Mega Force PCB?

[Retroman839]

I’m just wondering why wouldnt
That z80 come to life when you connected the +5v ?

Is it poss to measure the volts it’s getting when in socket ?

And measure what it’s getting when your adding resistor and clips ?

TTL 2.5 mhz z80 & 4mhz z80 need a strict +5v

Are you getting +5v at the chip leg ?

Cmos go down to 3.3v ..

 

[big10p]

I’m just wondering why wouldnt
That z80 come to life when you connected the +5v ?

Is it poss to measure the volts it’s getting when in socket ?

And measure what it’s getting when your adding resistor and clips ?

The CPU is behaving as expected. The problem is that it's constantly being reset by the watchdog. It looks like this is happening because of a fault in the memory addressing.

 

[ExZX]

Same seller that sold this Mega Force (Star Force) board has a non working Gaplus up for sale. He's honest that it's none working but once again he seems to know exactly what he's doing. That Gaplus is covered in customs and very likely a basket case

 

[big10p]

Same seller that sold this Mega Force (Star Force) board has a non working Gaplus up for sale. He's honest that it's none working but once again he seems to know exactly what he's doing. That Gaplus is covered in customs and very likely a basket case

Good detective work on where I got it. Lol

Pretty sure mine's a dud, but I got it mostly as a first step in board repair, and I've already learnt a fair bit. Wasn't going to buy but seller offered it for 100 quid. Don't think there's any customs on this, so there's a slim chance I can get it going at some point. Just getting the CPU going and get any signs of life on screen will be a win for me. Lol

 

[ExZX]

Good detective work on where I got it. Lol

Pretty sure mine's a dud, but I got it mostly as a first step in board repair, and I've already learnt a fair bit. Wasn't going to buy but seller offered it for 100 quid. Don't think there's any customs on this, so there's a slim chance I can get it going at some point. Just getting the CPU going and get any signs of life on screen will be a win for me. Lol

I saw it when it went up mate. Seemed to have some kind of water mark near the edge connector (could just be my poor eyesight of course). I just thought it looked like it had been stored badly. Could be wrong. Good luck with it

 

[big10p]

GitHub is confusing. Anyone know where the source code for Star Force is on there? There seems to be a directory for every game manufacturer except Tehkan. Looked in Tecmo, but no Star Force source there either.

 

[Jacmar]

GitHub is confusing. Anyone know where the source code for Star Force is on there? There seems to be a directory for every game manufacturer except Tehkan. Looked in Tecmo, but no Star Force source there either.

mame/src/mame/tecmo/senjyo.cpp at mame0284 · mamedev/mame

MAME. Contribute to mamedev/mame development by creating an account on GitHub.

github.com

 

[Georgian2]

GitHub is confusing. Anyone know where the source code for Star Force is on there? There seems to be a directory for every game manufacturer except Tehkan. Looked in Tecmo, but no Star Force source there either.

I use arcadefraidi.com for romident and click the link on the right hand side. This way i always land on the right mame driver for the romset that is on the pcb.

 

[big10p]

Thanks. I didn't know game source can be bundled together like that. I'm just trying to identify what the various ROM/RAM on my board is used for, specifically.

I've got the watchdog to shut up (mostly) and have red garbage on screen, with the occasional blip and chirp from the speaker. Still think there's memory addressing issues, but unsure how to proceed, really. I've been using the opportunity to learn how to use SLICE, but I think I'm getting a lot of false errors, due to something wrong up/downstream.

I don't know how to read schematics fully, but it's turned out that the Bomb Jack one has been useful, as it's very similar to my board, if not identical (different IC positions and pin outs, but the same or similar logic).

 

[big10p]

Bit of progress. Was checking the address bus connections from the main CPU that go through three ls367 hex buffer/driver chips. Found that the A8 pin on the CPU didn't have continuity to any of the ls367s. Traced back and found a tiny break in the trace that I could only see properly after removing the solder mask and viewing on my microscope.



I now seem to have both CPUs running solidly without any watchdog kicking in at all, but still just a screen full of garbage, but at least it's starting to look like graphics a bit, now. lol

 

[big10p]

Does anyone know what the N82S123 prom on the top board at 6C is for?

I noticed it was never getting read due to it's address lines being connected to a 4bit counter whose clock pin was never getting pulsed. I traced the clock pin back to an output from the Z80-CTC which seemed suspect when probing. I piggybacked the CTC which restored the 4bit counter clk and got the prom going, but nothing seems to have improved, visually or sonically.

I was hoping it was a colour/pallete prom or something, as I currently just have a monochrome display, but pretty sure I can see the game running through the screen corruption.

 

[favouredson]

Does anyone know what the N82S123 prom on the top board at 6C is for?

According to MAME, it is the DAC waveform PROM.
It is used for all the games on this hardware.

 

[big10p]

According to MAME, it is the DAC waveform PROM.
It is used for all the games on this hardware.

Thank you.

 

[big10p]

Heh, the monochrome display turned out to be dodgy video connections on the JAMMA adapter I got with the board. I was chasing a fault that didn't exist. I did learn from investigating the VRAM/colour RAM circuit, though. I would love to know what format data is stored, so when I view RAM contents in SLICE, I'll have a better idea if the data looks valid. My mind boggles at the fact VRAM is only 1k, of which, only 256 bytes is ever used! Lol

I replaced the Z80-CTC as it was indeed faulty, but I'm not looking any further into faults in the sound circuit until last.

I have a general question: how do I identify what all the various RAM/ROM chip are being used for, specifically? Romident tells me which ROMs are for graphics (gfx1, gfx2 etc.), but not what kind of graphics (tile, sprite, text). MAME source lists all ROM/RAM function and address range, but not board location, unless I'm missing something?

 

[Georgian2]

When in doubt, I pull an eprom out and see what part from graphics is missing.

 

[big10p]

When in doubt, I pull an eprom out and see what part from graphics is missing.

I was thinking about shorting address pins to see what gets affected, but wasn't sure how sensible the idea is. Lol

 

[big10p]

Almost there, I think. Phew, what an effort.

Things improved a lot, after finding this via between the cap was blocked with crud, and didn't have continuity with the via to the left of it (black dot). That connection carries the CPU read signal to a certain section of the top board.

I also discovered that replacing the faulty 6148 VRAM with 2114 wasn't a good idea. The 2114 isn't fast enough for the job. In my defence, I did replace it before I found out it was VRAM. There's quite a lot of 2114 on this board.

 

[sukhbir]

From memory those 6148 srams in my board set were the sprite line buffers which were 80ns access time, 3 of them were bad and I replaced them all with new srams at 70ns.

 

[big10p]

From memory those 6148 srams in my board set were the sprite line buffers which were 80ns access time, 3 of them were bad and I replaced them all with new srams at 70ns.

Yes, this board seems very similar to the Bomb Jack schematic, which has been very useful.

The faulty ones on mine were 6148LP which I've now replaced with same. Interestingly, the ones on my working 3-board Star Force are 6148HP 55, which I assume are 55ns.

 

[Georgian2]

You can replace 2114 with 2148 but not the other way round. 2148 is much faster.