Thanks. I'm still confused why my logic probe shows pulsing on another game board that has the same 4mhz crystal as this one. I'll check the caps if I get nothing on the scope.
Mega Force PCB?
- Thread starter big10p
- Start date
Ok, I've checked the caps and resistor connected to the clock and they're all fine. I couldn't SLICE the ls367 as some of the pins stubbornly refused to pass the floating check. Could this be further indication the ls367 is bad?
Here's a scope reading of the clock. Definitely not right. The frequency is bouncing around all over the place.
Just for contrast, here's a reading of the 12MHz clock on the bottom board, working fine.
I've ordered a replacement crystal, just in case, but I think I'll have to try replacing the ls367 and maybe the couple of ls74s that look like they may be in the mix, too. Any other advice appreciated.
Here's a scope reading of the clock. Definitely not right. The frequency is bouncing around all over the place.
Just for contrast, here's a reading of the 12MHz clock on the bottom board, working fine.
I've ordered a replacement crystal, just in case, but I think I'll have to try replacing the ls367 and maybe the couple of ls74s that look like they may be in the mix, too. Any other advice appreciated.
I was considering that, but if crystals hardly ever fail, I thought it would likely be wasted effort. lol. I think I will give it a try, though. Ta
It could be that some of the pins of the LS367 just aren't used. Check for tracks on the PCB. If a leg has no tracks going to them they will show up as floating.
Slice will then ignore those legs as they are irrelevant. If a leg has tracks going to it but it's still showing as floating it could mean the component the leg it connected to is faulty or you have a damaged/broken track.
Ah, right. I'll check. Seem to remember reading that in the manual now, but I have a memory like a sieve. Thanks.
Not sure if SLICE is going to be much help at this stage, without a clock, as all I/O seems to be stuck high or low at the moment, but worth a try.
Looks like only pin 9 of the ls367 isn't connected to anything. After a fair amount of clip wiggling, I got all pins to pass the float check in SLICE, including pin 9! Hmm, anyway, this is what SLICE shows. Basically, all pins stuck high or low.
I think I will try swapping out the crystal with one from another board, if only to rule it out...
I think I will try swapping out the crystal with one from another board, if only to rule it out...
It's connected to pin 1 which is the activation pin, I think (active low). Pin 3 goes to the CPU clock, and pin 2 goes to the clk2 pins of two ls74s.
Pity there's no schem, but I read that the Bomb Jack one is similar. I've been trying to decipher it. It does seem to show a tc40h00p being used in the same way.
Is there a cheaper alternative to a tc40h00p that I can buy if I need to replace it? They seem quite expensive.
Is there a cheaper alternative to a tc40h00p that I can buy if I need to replace it? They seem quite expensive.
On the bombjack schematics it's actually a TC40H004. Is that the chip on your PCB? Maybe the markings are hard to see?
Anyway the TC40H004 is a set of six inverters. You need to see if there's an output on any pins that have a signal on the input.
I.E. If there's a signal on pin 1 there should be an inverted signal output on pin 2.
Anyway the TC40H004 is a set of six inverters. You need to see if there's an output on any pins that have a signal on the input.
I.E. If there's a signal on pin 1 there should be an inverted signal output on pin 2.
Ok, I couldn't get any sensible readings on the TC40H004P so I pulled it, fitted a socket, and replaced it with a CD4069UBE, which looked like a good, cheaper alternative. At about £15-20 a pop (with shipping) for a TC40H004P, I don't want to start throwing money at this board until I get some sign of life, as I'm starting to think I've bought a complete lemon. Lol
Anyway, I now have a working clock, but the CPU is still constantly resetting. I've managed to trace the pulsing signal on the CPU reset pin back through a couple of other ICs to pin 6 of a 74LS393 at 6E. This is apparently a dual 4-bit counter (which is what's shown as 5N on the schem above, I think), so I have a bit more reading up to do before chasing things any further.
To be continued...
Anyway, I now have a working clock, but the CPU is still constantly resetting. I've managed to trace the pulsing signal on the CPU reset pin back through a couple of other ICs to pin 6 of a 74LS393 at 6E. This is apparently a dual 4-bit counter (which is what's shown as 5N on the schem above, I think), so I have a bit more reading up to do before chasing things any further.
To be continued...
That counter will be tied to something that constantly resets it if the software is running correctly.
If the game crashes the CPU is then reset so the game restarts. This is called the watchdog.
You now need to see if the CPU eproms,ram and other associated circuitry is working. Since there should be activity you can start slicing the LS chips.
If the game crashes the CPU is then reset so the game restarts. This is called the watchdog.
You now need to see if the CPU eproms,ram and other associated circuitry is working. Since there should be activity you can start slicing the LS chips.
I'm pretty sure you could also use a 74HC04 rather than the TC40H004P or the CD4069UBE. (Might even be a better alternative than the CD4069UBE)
74HC04 cheap and available.
Unless I'm wrong ...
edit.. just checking the above and appears I may be wrong, think you want an unbuffered inverter, so a 74HCU04 or your CD40469 so looks like you made a good choice!
74HC04 cheap and available.
Unless I'm wrong ...
edit.. just checking the above and appears I may be wrong, think you want an unbuffered inverter, so a 74HCU04 or your CD40469 so looks like you made a good choice!
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Ah, thanks! I know the eproms are good as I pulled and dumped them all to replace lots of sockets and romident to find exactly what version of Star Force it is.
The reason I said this board might be a lemon is because I pulled all the socketed SRAM to replace some sockets and clean the corrosion off the legs, and they all tested as bad. ☹️ I'm not sure if I killed them all cleaning them up, or something very bad has happened to this board, like being struck by lightning or something. Lol
I'm new to all this so tried to get something as close to the original IC spec as possible, but duly noted. 👍
Tbh, I'm not even sure why a CMOS chip is being used, when a TTL looks like it would do. But I'm a newb so figured it best to stick to CMOS.