Other reasons for White or solid colour screen

Discuss hardware-related topics, such as development cartridges, CopyNES, PowerPak, EPROMs, or whatever.

Moderators: B00daW, Moderators

lidnariq
Posts: 9774
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Other reasons for White or solid colour screen

Post by lidnariq » Tue Oct 13, 2020 8:05 pm

The CPU and PPU get their clocks from different places on the crystal circuit: the CPU is buffered through a transistor, whereas the PPU is just high-pass filtered.

... just for stupid giggles, what happens if you use the NTSC CPU and PPU with a 26.6MHz crystal instead of the 21.5MHz crystal they should have? (Resulting video will not be valid, it'll be 75Hz vsync and 19.5khz hsync)

VajskiDs
Posts: 16
Joined: Mon Oct 12, 2020 6:07 am

Re: Other reasons for White or solid colour screen

Post by VajskiDs » Tue Oct 13, 2020 8:05 pm

https://imgur.com/gallery/nqqi6Ya

And my original working OpenTendo

https://imgur.com/gallery/bsUlts9
Yeah I’ve got the PPU in the cpu slot in the picture

Such a shame because these come out so nicely in blue with fresh components. Board doesn’t look this neat anymore. Crystals being changed, trimmer caps being changed, added the inductor cos “why not”

I even tried 8kb Sony ram as WRAM with additional address lines and so forth tied high/ low with 1k pull up/ pull down wired in (28 pin)
Last edited by VajskiDs on Tue Oct 13, 2020 8:09 pm, edited 1 time in total.

VajskiDs
Posts: 16
Joined: Mon Oct 12, 2020 6:07 am

Re: Other reasons for White or solid colour screen

Post by VajskiDs » Tue Oct 13, 2020 8:07 pm

Haha not sure, my Telly might have a seizure.
Yeah I’ve tried new transistors and stuff it’s easy to trace components used for different circuits when you have the kicad files to open up.

I was thinking something similar with “what would happen” but it was using a PAL CPU but because of the different crystal divisions i think that would be silly and non operable at all

lidnariq
Posts: 9774
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Other reasons for White or solid colour screen

Post by lidnariq » Tue Oct 13, 2020 8:15 pm

Basically, since you keep on getting a good solid-colored picture, that implies the PPU and crystal are more-or-less working correctly. So something in the process of:
* 21.5MHz getting to the CPU
* /RESET getting to the CPU
* CPU itself
isn't working.

In the US, I can pick up the 5x overtone of the 21.5MHz crystal using an FM radio. (If you have a shortwave radio handy, that's a better choice: it can directly receive the accidental emissions of the crystal, instead of one of its overtones.) Tune your shortwave radio to 21.477MHz (or as close as you can), or your FM radio to 107.4MHz, and wave the antenna really close to the place you want to see if there's clock present. (You may want to cover the antenna with something non-conductive.) You should suddenly get a very clean silent signal, maybe with a little bit of mains hum.

The reason I suggest trying the 26.6MHz crystal is: maybe the two crystals output slightly different voltage ranges, and although both are good enough for the PPU, maybe the signal is slightly out of range for Q3 and the CPU's input. Maybe just using the 26.6MHz crystal will move the average voltages around to a value that works to get the clock into the CPU, and it'll end up "just" overclocking the CPU and PPU.
VajskiDs wrote:
Tue Oct 13, 2020 8:07 pm
I was thinking something similar with “what would happen” but it was using a PAL CPU but because of the different crystal divisions i think that would be silly and non operable at all
It'll run fine. Some trickier games will have problems, but many games will work fine, despite the CPU running at 1.4MHz instead of 1.8MHz.

VajskiDs
Posts: 16
Joined: Mon Oct 12, 2020 6:07 am

Re: Other reasons for White or solid colour screen

Post by VajskiDs » Tue Oct 13, 2020 9:48 pm

Heres your PAL crystal with NTSC ppu/ cpu
Image
This wasn't easy to get on screen I had to roll the trimmer cap back and forth

I tried the PAL CPU and got nothing.
No radios unfortunately! we used a similar method to track down a bad set top box that was taking out an entire end of streets internet.

AM radio in a vehicle to find the premise and a handheld to go through houses power outlets to find the set top box lol

lidnariq
Posts: 9774
Joined: Sun Apr 13, 2008 11:12 am
Location: Seattle

Re: Other reasons for White or solid colour screen

Post by lidnariq » Tue Oct 13, 2020 10:19 pm

VajskiDs wrote:
Tue Oct 13, 2020 9:48 pm
Heres your PAL crystal with NTSC ppu/ cpu
I assume that image didn't change with time? It was always that "mostly white with periodic horizontal black bars" thing?

Sounds like the NTSC CPU didn't get a clock signal.

Do you have the ability to run a custom ROM? (such as a flashcart or a socketed cart?) Do you have a breadboard and some kind of programmable ROM? Random other components? I could write a simple program that should let you test the CPU in isolation.

VajskiDs
Posts: 16
Joined: Mon Oct 12, 2020 6:07 am

Re: Other reasons for White or solid colour screen

Post by VajskiDs » Tue Oct 13, 2020 10:25 pm

I have a knock off ever drive but it went bad after I tried to update it thinking it was genuine (“high quality clone”)

Anyway I tried a few firmware’s and found a DIY jack (similar but different) link on YouTube - that firmware at least woke it up but it would be very picky with what it would and wouldn’t load.

What it did load would usually last 5 to 30 seconds until a graphical glitch/ lock up.

This was hardwired into my other nes cos I got over games not loading- anyway I took to it with a heat gun thinking I’d reflow the legs cos I wasn’t 100% sure it was a firmware issue (this has fixed many dodgy Chinese eBay electronic purchases, with an Iron usually). Anyway, Lost some unknown value caps on opposite side of board and stuck some 22nf on there.

Seller reckons cap value doesn’t matter and was nice enough to send the proper FW but it looks like a pile of spaghetti and I don’t wana waste my time turning it back into a regular Everdrive that will probably start locking up again.

VajskiDs
Posts: 16
Joined: Mon Oct 12, 2020 6:07 am

Re: Other reasons for White or solid colour screen

Post by VajskiDs » Tue Oct 13, 2020 10:27 pm

Oh and yes it’s clean pure white with that PPU and correct crystal/ other PPUs generally do light green.

It just does that interference white/ black with the pal crystal. A reset or power cycle and it won’t output any picture at all until fiddling with the trimmer cap

Also a TL866II+ but used last chip for sms bios. Some more on the way. I’m fairly certain it’s clock too. That should be present regardless of if CPU is socketed too...

VajskiDs
Posts: 16
Joined: Mon Oct 12, 2020 6:07 am

Re: Other reasons for White or solid colour screen

Post by VajskiDs » Thu Oct 15, 2020 4:12 am

Bought a famicom game so I can use the AV famicom as a test bench for the NTSC CPUs when it gets here :)

Post Reply