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KC9UDX
August 22nd, 2014, 11:54 AM
Unbelievable!

My C64, which I don't really think of as a "Vintage Computer", it's just my everyday computer, just now DIED! I saved and ran a program I'm working on, and :crazy: the video display is scrambled and it crashes reading from disk. Either my VIC-II gave up the ghost, or the heavy-duty power supply called it quits. (The one that came with the 1764 REU)

:crazy::o:-?

Well, if it's the VIC, I'll miss the original colours, but I won't miss the sparkle...:-|

KC9UDX
August 22nd, 2014, 01:29 PM
20025
I wonder if there's a bad xtal...

20026
Naturally, every time in my life I start to get somewhere with a program, it gets corrupted.

KC9UDX
August 23rd, 2014, 09:41 PM
Well, I found the problem, or at least, the start of the trail. Dot clock is 5454kc, should be 8180kc. That frequency is generated by U31, a 74LS629 voltage controlled oscillator. The controlling voltage is about 2.1V. If I bring it up around 3V, the system comes to life. The control voltage comes from U32, an MC4044 phase-frequency detector.

Does anyone know what the controlling voltage for the VCO is supposed to be? In other words, do I have a bad U31, or a bad U32, or related components? This is a Rev A motherboard; and it looks as though the VIC II is not original, but it's a 6567 Rev 5.

KC9UDX
August 24th, 2014, 05:35 PM
Well, after checking everything I could check without desoldering anything, I made the bold assumption that the MC4044 is on the way out. I shunted R16 with a potentiometer, dialed it in, and buttoned the machine up. This may speed the demise of the MC4044, even if it isn't the real problem. But, at least my main everyday computer is back working and when it dies again, I'll have a good handle on the problem.