2247A Readout issue


As shown on the picture attached  one of my Tekscope  2247A givestrouble shooting with no readout display at power on. Only a vertical read outline . Intensity of readout vertical line change with action of front panel pot .

Fortunately all the front panel commands are actives both pushbuttons functions orstep attennuators and voltdivisions OK.
Photo shows trace CH1 and CH2  OK .  Vertical Pos. command OKHorizontal Time base OK No fail in the trigger  led code table (Service Manual )Only the read out that is not displayed full screen .
Supplies voltage corrects within Service Manual  A16 processor board.
Readout position mixer U2415-2414  give correct vertical and horizontal waveform before amplifiers .
But if  Field & Mixer Control U2411  receive data at  R0-R7 inputs theMIX0-1-2-3 outputs  are locked at 0V with no waveforms so the followingReadout position mixers  don't receive waveforms characters to be mixedwith the Hor and Vert ramps .
Replaced U2411 with no effect. . This control latch seems to be activated witha MIX STB CLOCK  pin 11  ( that comes from the READOUT processor U2400) .But there is no wave form in the S. Manual for the kind of pulse to command thisU2411. Seems that it is locked
If anyone has suggestions because this scope is a first hand clean circuitry ?
Thank you for further infos . 



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