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PB1400 Power Supply Repair and Rebuild


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I've got two PB1400 power supplies, one works fine and the other died on me years ago. I decided to open the non-working one to get the lay of the land and see if I could repair it. After disassembly, I first noticed what is probably a common point of failure on these: the output cable leads wrap around this massive resistor. From the look of it, this resistor gets super hot, so it lead to hardening and cracking of the wire insulation. The resistor appears to get so hot that it roasted a bit of the board underneath. Maybe that happened via a short by the cable after the insulation was cracked...but that is speculation on my part. I would like to replace this resistor, but I am uncertain as to its true resistance because the colors are somewhat faded. Anyone have any insight into what it is?

 

I will be replacing the capacitors on here also once I figure out their ratings.

 

 

IMG_5326.jpg

IMG_5327.jpg

IMG_5329.jpg

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Can anyone at least tell me what the resistor value should be? I read 250 ohm on my meter, but I have no way to know if that is correct as it doesn't make sense with the band colors that I can see. Anyone have a clue?

resistor.jpg

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Looking at another resistor in there, the tolerance band is the one at the end that is towards the center, so the burnt black one on there is likely gold for the tolerance measurement.

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So far, I found three electrolytics and one of those safety film caps. I think that is all that is in there, but I still need to figure out that resistor to repair the damage.

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1 hour ago, LaPorta said:

Looking at another resistor in there, the tolerance band is the one at the end that is towards the center, so the burnt black one on there is likely gold for the tolerance measurement.

5% is probably right... that looks like 24kO, no?

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Posted (edited)

I would venture 24kohm as well, not sure why it tests at a factor of exactly 10x less than that, though. 

Edited by LaPorta
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Yeah that does seem far too convenient. I suppose it's possible that the multiplier is (was) actually brown, which would make it 240 Ohm... If the gold baked out like that, then maybe the brown did as well. Not exactly definitive or useful though.

 

I can only assume you considered looking inside the working PSU... is it just too destructive?

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I tried to open the good one, but it did not open the same way as thi sone did - the seal appears much more intact. I didn't want to jeopardize the working one and not have any usable power supply.

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I found similarly charred resistor when recapping my Quadra/LC 630 power supply. I couldn’t figure out a replacement by decoding the bands that made sense (color change over time/heat?) so I just proceeded with a recap of the electrolytic caps and it turned out fine. I wondered if the resistor ended up with two much voltage/current when the caps failed, or maybe cap goo leaked onto a normally hot-ish resistor not meant to deal with that and burned a bit.

 

 

 

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What it really appears to be is a very poor design: those leads that lead to the output wire wrap around the resistor. It generates so much heat that, over time, the wire insulation hardens and cracks. I think this lead to the wires contacting either each other or something else, thereby shorting.

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