Marshall VS100 blowing fuses

[brokenbones]

My amp had a blown fuse. I go to the hardware store to replace the blown fuse with a new one. Come to find out that the blown fuse is a 250v fuse while on the amp it says "230v or 150v". Did the previous tech "hotwire" this amp? So I bought a new 250v fuse and and the amp immediately blows the fuse. Everything I've read so far suggests its either the tube or valve. Does this sound correct? Or, should I find a 230v and try that? Any suggestions? -Eric

[Enzo]

If printed on the amp you see something like 120v 4A and 240v 2A, that means when the amp is running on 120 volts as in the USA, the main fuse is a 4 amp, and when running on 240 volts as in Europe and elsewhere the fuse is 2 amps.

The voltage printed on the fuse is not the issue. If you need a 4 amp fuse, then most any 4 amp fuse will be fine. Just be aware there are regular faast blowing fuses and slow blowing fuses, also called timed or time delay fuses. But you MUST use the proper AMP rating fuse. Forget the fuse voltage. The vvoltage printed on the amp is for the amp.

Nobody likely "hotwired" anything. The fact the fuses blow means they are not bypassed.

It is real unlikely the tube is blowing the main fuse.

Much more likely you have a blown output stage, meaning shorted output transistors.

[brokenbones]

I hooked my Multimeter up to all three of the transistors and tested them. On two of the output transistors, two of the three "arms" had a 0 rating. The other transistor had one bad "arm". I'm guessing these need replaced? Is there anything else that should be tested/replaced? How hard is it to replace the transisitors?

[Enzo]

Not sure what you mean "one bad arm."

DOn't measure resistance to ground, measure resistance between the legs. If two legs measure zero ohms between them, then that transistor is probably shorted.

I don't find it hard to replace transistors, but then I do it for a living. it is a matter of unsoldering and removing the old and soldering the new in its place.

[rockintothebone]

even i got that problem but ..is there anyway to figure the problem without unsoldering anything?

[Enzo]

Well no, not really. There are no socketed parts to speak of in there. Fuses you can replace, but until you find and repair the reason they are blowing, they will continue to do so. ANy repairs to be made will require soldering of parts.

[rockintothebone]

thanks for replyin mate...
i realise i have to solder to repair it
but how exactly do i find the problem first?
are there any steps to follow ?
(for example check the resistance between two points of the circuit)

[Enzo]

Well, if you are blowing fuses, then I would start by checking the power transistors. See if any two legs are shorted together.

[rockintothebone]

i tried checkin the resistance.. two legs of the t9 and t10 showed 4ohms

ill link you to the schematics .

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

[rockintothebone]

heres the picture of the circuit

[Enzo]

REmove them and check them off the board. It appears they are shorted.

Are they the BDV65s? Or are they TIP142/147?

[rockintothebone]

according to the schematics it says t9 and t10 are bdv65
and t11 & t12 is bdv64
i wonder ill unsolder them and let you know

[Enzo]

If you need to replace the BDVs, you can use the TIP142/TIP147 pair of transistors. Make sure to get the same package type.

[rockintothebone]

i managed to desolder t12... its a marshall t64 ...st microelectronics tip147..

now t11 has higher resistance...but th t9 and t10 still have low resistance between the legs

[rockintothebone]

I managed to remove t10 and t12

so now the t10(Marshall T65) and t12(Marshall T64)(ST Tip147)

the T64 has a low resistance of 0.06 ohms between the collecter and emitter
and
the T65 has a low resistance of 3.4 ohms between the collector and emitter

i connected the circuit back without these two transistors and the fuse didnt blow...



now..
can i replace these two faulty transistors with ANY TIP142 and TIP147 ..or shud i get certain ones?