Hi people !
Still trying to bring my C64 to life again.
When I bougth this one (it's the second I have) it was suppossed to be fully working, but it doesn't work at all.
The failure is very strange, since it isn't the clasic blank screen, o garbage caracters.
Instead of this, when I turn on the computer, I can only see lots of short lines. Also, the TV produces a high frec noise comming from the CRT.
Turning on and off, sometimes I get some sort of image with the commodore screen barely visible, with lots of lines. Sometimes the image contain two comodore screens fited in the CRT !!!, and of course, out of scale.
The dataset sometimes work. When I turn on, the motor runs, stops for two second and run again. Sound like the CPU is working after all.
ViC II is fine, since I tested it in another C64.
A clue for the problem ?
Lest see:
MPU frec is around 750KHZ !!!!!!!. The color clock seems to be fine (I put seems, because is very difficult to measure with my scope).
Dot clock is wrong. About 6Mhz and so is the MPU clock.
The 4044 phase detector has the 1Mhz signal coming from the color clock across the 74139 and 7474. And also the phase0 clock from the VIC II which is the MPU clock and which the dot clock divided by 8. This frec is aroun 750Khz.
So the 4044 has two signals which don't have the same frequency. The voltage value to the dot clock VCO is 2.6V.(The board is from 1982)
This control voltage is not enough to correct the VCO.
I tried to replace the 74ls629 (dual VCo) and the 4044 (pll) without any success. The same sympthom.
Just for fun, changing a bit the timing cap in the dot clock VCo, allow the C64 to display an "image", which was the area of the upper-left corner enlarged to the whole screen. The characters were enormous !!!!
Clearly, the machine works, but as long as there's no relation betwen the color and dot clock, no image is expected. Also, the MPU runs very slow.
Another question,
The C64 is PAL-N. Could somebody tell me why it has a 14.318 XTAL ?
A bit strange too.
Many thanks for reading this post and hope you can help me to fix this 64.
Regards
Pedro
Still trying to bring my C64 to life again.
When I bougth this one (it's the second I have) it was suppossed to be fully working, but it doesn't work at all.
The failure is very strange, since it isn't the clasic blank screen, o garbage caracters.
Instead of this, when I turn on the computer, I can only see lots of short lines. Also, the TV produces a high frec noise comming from the CRT.
Turning on and off, sometimes I get some sort of image with the commodore screen barely visible, with lots of lines. Sometimes the image contain two comodore screens fited in the CRT !!!, and of course, out of scale.
The dataset sometimes work. When I turn on, the motor runs, stops for two second and run again. Sound like the CPU is working after all.
ViC II is fine, since I tested it in another C64.
A clue for the problem ?
Lest see:
MPU frec is around 750KHZ !!!!!!!. The color clock seems to be fine (I put seems, because is very difficult to measure with my scope).
Dot clock is wrong. About 6Mhz and so is the MPU clock.
The 4044 phase detector has the 1Mhz signal coming from the color clock across the 74139 and 7474. And also the phase0 clock from the VIC II which is the MPU clock and which the dot clock divided by 8. This frec is aroun 750Khz.
So the 4044 has two signals which don't have the same frequency. The voltage value to the dot clock VCO is 2.6V.(The board is from 1982)
This control voltage is not enough to correct the VCO.
I tried to replace the 74ls629 (dual VCo) and the 4044 (pll) without any success. The same sympthom.
Just for fun, changing a bit the timing cap in the dot clock VCo, allow the C64 to display an "image", which was the area of the upper-left corner enlarged to the whole screen. The characters were enormous !!!!
Clearly, the machine works, but as long as there's no relation betwen the color and dot clock, no image is expected. Also, the MPU runs very slow.
Another question,
The C64 is PAL-N. Could somebody tell me why it has a 14.318 XTAL ?
A bit strange too.
Many thanks for reading this post and hope you can help me to fix this 64.
Regards
Pedro