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Ah, I had missed the CVT aspect. Although, I wonder how that extra winding, being disconnected, could be interfering at all. That got me thinking, though, about the fact that you have two transformers working in (possibly) close proximity. How nearby is the 220-110 transformer to the 110-15 one...

OK, so the output of that transformer should be a sine wave 50Hz signal. That makes the odd waveforms more curious. Sounds like some "bad" components someplace? Could be diodes, too.

Is your UK to USA step down really a simple transformer? Or is it a modern device that probably uses a switching power supply? I ask because I used a 12VDC-to-120VAC 60Hz power inverter to fake a telephone ring signal (modified it to drop down to 20Hz). The output waveform of those devices is...

The mainboard looks clean and in good condition, and I don't see any obvious problem areas. I do notice that the CPU appears to have been replaced, at some point over the life of the system. One thing we used to encounter some times was that a pin would get bent over and make partial contact...

It does appear to be a first-gen mainboard, and while the photo of the back is difficult to read it does appear that pin 21 is connected to +5V as shown in the schematics. A couple ideas of what might have happened: A) Something got damaged when removing the original ROM. Maybe carefully...

I can't think of any reason that the 28C16 would not work. But I'm assuming you have a first-generation mainboard. Maybe we need to see a photo of your mainboard to confirm what model it is.

So the debug ROM will leave garbage on the screen, as it does not initialize most of the hardware. You interact with the monitor using the serial port, typically connected to a PC (running a terminal program). I assume you meant you tried 81-149, since I've never heard of 81-159. With the...

Keep in mind that, from the perspective of a business trying to computerize - people trying to use a computer, DOS was not any sort of improvement over CP/M. Even the FAT filesystem had plenty of issues in the beginning, and it was fairly easy to corrupt a diskette. But users had no visibility...

I'm not sure the Zilog Z800 was even relevant. DRI was already making CP/M for other platforms, x86 and 86K and Z8000. They were poised to go with the market, but IBM decided the direction and made it difficult for any would-be competition.

There is some truth in that, but I think calling it "standardization" is being kind. Standardization implies a cooperation between dissimilar entities to agree on some common way of doing things. The IBM PC was not cooperation nor any sort of collaboration. It was simply a case of IBM doing a...

It was my experience that the name IBM carried a lot of weight, and when they announced they would be making a "PC" there was a drop-off in sales of all (other) micros from then on, regardless of whether they were 8-bit or 16-bit. And when IBM finally started shipping, there was very little for...

The standard CP/M BDOS will write the whole directory entry when any extent gets filled. So if a directory entry contains multiple extents, it gets written multiple times while filling up. Max "lost" is 16K under that scheme. As far as the definition of "lost", well... split that hair if you want.

Technically, the file is not closed. When the "extent" is full, it is closed and a new one opened. That extent may or may not represent the entire directory entry, but either way the directory entry (partial or full) is written to disk. This means that a file that is written but not closed will...

Measuring PSU output without a load will give higher readings, at least for old linear power supplies. Although, I expect that the load on the high voltage is so low that it probably does not drag it down much. 100 VAC (RMS) would be about 140VDC (rectified and filtered), taking into account the...

I'll just compare that to what I saw inside Wang programmable calculators from the same era, for much higher prices. You're not going to see the "quality" of fabrication that was seen only 5 years later. Even the PCBs look crude by modern standards, before any solder is applied. I don't believe...

FYI, I was able to get 28C16 EEPROMs, which might work better in modern programmers (doesn't require the odd voltages). Of course, legacy programmers can't handle EEPROMs. But the 28C16 should work in the Kaypro II just fine.

According to previous analysis, the characters produced by the "empty EPROM" test would not fall into the category of ones that get "corrupted". So, while this test confirms that basic video functions are working, it does not prove video RAM (or whatever is corrupting those characters) is...

My first thought is bad video RAM, but more might be revealed by using the monitor to try some patterns. As SomeGuy mentioned, being able to plant various data in different locations might reveal a pattern to the errors. There is "fill" command that allows you to put the same character in all...

You could try using the debug monitor ROM I created, which allows you to do specific things, like write to the video RAM directly. If you have the ability to burn a ROM, and can hook up a terminal (or PC terminal program) to the serial port then you might try that out...

(MicroSoft) M80 will accept Zilog mnemonics (when z80 mode selected). You should be able to find the manuals out there, too. You will also need L80 into order to link into a COM file. Not as easy to use as DRI tools in that respect, but if you must use Zilog mnemonics then probably the way to go.