Failed yoke on Xerox 6085 monitor

[compu_85]

After years of thinking it would never be possible, I've acquired a Xerox 6085 "Daybreak" computer and 19" monitor from 1986. This was as very high end system for its time, and runs the same software as the early 80s 8010 Star computer, generally regarded as the first computer you could buy with graphics and a mouse.



The monitor is special. It's 37kHz interlaced, with a P7 type phospor to reduce flicker. Mine is the 19", there was also a 15".

Unfortunately, the glue they used is the kind that turns brown / conductive / corrosive. On the yoke it holds some of the alignment magnets in place, and some edges of the coils. In these areas it ate through the enamel on the windings. Additionally, the coil was fitted very tightly on the tube neck. Some dag actually got embedded in the yoke wires, and made shorts in these areas.



I've started carefully peeling the wire apart, and the bad enamel goes deep into the windings. I just don't think there'll be any fixing this



So what to do? Finding another identical yoke will be basically impossible. I'm wondering if the yoke from a B&W set could be "close enough" to work? The tube has the "large neck" like like an earlier tube, not the "small neck" like a typical 80s 9" or 12" mono tube.

I'll keep picking at the yoke but my confidence on getting it to work again is very low. Considering this damaged happened while it was sitting it also makes me concerned about any other monitor, so getting a parts chassis might not help either.

Thanks,

-Jason

[MIPS]

I'm following along on this adventure on the side as well so I am aware you've tried a few other things but you don't have an actual yoke tester.
You applied a DC voltage on a fixed current through the windings in the hopes of locating a hotspot but on the thermal camera it was quite uniform all around.
What is the diameter of the neck? Inch and a quarter? What is the model of the tube?
I just find it weird that so much of the neck is shorted out.

For context, initial tests found the HOT was extremely hot and the system power supply (which also powers the monitor) was being dragged down when the monitor was connected. Compu_85 pulled the HOT (C3412) and did not find it shorted. Likewise he found that a dummy load on the PSU didn't cause the pulldown, so it's absolutely the display. With the HOT installed and the yoke disconnected he doesn't see a heavy load on it and the PSU behaves. All we could think of was the yoke was funky and that's why we pulled it and started looking for shorts. Could be a dead-end but we're not totally sure.




Edited: ah, he ninja'd me below.

[compu_85]

The neck is 29mm.

I've used my cheepie component IDer, the horiz. coil measured .9 ohm and .13 mH.

The initial problem was the monitor seemed dead. B+ from the switching supply was getting pulled down from 83v to 35v. HOT was getting... hot. Removed the HOT and tested it, seemed fine with a meter transistor check. Unplug the horiz coil and the monitor is much happier. B+ is normal. No HV though.

FWIW, here's a view of the chassis on this beastie:



You can see some of the glue which has turned brown / conductive. I've removed it since taking this photo.

Higher resolution version of that photo, and the back side of the PCB here: http://compu85.net/stuff/xerox/6085/monitor/

-J

[Electronic M]

If you have a SS scope with a calibration osc in it you can rig it up as a yoke ring tester by ringing it with the calibration osc and counting the rings on the scoop screen.

[zeno]

If you look for another yoke this may help. Look for the
FCC ID ## or type acceptance ## on the back. Go to FCC.GOV
and look it up. That will give you the actual MFG & model ## of
the set. Looking at the FBT it says Sampo so it may be them.
They were selling TV's about this time. This is the way most yokes
went BUT it was quite rare in SS sets. Maybe one per year.
FBT's went L & R.

73 Zeno
LFOD !

[compu_85]

Originally Posted by Electronic M

If you have a SS scope with a calibration osc in it you can rig it up as a yoke ring tester by ringing it with the calibration osc and counting the rings on the scoop screen.

Neat idea! I just got a Tek 2440...

The two halves of the vertical coil ring basically the same:
http://compu85.net/stuff/xerox/6085/.../vert_ring.jpg

The horizontal coil however...
http://compu85.net/stuff/xerox/6085/...horiz_ring.jpg

I think the whole monitor was OEM'd by Sampo.

-J

[compu_85]

I jumpered in the yoke of a 17" color CRT and the Xerox chassis seems to come up! Proper B+, and I hear high voltage. Very good news. My plan is to find the coils from a junky svga monitor and try to swap in the horizontal coils. I just have to find one...

-J

[JohnCT]

Originally Posted by compu_85

I've started carefully peeling the wire apart, and the bad enamel goes deep into the windings. I just don't think there'll be any fixing this

There might be. I've fixed a lot of yokes like this (Mitsubishi PTVs used to eat yokes like Doritos and then they became unavailable).

Keep separating the wires with a sharpened awl until you have them apart. I've had them go three layers deep but yours doesn't look that bad.

Even though there's several hundred volts on the windings there's very little between adjacent windings, so if you can get them separated, you can add a little liquid insulation. The arced winding don't need to be all that far apart because there's little voltage between adjacent windings, but you have to insulate your repair from the tube if the aquadag extends deep down the neck.

I used to use a very low viscosity cyanacrolate which would seep into the windings because of the low surface tension. Sometimes it took 10 or more coats to get it fully insulated and bonded, but I had great luck doing yokes this way.

John

[compu_85]

Originally Posted by JohnCT

...I used to use a very low viscosity cyanacrolate which would seep into the windings because of the low surface tension. Sometimes it took 10 or more coats to get it fully insulated and bonded, but I had great luck doing yokes this way.

So you'd pull the yoke apart, coat all the wires a couple times, then press them back together and give it another coat so it "holds form"?

-J

[JohnCT]

Originally Posted by compu_85

So you'd pull the yoke apart, coat all the wires a couple times, then press them back together and give it another coat so it "holds form"?

-J

You don't need to make it pretty and put the windings back exactly where they were originally, just make sure the compromised windings are isolated from each other a bit (you don't need a big separation here) and then coated. If the repaired windings will get crammed against the neck of the tube when the yoke is mounted, then you can add a coating of something like Corona Dope over the windings, or clean the tube and put a couple of layers of black electrical tape on the tube (you don't want the adhesive side to touch the yoke windings.

John

[compu_85]

I've ordered a NOS 110* mono yoke, RCA PN 109532-A. The shape of the windings looks like a perfect match.



My plan is to swipe the horizontal coils out of this and put them in the original Xerox bobbin, with the Xerox vertical coils.

Does anyone happen to know what the original application for the RCA yoke was?

Thanks,

-J

[damen]

109532 was used in the KCS-138 chassis with a 19AXP4 110 degree CRT. May have been used in others also.

[compu_85]

Ooo so another 19" tube. Cool!