Sharp MZ80-A

[centrespot]

Originally Posted by SiriusHardware

A good call by Owen, although that's bad news about what appears to be a vertical deflection coil fault. I don't think this can be a symptom of a fault on the tube itself and the difference in the resistances of the vertical deflection coils is a bit of a smoking gun, especially when the horizontal deflection coils both measure exactly the same.

Are those results definitely the right way around? Normally if coils fail with a changed resistance (as opposed to open circuit) they tend to go lower resistance due to shorted turns, so if you had presented those results and asked me to guess which was the 'bad' one I probably would have said that it was the one with the lower reading vertical deflection coil.

For it to have gone higher resistance I would be looking at the solder terminals where the wires from the PCB land on the tags on the deflection yoke - assuming you measured those resistances at the 'DY' plug which normally plugs into the PCB, try measuring the resistance of the coil where the copper ends of the coil are soldered to the tags on the coil assembly. If you are lucky you may just have some resistance in the leads or the plug / socket connections going between the PCB and the deflection yoke, or the soldered connections on the deflection yoke itself.

There is a known problem on some Philips monitors where little black spongy rubber wedges were inserted under the edges of the deflection coils next to the tube, and what sometimes happens is that they absorb and retain moisture which then corrodes the part of the deflection coil which the rubber wedge is in contact with - if you have anything like that on yours look carefully for any signs of corrosion, especially green 'rust', on the coil assembly where they are in contact with it.

There are a lot of TV / CRT experts here, possibly more on this site than anywhere else in the world, but not all of them visit this computer subsection so if you get to the point where you are considering either trying to replace the deflection yoke or have it rewound then it might be advisable to start a new thread about that in the 'Vintage Television And Video' section.

I measured the resistances at the solder joints and the plug.

No corrosion, no rubber wedges.

The yoke seems in good condition and looks quite complicated to rewind.

I will do as you suggest and see if there is anyone willing and cheap enough to undertake a rewind, alternatively I could look at fitting a 10" LCD instead.

Thank you for your advice and patience throughout this issue.

[SiriusHardware]

Even between the solder tags that the ends of the resistor go to, it was still higher than expected there? Put one meter lead on one leg of the vertical deflection coil and one meter lead on one leg of the horizontal coil and measure the resistance (be careful not to accidentally measure yourself). It should be infinite / open circuit but if you see an actual resistance between those points that would suggest a breakdown in the insulation between the two coils, which shouldn't be electrically connected at all.

The TV / CRT specialist guys might be able to suggest a contemporary black and white TV or CCTV monitor with the same sized tube and tube neck which you can 'borrow' the deflection yoke from - I'm showing my limited knowledge here when I say that I think these aren't usually very interchangeable because they are usually specifically made for the TV or monitor they are used in, but I may be completely wrong there.

If you do decide to try to have the vertical yoke rewound I would first take the extra step of transferring the suspect yoke onto the tube of the 'good' monitor and prove beyond all doubt that it is the yoke, because getting that rewound could be quite expensive and you want to be absolutely sure that it is faulty before you spend any significant amount on it.

You could also set an auction website 'watch' for another 'spares or repairs' 80A monitor, ideally one with a broken tube (therefore very cheap) which you can harvest the scan coils from.

Good luck, and let us know if you make any progress.

[Philips210]

I could be wrong but the field coils may be wired in parallel. The higher resistance reading could be that from just one coil. Therefore the other is either open circuit or there's a problem around the solder lugs where the windings are connected.
If there are two wires on each tag then perhaps carefully unsolder them and check for continuity across each coil.

Regards,
Symon

[ortek_service]

Originally Posted by Philips210

I could be wrong but the field coils may be wired in parallel. The higher resistance reading could be that from just one coil. Therefore the other is either open circuit or there's a problem around the solder lugs where the windings are connected.
If there are two wires on each tag then perhaps carefully unsolder them and check for continuity across each coil.

Regards,
Symon

Yes, I also thought that could be the case, and was just about to suggest that, as it could explain why a higher resistance than the working 'reference' one, was measured - Especially as the suspect one's resistance appears to be virtually double that of the OK one.
(I presume those resistances are including a fixed-offset from DMM's lead and that hasn't been deducted / it hasn't got a 'zero' adjustment that old analogue ones had / haven't got access to rather-more expensive bench DMM with 4 terminal 'Kelvin' measurement ability to automatically calibrate-out lead resistances).

But from the attached photo, I can't quite see if there are two of the enamelled copper wires from the coils going to each terminal (I can only see two wires from the middle coming out - with one going to each terminal?)

Of course, it's really the inductance of these that's the more important thing - but more difficult to measure as usually quite frequency-dependent. However, the inductance is usually is proportional to winding resistance given the same gauge wire on both. But if the magnetic material inside the deflection-yoke (that coils are wound on) cracked, that could affect their inductance without affecting their resistance.

If there is only one coil wire going to each terminal, then it might be worth re-soldering the joints, in case one has gone a bit dry.

It was rather handy having a working one to compare with, as it looks like the original Service Manual didn't have any details on these coils, in order to verify them. They do have some part numbers on them, but I can't see if there's any manufacturer's name anywhere, as that may aid finding a replacement. It could be the CRT manufacturer also supplied the coils (attached and maybe also separately) - I recall Mullard(Philips) had deflection coils, as well as CRT's, in their old databooks. Although, this is more-likely to be from a Japanese etc. manufacturer if it was made, as well as designed, in Japan / the Far-East back then.

[Philips210]

Hello again.

I assume the two CRTs/scan coils are identical to each other.

As an example, I was thinking about some Hitachi tubes that have differences in the scan coils. The CRT itself being the same but the suffix to the CRT type number would have a designation something like TC01, TC09 etc.

It was just a possible thought that the CRT in the monitor may not be the original one and has a different scan coil.

Regards,
Symon

[SiriusHardware]

For information, centrespot has now opened a thread about this in the Vintage TV and Video section.

https://www.vintage-radio.net/forum/showthread.php?t=207096

[SiriusHardware]

And it's now solved! See the thread linked to in the previous post.

[ortek_service]

Yes, in summary further examination + photos, revealed there was indeed two separate paralleled at the tags vertical-deflection windings.
And one of the Enamelled Copper Wires from these had snapped, a short distance from the tag - But it had been possible to make contact and re-solder a connection slightly further into the winding, restoring correct operation of these.