In case you cannot tell from the subject, I am a n00b when it comes to Vic-20s. I will admit, since the early 80's as a kid I have dreamed of having one (not sure why since I had a TI994a), but here I am in my early 40's and I snagged a broken one off of eBay with the intent on resurrecting it.
A little info on the Vic I have. It is an older style with the two prong AC adapter.
I had hoped that only a fuse burned out and upon opening it I was greeted with just that. I replaced the fuse and fired it up. I had to piece together a video cable since I did not have an RF modulator. The red light was on but I only received a blank screen. I know that SOMETHING was happening since my composite input portable DVD player went from a blue screen to a black one.
I did a little reading on the subject and checked some voltages. I took the heat sink off of the full wave bridge rectifier and put some more heat grease on it, screwed things back together and read some voltages. It appears that all the chips were getting the recommended 5 volts. I viewed the ripple on the voltage with my little handheld 5 MHz scope and saw that there was nearly no ripple. The big fat juicy cap in the power supply also seemed to be functioning properly.
I turned the Vic off and set my probes up to look at the output pin of the 555 timer used to reset the CPU. When I turned the machine back on the pin was high and stayed there. (Keep in mind I checked this pin before the inverter). I suppose this is part of the problem. Can a bad cap in this circuit cause this behavior? Also if I take pin 3 from the 555 and ground it through a 330 ohm resistor will that trigger a logic change in the reset circuit?
Next I moved on to look at the clock signal. I suppose the 14Mhz signal going into the the 6560 gets chopped down to around 1Mhz coming out of the 6560? Is this correct? Also there is a variable cap near the Xtal does this adjust frequency or does it help initiate oscillation?My scope could not see something that high in frequency so I am not sure that the clock is functioning properly.
I did check the clock going into the 6502 and only saw what appeared to be around a 1 volt P-P sine wave. Is this correct? When I looked at the data sheet for the 6560 it states that the clock should be a solid 5 volt signal. My question here is. Is the clock signal supposed to be a 5 volt square wave running at approx 1Mhz at the 6502?
Can anyone comment on the questions in red? I try to probe the machine for at least an hour each night to learn a little more, I'd really like to get this thing working since I am getting sick of playing with a VIC emulation
Any help would be great
Thank You
