Marshall g15rcd replaced lm1875t now has loud hum, no signal output

SOURCE: Hi, I have a Marshall 8240. The Reverb output

probably the reverb tank These coils are very thin and tent to break easily

SOURCE: Greetings. My MG 250 DFX

I believe the design of this results in what you are seeing... Check each speaker INDIVIDUALLY plugged into the MAIN output as plugging into the second jack shifts the tap of the output transformer to the 4 ohm tap which results in a 6 Db drop but now you have two speakers... the apparent level change will be almost unnoticeable.. Do comparision of the speakers individually using the MAIN jack. This is just the characteristics of the beast...

SOURCE: When I turn the amp on I hear a blop blop blop etc

This sounds like a DC feedback problem in the power amp. Check all the resistors R86 through R96. Verify that capacitor C80 is healthy. The right tool to solve this is an oscilloscope.

SOURCE: I got marshall 410h bought it from USA I thought

Likely destroyed the rectifiers AND the filter capacitors. Replace the capacitors and the rectifiers. There are a lot of them. There are fuses so some of those may have saved some parts.

D10 through D14 and D16,D17 and D20,D21 Some of these may have survived so test each.

C51 and C52, C26,C34 probably gone. Check the Bias supply is working at expected bias voltage.

Your schematic: http://elektrotanya.com/marshall_jvm410.pdf/download.html

SOURCE: My Marshall Haze 40 Amp started making a very loud

may be one of the electrolytic capacitors in the power supply has become leaky...this is a common problem among amplifiers...so you let it cool down..then after a period of time the capacitor heats up and then causes the hum...the power supply caps filter out the 60 hertz ac line noise...which is probably the frequency of the sound it is making...this is a good indication of these caps going bad....another thing you can check is to see if the amp is properly grounded(eg: the ground pin on the plug is present) and that you dont have an open cord or input