Hi All
I repaired one amp channel which only had one bad output transistor. I used the OnSemi MJ21193/4 pair and up she came, sounding great. The DC offset adjusted just fine but when I went to adjust the bias it wouldn't go below about 15mV when fully CCW. I checked the other channel which had the original Fairchild outputs and whaddya know, the same thing. I double checked to see if I was on the emitters since I know the service info listed the wrong test points and I was. As the pot is rotated clockwise it takes about a 1/5 of a turn before the bias starts to rise which seems like normal action. Oh, I also adjusted the +35 supply.
So on the channel that was already okay, I went ahead and using the best hfe matched NPN/PNP MJ2119x's, replaced the outputs. The problem still remains on both channels. The distortion measures the same on both channels and is what I expect at 1 watt with the covers off (.14%) and it clips darn near equally on each channel into 8 ohms at 91 watts. At just under clipping the distortion on both channels was the usual about .03%. i did a total re-cap and double checked that the ones in the amp are in correctly.
Any clues as to what I should try to look for? :scratch2:
I repaired one amp channel which only had one bad output transistor. I used the OnSemi MJ21193/4 pair and up she came, sounding great. The DC offset adjusted just fine but when I went to adjust the bias it wouldn't go below about 15mV when fully CCW. I checked the other channel which had the original Fairchild outputs and whaddya know, the same thing. I double checked to see if I was on the emitters since I know the service info listed the wrong test points and I was. As the pot is rotated clockwise it takes about a 1/5 of a turn before the bias starts to rise which seems like normal action. Oh, I also adjusted the +35 supply.
So on the channel that was already okay, I went ahead and using the best hfe matched NPN/PNP MJ2119x's, replaced the outputs. The problem still remains on both channels. The distortion measures the same on both channels and is what I expect at 1 watt with the covers off (.14%) and it clips darn near equally on each channel into 8 ohms at 91 watts. At just under clipping the distortion on both channels was the usual about .03%. i did a total re-cap and double checked that the ones in the amp are in correctly.
Any clues as to what I should try to look for? :scratch2: