commando logic probe findings

[stevebm1]

had a bit of spare time earlier,so dug out another commando boot from the repair pile,it boots to a garbage screen,poked around a bit with the logic probe,and found no clock signal was getting to one of the z80's pin6 it was stuck high,traced this back to a fujitsu 74ls32 at 5h,removed it,added a socket and a new 74ls32,and I now have some sort of signal going to pin 6,but the game still boots to garbage.there are 2 z80's on the board,but they both appear to have different results on the clock signal with the logic probe,is this normal?,any pointers as to where to look next?


the probe shows all 3 lit on one z80

but only 2 leds lit on the other one

at 3k which feeds the 74ls32 I replaced,im getting this

 

[Lurch666]

Logic probe only really shows the presence of a signal. You really need an oscilloscope to see if the signal is correct.
I have a working bootleg commando. I'll dig it out either after work tonight or tomorrow morning and see what it looks like on mine.

 

[stevebm1]

that would be brilliant,thanks mate,sadly I don't own an oscilloscope

 

[Lurch666]

My PCB is a slightly different configuration from yours but I'm getting the same on both points with my logic probe.
Looking with my oscilloscope shows both signals are identical.
No idea if this is what is affecting your PCB but if yours is like mine and lousy with fujitsu chips piggybacking any you have spares for could help.
Have you checked the program roms and the reset for the CPUs?

 

[Lurch666]

The second CPU near eprom 2 is just for sound I think.
I disabled it and the game still ran albeit without sound.

 

[stevebm1]

thanks for taking the time to help me,this board is the set 3 bootleg,all roms tested fine in mame,if by testing the reset line you mean do the z80's flash low,but then go high on power up on pin 26,then yes they both do,I will check the fujitsu chips as you suggested,and report back

 

[stevebm1]

No activity on most pins of the 74ls20 at J3,would you be kind enough to probe yours and let me know if its similar to mine please

 

[Lurch666]

If this is the one you are referring to there no activity on pins 1-6 but there is data on pin 8 (output) and all the input pins 9,10,12 and 13.

 

[stevebm1]

,yes that's the one,thanks for that,guess,I need to replace that chip then

 

[Lurch666]

If you are seeing nothing on the inputs then you need to see where those inputs originate.

 

[stevebm1]

thanks for the tip,I do have the schematics luckily,which makes life a bit easier

 

[Lurch666]

Looks like 3J is just for sprite select.
Put pin 8 to GND and my PCB still booted but without sprites.

Since it's not running code you need to check around the main CPU.
9J and 9L handle the selects,6N is ram select.
Have you checked the Z80 and ram at 7M is good as well?

 

[stevebm1]

the z80,is good,but I have not checked the ram at 7m,how would I go about testing that?

 

[stevebm1]

On closer inspection of 7m,looks like something was once soldered between pins,but has been removed,not sure if it’s a cap or resistor

 

[Lurch666]

If the ram is in a socket then swap out with a known good replacement.

 

[stevebm1]

Sadly it’s not,I can desolder it,but it will take a little while to do

 

[Lurch666]

You could check for activity on the chip select. All data and address pins will be active even if the chip isn't working since they are connected to the eproms.

 

[cliff_poole]

It looks like your logic probe is set to CMOS. Try switching to TTL, see if that makes any difference to the readings.

 

[stevebm1]

makes no difference readings are the same,the ram chip has activity on every pin,be it high low or pulsing,sadly I have none spare,so no point in desoldering it

 

[stevebm1]

the annoying thing is I swapped a working bootleg for another pcb a couple of weeks ago,so don't have a working set to compare readings with,I was hoping to get this one working,but feel like I have hit a dead end at the moment