NES front loader - Yellow solid screen

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bad_boo
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NES front loader - Yellow solid screen

Post by bad_boo » Fri Oct 09, 2020 5:17 pm

Hello,

Hope someone could guide me into the right direction.

I have this NES that was working decently. But I tried to do the stereo mod and now I am getting a solid yellow screen.

Image

I have undone all the soldering from the attempted mod, I have disabled the lockout chip, I have changed the 2200 uf cap. Also I have swapped the 72 pin with the other broken NES that I have, and no change.

Then I have turned on the console without the 72 pin hooked up, and still get I same yellow screen. No difference.
Also I can keep pressing reset, and sometimes I get some gray and blue screens.

I saw on another forum that the SN74LS139N chip could be a reason for gray screens. I think its state may not be the best though. But I could try ordering one off ebay and try.....( when I got this console, had some rust problems)
Image


After that, I would be running out of ideas, so any input would be a great help.

Thank you

lidnariq
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Re: NES front loader - Yellow solid screen

Post by lidnariq » Fri Oct 09, 2020 5:57 pm

You've got a lot of corrosion problems on that PCB and it's definitely not helping things. You'll want to clean that up and apply something to slow down future corrosion.

But that said, even with the corrosion, I think that pin on the 74'139 is still connected to the mainboard - you can test with a multimeter - and if it is I don't think it'll be a problem.

Which guide for the stereo mod did you follow? I fear you might have fried the CPU, especially if you're not accustomed to soldering.

My standard advice for this is to check for changing voltages on specific pins on the CPU. If you have a multimeter with a frequency or logic probe, that's easiest, but you can also use a bunch of LEDs and a resistor.

(Check A0 and M2 - https://wiki.nesdev.com/w/index.php/CPU ... escription )

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bad_boo
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Re: NES front loader - Yellow solid screen

Post by bad_boo » Fri Oct 09, 2020 6:12 pm

lidnariq wrote:
Fri Oct 09, 2020 5:57 pm
You've got a lot of corrosion problems on that PCB and it's definitely not helping things. You'll want to clean that up and apply something to slow down future corrosion.

But that said, even with the corrosion, I think that pin on the 74'139 is still connected to the mainboard - you can test with a multimeter - and if it is I don't think it'll be a problem.

Which guide for the stereo mod did you follow? I fear you might have fried the CPU, especially if you're not accustomed to soldering.

My standard advice for this is to check for changing voltages on specific pins on the CPU. If you have a multimeter with a frequency or logic probe, that's easiest, but you can also use a bunch of LEDs and a resistor.

(Check A0 and M2 - https://wiki.nesdev.com/w/index.php/CPU ... escription )
Thanks so much for your reply.
Yes, that rust has been a problem, but like you said, maybe it is not the main issue as it was working fairly well before.

I followed this guide for the mod: http://www.retrofixes.com/2013/09/nes-s ... -easy.html, But for sure, it was due to my soldering skills that made this issue.

I have a multimeter, I can test what you explained.

Also I think what you say makes sense, could be the CPU. If it is that, is it something I can just change? or would I need a broken donor NES to get it?

lidnariq
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Re: NES front loader - Yellow solid screen

Post by lidnariq » Fri Oct 09, 2020 6:21 pm

You can buy 3rd party clones of the NES's CPU, but let's see what your test results are first.

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bad_boo
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Re: NES front loader - Yellow solid screen

Post by bad_boo » Fri Oct 09, 2020 6:55 pm

Will check that, in that case.
Thank you

hiju123
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Re: NES front loader - Yellow solid screen

Post by hiju123 » Tue Nov 17, 2020 4:57 pm

Hi. Did you get the issue sorted out? I have the exact same problem with the solid yellow screen with and without the cartridge. Disabled the lockout chip as well. Still a no go.
[Removed post-widening quote - MOD]

lidnariq
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Re: NES front loader - Yellow solid screen

Post by lidnariq » Sun Nov 22, 2020 12:46 am

Here's my first steps of trouble-shooting a cantankerous NES:

* Does your TV think the output video is valid? (For example, a solid yellow screen?) If so, this means that you know the 21.5MHz or 26.6MHz clock works, and the PPU at least partially works.

Subsequent steps will require a logic tester of some sort. I favor this simple one, but anything will work.

The next steps are:
* Check if the CPU /RESET pin is high (pin 3)
If /RESET is not high, use one of the various schematics to figure out why.
* Check if the CPU M2 pin (pin 31) is oscillating (i.e. your logic tester should show both high and low, or my "two LEDs" one should light both LEDs)
If /RESET is high, and the 21.5MHz clock is arriving at the CPU (pin 29), and M2 is not oscillating, the CPU is broken.
* Check if the CPU A0 pin (pin 4) is also oscillating.
If things are almost working, A0 will oscillate for a little time and eventually stop at high or low. It will restart when you press the reset button.
If A0 never changes between high and low, the CPU is broken.
If A0 keeps on oscillating, but the PPU never starts drawing a valid picture, then either the CPU's RAM or the PPU could be broken, or much less likely, the 74*139.

Separate tests that can also be useful:
* Does the power LED blink in the standard "failure" way without a cart? Then you know that the 4 MHz clock works, and the CIC lock at least mostly works
* Does the power LED not blink if you have a cartridge in, even if nothing else works? Then you know that the CIC fully works (unless deliberately disabled) and the 74HCU04 mostly works.

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