Re: 7704A Vertical Amplifier Issues


n4buq
 

Hi Mark,

Okay on the parts. Apparently I was searching for a 2N4152 for CR4419 instead of a 1N4152 (which is readily available).

Is it acceptable to check the voltages at the IC without Q4420 connected? It would be good if I could go ahead check that now and get something ordered if necessary.

You mentioned a 2N2222A as a sub for Q4420. Isn't Q4420 an NPN? I have some 2N3906s on order (thought I had a few in my parts bin but apparently not).

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <orangeglowaudio@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, September 21, 2021 10:34:55 PM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues

Barry,

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.

Mark





Join TekScopes@groups.io to automatically receive all group messages.