Re: 7704A Vertical Amplifier Issues

Mark Vincent


You can test from the two bases of the outputs to the collectors. That may indicate the output IC is bad. Q4420 is a 2N3906 and CR4419 is a 1N4152. A 2N2222A and 1N4148 or 1N4937 will work as subs for these two. At the emitter of Q4420 should be 15,6V. The 33 and 5600 ohm resistors should be 1W. The bias voltage at R4416 should be 10,1V. The output pins should be 36V and 37V when working. Since the 33 ohm resistor burned, it is likely the IC is bad. If the 50V supply went too high, that will short the output IC. There is a seller on "that site" that has some known working boards from this model. Ask if he has the vertical output board. There is a NOS IC on there also. See if the voltages at the pins are correct. These can be done at ends of resistors. The four 200 ohm resistors should have 9,3V at the IC. Voltages that are way off, will also mean the IC is bad. It looks like the parts you ordered made the power supply work.


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