Moon Cresta bootleg PCB

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mattp

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NivagSwerdna said:
Do you have a Logic Probe? Is the Watchdog counter getting nVBLANK?

I think you had a stable picture so most of the video circuitry should be OK.

If your board is like a Galaxian then...

View attachment 21451

You want to know what is going on at pins 1,2 and 6 of the 74393

It's just a counter... causes a reset (on pin 6) if it doesn't see a CLR (on pin2) before too many nVBLANKs on pin 1.

PS
You can consider disabling the Watchdog... but obviously if you have a CPU, ROM or main Memory issue it won't help.
Click to expand...
 
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mattp

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All the signals around the 393 look good and clean, I’ll try disabling watchdog.The boards are very similar to Galaxian so I can make comparison.
 
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mattp

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74393 is never reset(pin2=lo).working back WDR is inactive, and the signals it comes from look ok. I notice the board behaves the same if the 5 256x1 SRAMs are removed. I don’t have replacements.
New SRAM ordered, and parts to run from single Eprom because the Eprom expansion PCB blocks access to the address decoders. Any idea if there is some documentation for this ?
 
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NivagSwerdna

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1718051131603.png

So probably the CPU isn't coping well due to system ROM and/or System RAM... but you can follow it back a bit further.

You can see that the WDR line is controlled by a READ to an address... 0111 1xxx xxxx xxxx i.e. a read to $7800 would do it.

You can also see if Interrupts are being serviced...

1718051871628.png

Looking at 6F should be informative... you should see pulses at pin 9 and pin 10. pin 10 is the "I'm done with this interrupt let me know on next VBLANK signal" and pin 9 is the "We've had a VBLANK and were enabled... let's go again signal"

Ignoring graphics... the CPU really only cares about the ROM and system RAM... that's the ROMs and the RAM and 7N and 7P.

1718051716211.png

Let me know if you already know this... and I will leave you to it.
 
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mattp

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Ah so you have to advance a page to see the next message here. I get it.
Thanks for all the info, I didn’t know all this infact I don’t know much about this circuit yet still learning the operation.
A13 is inactive so no WDR.
 
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mattp

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Ha ! Found it thanks to your pointer. PCB tracking error, the NMION signal was tracked to the wrong output pin on a 259. Linked it to the pin as per the circuit and my board is alive.
I had changed that chip and removed the solder bridge that it needed without noticing it was present. To compound matters the other board I purchased had the same issue ! Thanks Nivag I could have spent another week looking for that 👍
 
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NivagSwerdna

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There is something to bear in mind... The Galaxian and Moon Cresta memory maps are not quite the same.
I showed you a Galaxian schematic... For mooncrst the Watchdog is actually at B800 i.e. 1011 1xxx xxxx xxxx but it looks like you got close enough.
I cannot remember the detail of how the board is bodged to achieve that but you are in the correct area of the PCB to explain it.

If you haven't seen it you might enjoy my previous fixlog....
Thread 'What was I thinking! Moon Cresta Bootleg Journey' https://www.ukvac.com/forum/threads/what-was-i-thinking-moon-cresta-bootleg-journey.74484/
 
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mattp

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One of my boards was tracked for the Galaxian Watchdog address and had the same bodge to the address decoder as yours to fix the watchdog address. I liked your fixlog, that was a good repair on a truly horrible PCB. Very satisfying to bring things like that back to life. If I see tracks like that I use copper wick and flux to pick off the solder, it’s less likely to cause mechanical damage than a desolder pump.
I got some minor fixes to finish and then there will be 3 fully refurbished tables here after sitting around for a couple of years. I’ll post some images and boasting on this site.
 
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mattp

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An interesting web page, thanks for the link. This guy has passed many sleepless nights.
I like your device, perfect for fault finding game boards and very compact. I guess you have debug output on the PC. Is the emulator able to run game software ?
 
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mattp

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Hello Nivag I am hoping your offer to program a couple of 82S123 PROMs is still open. I have been fixing my two moon cresta boards and I think they are all working except for this part. I have two chips from the ebay chap you pointed me to and would be grat if you would help out.
 
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NivagSwerdna

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mattp said:
Hello Nivag I am hoping your offer to program a couple of 82S123 PROMs is still open. I have been fixing my two moon cresta boards and I think they are all working except for this part. I have two chips from the ebay chap you pointed me to and would be grat if you would help out.
Click to expand...
PM with my address sent. No worries
 
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Geekfix

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mattp said:
Hi folks, been reading your forums and hoping I might get some thoughts as to what might be wrong with my 2 faulty bootleg Moon Cresta PCBs (both with a display fault).
I am new to arcade boards and can see these 80s designs are complicated and could really do with pointing in the right direction.

So, my MC stopped working and I couldn't mend it so I did the sensible thing and bought another identical faulty one for comparison. It's good to have a reference. After some further "mending" they now both have identical faults.
I have tried all the normal easy fixes eg PSU, changing EPROM and CPU sockets, testing the various RAMs (by swapping them into my Galaxian board) , checking the EProms have good CRC, changing the colour PROM, scoping the video signals etc and sorted a few things out. I have run the MC test EPROM and can see flashing, corrupted something on the screen which makes me think it's saying "testing RAM" (like the Galaxian test ROM does) and so probably running code.If I install the MC EProm set there are stars in the background with garbage overlaid (see pic) and the watchdog triggers at 120ms intervals. My hunch is there is a software issue and I would rather not start changing ICs wholesale
If anyone can help I would be super-grateful
Click to expand...
Do you have a better picture showing the details of the checksums or details of the checksums?
 
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