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Great news! Congrats on getting it fixed!

Yep-- 22kHz. The reason you can't use an audio amplifier is that it's a sawtooth wave, which to get the correct wave shape would need a whole bunch of harmonics  above that, going into the hundreds of kHz. Or looking at it another way, the yoke is a big inductor. Getting it to change current...

Conductive epoxy might work, but turns out a tiny tube of the stuff is more expensive than a whole new machine! Conductive paint is cheaper, but I don't know if it would have enough strength or long-term resilience. I might try to solder it. First, get a flux pen and put a healthy amount of...

Interesting mystery! Yes, I'd go for swapping UF8 (and maybe UG8 while you're at it). More specifically, I'll take a wild guess and put my bet on pin 2 of UF8-- check that there's continuity between UF8 pin 2, UF8 pin 12, and UG6 pin 15. If those connections are good then definitely replace...

It's a loss of vertical deflection. Could be the analog board, even the yoke connector, but just as likely caps on the logic board. I had an SE/30 with this problem (no VSYNC from logic board) which went away with recapping. Also check UF8 and UG8 which are involved in generating VSYNC. UE8...

Yeah I think a simple divide-by-2 will work well for 832x624 (49.7kHz horizontal). For Mac 640x480 (35.0kHz) or VGA (31.5kHz) it should probably be 3:2. Or at the least, there will need to be other changes to the flyback circuit to make it happy at 16-17kHz. Longer scan lines means more current...

A little bit more progress. I added a voltage reference (TL431) in the video comparator circuit to get a more stable threshold and tightened up the wiring. That seems to have eliminated the vertical bars on the left side of the image.   For the video signal, I also took U1 (the 7438...

Finally, since it was all set up anyway, I tried a Mac 13" 640x480 signal (same resolution, 10% faster scan rate): No problems on vertical sync at all (just like techknight predicted). The picture is a little shorter but could easily be adjusted with the trim that's already on the analog...

First resolution-changing experiments! I decided the best way to test different resolutions with the existing display was to keep the 1-bit video circuits for now and build a high-speed comparator circuit to turn a VGA-level signal into a logic output. I used an AD8561 comparator for this, with...

The schematic is in Post #5 in this thread, based on the principles at the link in Post #2. It's taken a while for me to understand how these flyback circuits work because the principles are pretty different from other amplifier circuits. (I do have a background in analog electronics, but not...

Yep, circuit design 101 (though still quite easy to forget). :) Here is the schematic, though the circuit isn't in front of me at the moment so there may be an error here. It's nothing too elaborate, basically the first half creates a pulse of variable width in response to a falling edge of...

Today I worked on the image horizontal centering. As you'll see in the earlier pictures, the image is shifted off to the right. That could be due to one of two things: either the video signal is shifted in time relative to the HSYNC pulse, or there is a DC current through the yoke which shifts...

Flipped pixels like this would pretty much have to be a digital issue. There's just no way an analog failure in the flyback, yoke or CRT would produce clean pixels with the wrong value like this. If the same logic board works in your other SE/30, then check the voltage levels, especially +5V...

Well real life has intervened for the moment, so I haven't had time for any more experiments... mostly. I did order one of these 9" "Super VGA" CRT monitors to test: http://www.ebay.co.uk/itm/121770406890 It is advertised as 800x600 resolution, but so far as I can tell it is plain vanilla...

Yes I'm pretty sure that's right. 640x400 = 32,000 bytes, so two pages would fit. The pixel clock would need to be 23.3MHz to get that resolution at 60Hz. I wonder if that's within a possible overclocking range? Maybe with a socketed board with faster CPU installed? Beyond that, you're right...

Good thought on the phase-locking techknight. I just did a quick test running the AB from a free-running oscillator at (approximately) the same frequency and duty cycle, while driving the yoke from the actual HSYNC signal. The result isn't terrible, but there is a faint vertical ripple that...

Yes, that's the plan. From everything I'd read and tried, it just seemed unlikely that the flyback circuit could be pushed far enough off frequency to support any other useful resolution. So instead, decouple them. It will be interesting to see what kind of intermodulation artifacts might pop...

Finding a yoke replacement is tricky because it needs to handle quite a bit of current, at least several amps. My candidate replacement was Vishay IHA-305, a 100uH inductor rated at 4.2A. It has a DC resistance of 0.07 ohms so it's not far off from the original yoke itself. The yoke coil...

The first power-up test! For now, the same HSYNC signal drives both circuits. Powering up the Mac without the horizontal coil attached, of course the result is a bright vertical line on the screen. Then bringing up the voltage on the yoke driver circuit, the display gradually widened until at...

So the plan is to disconnect the horizontal coil on the yoke from the main analog board and connect it to a new drive circuit which does the same thing. The first goal is just to get a decent image on the screen with these two separate circuits, which should prove the concept. Then it will be a...