Tektronix 7D20 - need some help

David Popp

Hello everybody.

I have recently bought a 7603 with the 7D20 digitizer in it.
The 7603 works fine but the 7D20 gives me some grieve. It worked well enough but
sometimes goes into a failure mode where the signals on both inputs disappear
from the display. Both traces then start jumping around the screen vertically, the
vertical position pots are non functional when the scope is in this state.
The self test gives errors 65432 and 654321 in this state.

I have traced the input waveform up to the switching transistors on the A5 board.
This seemed a logical place, as both inputs are affected at the same time and both
channels have to go through this switching stage.
The original eight PNP transistors Q810 - 813 and Q820 - 823 were 2N3906 types
that I replaced with 2N4403s, as I had those to hand and at a quick glance at the
datasheets they should have worked fine.
Now the problems have become more severe, I now kind of have both traces on
the screen at the same time. The vertical position knob of one channel also affects the
vertical trace position of the other channel. There's also a waveform visible between both
traces, a kind of mangled triangle wave it seems, sort of clipped on the top and bottom
and not at all regularly shaped.
ATM I'm using the scope with the CCD's removed as I imagine the constant
disassembling might hurt them. I do not have any extenders, of course, so all I can do
to troubleshoot is solder wires on the A5 board, reassemble and measure with the
7D20 slotted in the 7603. Very tedious indeed.

The circuit diagram I'm referring to is somewhere around page 257/260 in the manual
that's available from BAMA.

I hope someone can help, I will take pictures of it if that helps.


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