfection - just try not to disturb the windings too much. Assuming there are wedges use a blade to remove them from the CRT bowl and mark the neck so you can get it back as close as possible. Then you can remove the problem one and deal with the damage.
Glyn

[Argus25]

I don't have the circuit for this monitor, and it seems very likely the scan coil short issues,,,but there is another problem that could do this. THe scan coil can be returned to either ground, or the power supply in some sets and can often have a coupling capacitor (S correction capacitor) in series with it, depending on the design, if that capacitor shorts out it draws current and loads the supply/output stage. So if there is a capacitor there it's worth a check. I see you checked the caps but it could possibly be one that got overlooked.

If you test the yoke's H coils in transistor sets typically the DC resistance is low, but you can check them for shorts on an inductance meter and compare with another yoke from a similar sized set that uses a similar power supply voltage.

[Keith]

I have now removed the scan coils and can confirm that the problem is indeed a burned corner of the winding that was in contact with one of the rubber wedges (I would never have guessed this scenario so thanks to McMurdo and 1955APREN). I'm going to attempt to rescue this, based on Glyn's advice, but I'm not entirely confident of success. I'll post some pictures shortly. Thanks to all respondees.

[Lloyd 1985]

Ah yes, I had this problem on an old Saisho TV, I couldn't repair the scan could in the end, so I swapped them with a more modern set, worked a treat! Even the convergence was pretty close, close enough to only have some errors in the top right corner, and only noticeable on Teletext.

Regards
Lloyd

[Clydeuk]

I had this problem many times in the 90's. I never tried repairing the coil as there seemed to be too many parts of the winding burnt which I thought would make it difficult to repair. I always kept the yokes off duff sets to transplant into any of these I came across.

I never had any problems with setting the purity or convergence. I always used the rings off the set and marked their position before removing. Minor convergence errors could usually be corrected using the rings and careful manipulation of the yoke in each corner, wedging into the best position (I kept wedges off old CRT's as well, so as not to reuse the problematic ones)

I can't remember the resolution of the Commodore monitors, but I think they have a smaller dot pitch than a TV of the same size. I might still have one in my parents shed, was working fine when it was put away. Can't remember if we threw it in a shed clearout though.

You seem to have been fortunate with this one. In all the portable TV's I saw with this problem the Horizontal output transistor had shorted. It got so common that when a 14 inch set came in with a failed output transistor the first thing I would do is remove the wedges one by one, the evidence was clear to see on the wedge.

[Welsh Anorak]

It's worth a go - you've nothing to lose. However what you can get away with on a bedroom TV isn't the same with a monitor.
Glyn

[1955APREN]

Hi Keith
I have just had look in my shed and located a A34JXV70K53 crt with scan coils
as far as I know this working ok . It was taken out of a 14 ctv . The tube was made by orion > I do not know if this could be made to work in your monitor
but if would like to try please send me a PM
Kind regards Derrick

[Maarten]

It may work but it won't have the same small dotpitch so details will be lost.

[Keith]

Thank you for your kind offer, Derrick, but the CRT fitted here has the part number M34EAQ01X (made by Philips). Maarten seems to think yours is a lower resoloution tube (although possibly the scan coil is the same or thereabouts).
I'm presently persevering with an attempted repair. Having removed the charring (just one corner), 7 turns of the winding are o/c and a few more have lost their insulation. I will bridge the breaks and attempt to insulate the bare sections. As someone said, what have I got to lose!

[Boater Sam]

If you can get continuity and separate the cleaned windings, space the uninsulated parts with a thin mylar or paper sheet, and give them a coat of varnish to hold them together. With care I think this will work, I have saved transformer windings this way.

[Maarten]

If the "new" winding is physically too far from where it used to be located, it could give (small) geometry errors.

[Keith]

Well - this turned out to be a labour of love!

Having marked the yoke and magnet assembly as best I could, they were both removed to inspect the extent of the problem (see photos). It didn't look too bad at first sight, but having cleaned off the black charring, it was clear that a lot of insulation had been lost and quite a few windings were burned through.

Nevertheless, I thought I'd have a go at a repair. Taking lots of pictures I dug out all the broken ends and lifted the wires that had lost their insulation. After I thought I'd repaired them all I tried to check for continuity. This turned out to be much more complicated than I thought as each half of the X winding is split into three parallel sections!

It turned out that I had initially got some wires crossed over but eventually I got consistent resistance results for all the sections. Then it was a case of slipping small pieces of mylar (?) packing tape between each repaired/uninsulated wire and trying to re-pack them down approximately as they were originally. I also insulated the rubber wedges before re-assembly.

I have to say that I wasn't too optimistic, but amazingly, it worked. The line output stage came back to life and I had a reasonably pure raster, albeit with some convergence errors. I'm going to run it for a while before attempting more adjustments and will post a picture of the final result. Luckily, my son only uses it for a low res graphical application so, if it continues to work, it should be acceptable.

[1100 man]

Hi Keith,
Very impressive even to attempt repair!! (bonkers of course, but impressive!) How on earth did you manage to not get crossed windings? Surely resistance readings would not be accurate enough to tell if you had connected to an adjacent turn?
Very well done that it actually works
All the best
Nick

[John123]

Great success! I think that's the way to go as encasing the repaired windings in some sort of epoxy or varnish may prove to be unreliable in the long-run, and could even make any sort of secondary repair attempt impossible.

Perhaps some of those little tube magnets like what were used in Sony Widescreen's could be used to bring in the geometry..but as you say, if it's acceptable then probably best to leave as is.

[mark_in_manc]

Goodness me. I second Nick's question - how on earth did you work out what goes with what??