CurtZHP
Well-known member
I've got this Symetrix 528E sitting on the bench with a busted mic input stage. The line input works fine, as well as all the other processing stages. The mic input will not pass a signal, but the "clip" light stays on. The radio station I'm fixing this for says the DJ grabbed the mic boom for the mic this was connected to, got a static shock, and that was the last they heard from it.
Popped the cover. Of course, it's an SMT nightmare! :'( :'(
Looking at the schematic....
http://www.symetrix.co/wp-content/uploads/2016/02/528E_B01-1998.pdf
At first, I suspected the op amp (NJM2068M) took a hit. I'm measuring 15VDC on the output pin. (That would explain the clip light...) I'm measuring about -10VDC on pins 2 and 3, which sort of makes sense, given that the only things between those pins and the -15V supply is a couple resistors (R8 and R9).
What I'm thoroughly confused about is what happens just upstream of the chip. Q8 and Q9, a pair of 2N4403's, appear to provide the balanced input to the chip.
I somehow managed to replace the NJM2068 with a plain old 5532 (same pinout, etc.), but the symptoms remain; so now I'm wondering about those transistors. But I'm sorry to admit I have no clue what is going on around them, especially where Q10 (TIP30A) is concerned. That's obviously the phantom supply, but what else is it doing? And I've never seen that sort of gain control before.
I suppose I could blindly replace components, but I rather think I should learn what I'm looking at here first.
Help?
:-[
Popped the cover. Of course, it's an SMT nightmare! :'( :'(
Looking at the schematic....
http://www.symetrix.co/wp-content/uploads/2016/02/528E_B01-1998.pdf
At first, I suspected the op amp (NJM2068M) took a hit. I'm measuring 15VDC on the output pin. (That would explain the clip light...) I'm measuring about -10VDC on pins 2 and 3, which sort of makes sense, given that the only things between those pins and the -15V supply is a couple resistors (R8 and R9).
What I'm thoroughly confused about is what happens just upstream of the chip. Q8 and Q9, a pair of 2N4403's, appear to provide the balanced input to the chip.
I somehow managed to replace the NJM2068 with a plain old 5532 (same pinout, etc.), but the symptoms remain; so now I'm wondering about those transistors. But I'm sorry to admit I have no clue what is going on around them, especially where Q10 (TIP30A) is concerned. That's obviously the phantom supply, but what else is it doing? And I've never seen that sort of gain control before.
I suppose I could blindly replace components, but I rather think I should learn what I'm looking at here first.
Help?
:-[