Power up problem with alesis rs300 (ra300)
you have more than likely damaged some transistors.
the resistors have burned because of excessive current.
there are 2 typesof transistors,bipolar and mosfet
a bipolar transistor has 3 pins.1) base 2)emitter 3)collector.
an NPN is tested with the + meter lead on the base , and a PNP transistor with the - lead on the base.
base to collector should read .5 to .6 v drop in one direction and open in the other with a digital meter on the diode check function.
base to emitter about the same,but slightly higher than the collector.
if base to collector is .5v then base to emitter is .52 or so.it is always higher than the base collector number.
between emitter and collector will read open in both directions.
if there is a short on any lead combination,the transistor is blown.
npn transistors operate on the positive rail power supply,and pnp on the negative rail.
typically a 150 watt amp will run + - 65vdc.
the transistors running on the plus side are npn and on the negative pnp.
most pnp output transistors are green.,but some are black,
so we must go by the numbers on the transistors, eg.A1076 is a pnp,B755 is a pnp and all transistors that have a C or D eg. D424 are NPN transistors. all A and B numbers are PNP and all C and D numbers are NPN.
MOSFET transistors have 3 pins 1)gate 2)drain 3)source
the source goes to power like the collector does. the drain is like the emitter,and the gate switches it on like the base does.
testing a mosfet is a different procedure.
a mosfet with k then a number e.g. k135 is a positive device like npn.
a mosfet with j and a number e.g. J50 is a negative device like pnp.
a mosfet reads open source to drain both directions.
for a positive (k number device) the plus meter lead goes on the source.,the black meter lead on the drain. on the diode test function.
jump a wire from the source to the gate for 1/2 second and remove the jumper while leaving the other two meter leads connected to the source and drain.The mosfet should switch on and the meter should show a short.that means the mosfet switched on.About 5 seconds later,the resistance from drain to source should go open.
for a p channel mosfet,it checks the same,except that the black meter lead goes on the source and gate .
a mosfet will stay switched on longer than the voltage applied to the gate,if it is driving a high impedance device such as a meter.
a bipolar transistor will not stay on when voltage is removed from the base lead.
i hope this has been of some help.
Sep 09, 2009 |
Alesis RA-150 Amplifier