Marshall 80RCD Boost channel is distorted and low volume
It could be a couple issues in that section. Start by cleaning the contacts and switches to make sure it is not dirt. Also check to make sure the plug you are using is right for the jack- just to make sure everything is connecting as it should and nothing is inadvertantly getting shorted- such a 2 pole plug plugged into a 3 pole jack.
If those are all okay, the issue is in that boost channel section. Check to see if you are getting both power rails at the IC's, measure on a 4558 chip pin 4 to chassis should be -15 volts and pin 8 should be +15 volts to chassis. if you do not have this voltage, check the wiring, check the PC board for bad solder joints and check capacitors. Also check the physically larger resistors.
If you have the voltages, put an audio source on the input and set up a headphone with test leads, or better- an impedance matching transformer with test leads on the high impedance winding- put one to ground, with the other lead, move from IC to IC to locate the area to look at. You can get a data sheet for the 4558 here: http://www.ti.com/lit/ds/symlink/rc4558.pdf It is a low noise version of the 1458 family of op-amps. Check each IC at the inputs and the output, once you locate the stage where inputs are clear and output is garbled, that IC may be the fault, or something in the vicinity- usually it will be the IC. You can get any configuration of the 4558 you need through Mouser or Digikey, and Digikey does ship worldwide http://www.digikey.com
Once you get the new IC in, you should be groovin in no time.
Jan 30, 2012 |