Sharp MZ80-A

[SiriusHardware]

I think that 'ringing' on the output waveform is probably real because it is quite a good fit for the visual effect being seen on the screen.

Normally digital scope artifacts happen when the sample rate is not significantly more than, the same as, or less than the frequency of the waveform under observation but here the vertical output waveform is probably no more than 50 or 60Hz, so the scope sample rate should be many times higher than that.

Also, other test points have been looked at using the same scope, probe and earth arrangement and the waveforms there look as they should.

[ortek_service]

Well if that 'noise' is actually ringing / an HF oscillation, then may be best to expand the 'scope timebase / up it's sampling rate to prevent aliasing, to see it more-clearly.

As to possible cause of ringing / HF-oscillations, the schematic does show a 'damping' resistor fitted in parallel with the vertical scan-coils and located on these that could have gone o/c. But as it's in parallel with the low-resistance coil, would need one end de-soldering to check it.
Unfortunately the schematic doesn't show it's value / wattage (I presume it was ready-fitted by the scan-coils manufacturer), but hopefully it still has markings on it (or could isolate the one on the other MZ-80A monitor, and measure that)

Otherwise, it's probably as case of measuring all the resistors and capacitors around the vertical field output IC - particular those in the feedback loop / frequency-compensation parts - as well as the output drive path (and maybe supply de-coupling. as might be some unintentional feedback from output drive to input if supply rejection isn't too good).

It would be good to have a datasheet for this IC, being as schematic only has pin numbers. But it seems to have a rather cryptic (Sharp?) RH-IX0015TA part number (not too clear what are 1's and I's or O's and 0's on this schematic, due to the font they've used). But maybe it's clearer on the part itself.

I did find that an IX0015 is equivalent to a still quite obscure VPC1031N-UPC, from (A similar PDF-version table attached) this list: https://www.***********/doc/208340752/Tabela-Substituicao-Ci-Sharp etc
(Whereas other IX00nn parts are equivalent to more-conventional TA7nnn / SN74nn etc IC's)

However, it looks like there was a bit of a typo in that equivalent, and it may actually be a rather more common NEC uPC1031 (as used in Amstrad CPC monitors). UTC version Datasheet (also a copy attached) at:http://www.unisonic.com.tw/datasheet/PC1031.pdf

And this was also mentioned in that eevblog frame-collapse thread: https://www.eevblog.com/forum/repair/repairing-the-monitor-of-a-sharp-mz-80a/ (Although if their replacement didn't actually work, that might call into question whether it really was a direct-equivalent)

[centrespot]

Originally Posted by ortek_service

Well if that 'noise' is actually ringing / an HF oscillation, then may be best to expand the 'scope timebase / up it's sampling rate to prevent aliasing, to see it more-clearly.

As to possible cause of ringing / HF-oscillations, the schematic does show a 'damping' resistor fitted in parallel with the vertical scan-coils and located on these that could have gone o/c. But as it's in parallel with the low-resistance coil, would need one end de-soldering to check it.
Unfortunately the schematic doesn't show it's value / wattage (I presume it was ready-fitted by the scan-coils manufacturer), but hopefully it still has markings on it (or could isolate the one on the other MZ-80A monitor, and measure that)

Otherwise, it's probably as case of measuring all the resistors and capacitors around the vertical field output IC - particular those in the feedback loop / frequency-compensation parts - as well as the output drive path (and maybe supply de-coupling. as might be some unintentional feedback from output drive to input if supply rejection isn't too good).

It would be good to have a datasheet for this IC, being as schematic only has pin numbers. But it seems to have a rather cryptic (Sharp?) RH-IX0015TA part number (not too clear what are 1's and I's or O's and 0's on this schematic, due to the font they've used). But maybe it's clearer on the part itself.

I did find that an IX0015 is equivalent to a still quite obscure VPC1031N-UPC, from (A similar PDF-version table attached) this list: https://www.***********/doc/208340752/Tabela-Substituicao-Ci-Sharp etc
(Whereas other IX00nn parts are equivalent to more-conventional TA7nnn / SN74nn etc IC's)

However, it looks like there was a bit of a typo in that equivalent, and it may actually be a rather more common NEC uPC1031 (as used in Amstrad CPC monitors). UTC version Datasheet (also a copy attached) at:http://www.unisonic.com.tw/datasheet/PC1031.pdf

And this was also mentioned in that eevblog frame-collapse thread: https://www.eevblog.com/forum/repair/repairing-the-monitor-of-a-sharp-mz-80a/ (Although if their replacement didn't actually work, that might call into question whether it really was a direct-equivalent)

The MZ-80A manual gives the IC as a uPC1031 H and this is a datasheet I found on google

https://www.datasheets360.com/pdf/6588144762620559109

[centrespot]

I've checked my stock of IC's and found these

[centrespot]

The screen with the foldover issue has this IC (see photo)

[centrespot]

The screen with the centre spot / horizontal line has this IC (see photo)

[ortek_service]

Originally Posted by centrespot

??
>>
The MZ-80A manual gives the IC as a uPC1031 H and this is a datasheet I found on google

https://www.datasheets360.com/pdf/6588144762620559109

That's strange, as the Schematic in that Service Manual only shows Sharp RH-IX0xxx part numbers, so I guess uPC1031H is listed on a parts list elsewhere? - Maybe Sharp decided to just use standard-marked parts, rather than make their own version / have them made for them by NEC, but marked with custom Sharp type numbers.

Originally Posted by centrespot

I've checked my stock of IC's and found these

Well that's rather useful, to have that many unused ones. Although I did note these have an extra '2' suffix, so wondered if that may be an updated version or maybe a slightly-different variant (Linear Technology used to be rather fond of having -1, -2 & -3 etc suffixed IC's, that did work differently)

Originally Posted by centrespot

The screen with the centre spot / horizontal line has this IC (see photo)

Well, that one has the extra '2' suffix. And it also appears that it's not as tall as the (original?) non-'2' suffix version. So is this not actually bolted to the heatsink (may have been previously changed?). or is there some extra bracket behind it?
- If it's not heat-sinked, then that may cause it to fail / do odd things when too hot.

[SiriusHardware]

Great that you have all those ICs to call on, and are they all seemingly new-old-stock, or are they 'pulls' (do they look as though they have been desoldered from equipment?)

It looks as though the H / H2 may signify the difference in the package, with the original 'H' being considerably taller, and with that centre hole punched out in the IC's heatsink which is a good match for the heatsinks in both monitors.

The one in the 'spot' monitor appears to have been fitted without any attempt to heatsink it at all and that may have led to the demise of that IC.

Suggestion:

Concentrate on the 'spot / horizontal line' monitor for now. I am assuming that you will already have either tested or substituted all of the capacitors which were referenced in the previous posts, if you haven't, do that first, then look through the scope test points. We need to determine whether the frame collapse on that monitor is due to a faulty passive component or the IC, and then we'll move forward from there.

[centrespot]

I've made progress on the screen with the centre spot / horizontal line issue.

Pin 2 of the IC was 0v and pin 10 was 11.4v

I had 11.6v on one side of R2013 and nothing on the other side so I replaced it.

I now had correct voltages on both sides of R2013 but still nothing on IC pin 2 but I did have the blank screen again (no horizontal line) and the centre spot was there on powering off.

I re-soldered the IC pin 2 joint and bingo ! I now have a fully working screen.

[SiriusHardware]

That's obviously great news and a big step forwards - however I do have some concern about that IC not being adequately heatsinked because it is not bolted to the heatsink the way the larger IC would have been.

If you can think of a way to do it, consider extending the metalwork of the large heatsink which is currently arching over the top of the IC but not touching it so that the heatsink of the IC can be bolted to it to keep it cool.

Looking at the area around these ICs in both monitors, do the boards look identical in every way except for the fact that one IC is tall and the other is not?

[centrespot]

Originally Posted by SiriusHardware

That's obviously great news and a big step forwards - however I do have some concern about that IC not being adequately heatsinked because it is not bolted to the heatsink the way the larger IC would have been.

If you can think of a way to do it, consider extending the metalwork of the large heatsink which is currently arching over the top of the IC but not touching it so that the heatsink of the IC can be bolted to it to keep it cool.

Looking at the area around these ICs in both monitors, do the boards look identical in every way except for the fact that one IC is tall and the other is not?

Yes they are identical

I've added a temporary additional heatsink to the working monitor.

[SiriusHardware]

Yes they are identical

Good, well, my reason for asking was to determine whether you should be able to use a 'small' IC in place of the 'tall' one. If the only difference between the two PCBs is the size and shape of that IC then that suggests that they are pin for pin compatible.

Well, now to your other monitor, the one with a partial / folded frame collapse.

If you are confident that you have eliminated all of the capacitors in that area of the circuit as a possible cause, and also those resistors in series with the supply to the chip - then, as you appear to be fortunate enough to have not just one, but more than one replacement 'small' IC, I would suggest trying one of those in place of the original 'tall' IC to see if that restores the raster to normal height.

If it does get it working, great, but the same caution re: The need for a heatsink to ensure long term survival of the IC will apply there too.

[centrespot]

IC changed but it hasn't made any difference to the foldover issue.

I will check all resistors and capacitors in the vertical circuit again.

[SiriusHardware]

Well, at least you were in the lucky position of having a chip to try and you've now been able to rule the IC out. It might be better to put the exonerated 'tall' chip back in now, as that one was properly mounted on the heatsink.

When you've rechecked all relevant capacitors and the resistors inline with the power, could you just remind us, by part numbers, of all the ones checked so far? We may have missed one somewhere. I'll have another browse through that section of the diagram.

[SiriusHardware]

Another thing to try, if you haven't already looked at them: Make a note of the exact current positions of the V-Hold, V-Size and V-Lin preset controls (Take photos?) and then try varying each one of them slightly either way (several times) from their current setting before returning them to their current settings. Does making a small adjustment to each of those internal controls in turn have zero effect, a small effect or a dramatic effect?

[ortek_service]

Yes, as it seems the IC's haven't been failing in these, it should now be safe to swap in the original taller non '2' suffix IC into the one with field foldover, to try. Just to also check there isn't some other difference between the '2' and non '2' suffix ones besides the height of body / tab.

And to save too-much swapping back & forth, you could just leave that in place, fitting a '2' suffix one to the now-working monitor, as a double-check.

Rather than having to try to extend the heatsink, it may be simplest to just bolt it to the heatsink and extend the legs if required - by laying some solid-core Tinned Copper Wire (can use off-cuts from leaded resistors legs etc.) on top of each, so they overlap, and soldering in place, before inserting into PCB. Sliding some tight sleeving / using heatshrik over solder joint joins, may also be a good idea, to prevent them coming apart if soldering the end of the wires in the PCB a bit too long.

BTW, Did you try disconnecting the parallel damping resistor on the vertical scan-coils, and measuring that, in case it had gone high?
You might also try swapping the scan-coils over between the monitors, if not glued to the CRT (or swap the monitor PCB's over, if easier - taking care to discharge CRT's final-anode cap to ground through a resistor and well-insulate probe lead,before disconnecting), just to check there isn't an issue with these (Could have shorted turns etc, but may be able to accurately measure their resistances to compare)

[SiriusHardware]

I did consider the idea of raising the chip up onto the existing heatsink and extending its legs, but wondered whether extended leads might have unexpected consequences with regard to stability, etc.

I agree it's a good idea to check the relative resistances of the vertical scan coils on both monitors - ideally with them unplugged from the PCB. It seems to be a 4-way plug in connector (Lettered 'DY' for 'Deflection Yoke' on the layout diagram) and I think the vertical coil is on the two pins which have a bigger gap between them than the others.

[centrespot]

I think I swapped the tubes around in the early days of testing, when I swapped them around the foldover issue stayed with the circuit board but I will try again today and test the coil resistances while disconnected and the ohms of the parallel damping resistor.

[centrespot]

Originally Posted by SiriusHardware

I did consider the idea of raising the chip up onto the existing heatsink and extending its legs, but wondered whether extended leads might have unexpected consequences with regard to stability, etc.

I agree it's a good idea to check the relative resistances of the vertical scan coils on both monitors - ideally with them unplugged from the PCB. It seems to be a 4-way plug in connector (Lettered 'DY' for 'Deflection Yoke' on the layout diagram) and I think the vertical coil is on the two pins which have a bigger gap between them than the others.

There is a difference in the resistances of the tube coils and when I swapped the tubes around the foldover issue stayed with the tube.

The good tubes resistances are 2.9 and 0.7 ohm
The bad tube resistances are 5.0 and 0.7 ohm

I removed the parallel damping resistor and it measures 410 ohm (colour code is 390ohm 10%)

What would be the way forward now ?

[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.