Old computer fuse blows... - Page 1
Forum section: electronics / repair

author: alank2
author_rank: Super Contributor
timestamp: July 10, 2017, 09:20:24 pm
content: I turned on a 35 year old computer and it actually created a video signal about about 1/2 second before going off. I opened it up and found a fuse that is blown. It is one of the 1/4" by 1 1/4" round variety. It is labeled SOC ST6 1.6A. You can definitely see the break inside it. The computer says 25W and it is for 120V power. It also says 0.3A on the bottom sticker. I want to proceed carefully because I want to try to bring this computer back to life. Could it just be an old fuse that was weakened over the decades, it may not have been powered for 3+ decades. The inside of the computer looks clean and nice. Caps look good, but I haven't tested any of them. Should I try getting a replacement fuse near 1.6A and give it a shot while measuring the current going through the fuse?

author: alm
author_rank: Super Contributor
timestamp: July 10, 2017, 09:31:56 pm
content: Fuses can blow from fatigue, so after a cursory visual inspection for obvious defects (loose wiring, burned parts), putting in a second fuse with the same rating and turning it back on is a reasonable test.

author: Armadillo
author_rank: Super Contributor
timestamp: July 10, 2017, 09:36:26 pm
content: Quote from: alank2 on July 10, 2017, 09:20:24 pmYou should be measuring for any obvious shorts of the power supply rails.

author: alank2
author_rank: Super Contributor
timestamp: July 10, 2017, 10:25:01 pm
content: I put 2A fuse in there and have it plugged into a kill a watt wall meter. It was drawing 2.47A, but it came up with a display and responded to the ENTER key. It sounds like I need to disassemble it further and investigate a shorted rail/bad cap/etc.

author: alank2
author_rank: Super Contributor
timestamp: July 10, 2017, 10:49:45 pm
content: It is in the PS board. I disconnected everything from it, still draws 2.5A, although once I powered on and the killawatt went from 0.03 to 0.19 and then a second later to 2.5A.

author: alank2
author_rank: Super Contributor
timestamp: July 10, 2017, 11:12:22 pm
content: Of the 4 fuses, the one in the lower left that has the darker leads seemed to generate a lot of heat with the thermal camera so I clipped it. It tests ok, one way open, one way not with my fluke meter, but I wonder if it fails under load. I cut it and the high current is solved. I guess I need to find a replacement for it.

author: Nusa
author_rank: Super Contributor
timestamp: July 10, 2017, 11:53:52 pm
content: You had me confused, then I figured out that when you say fuse, you mean rectifier diode.Diodes are cheap. Get enough to replace the entire bridge, not just that one.

author: alank2
author_rank: Super Contributor
timestamp: July 10, 2017, 11:56:02 pm
content: You are right, sorry about that.The diode does test good. I tried replacing it with 5 1N4004's twisted together as I don't have any power diodes and it still drew the 2.5A.How can I test the transformer winding? It is in the upper left of the board. The right two "labeled red" have 8VAC on them, but they are very low resistance (0.39 ohms).

author: RJFreeman
author_rank: Regular Contributor
timestamp: July 11, 2017, 12:22:29 am
content: Quoteeasiest way is to measure the output voltage, see if it is about what you expect.although I am just trying to work out what is what. I am guessing those diodes are for the main 5V rail, which has AC from the connector upper left, then a 3Amp fuse, caps and three pin connector to a power transistor.Unplug the output connector and measure the current draw.

author: alank2
author_rank: Super Contributor
timestamp: July 11, 2017, 01:14:02 am
content: It was the upper left diode - it was shorted. I clipped all 4 of them to test them and only the upper left is bad. I soldered the 3 cuts back into place and ran some jumper wires to 5 1N4004's paralleled together and it works perfectly with 0.23A draw. Voltages on 5V, +12V, and -12V all look very good. I ordered some more diodes from Mouser tonight so I'll swap all 4 out on the power board tomorrow or Wednesday! Thanks for the help!!!!