Marshall JCM 900 4100 blowing fuses

[Hapa]

I pulled the trigger and ordered the new PT,and while I await its arrival I will construct a LBL. The filter caps test very close to perfect but I’m wondering if I should replace them anyhow,due to age (dated 24 93)

 

[Purgasound]

If you're going through all that trouble, then yes it would be a good idea to change all the electrolytic capacitors including the smaller ones on the board. I haven't encountered any issues with failing electrolytic caps on 900's yet but I consider it preventative maintenance.

The LBL is going to help and will be necessary for the initial startup procedure. You'll have to diagnose why the PT went in the first place.