A strange Commodore 16 reset line


I’ve mentioned a few times that my Commodore 16 has some strange boot behaviour, though its gradually getting worse. The machine won’t power on immediately, but doing a power cycle after a couple of minutes (usually) brings it to life. The hardware reset switch also does nothing.

I checked the power regulator and VCC on the CPU and confirmed 4.90 V on my cute little ZEEWEI scope. I thought that might be a bit low, but I checked my fully-functional Plus/4, and it operates fine at 4.91 V.

I next tried tried substituting the PLA, CPU, and TED one by one from the C16 to the Plus/4, all of which worked perfectly. I also have a 64K memory upgrade below the TED, which I removed and re-added with no difference.

Given the reset button also doesn’t work, I decided to do a bit of digging on the reset line. I’ve watched enough 8-bit Commodore repair videos to know they typically start low for half a second(ish) before going high. I wasn’t sure if this applied to the 264-series of machines, so I probed pin 40 on the 8501 CPU in my Commodore Plus/4, and got a reset pulse of about half a second before it went high, as expected.

Photo of my tiny scope showing the reset line high on pin 40 of the CPU inside this C16.

But here’s where it gets interesting (or not, I’m not sure)! I probed the exact same pin on the Commodore 16’s 8501 CPU (shown above), and the signal goes high immediately when the machine is started, and stays there. Sometimes the machine boots on first try, other times I need to power cycle it a few times. The only constant is the reset line stays pinned at 4.90 V, seemingly regardless of what I do.

When I’m able to boot into Rob Clarke’s excellent 264 diagnostic cart, it passes with (literal!) flying colours. So I’m wondering if its something as simple as a timing issue? But I’m not even sure what components are responsible for setting the duration or width of that reset pulse on the C16 specifically.

Anyone got any ideas?

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