Original NES power issues

[sli4]

I recently turned on my old NES to find the red light won't go on at all. When powered on it shows black bars on the tv and a loud buzz through the tv speakers. Powering it off, the buzz goes away and tv goes back to normal (white snow).

I tested the power supply itself and it is outputting 9V as it should - first two solder joints on the board at the power port show the same.

Here's where it starts getting funky - When powered on, one side of the 7805 voltage regulator is showing 18V and the other is about 8.7V (center 0).

On the big blue connector I get 29V on the Brown (to power button) cable when it's off and 0V on the Red (power button return) cable. Pressing power on, the brown drops to 18V and the red goes up to 18v while the orange (voltage to reset button) goes from 0 to 8.7V, Yellow (power to LED) goes from 0 to 1.8V and the white (Reset button return) goes from 0 to 8.7V. If I hold down the reset button while it's powered on the Yellow (power to LED) goes up from 1.8V to 8.7V. I recall under normal operation the red led light goes off when you press the reset button which probably accounts for the increase in voltage from 1.8 to 8.7 there....

Both leads connected at the LED read 8.7V when powered on.

The 7805 should be dropping it down to 5V but I'm not seeing that anywhere. And I can't figure out where the 18V or 29V is coming from - so this doesn't seem right.

All of this was tested as AC on my multimeter as the power supply is a 9V AC power supply.

Any ideas?

One thing I can rule out is a dead LED as well. When I touch the two pins circled in yellow in the attached photo together the light goes on solid. The first circled pin is 8.7V second is 0v

[CapLeaker]

Chit... wrong NES... hang on. You should have 2 voltages. 9VAC coming in, going through the bridge rectifier, then your get 13VDC and it goes to the 7805 regulator, 13VDC in and 5VDC out. Set your DMM to DC after the bridge rectifer!

[sli4]

Update: I tried a DC power supply and the red light started blinking (no game inserted) when powered on. Could this be a rectifier issue?
…this is beyond my knowledge and I'm learning of these components as I go, but I can probably handle the soldering if needed.

[CapLeaker]

Update: I tried a DC power supply and the red light started blinking (no game inserted) when powered on. Could this be a rectifier issue?
…this is beyond my knowledge and I'm learning of these components as I go, but I can probably handle the soldering if needed.

A blinking red light with a cassette can be all kind of things. Mostly it is bad contacts on the 72 pin header

No cassette = blinking red light
What PSU did you use now?

[sli4]

Thanks for the response CapLeaker!
yeah there's currently no cart in the machine so I'm not worrying about the 72pin (yet haha).

Up until now I've been using the original nes-002 AC power supply and a third party AC. Both rates at 9V 1.3a

The blinking light came on when I tried a 10V 700ma DC power supply (I would not use this long term as its lower a peerage and higher voltage than the NES spec.

on the 7805 I'm reading 8.3VDC on the in and 4.3VDC on the out when using the AC power supply.

using the DC power supply I get 9VDC in and 5VDC out.

[CapLeaker]

Yeah, but your second voltage wouldn't be right. 9VAC x 1.414 = ~13VDC. If you put in 9VDC, then you have to subtract the bridge rectifier voltage drop and you get 8.3VDC. Look at the schematic, it needs 2 voltages, one 13VDC and a 5VDC from the 7805 regulator. That means you'd need a 14VDC PSU, not a 9VDC PSU. An AC voltage through a rectifier is always times 1.414 more on DC.

[CapLeaker]

Read this: https://www.badcaps.net/forum/troubl...d-no-audio-fix

[sli4]

So would this be an issue with the rectifier?

10VDC power supply is yielding 9VDC going into the 7805 and 5VDC coming out. If the rectifier is not doing anything this would be about right with roughly 1V drop? I think the 7805 works as long as there is more than 8V going into it and I believe If the rectifier was working then I'd need a ~13VDC supply to compensate, correct?

The AC power supply at 9VAC is yielding 8.3VDC going into the 7850 and 4.3VDC coming out. Which is not right.


I checked that thread as well. Hopefully I don't have to replace that chip!
good work with the mod/demod. I have to say I'm pretty new at reading the circuit diagrams. So it takes me a while to figure out everything going on but I do see what you're talking about with the need for 2 voltages; one in and one out of the 7805. I'm thinking in terms of 3 voltages. The psu input, what should be going into the 7805 and what should be coming out.
Once I get the power situation sorted I'll make sure the av stuff works too.

[CapLeaker]

The rectifier is doing not much, because you feed it with DC!
So either you feed the NES with a 9VAC power supply or you need a 14VDC power supply. The diodes in the rectifier have a 0.7VDC loss, so you should get 13.3VDC, just like when you feed it with 9VAC.

[sli4]

Sorry if I'm confused.

In previous posts I've mentioned voltages with both AC and DC supply options.

the AC 9V supply option is yielding 8.3VDC going into the 7805 NOT the 13VDC it should be.

So do you think the rectifier needs replacement or could something else be causing the mismatched voltages you're talking about?

[CapLeaker]

Sorry if I'm confused.

Yeah, me too. You need that 9VAC 1.3A supply. Like I said, 9VAC x1.414 = ~12.8VDC. You can't have 8.7VDC there with the AC PSU, unless you are using a 6VAC supply

Put your DMM in AC mode, plug the PSU in and check the AC voltage in the barrel. If that is o.k, well maybe there is a problem with the bridge rectifier.

Here is a vid for you to watch: https://www.youtube.com/watch?v=MiKiazW5brI

[CapLeaker]

Maybe search for something like "how to test a bridge rectifier" on youtube.

[CapLeaker]

I wonder what pints do you use on the power supply as ground reference with your DMM?

[stj]

the biggest problem for beginners is the power stuff is in the can so you have to desolder the RF block to do work on it.
my nes is now 5v

[sli4]

I wonder what pints do you use on the power supply as ground reference with your DMM?

I checked the voltage of the power supply by putting one of the meter leads into the barrel and the other on the outside of the barrel. It shows proper 9VAC there.

when testing pins on the actual board I'm using the chassis as ground.

I could absolutely be wrong but I think to test the rectifier I'd have to desolder the power rf box from the main board to get inside to where the rectifier is. As STJ said - that's not going to be fun to desolder and redo. Though I will say I have more experience with a soldering iron than reading circuit diagrams.

I was actually watching that video earlier today. Thank you for sharing it though. It gave me the tip on where to get a replacement without it taking forever and costing an arm and a leg in shipping!

stj that's a really neat 5V mod. I'm interested how you did it - though I want to keep my NES as close to its original form as possible.

[CapLeaker]

Lemme guess… you disabled the RF out section which uses the 13VDC, and use the AV out and the rest of the NES on 5V USBC only. Piece of cake.

[stj]

disabled the RF would be an understatement - i think i stripped it out.
my original plan was to go RGB but the arcade version of the gfx chip is stupidly expensive if you find one
i also disabled the protecion chip - or removed it, i cant remember which

[sli4]

I ordered a new bridge rectifier... should get in this weekend. I'll comment back when I get that sorted.

[sli4]

I wanted to ask about some new findings before I started ripping apart the power/rf box.

When the NES is powered off and I plug in the 9VAC power supply the bridge rectifier seems to be doing its job. I get 9VAC at the two diagonal AC pins, and I get 13VDC at one of the other pins that I'm assuming is the negative pin. I get 0 volts at the what I assume is the positive pin though. Picture attached.

When I press the power button that 13VDC drops to 8.3VDC

So does that mean the rectifier is working and something else is causing the voltage drop? Or does that mean it is NOT working since that one pin has 0V whether it's on or off.

this seems to be the main problem. Whatever is causing that voltage drop is putting the input voltage to the 7805 lower than its working threshold. So the 7805 is spitting out under 5 volts which does not seem enough to fully power the NES.

[stj]

does the 7805 cook?
it could be a bad 7805, or it could be a part downstream overloading the 5v
also it could be a problem with the caps after the rectifier, but as you tried with dc thats probably fine.