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PostPosted: Wed Jul 22, 2015 8:58 am 
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So I have an NES-101 that, as the title says, is getting a low voltage both before and after the 7805 regulator. About 2.5 volts too low on each side. I've replaced the regulator with a known good regulator to no avail, and I've also replaced the filter cap thinking that maybe it had dried out and was showing high resistance. No luck so far though. Any one have ay suggestions on where to check around next?

Also, new here and this is my first post. I'm sorry if I violated any rules, and I'm looking forward to being a part of this forum.


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PostPosted: Wed Jul 22, 2015 11:35 am 
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78xx ICs have a minimum headroom below which they cannot operate correctly. Usually they need at least ≈2V of headroom, and IIRC the unregulated voltage going into my NES is huge (something like 14V), but if you aren't using the original power supply, maybe that's it?

Only other thought is, maybe something is drawing TONS AND TONS and current on the low side. It'd have to be awfully huge to drop the 7805's output by 2.5V, such that you should have an easy time noticing something getting stupidly hot.

We don't have a NES-101 schematic, but there isn't really that much to the NES(-001)'s power supply: http://console5.com/wiki/File:NES-001-S ... Switch.png – it's just filter caps, full-wave rectifier, more filter caps, power switch, more filter caps, 7805, more filter caps. Not even an on-board fuse.


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PostPosted: Wed Jul 22, 2015 11:53 am 
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lidnariq: thanks for the input. I've tried both a 12v DC 1250ma adapter and 9v 2A DC adapter and both exhibiting similar drops of about ~2v on the high side and getting about 2.7V on the low side of multiple different 7805's. The 7805 is getting burning hot within seconds, obviously. Do you think a short to ground could be the issue? The bridge rectifier gets decently warm, warm enough to feel hot to the touch but not instantly burning kinda hot like the regulator. I haven't noticed any other components getting extremely hot quickly, but I haven't kept the unit on for more than a few seconds due to how hot the regulator gets.

I'm guessing current draw is the is issue here, maybe something shorted to ground? so any suggestions on how to start hunting down the culprit?


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PostPosted: Wed Jul 22, 2015 12:16 pm 
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Yeah, that does sound like a short somewhere. I don't suppose you can measure current going into the whole thing?

If you have a current-limited bench supply, what happens if you remove the 7805 and try to inject 5V directly?


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PostPosted: Wed Jul 22, 2015 12:37 pm 
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I don't have current limited bench supply sadly. I do have a fluke 117 which can read up to 20 amps though. I also have a 5v DC wall adapter, ad several female solder lugged barrel jacks around. I could basically whip up a 5v DC probe and then run the DMM in series and check the current. Anything over about an amp should be more than the NES sees right? I think the 7805 is only rated for 1.5, so if it's over that then there's definitely a current issue.


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PostPosted: Wed Jul 22, 2015 1:20 pm 
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Under normal conditions, the NES only draws about 350mA. You're well past that :)

I'd probably start by trying measuring resistance between the power rails at each of the ICs, and see if there's any noticeable difference at different locations on the mainboard. (My working NES-001 shows a resistance from Vcc to Gnd of 255 ohms)

You could fake a current-limited power supply with a small (≈3Ω) resistor or a 5V-6V (e.g. #909) light bulb; the NES runs correctly down to about 4V. (A little below that, the PPU's pixel counters stop working). Figuring out what exactly is shorted out if there's nothing visibly obvious may be a bit annoying, though. Sometimes you can move where you inject power around and measure voltages at other points to figure it out.


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PostPosted: Wed Jul 22, 2015 1:34 pm 
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I'll start by testing the resistances from each VCC pin to ground. I remember coming across a pinout for each of the PPU, CPU, and SRAMs a day or two ago when I was researching the issue. I'm hoping it is simply a bad cap or failed resistor somewhere, and not something worse like all the ICs got toasted some how.


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