To add to my Lucky Moron PCB Repair series, I decided to start this sub-chapter called Boot Camp. Previously I fixed things that were just sort of not-working, but now we'll really get into it where the board doesn't even boot. To add to the fun, I don't actually know what I'm doing and don't have the solution worked out just yet. So if you're new to this, enjoy my flailing and probable failure. If you're good at this, chime in and let me know if I'm on the right path. Remember I only have to be right once!
First up, my previously working Gundam EX Revue seta2 board. It booted. Now it don't boot. Lucky for me I have the same PCB running Guardians, so I can cheat a little.
1) Power. Check. Chips are getting lovely 5v.
2) Program Roms. Check. Verified with MAME I'm running this one
3) Work Ram, I didn't see anything immediately wrong with the LH52B26D-70LL when logic probing
4) Clock, clocking. My oscilloscope shows me a nice clean strong signal.
5) Reset. It's resetting(!) Reset on this Toshiba CPU is active low, and it bumps low every second or two....
Is that a watchdog? What a weird name. So it barks "reset" because something in the circuit is broken? Why even build that? My korean Guardians doesn't look like it has one, just saying... Anyway I wrote out a whole schematic - look at it really! It's dope! I also compared the inputs and outputs with my Guardians pcb. And I think I found something!
The reset signal seems to come from U23, 74LS161. That's a counting chip as far as I can tell. It gets a clock signal from a little loop between a 74HC14, a resistor, and a capacitor. How? No idea, but that little loop generates a clock-like signal. The chip counts based on that clock. It just counts and counts until something resets it, and if nothing resets it - it freaks out and pumps output signals (namely the one that is resetting the cpu). Normally there is a combination of a few signals that decide if the counter resets, but on my board all the other options are grounded (i.e. out of the equation). There's only one thing that sends a reset signal -- MR, pin 1, Master Reset.
On my working PCB I can see MR get pulled low periodically. On my non-working PCB, MR stays high. So the watchdog counter is never reset, and my pcb gets a jolt from it every time the counter fills up.
So, what generates the periodic reset signal, you might ask?
A GAL16V8 at U51
Anyone got a dump? KA-101, might be used on some Guardians boards too, but sadly not mine.
First up, my previously working Gundam EX Revue seta2 board. It booted. Now it don't boot. Lucky for me I have the same PCB running Guardians, so I can cheat a little.
1) Power. Check. Chips are getting lovely 5v.
2) Program Roms. Check. Verified with MAME I'm running this one
3) Work Ram, I didn't see anything immediately wrong with the LH52B26D-70LL when logic probing
4) Clock, clocking. My oscilloscope shows me a nice clean strong signal.
5) Reset. It's resetting(!) Reset on this Toshiba CPU is active low, and it bumps low every second or two....
Is that a watchdog? What a weird name. So it barks "reset" because something in the circuit is broken? Why even build that? My korean Guardians doesn't look like it has one, just saying... Anyway I wrote out a whole schematic - look at it really! It's dope! I also compared the inputs and outputs with my Guardians pcb. And I think I found something!
The reset signal seems to come from U23, 74LS161. That's a counting chip as far as I can tell. It gets a clock signal from a little loop between a 74HC14, a resistor, and a capacitor. How? No idea, but that little loop generates a clock-like signal. The chip counts based on that clock. It just counts and counts until something resets it, and if nothing resets it - it freaks out and pumps output signals (namely the one that is resetting the cpu). Normally there is a combination of a few signals that decide if the counter resets, but on my board all the other options are grounded (i.e. out of the equation). There's only one thing that sends a reset signal -- MR, pin 1, Master Reset.
On my working PCB I can see MR get pulled low periodically. On my non-working PCB, MR stays high. So the watchdog counter is never reset, and my pcb gets a jolt from it every time the counter fills up.
So, what generates the periodic reset signal, you might ask?
A GAL16V8 at U51
Anyone got a dump? KA-101, might be used on some Guardians boards too, but sadly not mine.
Last edited: