Hidden Problem!!!
So here's the story...
I had this amp all back together and figured I was about done. Just do the bias adjustments and such and I'd
be good to go....
It was the "and Such" part that things start to go bad... I adjusted the bias (7mv) on both channels without
problems. Sounded fine... I hooked up the 8 ohm load resistors and ran a 1k Hz signal through it to measure
the gain. One channel was producing a whopping 25v (39 watts RMS). The other was clipping at 18v (19
watts RMS). Hummm..... I did some voltage checks and found that I was down 10v (24v should have been
35v) on the collector side of the output transistors....
I figured this was a transistor issue, So now I had to take the unit partially down again to get access to the
board. While it was down, I re-check the resistors and diodes. Did this once... but ya just got to be sure... I
could have missed something. I found all to be well, so I was sure this was transistor related issue. I spent
some time with the schematics and narrowed this down to three of eight transistors in this channel. I had a
pair of exact replacemets for the current limiters (suspects) so I swapped them out ....
I had this amp all back together and figured I was about done. Just do the bias adjustments and such and I'd
be good to go....
It was the "and Such" part that things start to go bad... I adjusted the bias (7mv) on both channels without
problems. Sounded fine... I hooked up the 8 ohm load resistors and ran a 1k Hz signal through it to measure
the gain. One channel was producing a whopping 25v (39 watts RMS). The other was clipping at 18v (19
watts RMS). Hummm..... I did some voltage checks and found that I was down 10v (24v should have been
35v) on the collector side of the output transistors....
I figured this was a transistor issue, So now I had to take the unit partially down again to get access to the
board. While it was down, I re-check the resistors and diodes. Did this once... but ya just got to be sure... I
could have missed something. I found all to be well, so I was sure this was transistor related issue. I spent
some time with the schematics and narrowed this down to three of eight transistors in this channel. I had a
pair of exact replacemets for the current limiters (suspects) so I swapped them out ....
Wahoo!!! Voltage on the board was correct and the two channels performed in perfect harmony. 78 watts
peak (39 watts RMS).
peak (39 watts RMS).