7834 intermittent unstable readout. Readout wobbly in vertical direction - replaced socket on vertical interface


FAZ
 

I hope this can help somebody. The readout on my 7834 was wobbly. When a signal was displayed by any of the vertical amplifiers the readout would sometimes flicker up and down. Gating the readout made for a more stable display, but the characters were not aligned vertically. The vertical signal to be displayed was being coupled into the readout.

I found the section in the 7834 service manual that mentioned the x-y inhibit from the readout system, so I wanted to make double sure that the x-y inhibit signal was fine. I checked pin 13 of the timer IC U3433, which on the A13 readout board was marked as just 33. I observed that signal while the readout was functioning normally, then waited for the intermittent waviness. No relationship, so I followed that same signal through the manual to the A8 vertical interface board and it appeared the same. On the vertical interface board are two transistors, Q4633 and Q4652 that are responsible for receiving those x-y inhibit pulses. They tested fine, and even after cleaning the leads the intermittent waviness would still happen. That pair of transistors just receive the inhibit pulse. The action really happens downstream in transistor array U4641. Just wobbling it while the scope was on made the readout even worse, with about two centimeters between some of the characters and adjusting the position on the vertical amplifier would take the entire readout with it. Removing the array, I tested each transistor. They were all fine and very uniform as expected. Pressing it back into the socket and spraying contact cleaner didn't help. I noticed that pressing down one end of the IC would lever the opposite end out. Doing that, I managed to get an intermittent trace and stable readout haha. I swapped the socket and steel wooled the pins on the transistor array. Now all fourteen connections are held firmly in place and I haven't had problems since.

Hope this helps if anybody has a similar problem. I was worried it would be something power supply related and didn't want to go down that rabbit hole again

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