MG100DFX stuck in standby

[Jamlot]

Jamlot​

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Joined 2024

• Today at 10:11 PM
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• #1

I recently bought a Marshall MG 100 DFX, it was going cheap and sold as not working. I don't know much about electronics but was hoping for an easy repair, fuse, transformer etc. After watching YouTube videos and reading online forums I've been able to conclude that the amp is stuck in standby. I've replaced all of the standby cct and the power chip but it's still stuck in standby.
Looking at the data sheet for the power chip it needs min 3.5v but I'm only getting 0.68
If I put a short between the regulated 5v and the standby cct input the amp works, so I just need to fix the standby cct.
I've made a drawing of the voltages I'm getting so I'll try uploading it
Any help would be greatly appreciated
Thanks

 

[llwhtt]

IIRC depending on the original problem sometimes a circuit trace will burn keeping the amp in stby. Check the traces carefully.

Craig

 

[Jamlot]

What's the point of the standby cct anyway? There is no switch, why don't I just bypass it or put a smaller resistor in to get the voltage above 3.5v?