Mission Amplifier Repairs

Mission 772 Stereo Power Amplifier

Came across one of these with a faulty channel, a Stan Curtis design from a while ago. Looked at the circuit driving the TO3 transistors, as he states on his web site the three in each side of the HT are in series and implemented as Carver did amongst others in the M400 to switch higher voltages to the main amp, he managed to do this just using a handful of resistors.

The faulty channel needed all the T03 transistors replacing along with the drivers Zener's and some tantalum capacitors. Anyone familiar with these will notice the addition of resistors on the transformers, speculation leads me to think they are there to protect the amp when it goes into melt down mode. The amp seems susceptible to interference to external blips on the mains, whether this is because the main filter capacitors have lost capacitance, I don't want to buy 4 of the computer grade ones to find out.

Mission 777 Stereo Power Amplifier

One channel had blown resistors and transistors; 2SB646, 2SD666, 220 ohm, 390 ohm. I replaced these with great difficulty as the holes are through plated, also replaced the o\p FETs. The o\p DC level on one channel was slightly higher than usual, so made a small PCB to adjust the DC level and set the quiescent current at the same time. I also fitted a soft start to the Mission 777 power amp to save the mains switch from welding itself together.