Electronics > Beginners
Troubleshooting Microprocessors
bostonman:
The bad quality schematics I have don’t seem to show PHI0.
Where is it?
james_s:
--- Quote from: techman-001 on December 28, 2019, 08:42:31 am ---My point is that there are two ways to fix this old technology, the fast way with a few simple tools and lots of experience, or the *really* slow way with a workshop full of hitech gear, schematics and every possible aid.
--- End quote ---
The first way is obsolete when dealing with vintage gear, there isn't the volume to get that kind of experience and the stuff is rare enough that you don't hack and slash or just give up if you can't fix it in a few minutes.
Then of course there is a whole spectrum in between those two extremes. I have found a happy medium with a modest collection of tools and gear and schematics when available.
james_s:
--- Quote from: bostonman on December 28, 2019, 02:45:12 pm ---
--- Quote ---If the address lines are not showing any activity at all (provided you've identified them correctly), that is a very bad sign.
--- End quote ---
Is it a bad sign because the failure is catastrophic or complex to find? This was a working C64 a few years ago, placed it in "storage", and now it doesn't have a display. As for the voltages, the power supply was built by me and works on two other units; it also worked on this C64 before I placed in storage. The 5V is a bit low though (I used a 2A or 3A regulator). With no load it's 5V, at power on it's 4.9xx V, and after the inductor (L5) it's 4.88V, but two other C64 units work fine with this power supply.
During power recycles, sometimes the address lines will show some activity (although I don't know if it's correct as the signals just jump around). Also, from what I can tell, the clock signals are present such as 14.3MHz and 8.18MHz (at U30 and U31 respectively).
I'm trying to avoid fast and hacking methods to fix this C64 for a few reasons: It's my original C64 and I'd like to keep it in tack without hacking the board, it was working before placing it in storage and now it doesn't work indicating (hopefully) one component has failed, and replacement chips are hard to get (I'm trying to avoid drop in replacement stuff designed within the last few years such as the PLAnkton which is a PLA replacement).
Thinking it was the PLA (U17), I unsoldered it using a hot air pencil and for whatever reason, the pin broke from the body of the IC. So now I'm avoiding the option of randomly removing ICs; and I don't have a vacuum desoldering station (just good old solder braid and the heat pencil).
--- End quote ---
On the contrary, I'd say it's a good sign because it points to the problem being something relatively simple to find. It's a lot easier to troubleshoot something that is completely dead than something that behaves in some bizarre way.
I'd check the CPU clock and reset lines first, you could have a bad clock, stuck in reset or it's possible you have a bad CPU. IIRC the C64 uses a custom derivative of the 6502 and failure is common due to one of the pins being routed directly to the expansion connector.
atmfjstc:
--- Quote from: bostonman on December 28, 2019, 06:36:21 pm ---The bad quality schematics I have don’t seem to show PHI0.
Where is it?
--- End quote ---
Check those links I provided. PHI0 is pin 17 of the VIC-II. On the C-64, the CPU is not clocked directly by the oscillator. Instead, the oscillator clocks the VIC (on pin #21, DOT CLOCK), the VIC divides the frequency by 8, and outputs a ~1MHz clock for the processor on pin #17, PHI0. This should go to the 6510's pin #1, PHI0.
This particular feature of the C-64's architecture means that if the VIC-II is removed or completely toast, the CPU will not work either, which might explain why there's no address activity.
Thus, you should check whether a 1MHz clock is being output on pin #17 of the VIC, and if yes, whether it is actually getting to pin #1 of the 6510 (could have a broken trace, who knows).
bostonman:
--- Quote ---OK, so the COLOR (14.3MHz) and DOT (8.18MHz) are going into the VIC-II chip. Now check whether the 1MHz clock for the CPU is being output on pin PHI0.
--- End quote ---
I took pictures to make it easier than going by my description. Hopefully the file names are posted.
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