LisaList2

Quote from: mst3k on September 26, 2026, 11:27:58 AMI found a variety of specs

The original Apple schematic of the video board gives the resistance of the windings as
 Vertical: 4.6 ohms
 Horizontal: 0.62 ohms

So if you've reversed H & V, the measurements seem fine. It isn't impossible that the schematic has them backwards, so measure the good yoke's resistances for comparison.

I have seen an overheated horizontal winding connector, which might warrant replacement in some cases. Since the connector contacts are relocatable in the housing, it would be possible to make a mistake and swap vertical and horizontal connector configurations. Check the wire colours vs the good yoke to confirm they go to the same places in the connectors.

Did you verify the resistance between the windings and the ferrite is high? Unless the connectors are miswired, I'm predicting a shorted turn due to the magnet wire having been pressed against a sharp edge of the ferrite. See the section from one of your pics for a potential spot of the concept. The cuts through the varnish might be impossible to see; you'd have to find them by watching the resistance as you tweak the fragile wire away from the edge of the ferrite.

Quote from: sigma7 on September 26, 2026, 04:00:20 PMThe original Apple schematic of the video board gives the resistance of the windings as
 Vertical: 4.6 ohms
 Horizontal: 0.62 ohms

I checked one of the working Lisa yokes at the connectors and got 4.0 ohms on the red/blue and 0.5 ohms on the green/yellow.
Nearly identical to the 'bad' yoke.
I guess I have them backwards?  Everywhere I have looked the green/yellow is ID'd as Vertical and the red/blue as Horizontal.  I must be looking in the wrong places...  If the schematic is backwards that works too.

My V and H resistance specs came from searching the Clinton CRT, not the Apple spec.  Not sure why they would be so different but way off from the quoted Apple spec.

Quote from: sigma7 on September 26, 2026, 04:00:20 PMDid you verify the resistance between the windings and the ferrite is high?

I get OL from any connector or connector attachment point to the ferrite core.  Are you saying I should remove some insulation from the windings and check that resistance to the ferrite core?

I'll give the windings a look see with the magnifier to see if there is something visible around the point of contact with the ferrite.

Thanks too for helping me try to get this thing working.  My eyes are bleeding from searching to find someone who repairs yokes or locating a replacement. 

 

Quote from: mst3k on September 26, 2026, 06:50:34 PMI checked one of the working Lisa yokes at the connectors and got 4.0 ohms on the red/blue and 0.5 ohms on the green/yellow.

That agrees with the schematic.

QuoteI guess I have them backwards?  Everywhere I have looked the green/yellow is ID'd as Vertical and the red/blue as Horizontal.  I must be looking in the wrong places...
 
My V and H resistance specs came from searching the Clinton CRT, not the Apple spec.  Not sure why they would be so different but way off from the quoted Apple spec.

The Apple schematic says green/yellow is horizontal... and (if I am reading accurately) you have a good yoke where green/yellow is horizontal too. Those are definitive, so if your bad yoke matches them, there is a subtle problem (and if it doesn't match, please explain).

Other (presumably non-Apple) documents that have different conventions/specs just don't apply here. The tube (or other component) manufacturer typically documents a "reference design" that is a starting point for OEMs to incorporate the component in their product; having a working example helps minimize questions about aspects that aren't otherwise documented. Some OEMs will follow the conventions in the reference design, others (especially Apple around this time of NIH syndrome) will do something completely different.

In this case, the Lisa has unusual video with more lines and higher horizontal frequency than is standard, so the reference and other designs with more conventional video specs are bound to have differences.

QuoteI get OL from any connector or connector attachment point to the ferrite core.  Are you saying I should remove some insulation from the windings and check that resistance to the ferrite core?

No, no need to remove insulation; checking from the connector to the ferrite is sufficient. I suppose my theory is wrong.

Not knowing what else to suggest, I'd put the good and bad yokes beside each other and stare at them very carefully looking for differences. Doing some kind of experiment to isolate which winding is the problem might reduce the task by 50%.

More electronic equipment could narrow things down if you happen to know someone with a lab full.

I hope your eyes recover soon... I'm confident you won't find a suitable replacement from any place other than another Lisa.

edit: I agree it is curious that the colour code is plainly reversed rather than just different. I suppose that might be a coincidence or reflect a minor mistake at some point and they just went with it.

I thought it was dust, but it is green.  It has texture and it can't be removed with a Q-tip and a dab of IPA.
Appears to be corrosion between several of the windings.  Really hard to see, but it is there.

Quote from: mst3k on September 27, 2026, 05:38:55 PMI thought it was dust, but it is green.  It has texture and it can't be removed with a Q-tip and a dab of IPA.
Appears to be corrosion between several of the windings.  Really hard to see, but it is there.

Good find! Indeed, I can't see it. If it is green like corroded copper, then that probably indicates missing insulation (varnish) plus moisture.

I saw one of the spring clips holding the ferrite together is a bit rusty, so that may indicate water on the yoke at some point.

You could try a dab of vinegar to see if the green will dissolve in weak acid. Make sure you wash the vinegar off afterwards with lots of water, and then wash the water off with IPA and dry well to prevent rusting.

If there isn't much green, then separating the windings slightly may be sufficient to test the theory that this is the problem. If separating them does work, then you'll want to find some more reliable insulation than air.

A little vinegar, a good wash, a lots of IPA...

I guess the only thing holding those wires together was the corrosion.

Someone very dextrous and determined might be able to repair the breaks in laproscopic fashion.

As the windings don't really need to be tight/snug (loose/sloppy ones can be glued in place), the connections can be done away from the other wires, but the connections do need to be between the correct wires.

If it looks like the corrosion goes further than the few visible wires, then perhaps too much to fix.