Using a Logic Probe to Fix PCB's - Crystal Castles

jeffsgames

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As I continue to collect games, there are more and more problems I'd like to learn how to fix on my own. I picked up a logic probe a while ago, and I'm trying to comprehend the concept of checking IC's. I have a Crystal Castles that passes all of the RAM and ROM tests, but is screwing up when drawing/erasing the backgrounds and the gems on the screens.

Here's a video of the problem:



With some advice from bit_slicer, I started looking in the Bit Mode section of the board. I have the schematic pack so I know what IC's should be checked. I looked up some of the spec sheets for the ones I wanted to check. However, what I haven't put together yet is exactly what I should be looking for. From what I understand I need to check all of the inputs and outputs to see if they are high, low, or pulsing. How should I be looking at this? Should I list out the inputs vs the outputs based on the spec sheets and try to determine what the outputs should show depending on or/and for the chips? I just honestly don't "get it" yet on what I need to look for. Is this pretty tough and should I just give up, or is there some easy, common advice I can follow?

I hate to be such a newbie with aspects of this hobby since I've been collecting so long, but I guess I'm envious of some of you guys who can do some of this advanced stuff.
 
Fairly noobish to this myself. Hopefully someone else chimes in.... :)

My best stab: if you understand which ICs should be responsible for the problem, just swap them out with known good ones to confirm the problem. If you have the same ICs somewhere else on the board not causing any problems, you can use those.

I don't know that you'll actually gain anything useful from checking the lines with a logic probe, because you'd need to know what you're supposed to be seeing and if you don't have a good board to compare it with, you won't know what it looks like if it's good as opposed to bad.

You can basically check that there is a signal at all, which it looks like there should be as it "tries" to redraw but fails.

I think being able to verify the ROMs is a good first step, too.
 
I did reply via PM, but never got a response. I guess the issue is dead? :confused: In a nutshell, as best I can tell, all of the bits in the Bit Mode sections should be toggling. I also suggested reading up on the 6502 processor would give some insight on how the board works as a whole. Admittedly there's a lot to understand. The alternative is to poke around blindly on the board with a logic probe. :eek:
 
I did reply via PM, but never got a response. I guess the issue is dead? :confused: In a nutshell, as best I can tell, all of the bits in the Bit Mode sections should be toggling. I also suggested reading up on the 6502 processor would give some insight on how the board works as a whole. Admittedly there's a lot to understand. The alternative is to poke around blindly on the board with a logic probe. :eek:

Sorry Bit, I got your message and spent a ton of time in the bit mode section but no luck. There seems to be signal going to them, and they are registering pulses. I can definitely even hear a different pulse pattern when I actually launch a new game. I still can't tell if anything is bad there. The traces all look good. I'm at a point where I think I am going to give up on it. Can you send me a PM with your address and I'll send it to you for repair.
 
I feel your pain on wanting to repair boards more on my own. I also have a logic probe and it has helped at times, but it's just the understanding of what various IC's are doing and what I should be seeing that make it tough to learn.
 
I'm by no means any sort of expert, but for example, on a rom/eprom, you have address lines (labeled as a0,a1,etc) and data lines (d0, d1, d2). In most cases I would think if those chips are actively being used that they should at least show activity on all those lines. If one leg shows either a solid high or low that could mean a bad chip.

Logic chips (gates) are kinda like relays in a way, they look at one or more inputs then output based on a set of rules. Again the inputs/outputs should probably be active, if you see something stuck on high or low then its questionable, or if you see no activity whatsoever well thats a good dead chip indication.

If it were me and I sort of knew the area, I'd print out datasheets on any chips that there's a good number of so you have them handy, then just go from there.

You can also check for VCC voltage incase there's a bad trace killing off a section of chips.

Actually I have a question myself - how does one troubleshoot a board where the crystal/oscillator may be bad?

Again, I'm NOT a digital troubleshooter, but have been poking my head in a book the past few nights trying to fall asleep.
 
I'm by no means any sort of expert, but for example, on a rom/eprom, you have address lines (labeled as a0,a1,etc) and data lines (d0, d1, d2). In most cases I would think if those chips are actively being used that they should at least show activity on all those lines. If one leg shows either a solid high or low that could mean a bad chip.

If you can understand the how's and why's of a CPU's bus access cycle, and understand basic boolean logic and how basic flip-flops, counters, and memories work, then you can understand 90% of how a cpu-based board of that era works.
 
The video looks like the screen is not being cleared when switching from self-test to play mode.

It IS being cleared when switching from player 1 to player 2.

So, the hardware section to clear the screen seems like it's working, but not being called properly in the program flow. But the ROM test passes so it seems like the program is not corrupt.

I don't know. Let us know what the answer is if you find it.

Kerry
 
Actually I have a question myself - how does one troubleshoot a board where the crystal/oscillator may be bad?

Look at the inverter that connect to the crystal. You should see some mad pulsing on those pins. Sometimes a logic can't read the legs that directly connect to the crystal (a scope could)....but it can read the output side of the inverter.

Edward
 
Thanks - I think this may be my problem with my Video pinball...kinda works in test mode (screen not right though), but repeatedly beeps in normal mode. The screen is repeated which kinda tells me its looping/resetting somehow. I want to check the power supply with a scope first to see if that shows anything.

Edit: looks like a majority of this game runs off the H-Sync signal which is derived from the 12mhz crystal and a chip.
 
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Also look to be sure the correct type of inverter is used. It will usually be a '74H04' or '74HC04'. I had one flaky Centipede where some enterprising individual replaced the inverter with a '74LS04' instead. Not enough drive strength or speed to be effective I think.
 
Also look to be sure the correct type of inverter is used. It will usually be a '74H04' or '74HC04'. I had one flaky Centipede where some enterprising individual replaced the inverter with a '74LS04' instead. Not enough drive strength or speed to be effective I think.

Yes, an LS series at the crystal will almost always cause issues. Everything I've worked on (and I bothered to notice) has used an "S" series TTL (most stuff I work on is pre-1984). The "S" series TTL chips are getting difficult to find. Can an "H" or "HC" series be a substitute for a "S" series?

Edward
 
Yes, an LS series at the crystal will almost always cause issues. Everything I've worked on (and I bothered to notice) has used an "S" series TTL (most stuff I work on is pre-1984). The "S" series TTL chips are getting difficult to find. Can an "H" or "HC" series be a substitute for a "S" series?

Edward

Not always. It depends on the application. The 74HC04 (and it looks like the H crosses right to the HC) has a good deal more drive strength both sink and source than both the S and LS part. This is probably why it's chosen for the crystal circuit. However the S part is much faster than the HC part. If the S part is used in a timing-critical path then substituting with an HC likely wouldn't work.
 
Not always. It depends on the application. The 74HC04 (and it looks like the H crosses right to the HC) has a good deal more drive strength both sink and source than both the S and LS part. This is probably why it's chosen for the crystal circuit. However the S part is much faster than the HC part. If the S part is used in a timing-critical path then substituting with an HC likely wouldn't work.

Thank you! I'd always been told a "F" could be substituted for a "S". I've never needed to try it, yet. I don't really keep "F" series around.

Edward
 
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