Hi all, I have an Ampeg bass combo with a DC offset - and a twist?
The amp was making a very loud throbbing sound and if left switched on after a few seconds would blow it's T2.5A fuse. A quick test of the 4 Mosfets revealed 'Q5' to be dead short between gate and source, the rest tested good as did associated diodes.
So I replaced the IRFP140 and wound up the variac and the amp came to life but with a 500mV DC offset with no load connected. A voltage test across the four 0.33R source resistors showed one, 'R74' had 20V across it! The other had a few millivolts each, so I thought 'Q2', despite testing good must be leaking and turning on. So I replaced it (IRFP9140) but on turn on it's still measuring 20V across its resistor
I've yet to get to the bottom of the DC offset but am puzzled why Q2/R74 is conducting while the paralleled 'twin' Q7/R75 is apparently behaving as expected? The gate resistors and others are good
Many thanks