Repairing a 7704A - No Read-out...



Having spent some time repairing a salvaged 7704A I was almost able to fix it.
Recapping braught it back to life except for the read-out.

As far as I can see, the analog parts of the instrument are working but the read-out
is not. Using read-out capable plug-ins results in two vertical "I's" or | (bars)
centered at the top and at the bottom. Since the -15V was zero volts
(shorted tantal capacitor at the X-output board on top of the CRT)
and since the -15V is used by the TEK custom IC's I take it that the board is
bad. The signal levels on J3405 and J3404 (Y and X o/p) on the A34
read-out board is zero with some small ripple. Removing Q3416 described in the
service manual does not make the CRT display two rows of 0's.

What I would like to know is, if the lack of any of the supply voltages
causes the custom TEK-IC's to break. If they can handle such
an error condition, I will spend some more time. But not if the
board cannot be repaired.


Ulf Kylenfall - SM6GXV


Several signals that should be present from interface boards
were not present. Scope will be used for spare parts...