Marshall VS100 blowing fuses

[Enzo]

Look at the shape of your BDV part. You want a TIP part of the same shape - what we call its package.

TO - for Transistor Outline - is the shape designation. TO-220 is a smaller version. TO218 is a larger tab top type. TO248 is a large rectangular type. Then there is TO3P. The TIP142/147 parts come in all those packages.

I think your BDVs are either TO3P or TO218. Both are about the same size. One has the screw hole through the tab only, while the other has the body extended up farther with the screw hole through it.

ANd remember to keep track of which part replaces which. Don;t mix up the NPN and PNP parts.

[rockintothebone]

i replaced t10 and t12
the amp worked fine for a few minutes and blew the fuse again
now when i checked the power transistors
the t11s short..

how can this happen ?

[52 Bill]

the t11s short..how can this happen ?

If you replaced some of the original outputs with the TIP parts and left in some of the old parts, you set up a mis-match where the device at T11 had to work harder than the new part.

Try replacing all of the outputs with one type or the other.

Be sure to check the rest of the circuit (drivers, resistors, bias diodes, etc.) for problems as well.

[rockintothebone]

after a lot of hassle... finding all four transistors in the right package ... i replaced all the four (the body extended up farther with the screw hole through it)......as soon as i switched it on.... the t11 was short and the t10 had low resistance(30 ohmns) between two legs...

i desoldered the t11 and t10... but ..when i hook up my multimeter(20 k) the resistance continues to increase slowly... could this be a faulty capacitor ?

[52 Bill]

after a lot of hassle... finding all four transistors in the right package ... i replaced all the four (the body extended up farther with the screw hole through it)......as soon as i switched it on.... the t11 was short and the t10 had low resistance(30 ohmns) between two legs...

Did you check the rest of the circuit as well? Sometimes more than just the outputs will short when the circuit dies, and if you don't catch those problems then they will cause the new outputs to die again.

I would check all of the transistors and diodes in the power amp. Are you powering up the amp with a speaker connected? For now, don't. Do you have a light bulb limiter or a variac? Did you re-install the insulator washers on the output transistors when you replaced them? Did you use heatsink grease on the new outputs?

i desoldered the t11 and t10... but ..when i hook up my multimeter(20 k) the resistance continues to increase slowly... could this be a faulty capacitor ?

What are you hooking the meter to, the circuit or the transistors? If you put your meter on the board, you could be charging up a filter cap with the meter battery. This is why you get the rising resistance measurment.

[rockintothebone]

im not connecting the speaker ...i dont have a light bulb limiter or a variac...the previous transistor had no insulator washers ...and i didnt use heatsnk grease because there is still some left on the heatsink...

i kept hooking my multimeter to the circuit with the faulty transistors removed..

while i was checking the circuit i found something weird R16 , R17, R18 ,R20, R22 ,R23 each having a value of more than 100 ohms somehow had a resistance below 50 ohms..i desoldered R18 cause it had the lowest ohm rating than the rest and checked its resistance ...it showed 100 ohms.....it cud be minute mistake.. but how is it possible ...?

heres the schematics ...
http://www.drtube.com/schematics/mar...v100-62-02.pdf

thanks

[Enzo]

Because while soldered into the circuit, those resistors are essentially in parallel with each other, so the apparent value reads low.

[rockintothebone]

well i did check all the diodes and transistors they all seem pretty ok ..what sthe next step i need to follow.. i dont know any other way to check capacitors other than to look for leakage or buldging...and none of them are...
plz help