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I bet his A: drive is playing up and he is having problems booting...

That's a heck of a thing. Look for ROMs and dump them with a reader. There may be a monitor or other bootstrap. Gosh imagine getting CP/M running on it, assuming it's not a clone of something (like a Cromemco, as suggested). What a challenge!

Thanks Larry. I've replaced the HOT with a BU407 (same as the originbal part) and fitted a 2W metal film resistor at R478. I have the picture back and it is stable, but as before R478 is heating up and it is pulling over 1.4A. I've not attempted to soak test it as I know the resistor will go...

I swapped the BU406 out for another one, which promptly went up in smoke... Grrrr.... have ordered another, along with a 1.2 Ohm 2W metal film resistor. The 7W wire wounds that are the wrong value are going back. I feel like I'm chasing my own tail here...

New resistors arrived, they are 1.6R which is very annoying. Plugged one into the board and the VDU is running, but the resistor still gets very hot (enough to boil water on the end of my finger) and the image stays steady for longer than before, but ultimately ends up glitching but not as badly...

Righty ho, last cap for the PSU arrived. Popped it in and the PSU is humming along nicely. But.. the VDU is still drawing too much, making the PSU click. I still have the burned resistor to replace, but for now it’s pulling nearly 1.5A which seems a bit heavy.

Hi Larry Looks very similar to what I ordered. They are ex UK military radio parts, I think and likely rather BIG. New old stock though.

Yes, I see your point. I was thinking of the traces I posted showing horizontal / vertical signals (idea being to see if the H section was interfering with the V section). Meanwhile a bunch of caps arrived for the PSU apart from one of them, and the 1.2R resistor hasn't turned up yet, either. I...

I bought a wire wound part that I think should do the trick. It's a bit high rated (7A) which I may come to regret, but it'll do for a test. Not expensive, but I will seek a metal film resistor anyway. I'm pretty sure I did that but found no correlation (I posted the traces). Of course, I...

Regarding the resistor, I bought a wire wound part that I think should do the trick. It's a bit high rated (7A) which I may come to regret, but it'll do for a test. Not expensive, but I will seek a metal film resistor anyway. The Kaypro PSU was defective before - it was tweeting when the...

We did. The 12v rail from the PC-AT PSU looked OK on the scope, but I did try another PSU and it gave the same result. The picture only straightens with the original PSU connected, how odd..! Resistor is still overheating although it now reads ~4 Ohms, I suspect due to it getting hot in the...

Right, I'll stick my neck out and say the deflection issue is sorted. On a hunch I decided to try and power the VDU with the Kaypro PSU (hopefully you'll recall that I had it running with an AT-PC unit with the Kaypro itself on its own PSU - with the grounds connected on both supplies). And the...

Two traces. In each picture the magenta trace is TP3 that we have seen before. They are shown with TP8 and TP7 of the horizontal circuit, and there is no obvious correlation between them and TP3's weird plateu. So, scratch that idea then! TP3 vs TP7 TP3 vs TP8 (Click to enlarge.)

Well.. I tried to read the technical docs for the TDA1170S vertical defection IC but it made very little sense to me. What I'm looking for is "how do the discreet components around the chip generate the vertical yoke waveform?" - but I can't follow it. During today's trial and error session, I...

All diodes checked out OK, with the exception of CR407 as it is inline (part of the HT lead which I'm not messing with). Found a burned looking resistor R318 but it checks out OK - measures 8.6R, should be 8.2R. Replaced it, has not helped.

I need to amend those statements somewhat. Resistors haven't been checked yet. Only those caps I mentioned are changed. Only one diode has been tested. PSU is an AT-PC unit which seems to work. Has no more noise than any other, at least. PCB has been looked at but happy to do it again...

This last test also occurred to me just now as I was driving home (minus the load resistor). I probed at TP3 and got this: It's not exactly the same as before (no spire at the start) but we still see the level + rampdown that is indicative of the on-screen artefacts. Just to be clear, the...

There's no continuity between the H and V coils, thankfully. CR301 checks out OK on my diode tester. R305, the 1.5R you mentioned, is reading 1.5R on my meter. Both components tested with one leg disconnected from the board. Interestingly, the R305 on the other board is reading 1.6R. I'm a bit...

I think the DM40 schematic is for a 12" tube and the DM30 schematic is for a 9" tube like mine (though as we can see it is not quite the same, and in fact, my board isn't quite the same as the photographed schematic that Larry sent me as it is missing Q298). As you may know I have Superbrains...

About this. The specification for this resistor is 1.2R @ 2W. Given that wattage is the product of voltage and amperage, doesn't that imply that the circuit shouldn't draw more than ~166mA given a 12v power supply? Or am I being a bit thick?