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I've got an early 8080 on the bench with power amp and clean channel working great. Very low output from Lead channel but I've had the board out and the resistors/tracks for the 12AX& heater were in woeful state (someone in there before me) so I have new heater resistor coming which should restore full operation. They'd used three 68 ohm resistors to substitute for the correct 180 ohm, and the strain on the track with those resistors mounted vertically had broken off the solder pad!
But while I was checking the Drive channel I found something puzzling in the switching arrangement. My reading of the circuit is that the amp uses an M5201 switching op-amp to select between the clean signal at IC5A or the boost signal from the cathode follower 12AX7 at IC5B. The switching signal is generated from a Darlington pair in the simple circuit attached (together with the complete pre-amp circuit). If I measure the voltage at the collector of TR1 it changes from 14.3V with the channel select switch SW2 set to the normal channel (so the LED for that channel lights) to 0.6V when the Boost channel is selected.
All well and good but the switching signal is not taken from the TR1 collector but from the top of the resistor R31 connected to the collector. Because it's 47K and connected at the top (switching signal) end to a 4.7K to the +15V rail the signal at this point only moves between 15.3V and 14.1V between Clean and Boost. Could anyone please explain why the circuit is arranged like that and why the switching signal isn't taken direct from the collector? Can IC5 switch between the Clean and Boost channels with only that modest 1.2V difference in the switching signal? Maybe I'm understanding the channel switch arrangements wrong?
But while I was checking the Drive channel I found something puzzling in the switching arrangement. My reading of the circuit is that the amp uses an M5201 switching op-amp to select between the clean signal at IC5A or the boost signal from the cathode follower 12AX7 at IC5B. The switching signal is generated from a Darlington pair in the simple circuit attached (together with the complete pre-amp circuit). If I measure the voltage at the collector of TR1 it changes from 14.3V with the channel select switch SW2 set to the normal channel (so the LED for that channel lights) to 0.6V when the Boost channel is selected.
All well and good but the switching signal is not taken from the TR1 collector but from the top of the resistor R31 connected to the collector. Because it's 47K and connected at the top (switching signal) end to a 4.7K to the +15V rail the signal at this point only moves between 15.3V and 14.1V between Clean and Boost. Could anyone please explain why the circuit is arranged like that and why the switching signal isn't taken direct from the collector? Can IC5 switch between the Clean and Boost channels with only that modest 1.2V difference in the switching signal? Maybe I'm understanding the channel switch arrangements wrong?
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