[Resolved] Fixing my 1-chip SNES (Blk screen)

[RIP-Felix]

Background:
I intentionally purchased a broken 1-chip SNES off e-bay, listed for parts, for the purpose of restoration. I have successfully done this with an Atari 2600 and I learned alot along the way. It was fun too. So this is a follow up challenge for myself. At worst I'd be out $40, but gain experiance reworking and troubleshooting the SNES.

Here is some pertinent information:

• Model = SNS-CPU-1Chip-01

• It had been worked on previously. It had a botched recap installed. The through hole type kit used was appropriate and all the caps were healthy, but the installer did not pay attention to polarity. C9, C12, C13, C14, C15, and C16 were backwards. (Listed in case this could burn out a resistor in the line or something. IDK, it might shead some light on where to look for a damaged components if this could cause my ongoing issue.) Other than that, the board appears to have been given a new 2 piece 62-pin connector.

What I've done:

• I replaced the 7805 voltage regulator with a new 78S05 and placed a 470uF cap across the output/ground, to prevent the vertical bar in the center of the screen sometimes seen.

• I also went ahead and recapped with a Console5 SMD kit, being sure to get polarity right and checking the capacitor health (out of circuit of course). I also checked continuity of the 1.5A pico fuse, a BEEP confirmed it's health. The power LED comes on and games play. I think that definitively rules out the main power circuit.

• I read that 9v is sufficient to work with the 7805, but not quite high enough to prevent a rolling bar of static fuzz that crawls down the screen every 10 seconds. I was using a cheap 9v aftermarket adapter purported to work with both nes & snes. I used my CRT before getting the OSSC, which is why I didn't notice it before. Now that I have it hooked up to the OSSC on my big screen, it's there just like they said. So I hacked up a 12v 2A AC/DC adapter I had laying around. I reversed the polarity so that the center pin is negative, to match the SNES requirement (this is opposite to most adapters). After checking it with the multimeter and feeling for any heat in the wire, which would indicate a short, I tried it out. Works fine to remove the static bar. No change to my ongoing issue.

• I reflowed, and cleaned contacts on the 2 piece 62-pin connector (I think that was added by the previous attempt to fix this board). Then I confirmed each pin with a multimeter. Perfect!

My Ongoing Issue:
On powerup it black screens. After a few seconds I can hit reset a few times and It will startup. After the initial power fail, I can hit reset a few times to get it to come up and play fine. Every game I've tried runs perfect. No graphical glitches that would indicate a bad PPU, or CPU, other than the black screen on initial power up. It just takes about ten seconds after PWR ON before the reset button "trick" will begin working. On my OSSC LCD screen I notice that after a few resets it will show the 15KHz sync. Once it has been on and running for a while, if I turn it off then back on within 4 seconds, it will fire up no problem, but not after 5 or more seconds. After that I have to hit the reset button a couple of times to get it back.

Discussion:
I was hoping the above might fix my power/reset issue, which I'm starting to think is a problem with SYNC. I could bypass the RGB sync altogether with Voultar's 1CHIP/Mini THS7374 RGB Bypass Kit. It produces a clean, properly attenuated sync. If that's the problem, this might fix it. However, I'd like to find some diagnostic procedure to confirm before buying his kit. I do have a very cheap oscilloscope that might help, but I have very limited experience with SNES. So I'd need someone to point me toward a target pin, and what to look for, to diagnose.

Speculation and my questions for the peeps:

• 1) According to http://problemkaputt.de/fullsnes.htm#sn ... ryandiomap Upon power-up, APU RAM tends to contain a stable repeating 64-byte pattern. After Reset, the boot ROM changes. My question is if the APU RAM were corrupted, could it cause the system to initially fail to boot, but then work upon reset? This is a total shot in the dark. If the APU unit needed replaced, How could I tell? Can I scope it, or would I have to pluck it and replace with a known working unit to know? That's something I've not done yet in my reworking bag of tricks, but I'm not opposed to learning. Probably not on a 1-chip SNES. I'd need numerous attempts on junk boards first!

• 2) Are there any capacitors or resistors in the SYNC line that could cause an issue like this? Could using a RGB Bypass board like lord Voultar's possibly fix this?

[paulb_nl]

If it were just a sync issue then the game should be running and you should hear the game audio from the analog output.

Otherwise maybe the cic chip is not working correctly and holding the system in reset?

[RIP-Felix]

If it were just a sync issue then the game should be running and you should hear the game audio from the analog output.

Otherwise maybe the cic chip is not working correctly and holding the system in reset?

I'm not really sure. I thought it was a typical balck screen console, probably a bad CPU. I was probing around the CPU and SRAM looking for chaotic non repeating signals (which would indicate the game was running in the background, but audio/video not making it to the TV). No activity, just a voltage here and there, then I must have grounded the reset pin because after that I got the signals I was looking for (probably the address lines). I plugged it into the TV and indeed the console was up and running. That's how I initially discovered the reset trick.

So yeah, it may be held in a reset state on initial power-up. Why would it take a while for the reset to work? It will fail every time for 10-30 seconds after the power is first turned on (how long it takes seems to depend on how long it has gone without power). Then it will work. Almost as if there's a precharge time.

EDIT:
Just found this: https://assemblergames.com/threads/snes ... ure.56231/
I'm suspecting the APU, because the issues related to bad connections or a faulty APU sound similar, but not quite the same as my issue. I'll try re-flowing the APU pins just to rule out a cold solder joint and report back...Nope! Beautiful shiny contacts, no bridges. Same old behavior.

[RIP-Felix]

This morning I reflowed all the fine pitch chips (CPU, SRAM, etc.) with rosin flux, except three chips under the heatsink (laziness). They're all shiny and strong again. I even reflowed the 62-pin connector again with copious flux and new solder. Unfortunately this had no effect on the issue. Still refuses to boot on first PWR on, will come up as before after numerous resets.

It was worth a shot.

EDIT:
I'm looking for some hi-res shots of the motherboard, SNS-CPU-1Chip-01 specifically. The backside where all the tiny SMD caps is what I'm interested in. There are a few unpopulated pads on mine. They are probably unpopulated on purpose, but given the board's history of rework they could have been removed. IDK, I'm grasping at straws trying to find a way forward with this restore.

[Syntax]

Probably going to have to replace customs 1 by 1 until you get the fix your looking for.
Ive had a similar issue with gamegears before, Only working on power cycle.

[Arthrimus]

Maybe check the solder on the APU crystal. If it has an intermittent connection it could cause the system to fail to boot.

[rama]

I'd rather start checking out the reset circuitry.
There should be a 3 leg device with designation U11 on the board. This drives the reset signal once power is good.
From the description it sounds like this is not working correctly.

[RIP-Felix]

Maybe check the solder on the APU crystal. If it has an intermittent connection it could cause the system to fail to boot.

I just tried reflowing X2, no dice. I have wondered about it. It's a strong connection now. While I was at it, I got under under the heat spreader and reflowed all the chips. One thing I noticed, and this could be my imagination, is that it took less resets to bring the game up. It's working on the first or second attempt. Watch, now that I say that, it'll go back to normal.

I'd rather start checking out the reset circuitry.
There should be a 3 leg device with designation U11 on the board. This drives the reset signal once power is good.
From the description it sounds like this is not working correctly.

I reflowed it this morning to eliminate a cold solder joint and verified it's not bridged. That had no effect. Is there a way to verify if it's working correctly? I can measure voltage before/after power on, but Need to know what I'm looking for on each pin. I'm getting better at electronics, but gates, relays, what people mean by "pull up/down resistors, and etc are still areas for me to improve.

[RIP-Felix]

Update:
I just got a new rework station with hot air and a DC power supply. I went ahead and swapped U11 with one from a working SNS-CPU-RGB-02. Went smooth as butter with hot air, way easier than I thought. SMD doesn't scare me anymore! Anyway, the same behavior was persistent on the 1-chip. Black screen on power up, reset button syncs and the game start as normal. Also, the donor board still works fine. So U11 definitely wasn't defective.

A word of caution though. If your working on a SNES always keep at least 1 screw in the power switch, to keep it secure. Or use some electrical tape on the bottom side to cover the pins. Like an idiot, I let it dangle and then I knocked it off the standoffs while testing. It must have shorted on the RF shielding. When I went to turn it on...nothing (no PWR LED). After a bit of self pity (I took a break), I confirmed I'd blown the pico fuse then replaced it. Easy fix, but it gets the mind racing. "Will I have to Recap again? Did I just send 12V strait to my CPU and blow it up? Did I just F#$K up my 1-chip." It's best to take a break and come back with a multimeter after the panic subsides and reason returns. If nothing else, it get the heart pumping!

*********
From the available schematics, datasheets, and from Fullsnes documentation I've pieced together the following:

• Note: I'm using "x" to indicate components I've already ruled out by either replacing with working units or I have confirmed operation with a multimeter, reflowing, swaping out, and etc. "-" means I don't know or haven't ruled out yet. Also, I'm not listing every capacitor or resistor. Just the major components. I did perform a recap, so that's already ruled out.

Everything in the Power Circuit:
x 10V AC/DC Adapter (Center pin -)
x Barrel Jack Conn
x 1.5A Pico Fuse
x PWR Switch
x 7805 Voltage Regulator

Everything in the Reset Circuit:
x Reset Switch
- S-APU ("combines S-SMP, S-DSP, 64Kx8 Sound RAM, and possibly the NEC UPD6376 D/A Converter")
x X2 (24.576MHz ceramic resonator that feeds into the S-APU. I replaced with one from a working donor board.)
- S-CPUN A (combines CPU, PPU1, PPU2, S-CLK)
- S-WRAM B
x T529D (U11), "for resetting the system after detecting the voltage at the time of powering on".
x Cartridge Slot pin 26
- Expansion Slot pin 19

I may be missing stuff, but that's what I was able to piece together.

I've been wondering about the initial PWR cycle. What I'd like to know is how it works, but there is little to no information out there. Just from observation, when you start any SNES you'll notice a flicker (I assume it's the Reset pulse), followed by a short pause before the game starts up. Mine doesn't flicker at all when I turn on the power switch. Just still black, no sync on my OSSC is displayed. The Sync doesn't happen at all until I manually press reset. Then it flickers and the game comes up like normal. So it's like there's no reset pulse on power on. So @rama's suggestion sounds dead on point.

That T529D (U11) is very suspect. Like @rama said, it's job is to generate the reset signal after it "detects" power on (4.2v). It's DataSheet says that it's used as a "measure against erroneous issues at power on/off". I mean, bingo! That sounds like the right thing to check, but I've already confirmed replacing it doesn't fix the issue. So that suggests the CPU isn't ready to receive the reset pulse. In this thread @rama points out that there is another reset signal generated by the S-APU that the CPU is "waiting for". I've read a bad APU can cause the system to hang (black screen), but I've not heard of the reset button fixing it. I did swap X2 to rule it out. I also ran the Burn in Test Cartridge and it passed on everything, even though it sent my OSSC for a ride (kept dropping sync).

Perhaps my APU is going bad, but it's not dead yet? But then, why would the test cart give it a pass? Same if the CPU or SWRAM had issues? If my only way forward would be to try swapping the APU, then the SWRAM, and finally the CPU, perhaps it would be better to keep this board as a donor for when my working 1-chip goes bad. If I were to buy yet another for parts board off e-bay, it'll probably be an easier fix than this one. I would like to try my hand at replacing SM IC's, for the practice (that was the point of this purchase), but it's actually easier to get working 1-chip off e-bay than it is to find donor boards (let alone specific chips). Go figure...

[Link83]

Have you tried installing a SuperCIC to rule out issues with the original CIC?

[RIP-Felix]

No, I have been assuming it's okay, since the game does run fine after hitting the reset button. I figured if it were bad it just would always refuse to allow the game to run, not just on the initial power up.

I do have a SNS-CPU-RGB-02 with the same CIC part number (F411B), but I wasn't sure they are compatible across hardware revisions. I'll try swapping them to see if it has any affect & edit back...

EDIT:
@LINK83 YOU ARE THE MAN!!!

THAT WAS IT! It fired up immediately, no resetting. I went to check the donor board, to check if the same issue presents in it. If so, then that was a bad CIC. However, I must have had a cold solder joint on the CIC because now the donor board works too. WOW, I'm elated to have this solved. Thank you all for the help.

BTW: No SNES's were harmed in the course of this project. 4 were obtained, 2 were broken, all 4 are now working.