Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

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Liengamer
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Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 9:14 am

Recently I´ve found my second nes in my grandads basement and took it home. Later I noticed something when wantet to turn it on. The Power led just kept flashing really fast, even faster as when there is no game inside (cic). I began to poke around and so far I´ve discovered that I´m only geting 2,3v on avg on the 5V rail.
Im also geting no video most of the time but sometimes it get some flashing pixels on the screen and a weird buzing noise.
Thanks a lot for help Image
ps. its also a PAL nes because I´m from germany
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Nes 2v on 5v rail.jpg

Joe
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Joe » Wed Mar 03, 2021 10:42 am

It looks like something is drawing more power than the regulator or power supply can provide. Are any components getting unusually hot? Does the power supply work with your other NES?

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 10:58 am

Thanks for your reply... No components are getting unusually hot or anything. Now I had the idea that I'm going to measure the current draw on the 5v Rail to check if just the supply is broken or if something is broken on the main pcb. One strange thing that would agree with what you've said is when nothing is connected to the supply( I removed it before to check if it would fit into the other one) it gives me stable 5v but under load the voltage drops to around 2V. Now I'm wondering if something on the psu is broken and it can't supply the ~1A which the nes needs or if something is broken on the main pcb.

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 11:03 am

I've heard that the supply can be interrupted by a broken cic chip ( I've heard its common for them to fail after 35 years)
So that might be the solution?

Joe
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Joe » Wed Mar 03, 2021 11:28 am

Are you using a 5V power supply? The NES includes its own 5V regulator, you need to supply more than 5V for it to work correctly. (The original AC adapter outputs 9V AC, but you should see it working with as little as 7V DC.)

With no cartridge or controllers, the NES doesn't need 1A. If you do see it drawing that much, something is wrong.

I can't say I've ever heard of the CIC overloading the regulator when it fails, but I suppose it's possible?

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 11:31 am

Im using a replacement psu that outputs 9.3V so that is fine now I checked how much curent the main pcb draws and it draws with no and even with a cartridge around 200mA

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 11:34 am

Doesn´t the CIC control the reset line? That would explain why the voltage on the cpu changes so rapidly...

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 11:37 am

So my conclusion at this point is that the psu cant supply 200mA and it breaks down. I will test the psu seperatly now and find out at wich point the psu drops its voltage

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Quietust
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Quietust » Wed Mar 03, 2021 2:17 pm

The label on the bottom of my NTSC frontloader says it requires 850mA, and the original AC adapter supposedly provides 1.3A at 9V AC.

(the 3rd-party AC adapter I got with my NES supplies 800mA at 9V DC, though it still seems to work)
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Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 2:33 pm

I thought about further troubleshooting it by using my current limiting bench psu. But what is stil a bit strange to me is the fact that it draws the same amount of current no matter what (with or without cartridge inserted) wich is arround 200mA, then the psu only outputs ~2Vavg but when I remove it and place a load like a resistor that matches the resistance between 5Vin and Gnd it works perfectly fine and outputs the desired 5v... Almost as if it knows that something isn´t right with it.

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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by lidnariq » Wed Mar 03, 2021 2:36 pm

When I've measured it, the NES consumes somewhere around 300mA even without a cartridge.

The cartridge is unlikely to add a significant load to that until such time as it's actually working.

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 2:41 pm

ok so now ive tried turning it on with my bench psu and well so far nothing has burnt down but I´m still getting no video or anything that you could call "video" the only thing I can see is some garbled stuff on the Video output pin before it gets into the little box with all the ports on it wich to my knowledge should just be a really weak komposit signal.

lidnariq
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by lidnariq » Wed Mar 03, 2021 2:42 pm

You won't see anything work until the PPU gets at least 4V. It just doesn't operate at lower voltages at all.

Liengamer
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by Liengamer » Wed Mar 03, 2021 2:47 pm

thanks a lot so far... Now im getting 5V on all of the chips by utilizing my bench psu but and now even a really weak but stil some garbled composite signal on the video out pin.

lidnariq
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Re: Nes Power Led flashes realy fast+getting only 2,3Vavg on 5V rail

Post by lidnariq » Wed Mar 03, 2021 3:03 pm

The video output of the PPU is very high impedance, and needs one or two stages of current amplification / voltage buffering before it's usable. If the transistor on the mainboard that does this (Q1, 2SA937) is damaged, or the output stage inside the RF modulator, that can easily drag the PPU's output down to invisible levels.

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