The reset circuit on the 128 is driven by 2 555 timers (more accurately a 556, but same difference). There are a small handful of electrolytics on the various control and timing pins in that circuit, if they go bad that could affect it coming out of reset. I'd start by looking at the caps around the reset 556, check that they have good ESR, the right amount of capacitance, etc. It would probably be worth trying to scope out the various signals coming from, and within, reset, to help narrow it down, but the timing electrolytics would probably be a solid starting point even without doing that.
I don't know much about the C128 circuitry, but I know about the C64. If this were a C64, first thing I would to is connect a scope to the CPU reset line and do a lot of captures to see if there was a different behavior there when it boots, and when it doesn't. If the circuit is like the C64, maybe the cap that does the power on reset delay is bad, or maybe the CPU is getting a little senile and needs a little bit more delay time to gets its bearings right (=larger cap) to start working properly.
inserting nice comment here... It's always the journey that keeps me invested as you'll always have an opportunity to learn, challenge and explore the possibilities of endless outcomes! Look forward to the update!
The fact that it seems to boot reliably when cold STRONGLY indicates a temperature-related issue. Could be something as trivial as a capacitor. Some freeze spray might help narrowing it down. The Z80 is in control when the computer is powered on, and it obviously works. As griffinretro points out in his comment below, the fact that the system boots reliably to C64 mode every time implies that the fault lies with a component that's only initialized in C128 mode. That pretty much eliminates the reset circuitry as a possible suspect, as well as the VIC-II and the 8502 (and of course the Z80). In C64 mode, the RAM configuration (bank config 15) uses only RAM from bank 0, so the question is whether a flaky RAM chip in the upper bank would cause these exact symptoms. As RAM chips of this particular type are notoriously unreliable, that would be the first place I'd have a closer look with the oscilloscope probe. Another possible culprit would be the 8722 MMU.
To paraphrase the great Ron Burgundy “90% of the time it works 50% of the time”. Great video as always. I love your conversational style of videos it really makes me feel I’m part of the process.
Have you seen the power supply videos on Noel's Retro Lab's 2nd channel ("Noel's Spare Parts"). His problem was slightly different - C64 intermittently came up with no ready prompt. But it could still be a similar cause. Essentially, a missing output cap on a replacement switch mode PSU meant that the 5V rail did not appear cleanly, and that would cause it to not boot. On a 128, which also draws more power than a C64, maybe that could cause it to black screen. It's a thought, and worth trying a different power supply at least, if you have one.
Feels like a reset circuit issue, as in reset not being held low long enough. I don't know how the Commodore reset circuit works but usually it's a capacitor and maybe a 555 timer. P.S. Just looked up the schematic, yes it's a 556 timer (twin 555). Check the timing of the U27 and the ceramic caps etc. in that area.
I'd love to say - yes, it's this thing! But honestly I don't know. I did find a bad cap or two on the 128 that Lurch gave me ages ago, somewhere around the one you pulled out. But at the same time, it doesn't seem like your usual bad cap behaviour where they get better as they warm up. I think you're on the money with the bad sockets, the only question is which one. And as another side note, that 128 from Lurch eventually stopped booting due to a bad socket on the VIC-IIe. But it was also rusty AF in that area. Anyway you know where to find me if you want to pick my brain
Just went back and watched part of my old video - cringe. But the bad caps were at C101 and C80. However looking at the schematics, I'd suspect C91 and C92 as the only caps that could cause the issues you're seeing as they form part of the reset circuit. Perhaps the 128 is coming out of reset too early?
When you power on while holding the C= key down, the Z80 boot sector will start C64 mode directly rather than passing via the C128 KERNAL/BASIC to do so. Since it works reliably in C64 mode, my guess would be a marginal RAM chip in the upper bank (U46-U53) or something related to the C128 ROMs (U33-U35). The Z80 boot block and all the C64 ROMs are in U32. Also check that the power supply used can supply enough amps on the 5V line, the C128 draws a lot more 5V compared to the C64. The factory 128 brick is rated at 2.5A 5V.
I've not yet worked on a C128 (got one sitting here that needs work, but haven't gotten to it yet), so this probably isn't of any value whatsoever, but it's what comes to mind. I'd look at the RESET circuit timing. If it takes longer for the C128 to get everything settled down when booting into C128 mode than it does to boot C64 mode and the RESET is intermittently not holding long enough, that COULD explain your symptoms. If the C128 uses something like a 555 or 556 timer for reset delay, then the 555/556 or one of the passives around it (like maybe a capacitor) could be a problem. Like I said, though, I've not worked on a C128, so I'm probably talking right out my backside.
The power switch is a common cause for intermittent problems on the C128. Edit: Just realized the video is old and problem has probably been sorted by now.
One thing that frustrates me about many of these channels is the lack of actual investigation. Most rely on software diagnostics and/or trial and error and then hope for the best. Since the computer both works and doesn't work reasonably reliably, scoping the primary functions on the board when it works and compare to when it doesn't work is likely to be more useful (and educational) than just relying on pot luck.
Not a clue on this one but may be caps. I think the phrase you were trying to think of was "This doesn't bode too well for the other sockets" Love the streams keep it up. 👍
Im betting on the reset signal caps like others have said, would be cool to scope those lines a bit whilst switching on off to see what it looks like when it goes to the white line - perhaps it doesnt have long enough to discharge if swotched too quickly (i think the Amiga doesn't like thay, i think it confused Adrian a while back). Or its the sockets. Or the RAM. Or a MOS chip. Or gremlins 😂
It IS the best Commodore branded computer and my first. And I'm sure that it being my first micro does not influence my choice of it being the best one...
I had a similar problem with a Z80 computer, it turned out that it was the memory being temperature sensitive. You can test for this by either putting the computer in the fridge or using freezer spray on each memory chip or heating it up with a fan heater.
Just a thought but could it be that the reset circuit is caught between the thresholds making it not resetting the chips properly? With almost all devices it's recommended to wait 30s before switching back on again, allowing the caps to discharge and put the device in a proper known state. An oscilloscope probe at power and reset signal might give you some clues as to what's happening. Maybe the psu isn't stable before reset becomes inactive in all cases. And check the state of the cpu when it's hanging. Ie halt, reset signal etc and if it's still executing code. Also check if it's not actually the psu not powering up. I have an Amiga psu from icomp and doesn't like switching on/off quickly, it just stays off for the protection circuit to fuly discharge. After 30s it turns on again. It's not a good psu for troubleshooting or building a ReAmiga for example.
for 1 even if the ram passes the test there can still be a faulty ram that is missing a bit or 2 ... then i would check with a scope the ttl signals on the logic chips ... |I don't believe the rom is at fault it is or a flakey ram chip and good luck finding which one or a logic chip
I initially think caps too. The schematic shows the reset button before the 555 timer circuit (so it triggers the 555) If in black screen and you manually ground the reset pin on one of the CPU’s (& thus the other cpus as well) does ungrounding reset the computer back into life reliably? (But reset button doesn’t? Then 555 circuit) otherwise my guess is still caps
