Re: 7704A Vertical Amplifier Issues


I am happy to report that rebuilding the circuitry around Q4420 resolved the issue I was having with the Vertical Amplifier. I can now, for the first time since I got this scope, see a trace and fully adjust its vertical position on the CRT. I am somewhat elated.

Unfortunately, some of the 1/4W metal film resistors I ordered were slightly larger than the others (almost as big as a 1/4W carbon comp) and I didn't think there would have been room to install that circuit quite like I wanted to so I built it on a very small square of project board, mounted that using an existing hole in the lower-left corner of the amplifier board, and ran jumper wires as needed. A bit ugly, but it works. Once the smaller parts arrive, I plan to build it in the spare, but small, open spot next to U4413 (which, I am relieved to find, is working).

Thanks so much for all the pointers!
Barry - N4BUQ

----- Original Message -----
From: "Barry" <>
Sent: Thursday, September 23, 2021 8:38:15 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues


Thanks for the explanation. That really helps.

I checked C to B for the output transistors in the IC and those appear to be
good. I also checked E to B for the input transistors and those also appear
to be good (although the junction voltage showed to be about 700 mV which
wasn't low enough to cause the Fluke diode check function to "beep").

When I was seeing a trace and, apparently, R4419 was not overheating at that
time, it was "squished" vertically to about 1 cm high and off center. The
POSITION knob on the plugin would only move the trace up and down about 2 or
3 mm. I tried the centering pot (R4493) and that had little to no effect
and that still makes me worry that the IC is bad but I won't know for sure
until I get the Q4420 circuit reinstalled.

The parts for the Q4420 circuit are due in today so I should have that
together soon and can test the overall functionality.

Thanks again,
Barry - N4BUQ

----- Original Message -----
From: "Mark Vincent" <>
Sent: Wednesday, September 22, 2021 9:53:12 AM
Subject: Re: [TekScopes] 7704A Vertical Amplifier Issues


CR4419 is a clamp. Voltage at the emitter and one end of the 33 and 5600
resistors can go lower than 15,6-15,7V, not higher. It also sets the common
base voltage and current limit for the output stages. The voltage divider
the base of Q4420 sets that voltage at the base and emitter. If either/both
output transistors in the IC short from C to B, that will cause the 33 ohm
resistor to burn. Testing those two junctions will determine if the output
transistors are good or bad between those two junctions. One thing that can
cause the output to go bad is someone turning the bias control too high,
R4415 at too low resistance making the voltage higher than 10,1V at one end
of R4416. If that pot was not touched, it should be at the proper voltage.
Once you get it working, check that voltage and adjust as necessary. If you
want, you can turn the pot a little so the voltage will be bit low at first
then increase. Starting at a lower current will not hurt. You reduce the
chance of any damage with lower current. When set right, the vertical will
be correct. The centering control may need adjusting. That is done with no
vertical plug-ins installed. Any other controls for things may need to be
touched up slightly with another IC installed. See if the controls need
adjustment after it is working and a known type of signal is applied. If
trace is the way it should be, leave the other controls alone. The manual
will say how to calibrate.


Join to automatically receive all group messages.