I have a customer's PL700B with a real head scratcher. I am looking to the group to help me figure this one out. I thought I had seen everything.
I don't know if the amplifier ever worked previously. It already had WOA Capacitors and a WOA Blue Meter Illumination LED Board installed.
I replaced the original control board with a known good (bench amplifier tested) WOA Rev G1 Control Board.
At the same time, I sourced all new Onsemi MJ21196G NPN Output Transistors with new Bergquist Sil-Pads. The four RCA Driver Transistors tested good on my Sencore TF-151A Transistor Tester, so they would be re-used.
Bias came into spec at 350mV. All fuses stayed intact at initial Bring Up using a DBT and a Variac with only the RCA Drivers installed (kind of Class A operation)
Using a CD as a musical source, the Left and Right Channels were as clear as a bell. Of course, at a very reduced input level and Level control knob position.
I installed all 20 Output Transistors.
The Left Channel was Loud and Clear. However the Right Channel was severely distorted. I suspected a bad Output Transistor because no fuses were blown.
But this distortion was unlike anything I had ever experienced. At low levels (Level control knob positions), the audio was fine. Turning the knob slightly clockwise, the DBT would go full bright (SHORT), the Meter Lights would dim, and the crackling sound output was almost like it was being turned on and off rapidly at a high level of static.
Here is the weird part. I spent a day trying various troubleshooting methods. In the end, this is what I found out: With three (from the top- 1, 2, and 4) of the Output Transistors in the far right column (next to the input/output panel), removed - the amplifier behaved normally, both channels loud and clear. No bright DBT, no hum, and no distortion.
I have checked for proper rail voltages for these Output Transistors and all the others. I have checked for shorts to the chassis and shorts between Output Transistor pins.
All the current sensing .33 and .27 Ohm resistors check good. The two 1N4004 Flyback Diodes check good. The 150 Ohm and 10 Ohm Bias resistors are good.
I have ruled out the Power Supply and everything on the Left Channel. I have ruled out the Control Board, I even tried another known good Control Board, same results. I have ruled out the Level Control because with the three Output Transistors removed, it functions perfectly.
Clearly, I have an over-current situation when the full column of negative rail Output Transistors. My next course of action is to replace D15, the Flyback Diode. I suppose it could be breaking down under a load.
I am open to suggestions. Maybe I missed something?
I don't know if the amplifier ever worked previously. It already had WOA Capacitors and a WOA Blue Meter Illumination LED Board installed.
I replaced the original control board with a known good (bench amplifier tested) WOA Rev G1 Control Board.
At the same time, I sourced all new Onsemi MJ21196G NPN Output Transistors with new Bergquist Sil-Pads. The four RCA Driver Transistors tested good on my Sencore TF-151A Transistor Tester, so they would be re-used.
Bias came into spec at 350mV. All fuses stayed intact at initial Bring Up using a DBT and a Variac with only the RCA Drivers installed (kind of Class A operation)
Using a CD as a musical source, the Left and Right Channels were as clear as a bell. Of course, at a very reduced input level and Level control knob position.
I installed all 20 Output Transistors.
The Left Channel was Loud and Clear. However the Right Channel was severely distorted. I suspected a bad Output Transistor because no fuses were blown.
But this distortion was unlike anything I had ever experienced. At low levels (Level control knob positions), the audio was fine. Turning the knob slightly clockwise, the DBT would go full bright (SHORT), the Meter Lights would dim, and the crackling sound output was almost like it was being turned on and off rapidly at a high level of static.
Here is the weird part. I spent a day trying various troubleshooting methods. In the end, this is what I found out: With three (from the top- 1, 2, and 4) of the Output Transistors in the far right column (next to the input/output panel), removed - the amplifier behaved normally, both channels loud and clear. No bright DBT, no hum, and no distortion.
I have checked for proper rail voltages for these Output Transistors and all the others. I have checked for shorts to the chassis and shorts between Output Transistor pins.
All the current sensing .33 and .27 Ohm resistors check good. The two 1N4004 Flyback Diodes check good. The 150 Ohm and 10 Ohm Bias resistors are good.
I have ruled out the Power Supply and everything on the Left Channel. I have ruled out the Control Board, I even tried another known good Control Board, same results. I have ruled out the Level Control because with the three Output Transistors removed, it functions perfectly.
Clearly, I have an over-current situation when the full column of negative rail Output Transistors. My next course of action is to replace D15, the Flyback Diode. I suppose it could be breaking down under a load.
I am open to suggestions. Maybe I missed something?
![IMG_0006[1].JPG](/data/attachments/77/77369-35fee342a9d98f96a4d233c20ac98fde.jpg)