SNES rev 1 black screen at startup

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Poot36
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SNES rev 1 black screen at startup

Post by Poot36 » Sun Aug 02, 2015 5:57 pm

I have an original US SNES that when you turn it on you just get a black screen. I have a post on this forum http://www.eevblog.com/forum/repair/sne ... t-startup/ of what I have tried so far to get it to work. They did suggest to post over here for more help so that is what I am doing. The tools I have to try and fix this are a multimeter, a Tektronix 465 100Mhz 2 channel scope and a HP 1661C logic analyzer with 3 pods (so 48 usable channels). Do you have any idea why the SNES does not start up the same way each time (at least according to the logic analyzer)?

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Sun Aug 02, 2015 6:41 pm

It sounds like you've already:
* ensured that the CIC isn't getting in the way, so I assume that the /RESET pin on the cartridge edge is high, and the address bus is counting...
* ensured that (when present) the cartridge connector is making good contact.

It is possible for the SNES CPU to run code without the PPUs present, but you'll need a special program to do specific things given that there's no output methods available.

Do the controller parts do the (in-silicon) auto-read?

Are any of the parts getting noticeably warmer than other ones?

Unfortunately, where-as on the NES the different failures produce noticeably different symptoms, almost all failure modes produce the same black screen on the SNES.

Poot36
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Re: SNES rev 1 black screen at startup

Post by Poot36 » Sun Aug 02, 2015 6:56 pm

How would I check that the controller ports do the auto read and how would I check for that? Other then the regulator nothing appears to get very warm or much warmer then other components. I have attached a startup sequence that I captured with the logic analyzer. I can't remember if the cart was in the SNES or not in this capture (I think I was trying to get repeatable startups so there may have not been a cart in the SNES). I have labeled all of the pins that I was probing. You can open the file with any ASCII compatible reader. I do find it somewhat weird that the IRQ line never changes.
Attachments
LISTING.txt
SNES start up
(236.41 KiB) Downloaded 209 times

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Sun Aug 02, 2015 7:10 pm

http://wiki.superfamicom.org/snes/show/ ... nd+Pinouts
If the controller daughterboard were plugged in, you'd check to see if LATCH pulsed high at 60 Hz and for each then CLOCK pulsed sixteen times (at about 1MHz). This is done entirely in silicon, so it's only a very minimal test that the CPU die works.

These pins are directly connected to the CPU, ultimately, so you could use jwdonal's redrawn schematics to find where to tap the signals, also.

Looking at the first couple hundred lines of the trace you have, it looks perfectly plausible:
- reads from 100, 1FF, 1FE because power-on-reset is the same sequence as an interrupt
- reads from the power-on vector at FFFC, FFFD
- power-on vector points to $8000
- execution continues logically thereafter.

Poot36
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Re: SNES rev 1 black screen at startup

Post by Poot36 » Sun Aug 02, 2015 10:04 pm

Ok I have done the controler test and here is what I have found, the latch line stays low and all of the other signals on the controler port stay high. I have also reconected the resistor that I removed to disable the ppu chips. I have also reconected the CIC chip just incase there was some fault with it (there was no change the reset line was still high). The test results were the same with and without the cart. Does this mean there is something wrong with the CPU? Edit: I just tested this configuration on the logic analyzer and now I am not getting any data or garbage data (I don't think 0x50AA is a valid address) that loops.
Last edited by Poot36 on Sun Aug 02, 2015 10:35 pm, edited 1 time in total.

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Sun Aug 02, 2015 10:17 pm

Oh, no, I misunderstood: http://problemkaputt.de/fullsnes.htm#sn ... ticreading says that the software has to opt in first.

So whatever's going wrong is more complex than that :/

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Mon Aug 03, 2015 12:14 pm

I agree that $50AA is not an address where things should be poking; there's nothing there in banks $00-$3F or $80-$BF.

Looking at the LA trace you have from before, it looks like it's waiting forever polling something ... it just executes from $8034 to $8049 over and over. (What game were you testing with?)

Poot36
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Re: SNES rev 1 black screen at startup

Post by Poot36 » Mon Aug 03, 2015 4:24 pm

I was testing with Super Mario World. I also have Super Mario Kart, Super Mario All Stars and Super Mario All Stars Super Mario World. The Super Mario World game is what gave me the most success when I first got the SNES so I stuck with it. I do want to make a test cart with a flash rom so that I can try to activate the minimal set of chips in the SNES to prove it is somewhat working (eg just video or just audio or just controler ports) if that is possible. I currently have the 2Mb flash chip programed with a video test but am unsure as to which crappy cart to get to mod into a flash cart.

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Mon Aug 03, 2015 5:20 pm

Ah. In SMW, the loop from $008034 to $008049 is filling memory at $7F8002)... so the LA doesn't show anything there.

As far as I can tell, it looks like the CPU is ok, but who knows about the RAM or PPU...

Poot36
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Re: SNES rev 1 black screen at startup

Post by Poot36 » Tue Aug 04, 2015 9:06 pm

Ok, I have reattached all reset lines and have re disabled the CIC chip and have rerun the LA again and have attached the output. Listing is with out the cart in the SNES and Listing1 is with the cart in the SNES. Both of these readings had to be done by turning the SNES on and then selecting the RUN button on the LA otherwise I would just capture all zeros by arming the LA and then turning on the SNES. I am grabbing the trigger signal from the xin clock on the SNES.
Attachments
LISTING1.txt
cart
(216.38 KiB) Downloaded 202 times
LISTING.txt
No cart
(216.38 KiB) Downloaded 179 times

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Tue Aug 04, 2015 9:25 pm

In attachment #3690 ("LISTING1.txt") is waiting (forever?) while it hopes to upload data to the sound coprocessor.

In attachment #3689 ("LISTING.txt"), there's something weirder ... looks like something's repeatedly triggering the IRQ (writes to the stack at 01xx, followed by reads from FFFE and FFFF) and fetching garbage for the IRQ address.

Poot36
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Re: SNES rev 1 black screen at startup

Post by Poot36 » Tue Aug 04, 2015 10:01 pm

I have tested the sound module in a working SNES and it works so I assume that there may be a bad connection to the sound module? What other signals in the SNES should I probe? The B address bus? Or some more signals from the sound module? It kind of makes sense now that when I would bend the cart in a certain direction (when the SNES was still mostly together) that I could get a random sound effect to play and usually repeat with a garbled screen. I will try to test all of the sound module pins tomorrow and get back to you. I still do not like the IRQ line staying high when I have a cart in the SNES but that could be due to it crashing so early on in the boot stage.

lidnariq
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Re: SNES rev 1 black screen at startup

Post by lidnariq » Tue Aug 04, 2015 10:56 pm

Bad connections to the sound module would be an obvious thing to test...

Unfortunately, I'm beginning to run out of ideas for things to try without a flashable cart. E.g. trying something that didn't use the PPU or APU would be the next obvious test...

Does anything ever produce anything visible at all?

You could test random signals on the PPU... but I fear that there wouldn't be any PPU bus activity if the CPU never enables it.

Poot36
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Re: SNES rev 1 black screen at startup

Post by Poot36 » Wed Aug 05, 2015 9:21 pm

Well I found a broken /PAWR line going to the APU so I repaired that and the LA is still going through 0x2140 to 0x2144 address range and looping. I have noticed that the SNES now seems to be starting up in a consistent state so I think that is good. How I tested the connections to the APU was setting my meter in diode mode and looking for the protection diodes or transistors in the other chips. I am not sure if this is the right way to do it but it is fast. The SNES does not seem to change behavior when I pull the sound module. I have also attached a LA trace with triggering off the SYSCK to avoid the all zeros problem at power on.
Attachments
LISTING2.txt
SNES SYSCK trigger
(216.38 KiB) Downloaded 183 times

DogP
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Re: SNES rev 1 black screen at startup

Post by DogP » Fri Aug 07, 2015 3:12 am

Lately I've been repairing a bunch of Nintendo Super System PCBs (basically an arcade SNES), and have come across a LOT of "Black Screen" problems. Unfortunately, the majority of them have been bad CPUs. :( To get CPUs to fix them, I have to pull them from SNES systems (I try to take from dead or heavily damaged systems)... some of them have also had bad CPUs though.

I'd recommend making a cartridge of the burn-in test cart ( https://tcrf.net/SNES_Burn-In_Test_Cart ). Getting the menu to boot must be fairly simple, because it'll boot even on a lot of bad CPUs (which then typically lock up after selecting a test, or fail one or more tests in the Burn-In Test). Unfortunately, if you find out that your CPU is bad, the only way to get a replacement is from another SNES... and it's a 100 pin QFP, so replacing it requires more than the typical repair tools/skills.

DogP

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