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 [ 10 posts ] 
 68K Mac classic clone 
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
Hello everyone! This has been a project I've been working on for a while now. I figured I would share it's progress!

It is a 68K computer but meant to be a clone of the Apple Macintosh classic (or any of the macs of the same style). I want to make it as close as I can, even down to the CRT screen. It will use a 68000 processor, use 30 pin SIMM DRAM, and have a floppy drive and hard drive.

I decided to work on the screen first, since I have a fascination with CRTs and always wanted to get one working. I'm not going to use a pre-made board to drive the tube, I want to make a driver board myself and figure out how it all works. I also decided that instead of trying to figure out the windings to the flybacks I have, I was going to wind my own as well.

My first step is getting the tube to light up at the very least, just a dot on the screen. And yes I know I shouldn't have just a dot without deflection or else I could burn the phosphor, I'm being careful about that.

The tube is a 5 inch monochrome found in many portable TVs, I have found the tubes all use the same neck pinout which makes it easy for me since I have a lot of those tubes. So if I break one I have more that will work. Heater voltage is 12 volts, anode voltage I'm guessing would be in the 10 to 15 KV range. I don't have any tools to measure the anode voltage so I don't know the exact voltage, but thats my guess for a small 5 inch.

The flyback uses a 3d printed bobbin with a U core I tool out of a different flyback. The rectifier is a strong of 5KV diodes with high voltage capacitors in between. The primary is around 10 turns, the G2 winding around 150 turns, and an axillary winding thats 20 turns. The anode secondary is approximately 1100 turns. The whole transformer is cased in resin, with bubbles removed using a vacuum chamber.

The test circuit uses the reference schematic I keep seeing for horizontal drive, with a horizontal signal of 15 LHz generated by a 555 timer. I have the HOT transistor set to only let 2 amps pass as a start. Horizontal deflection is not yet implemented.

Testing this setup, I got a dim spot to show up on the screen. It's nowhere near as bright as it should be, but it's a start. I have a few ideas on why it's not bright enough.

My transformer anode secondary is not putting out enough high voltage. I believe it's because there aren't enough secondary turns, which I will fix in the next transformer. I also think it has to do with the capacitors parallel to the rectifier diodes. I think they are letting the negative pulses through, lowering the high voltage. I will most likely omit them in the second transformer, since most teardowns I saw don't have them anyway (one did have them which is why I included them, but the transformer I saw had them in a different configuration than I did).

The inductance on the primary is too low. From my understanding, flyback transformers need a higher inductance to store energy into the core which mine can't do (someone please correct me if I'm wrong on this). I can fix this with some tweaks to the bobbin size and more winding on the primary.

I probably don't have enough current going into the HOT. Since it's current limited to 2 amps, the primary voltage isn't getting the full 12 volts. Raising the base current would fix this, but then the primary pulls too much current due to its super low inductance.

The timing / retrace capacitor could be the wrong value, since this influences voltage output. I could also be missing other timing components that the reference designs don't show. Or it could be because I'm prototyping it on a breadboard which is already asking for trouble.

My next step is remaking the flyback to mainly have a higher primary inductance, more anode secondary turns, and the rectifier caps removed. But being this scuffed design made the tube show a faint dot, I feel I am learning a lot about the operation of these tubes. I will post more about the computer itself later, along with progress on the CRT and the new flyback. Also if anyone would like to share their ideas on why my high voltage is too low, I would greatly appreciate it!


Fri Dec 09, 2022 1:12 pm
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
And some pictures of the experimenting process!


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Fri Dec 09, 2022 1:17 pm

Joined: Mon Oct 07, 2019 2:41 am
Posts: 619
If all failes, you still have a TV. :)
How about doing a APPLE LISA clone, if you can get the roms?


Fri Dec 09, 2022 7:36 pm
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
Hey all! Time for a long overdue update.

I'm still working on the monitor, but I have made a lot of progress since last post! I did get sidetracked really bad and built an entire coil winding machine since I dont feel like hand winding a flyback anymore. So that machine took a huge chunk of my time, but it works very well!

My second revision of flyback is much larger than the first one as I'm using 36 gauge wire for the secondary. I have 38 and 40 gauge though so future flybacks I make will be much smaller. I also added more secondary taps for +-15 volts for the vertical amp, +-50 volts for the cathode and possibly a focus voltage if I feel I need it (although from what I've researched 5 inch tubes just tie focus to ground), and a 150 volt G2 tap.

The result? A much brighter spot on the screen! So bright actually that it burnt the phosphor a little bit. My measurements though show that all my secondary voltages are too low. I have a few ideas as to why this is. My core gap is much too big so i need to make that smaller. The filter cap I'm using for G2 and the secondary voltages could possibly be too small so the voltage isn't getting high enough. I could be saturating the core since there is no actual load on any of the secondaries, Since I have noticed G2 will start near 100 volts and slowly drop to 80 volts. My timing / safety capacitor is still wrong so my flyback driver is not properly tuned. Just a bunch of small things like that. But even with a suboptimal test circuit, I'm still getting a really good emissions!

One main thing I need to change is how I'm driving my flyback. I just have a 555 timer running at close to 15 KHz but at a 50% duty cycle. From what I've read I want closer to a 25% duty cycle. My new horizontal oscillator design will pass the 555 output into a transistor ramp generator and THAT is fed into a comparator to adjust the duty cycle between 50% to 0%. Lowering the duty cycle should also lower power draw, as my HOT does get very.... hot :D

I still don't have anything to measure anode voltage but I feel it is sufficient enough to put together proper deflection circuits for testing. I will post further updates as I go along!


Thu Mar 23, 2023 1:33 am
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
Pictures of the experimentation process! (plus a schematic of the current horizontal oscillator)


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Thu Mar 23, 2023 1:37 am
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Joined: Sun Dec 19, 2021 1:36 pm
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Location: Michigan USA
Neat project! How are you driving and controlling the stepper motors on your coil winding machine?


Thu Mar 23, 2023 1:05 pm WWW
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
mmruzek wrote:
Neat project! How are you driving and controlling the stepper motors on your coil winding machine?


The motors are controlled by a standalone ATMEGA328 and a pair of DRV8825 modules! I do plan on building more of these machines for those interested once I am able to source the 328 easily, the one I used was in storage from last time I stocked up.

It's not at 100% functionality yet, It's still slightly buggy and one feature is missing. But i got it working enough to make the flyback!


Fri Mar 24, 2023 9:02 am
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
So after some fiddling with test circuits I am finding my main issue right now is im not driving my HOT into saturation, so it's dissipating a lot of the current as heat. My horizontal oscillator isnt able to provide enough base current even with a current boosting transistor after it. I will try throwing in another one for more current boosting and see if that improves things.


Tue Mar 28, 2023 11:37 pm
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Joined: Thu Oct 08, 2015 11:57 pm
Posts: 74
I decided to try a mosfet based horizontal output stage and I now have full high voltage! I will definitely need to make a new flyback with more primary turns because the primary draws a lot more current than i want it to. I also have partial horizontal deflection too! I will need to make a beefier shell in the new one though because I had some small arcing to my fingers when i tried moving it. Perhaps using PLA for the outer shell is not a good idea, regardless if i incase the whole thing in resin or not.


Wed Mar 29, 2023 1:04 am

Joined: Sat Sep 03, 2022 3:04 am
Posts: 51
Have you tried JBWeld (Ferrosilicon "steel" filler, 3x permeability of air) ? Or bulked up with with crushed ferrite.

As for insufficient HOT base current, "ground" at +3V and let MOSFET drive the emitter instead. Cascode.

Never wound a horizontal flyback, so talking sideways like I know something I don't...


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Wed Mar 29, 2023 9:32 am
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