I have a 30 watt alesis spitfire amplifier that a friend gave me that is dead. The output transistors were smoked so I bought new ones and did the not so smart thing and replaced them without checking anything else. Well they are still in there, but I am not getting any output.
I believe the driver transistors could be either shorted or open, I used a set of headphones and a resister to trace my audio signal up to the drivers and out of the drivers and into the finals, but the voltmeter says the amplitude of the signal is smaller coming out of the driver, so a very small signal is reaching the finals(~1.2v in and .6v out)
Another thing I noticed was that the stage just before the drivers is running a little warm. Could this be because the drivers are messed up or because I was using headphones to trace my signal (I don't have a scope right now unfortunately.)
I am planning on just ordering all of the transistors in the power amp section and changing all of it. I figured it up and its less than $8 to buy all of them plus a few spares.
Sounds like a good diagnosis. Do you have a scope available? It will make it very easy to compare the waveforms at the input vs. the output of the drivers.
I loaned it to someone... I found a piece of software that allows you to use a soundcard to sample signals and display them much like a scope so I might try it. I will have to be careful to isolate the amp from my sound card as much as I can though.
guitar amplifier
Posted - Dec 08 2006 : 02:07:30 AM
