Tweet
Hey all,
I'm calling in for help on this Heathkit TA-17 solid state amp from the 70's. The power amp section is giving me some trouble. The original complaint is that it was "getting hot." Considering that the 2n3772 output transistors are rated at 150W max, yes, it gets hot. The owner had a box full of parts, including lots of all of the transistors this thing has, caps and resistors. I changed out the massive electrolytic on the power supply and the 4000/50v coupling cap on to the speaker output because they were visibly leaking. Everything was working, but I started trying swapping transistors with some from the parts box to try to get the idle noise down when -
R311 (should be 15r, was 33r) burned up. I found Q14 had shorted out. I replaced R311 with a 33r resistor and Q14 with a good testing one from the parts box. I now have low voltage measuring on the Emitter, and no voltage on the Collector. All voltages on the schematic are running about 4V higher that listed and were reading that way when it was working.
Voltage at the last coupling cap + side is ~34VDC
Voltage at the Base of Q14 is ~42VDC
Voltage at the Collector of Q14 is 0VDC
The output transistors are mounted on their own large heatsinks. I reapplied thermal compound to each one. The two thermal compensation diodes are also mounted to the heatsink of Q15 and test OK and are dropping ~1.5v.
I'm calling in for help on this Heathkit TA-17 solid state amp from the 70's. The power amp section is giving me some trouble. The original complaint is that it was "getting hot." Considering that the 2n3772 output transistors are rated at 150W max, yes, it gets hot. The owner had a box full of parts, including lots of all of the transistors this thing has, caps and resistors. I changed out the massive electrolytic on the power supply and the 4000/50v coupling cap on to the speaker output because they were visibly leaking. Everything was working, but I started trying swapping transistors with some from the parts box to try to get the idle noise down when -
R311 (should be 15r, was 33r) burned up. I found Q14 had shorted out. I replaced R311 with a 33r resistor and Q14 with a good testing one from the parts box. I now have low voltage measuring on the Emitter, and no voltage on the Collector. All voltages on the schematic are running about 4V higher that listed and were reading that way when it was working.
Voltage at the last coupling cap + side is ~34VDC
Voltage at the Base of Q14 is ~42VDC
Voltage at the Collector of Q14 is 0VDC
The output transistors are mounted on their own large heatsinks. I reapplied thermal compound to each one. The two thermal compensation diodes are also mounted to the heatsink of Q15 and test OK and are dropping ~1.5v.
Attached Files
Comment