Marshall mg100hdfx power chip

[jessadr]

I got the new power chip board in. Does it need to have the mica installed between it and the heatsink?

[52 Bill]

I got the new power chip board in. Does it need to have the mica installed between it and the heatsink?

Yes, and heatsink compound as well. Be sure the fan works and discharge the power supply before plugging the board in.

[soundguruman]

The heat sink is hot, (as in energized) the plastic fan insulates the heat sink. There is no insulator.
The best heat sink compound for that amp is the arctic silver, sold in computer stores, or at radio shack, etc...
Newegg.com - Arctic Silver 5 Thermal Compound - Thermal Compound / Grease
however, don't get the silver on the electrical connections, it's sort of conductive. (you can wash it with denatured alcohol)
anyhow, the silver dissipates heat much faster than ordinary zinc grease...
The best way to deal with this is to buy the chip already mounted on the board. It's a regular Marshall part.
However, the board chip assembly is a bit hard to find and order, but worth the extra price. I suppose the places that have it are charging a premium. It saves a lot of headache and circuit board work...
Then as far as cooling goes you can speed up the fan...or customize modify it...
The part that usually blows along with the output---is a zener diode in the power supply, that regulates the voltage for the standby pin...

Then what to watch out for is the speaker load. It can be no lower than 8 ohms, or the IC overheats.
Some people have swaped out the speakers and wired it 4 ohms, then the IC flames.

[jessadr]

So you say use an insulator ? 52 Bill said the heat sink is hot to the chip. Is there a mod like drilling holes in the chassie to get more air ?

[soundguruman]

So you say use an insulator ? 52 Bill said the heat sink is hot to the chip. Is there a mod like drilling holes in the chassie to get more air ?

No you do not use an insulator.
No, the speaker load is usually what makes it overheat.
You can modify the fan maybe...

[Jazz P Bass]

So you say use an insulator ? 52 Bill said the heat sink is hot to the chip. Is there a mod like drilling holes in the chassie to get more air ?

'Hot to the chip" only means that the back metal plane of the IC is at the same potential as pin #13, which is your B- rail of the power supply.
I am not at all clear on the use of a mica insulator.
Some Marshall models used them, some did not.
I believe SGM is refering to the model that uses four plastic isolators around the fan attachment screws.
There are little caps on them.
Yes, they isolate the B- from the chassis.
In that model a mica insulator is Not used.
But a small smear of silicone grease is used.
Was there an insulator attached at disassembly?

[jessadr]

Ok I guess it was installed correct . Going to check the diode and trasistor in stanby circuit

[Jazz P Bass]

The 'Standby" pin is pin# 9.
You will see a slow ramping up at power on, ending with 5 volts (more or less) on the pin.

[jessadr]

I don't have the 5 volts. I have not checked the voltage on both sides of d14 yet. Kinda close in there for my test leads. If it is bad is there a radio shack part (5.1 v 276-565) that can be used. Also can a sub be used for the tr2 transistor ?

[Enzo]

Find out which part is shorted, thenb we'll tell you what to get for it. Don;t forget the cap in parallel with the zener and transistor. It can short too.

[jessadr]

Boy you guys earn your money making a living working on these things. This thing will be a bitch to get apart.
I will need at least a d14 (bzt55c6v8), tr2 ( 2sc3198) ,and c98 220/35v in hand to start

[Jazz P Bass]

Parts: 78 cents.
Labor: 120.00

[jessadr]

All I got to say is without you guys, "that I think are genius's" there would be alot more stuff in the landfill. I really enjoy reading all the forums and tearing into these things that other people just lay aside. Sometimes I let alot of smoke out of stuff, but that is just more spare parts to me. Glad I am retired, but will never get all this stuff working.
Thanks for the help Getting back to tearing this thing apart. Next one is a VS100 8080

[52 Bill]

Yes, the heatsink is live but I prefer the insulator at the chip rather than trusting the fan mounting. Just me.

Did you just replace the IC or did you replace the entire loaded board?

[jessadr]

I replaced the whole board. Removed the fan cleaned and oiled it. The heatsink is insulated front the chassie. No shorts there. Just no voltage at pin 9