marshall vs100 blown fuses

[guitardude10890]

ok when i got the amp it didnt work and it time to fix it. well i found out that i have an electrical problem in the output curcuit. well i just reasently took out t1 from the curcuit and it still blow the fuse so i know the problem isnt after T1 (transister 1) and i was wondering what to check next. i can find a shematic for you at this web site http://www.drtube.com/schematics/mar...v100-62-02.pdf
please help me
donnie

[Jazz P Bass]

With the power off try testing T9,T10 & T11,T12 for a shorted condition.
These are the output transistors.
T1 is 1/2 of the differential pair input stage.

[guitardude10890]

i tested T9 T10 T11 and T12 and they all checked out fine what eles should i check

[Jazz P Bass]

I would start at the beginning.
The lower part of the schematic is the power supply.
There is also the preamp to consider. That is where the tubes are.
The IC's need + - 15 volts.
Check T 13,14 & 15 of the power supply.
All the diodes, specifically the bridge rectifier.
It looks like Conn 14 can be disconnected. If so disconnect it from the rest of the amp. That is your high voltage rails.
I do not know what equipment you have. A variac would be nice. An ammeter in series is always good.
Short of that a 60 watt lamp in series with one of the AC legs will help.

[guitardude10890]

ok i still need to test T 13 14 and 15 i already know all the diodes are good because thats the one of the first things i did and their all good and my dad put a new bridge rectifier in it so i know thats good so ill get on testing T13 14 15 thanks again Donnie T

[guitardude10890]

i have tested T13 14 15 and they all look good this thing is being a pain

[Jazz P Bass]

At this point you have to back up.
You have the schematic, right.
Start at the beginning.
Disconnect the load off of the power supply.
+L & -L are on one connector.
Disconnect it.
Power up the amp & check +L & -L.
These are your high voltage rails.
If o/k, move on to the IC voltages. +15Vdc & -15Vdc.
If o/k then you have to find exactly what is "loading down" the power supply.
Start there.
Isn't this Fun!

[guitardude10890]

hey everyone i have found the problem with my amp and now i need to get a new transistor for it. does anyone know where i can get transistor 12 on the schematic
http://www.drtube.com/schematics/mar...v100-62-02.pdf
if you can help please do thanks

[Jazz P Bass]

I find it highly unlikely that the one output transistor is all that is wrong.
I would check all around that area.
Resistors, transistors.
Here is a link for the BDV64
FindChips.com Search

[Enzo]

Replace a BDV64 with TIP147. Replace its mate. The two BDV65s can be replaced with TIP142.

[guitardude10890]

i have replaced all four output transistors and now the amp will turn on but all im getting out of it is a loud humming noise what would you suggest i do?

and again thank you for all the help.

[Enzo]

It is still putting DC on the speaker most likely. Pull the wires off the speaker and check if there is substantial DC there.

I'd be checking T6,7,8, and R12-15, at least.

[guitardude10890]

i checked the out put and had 38v dc and resistor 12-15 are good and transistor 6 looks good ill have to check the other 2 later

[zero cool]

If T12 was bad, make sure that R19 is still good. in fact check R24 and R15 also.

I have repaired many amps where just one output transistor has gone bad!

[zero cool]

Originally Posted by guitardude10890

i am getting 0.00 ohms on r19 r24 r25 and r21

Those resistors are .33 ohms so set your meter to the lowest scale possible. with the lead resistance added in you will probably see something like .6 ohms that's ok, just make sure they are not open or some value above 1 ohm.