Should I replace these dropping resistors

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themaninthesuitcase
Posts: 2
Joined: Tue Nov 30, 2021 9:43 am

Should I replace these dropping resistors

Post by themaninthesuitcase »

Hi everyone, I picked up this '93 Marshall Valvestate 8080 for a small charity donation sold as seen. It is externally in great condition and mostly sounds okay, but there is a bit of a strong ground hum that is constant no matter the volume so I wanted to change out the filter caps.

When I opened it up to check the physical sizes of the caps I noticed that the 2 big cement dropping resistors for the HT supply looked like they may have gotten a bit warm and might be worth swapping out.

Image

(180R 7W and are R112 and R113 on the 8080 schematic if your curious)

Looking at them they have clearly gotten warm, but it's a 7W resistor so it's dropping a lot into heat by design.

The Zeners that do the +/- 15v regulation for the rest of the amp look like they have gotten a tad warm as well so will order some new ones and lift them off the board as well.

I've never done amp work before, but am generally good with electronics work so just wanted to check I'm not replacing stuff for the sake of it given the reputation Marshalls PCBs have for lifting pads. I've watched enough Lyle Caldwell videos to hopefully do it correctly but the best way not to damage things it not touch it.
Last edited by themaninthesuitcase on Sat Feb 11, 2023 10:05 pm, edited 1 time in total.
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martin manning
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Re: Should I replace these dropping resistors

Post by martin manning »

I'd guess the resistors are fine, but check the solder joints and reflow them if they look dodgy. Replacing and raising the Zeners is not a bad idea.
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