need help with Tektronix 2235 intensity problem

[John B]

Recently purchased a Tek 2235 (does not have military option). It displays
both traces ok as well as a square wave on both channels. Both traces
respond to the 'A' intensity control, while the 'B' intensity control has no
effect. Has anyone else experienced this problem? Both the 'A' & 'B'
intensity control pots check out ok. Voltage at the base of Q814 varies when
the 'B' pot is turned as well as the output voltage on the emitter of Q814.
Voltages here are correct too. There is no voltage change at the node of
anodes CR818 and CR820 while turning the 'B' pot, but it does change here
when the 'A' intensity pot is turned. I've checked all related
diodes --CR818, CR820, CR805, CR817, CR816, CR680, etc., and related
resistors. Q586 is biased and stays on in "Ch1", "CH2", and "Both"
positions, at least as far as I can tell with the voltmeter. I don't see
any components with visible damage, but there is a place on the board that
is brown from heat around Q254, Q255, Q256 which is the Vertical output to
CRT section. I'm guessing that this is unrelated to the intensity problem.
any help or ideas would be greatly appreciated.

Thanks,
John

 

[John Fields]

Recently purchased a Tek 2235 (does not have military option). It displays
both traces ok as well as a square wave on both channels. Both traces
respond to the 'A' intensity control, while the 'B' intensity control has no
effect. Has anyone else experienced this problem? Both the 'A' & 'B'
intensity control pots check out ok. Voltage at the base of Q814 varies when
the 'B' pot is turned as well as the output voltage on the emitter of Q814.
Voltages here are correct too. There is no voltage change at the node of
anodes CR818 and CR820 while turning the 'B' pot, but it does change here
when the 'A' intensity pot is turned. I've checked all related
diodes --CR818, CR820, CR805, CR817, CR816, CR680, etc., and related
resistors. Q586 is biased and stays on in "Ch1", "CH2", and "Both"
positions, at least as far as I can tell with the voltmeter. I don't see
any components with visible damage, but there is a place on the board that
is brown from heat around Q254, Q255, Q256 which is the Vertical output to
CRT section. I'm guessing that this is unrelated to the intensity problem.
any help or ideas would be greatly appreciated.

 

[John B]

Thanks. I guess it helps when you can remember how the piece of equipment
you are troubleshooting is supposed to operate. I bought this to use in
place of my old 7000 series mainframe which I havn't used (as you can tell)
in years because it is too large to fit on my benchtop.