How to fix a NES mainboard without 10NES lockout chip ?

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dphirschler
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by dphirschler »

Everything works as you'd expect... except I get gray screen. I see the red power LED. The Reset button resets. What could be going on?

Darryl
lidnariq
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by lidnariq »

Red power LED is on, but off while reset is pressed? Video draws solid-colored screen, but is off while reset is pressed?
dphirschler
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by dphirschler »

OK, update on this Nintendo NES. I now believe the lockout chip removal was successful.

I'd long suspected this NES of having power issues, so I just replaced the 7805. Cheap and easy repair. Then I did further testing with it. Still got the gray screen, but since the lid was off, I fiddled with the cart and as I raised and lowered the angle of the cart, it suddenly came to life! Not sure if the trouble is where the 72pin meets the mainboard or where the cart plugs in, but I suspect it's where the cart plugs in.

But the lockout mod seems successful. I should be play testing this thing later tonight after I get the 72pin issue worked out.


Darryl
dphirschler
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by dphirschler »

Last update. I bent the pins on the 72pin connector and I have the NES working reliably now. Best of all, the lockout chip removal was a success! Thanks all.

If anybody else attempts this mod, it would be best to only change one thing at a time. Had I done that, I would not have been wondering which thing was causing my issue.

Darryl
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rainwarrior
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by rainwarrior »

I just wanna say I'm glad this thread is still here. Wanted to replace a potentially dead CIC and this had exactly the info I needed.

Also I'm surprised to find myself advocating for replacing the chip earlier in the thread, ha ha. I think my tolerance for desoldering work has drastically reduced in the years since.
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rainwarrior
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by rainwarrior »

I have a question about the 1M resistor. Since doing this modification I've noticed the reset takes considerably longer than it used to be (like ~1s instead of "instant"). Is this proportional to the large size of that resistor? Would a smaller resistor speed it up and/or be okay/appropriate to replace it with?
lidnariq
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by lidnariq »

Do you mean on initial power-up, or subsequent reset buttons? I don't experience any delay on mine (version 7).

After moving the 1MΩ resistor, there should end up being a RC time constant of 1MΩ (moved R1 so that it's now "just" a pulldown) and 100nF+10nF (C8 next to CIC and C17 next to P6), or 110 milliseconds. Since you're seeing something 10 times slower, it's got to be something else beyond just this change. (edit: braino, forgot C8)

What version of the NES mainboard is yours?
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rainwarrior
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by rainwarrior »

Both. When I click in the power button, the red light (and video signal) doesn't start for roughly 1 second. Same deal after a reset.

The board says NES-CPU-10 and 1987 Nintendo on it.

Incidentally, this is also an NES that has an intermittent self-reset problem... that's actually what led me to remove the CIC and try this. I had wondered if my original CIC disabling had somehow damaged it and caused it to just sometimes reset the NES. (I've also recapped the power supply... even tried replacing the CPU which I thought maybe my desoldering to install a CopyNES was the problem... but a new CPU changed nothing.)

Though... I don't think I ever replaced the 3 electrolytic caps on the main board. Maybe I should try those... Edit: turns out I had replaced those already.
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lidnariq
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by lidnariq »

It's remotely possible that R17 could be a problem - in the rev7 mainboard it's 0 ohm trace instead of rev10's 2.2kΩ resistor. But I believe it's in the wrong place for that to make any sense.

Do you have any test equipment? (Multimeter? Loose LED?)
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rainwarrior
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by rainwarrior »

I've got a multimeter, a cheap DSO, and a simple component tester.
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by lidnariq »

Since you have a cheap DSO... Can you watch the voltage on the 1Mohm resistor - at the 74HCU04 pin 1 is fine? If it discharges slowly (the roughly 1 second) then you may as well replace it with a lower value, even if it's not clear why this is different for you than for me
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rainwarrior
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by rainwarrior »

Holding reset, that pin goes to just under 5V, then smoothly/exponentially lowers to 0. The red LED comes on about 350ms after it starts to fall, at about 2.7V (just estimating this visually). Not sure at what threshold the CPU or PPU should start doing their thing... seems to reach 1V at about 1s after release.

I don't see anything "weird" about the signal at this pin, just a simple immediate rise when I press reset, and a natural looking decay when I release.

Edit: Tried using my cheap component tester (GM328) to measure the mentioned capacitors in-circuit, and my multimeter for the resistors. The results for the caps don't seem to make much sense to me (it does seem to work correctly for isolated capacitors I had on hand), and I'm not sure to what extent the rest of the circuit would affect them, so I dunno if they really mean anything. The resistors gave expected values.

C17: 103uF ESR 3.1o Vloss 11%
C8: 97uF ESR 3.1o Vloss 7.2%
R17: 2.2ko
R1: 1Mo

C8 says 105 on it (1uF?) and C17 says 1032 (0.001uF?) 103Z (0.01uF).
Last edited by rainwarrior on Sun Jan 08, 2023 3:52 pm, edited 3 times in total.
lidnariq
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by lidnariq »

C8 is substantially bigger than on mine, and would explain the problem you're seeing.

105 does indeed mean 10·10⁵ pF = 1 µF, and 1µF · 1 MΩ is 1 second. The laziest option is to just remove C8 from the PCB - that should still be ok.

(Other capacitor is "103Z", meaning 10·10³ pF and Z=-20%/+80%)
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rainwarrior
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Re: How to fix a NES mainboard without 10NES lockout chip ?

Post by rainwarrior »

The board next to C8 says 0.1uF and C17 says 0.01uF.

Oh, it's not 1032 but 103Z.

So my board just has a wrong C8 cap? Weird. I can replace that with a better value later.

Thanks for the help.
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