I recently acquired an inoperative SQ-80 and am trying to get it going, beginning of course with the VFD display. Here are the particulars:
(1) All power supply voltages present and within tolerances (according to service manual), both at testing points on power board and at connection points on display board…
(2) The self-test for the display is inoperative, i.e., no response to buttons (I have confirmed that the button switches are still good), regardless of connection or disconnection of four-lead ribbon to main board.
(3) VFD display itself seems in good condition and is receiving necessary power. By grounding a particular pin on the driver chip, I can get a “full light up” on the VFD, i.e., every segment on the display lights up simultaneously.
(4) I think that the timing crystal (4MHz) leading to the display driver chip is faulty. I am unable to read a signal coming from the crystal either with my multimeter, or my oscilloscope. I am measuring several ways: across the leads, from a lead to ground, from just beyond the capacitor on the lead to ground, etc.
So, my provisional diagnosis is: faulty crystal. Proposed action: replace crystal.
(1) Is there a particular way of testing the crystal in-circuit that I am overlooking?
(2) Is a crystal failure likely to be due to mere age? or should I be looking for a cause elsewhere on the board?
(3) Am I right to assume that replacing the crystal will “wake up” the driver chip, so that it will start the self-test mode?
Many thanks in advance! Best,