Help! Marshall 4203 blowing fuses, not PT

1/30/2006 11:30 AM

John L.

Help! Marshall 4203 blowing fuses, not PT Hi, I have a Marshall 4203 Combo Amp I bought new in the '80's. What a great amp that has served me well! Solid state pre-amp and tube power-amp give a great sound.     The schematic can be found here:     http://www.drtube.com/schematics/marshall/3203pwr.gif     The Power Transformer blew and I had a replacement made from Heyboer. I hook up the new PT and hit the power switch, the indicator light comes on and the tubes light up. But when I hit the standby switch, the fuse immediately blows. According to the schematics, the transformer feeds 347 vac to the stand-by switch, which then feeds to a grouping of 4 or 5 diodes, which I believe act as the rectifier portion of the amp. Do you think it is possible that some or all of these diodes may be bad and are allowing the current to backfeed or short out which is causing the fuse to blow? Maybe this is why the original transformer blew...   I am not an electronics expert, but I do know the basics, and I am learning more and more. Is there a way to test these diodes, or possibly track down the problem that's causing the short and the fuse to blow? Any info or ideas you may have would be greatly appreciated. Thanks.   Best Regards,   John L