300 ohms between 5v and ground on nes 001, is this normal

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ap4ss3rby
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300 ohms between 5v and ground on nes 001, is this normal

Post by ap4ss3rby » Sun May 19, 2019 9:16 am

Hello, this is my first post here. I have an NES that has a black screen. Every once in a blue moon the NES displays a picture and everything displays normally which tells me the CPU, PPU and RAM, and 72pin are all at least in some form are functional to some degree. I jave wondered why on earth would I get 300 ohms btw ground and 5v

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Ben Boldt
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by Ben Boldt » Sun May 19, 2019 10:38 am

300 ohms seems reasonable to me. I am not sure what a black screen could indicate (normally that would be a gray or green screen?), it could be a dirty or damaged 72 pin connector. Also check the large capacitor near where the power goes in. The top of it should be flat and not bulged up at all. You might also want to check the voltage of the 5V with the power on and verify that it is a stable 5V.

ap4ss3rby
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by ap4ss3rby » Sun May 19, 2019 10:53 am

No, I bypassed the 9v to 5v because the power regulator overheated consistently, and even before bypassing it it seems to work fine( save for the overheating regulator

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Ben Boldt
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by Ben Boldt » Sun May 19, 2019 10:59 am

Here is my thinking on the 300 ohms:

P = V^2 / R
P = 5^2 / 300
P = 0.083W

That doesn't seem enough to make the regulator hot. In your bypass, you are basically feeding 5 Volts directly into the NES? Does anything else get hot when you do it that way?

lidnariq
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by lidnariq » Sun May 19, 2019 11:12 am

My working NES, when off, has a resistance from Vcc to ground somewhere around 250Ω - https://forums.nesdev.com/viewtopic.php ... 48#p151348

At some point in the past I know I measured the CPU and PPU resistances after being removed, but I can't find my post now.

ap4ss3rby
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by ap4ss3rby » Sun May 19, 2019 11:40 am

Ben Boldt wrote:Here is my thinking on the 300 ohms:

P = V^2 / R
P = 5^2 / 300
P = 0.083W

That doesn't seem enough to make the regulator hot. In your bypass, you are basically feeding 5 Volts directly into the NES? Does anything else get hot when you do it that way?
No not at all, maybe the cpu and ppu, but not as much or as fast as the voltage regulator

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Ben Boldt
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by Ben Boldt » Sun May 19, 2019 12:03 pm

To be completely clear, I am talking specifically about the 7805 voltage regulator now.

From what I understand, the voltage regulator does put out 5V but it quickly gets too hot to touch. Is this correct? In this case, it suggests that the input side of the regulator is unstable, probably because of a bad capacitor or bad AC adapter. You can try replacing the large 2200uF 25V capacitor (highly recommended), and/or adding a ceramic capacitor from "in" to "gnd" on the regulator.

ap4ss3rby
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by ap4ss3rby » Sun May 19, 2019 12:14 pm

I will test it separately and the cap mentioned, and follow your advice if the cap is bad

ap4ss3rby
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by ap4ss3rby » Sun May 19, 2019 12:24 pm

The voltage regulator heated up the same with no load attached outside, still there is no telling if the cap is bad without some testing equipment(charging to 9v gave some sparks and that's it can't tell the current involved to calculate capacitance)

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Ben Boldt
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by Ben Boldt » Sun May 19, 2019 5:18 pm

What sparked? Are you saying that you removed the 2200uF capacitor, charged it to 9V, and then it sparked when you shorted it?

Voltage regulators will get hot like that if the input side does not have the right capacitance to Gnd. If you have any ceramic capacitors, try attaching one from input to GND and see if it can help prevent it getting hot.

ap4ss3rby
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by ap4ss3rby » Mon May 20, 2019 12:41 pm

Haha! I found it! Removing aka butchering the CIC chip made it work reliably.

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Ben Boldt
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Re: 300 ohms between 5v and ground on nes 001, is this norma

Post by Ben Boldt » Mon May 20, 2019 6:35 pm

That doesn't explain how your regulator was getting hot though.

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