15

Thank you @Raffzahn for the comment that inspired me to think of something blocking the video signal. The initial problem described in the previous question was that the ZX81 was showing a white screen. I have an old version of the ZX81, so there was doubt about the absence of the back porch in the video signal. This doubt was removed by adding the 555-based ...


15

That portion of the video signal is generated directly by the ULA; early ZX81s just don't generate a back porch, which was seemingly good enough for the majority of televisions but problematic enough to be fixed in a later revision despite the machine's commercial lifetime being only a year. As you've observed, the absence of a back porch causes some ...


14

Unfortunately, the reason for random freezes can be hard to identify as a lot of different failure modes can lead to this outcome. Try to get hold of a bootable burn-in test program that works with a machine of this vintage that will put predictable and repeatable pressure on the CPU, RAM and I/O and see if there is any pattern in it triggering freezes. Fx ...


9

The problem is in the geometry. Not in the physical disk geometry, but rather in the translation how the BIOS must translate the geometry for DOS when using disks larger than 1024 cylinders. The laptop, or very close model of it, was sold with an 800 MB drive, so we have to assume that the laptop properly supports disks larger than 504 MiB, i.e. disks with ...


9

I fixed it. I used a known working logic board from a different mac and a multimeter set to continuity to find the bad trace. first I tested the first 16 pins of the simm slot to the ground on the scsi port on the working board for reference resistances. Then I compared them on the bad board, I found that pin number 9 was open when it should have matched ...


9

The garbled screen is normal on startup, but should be replaced by the BASIC screen after a moment. There's a rather detailed troubleshooting guide, including links, at http://www.dasarodesigns.com/projects/troubleshooting-common-problems-with-the-commodore-pet-2001/ I'd suggest that you ignore the spare parts sales pitch at the beginning and work your way ...


7

65C02 is a CMOS version of 6502 (which was NMOS) chip. Judging from Wikipedia articles on these chips and few of the datasheets I found, it seems like it should work exactly the same way it did on 6502, i.e. held high during normal operation, pulled low to fire an interrupt. Are you sure it is not connected to anything? I'm not too familiar with Apple ...


6

Whew, I've got it solved! I took one of the disk images with Windows 95 installed on it, and examined it with a hex editor. I noticed that when the Windows installer program initially sets up the system files on the C: drive, it puts IO.SYS rather "far" into the filesystem, actually more than 100 MB inward. What I tried then was to perform a clean ...


5

I've been meaning to get around to this for ages, sorry. Tonight I stumbled across SAMS COMPUTERFACTS for the Apple IIe which I had downloaded from the Internet Archive. Here is the logic chart data for the CPU (IC UC4) from page 27. I've rearranged it for clarity. Pin 6 should be high, as you say. (Note: I haven't verified this data on real hardware yet.) ...


5

In such cases, the problem is almost always in one of three places: One of the memory modules. The motherboard itself. A plug-in device. To help with the testing, you should start be removing all non-essential devices form the system (CDROM, Network, HDD, other I/O). Just leave the keyboard, screen and floppy. Then re-run your tests. If it works, the ...


5

Possible reasons IMHO could be: Older computers and consoles sometimes do not work with modern TVs. Modern TV needs a more precise TV signal, especially the timing and voltage level, than older TVs. There is a problem in the N64 itself - maybe some passive parts (resistors or capacitors) in the video circuit are degraded by time. It looks like a video ...


5

One thing I can say for certain: installing an earlier version of MS-DOS could not have made anything worse (or better, anyway). Both MS-DOS 6.22 and 7.x (the version built into Windows 95) rely on BIOS interrupt service 0x16 to access the keyboard. But that same thing also makes the fact that the keyboard is fully functional during the POST stage ...


4

Workbench 1.2 is known to have some bugs, and it would be better if you could boot into Workbench 1.3 instead. Based on your description, I suspect your mouse could be malfunctioning. It sounds like the left-button might be "sticking", meaning that it registers as pressed after you release it or fails to detect the "click" when you press ...


2

You mention there are 4 2Mb modules. If I remember correctly, many 486 era boards would let you populate in banks of 2. If your board supports it, you can try removing 2 of the memory modules and running the system with only 2 installed. If the problem persists, it is likely to be one of the two remaining modules. If the problem goes away, then it is ...


2

This is anecdotal, so take with a big grain of salt... I have a C-64 power supply that was home-brewed for me by my Dad when I was a kid from the remnants of the original. My Dad, never being one to do anything halfway, did a fantastic job and added a 'voltage adjust dial' and analog meter to the finished supply. This means I can actually dial-down the ...


1

This is a logical–physical geometry mismatch. Like @Justme’s answer explains, the ‘logical’ geometry used by the BIOS disk interrupt services, and the ‘physical’ geometry reported by the disk controller are not necessarily the same. This is because the BIOS interrupt call interface and the hardware interface of the ATA controller establish different limits ...


1

Many vintage PCs have a PS/2 port that can be configured (via jumpers, BIOS, or both) to work as either PS/2 keyboard, PS/2 mouse, or combined keyboard+mouse with a Y-cable adapter. Having the port configured differently than what is actually connected can confuse software and drivers and prevent the keyboard from working. However, the keyboard might still ...


1

I would recommend getting decent PS2 cable with RF shielding. I had the same issue when connected to TV, even though console's audio was muted.


1

Who made your 65C02? Was it WDC? If it's WDC, then there are a few pins that should be held high with a 3.3k pull-up resistor. Such as the BE pin which is only on WDC versions. BE (Bus Enable) should be pulled up. Some example taken from http://wilsonminesco.com/6502primer/MysteryPins.html VPB (or VP): With WDC's 65c02, leave it unconnected. With ...


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