tommytones
Well-known member
Schematic
So I finally decided to re-rack my AM-16's into a nice case, new PSU, face plate, etc.. only to run into a very bizare problem. These units have been working perfectly for years and have never been altered or abused. They have been on the shelf for a few months but well stored...once again not damaged.
I powered up the box and proceeded to pass signal. CH-1 is working just as it should. I plug into CH-2 and low and behold, oscilation and mega noise! I probe around to find that Q7 has died. I change it...oscilation and mega noise still. All the DC voltages are fine...they match up very close to CH-1 and the values in the Langevin manual Voltages.
Suspect that another transistor is funky, I changed all of them. Oscilation and mega noise! Here's the bizare thing. After futzing with it for a while, I discover that if I touch my finger or my dmm probe or a screw driver to any point between the base and collector of Q2 & Q4 (left of R7 on the schem) everything settles down and the unit works perfectly :shock: :?: The only other way I can get the unit to work is if I load the input with some 600 ohm source. By this I mean if I plug my cheapo dynamic mic with a trafo or if I load the output of my test set (via impedence selector on the test set itself) the unit works perfect. But if I feed it any other lower impedence source, I get the oscilation and mega noise thing. I've tried loading at the XLR with a resistor but all it does is drop the level a few db...the oscilation and noise stay. Now, after several hours of playing with things, I'm beggining to see the same symptons on CH-1 arise.
I'm quite stumped on this one. What the heck might be happening here?? It's doubly frustrating since these units were working just fine :twisted:
So I finally decided to re-rack my AM-16's into a nice case, new PSU, face plate, etc.. only to run into a very bizare problem. These units have been working perfectly for years and have never been altered or abused. They have been on the shelf for a few months but well stored...once again not damaged.
I powered up the box and proceeded to pass signal. CH-1 is working just as it should. I plug into CH-2 and low and behold, oscilation and mega noise! I probe around to find that Q7 has died. I change it...oscilation and mega noise still. All the DC voltages are fine...they match up very close to CH-1 and the values in the Langevin manual Voltages.
Suspect that another transistor is funky, I changed all of them. Oscilation and mega noise! Here's the bizare thing. After futzing with it for a while, I discover that if I touch my finger or my dmm probe or a screw driver to any point between the base and collector of Q2 & Q4 (left of R7 on the schem) everything settles down and the unit works perfectly :shock: :?: The only other way I can get the unit to work is if I load the input with some 600 ohm source. By this I mean if I plug my cheapo dynamic mic with a trafo or if I load the output of my test set (via impedence selector on the test set itself) the unit works perfect. But if I feed it any other lower impedence source, I get the oscilation and mega noise thing. I've tried loading at the XLR with a resistor but all it does is drop the level a few db...the oscilation and noise stay. Now, after several hours of playing with things, I'm beggining to see the same symptons on CH-1 arise.
I'm quite stumped on this one. What the heck might be happening here?? It's doubly frustrating since these units were working just fine :twisted: