# SQ-80 blank VFD: timing crystal prob?

**URL:** <https://forums.syntaur.com/t/sq-80-blank-vfd-timing-crystal-prob/2119>\
**Category:** Synth Repair\
**Created:** [August 3, 2020, 7:46pm UTC](https://forums.syntaur.com/t/sq-80-blank-vfd-timing-crystal-prob/2119 "2020-08-03T19:46:28Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![wacotrumpet](https://forums.syntaur.com/user_avatar/forums.syntaur.com/wacotrumpet/32/717_2.png) [@wacotrumpet](https://forums.syntaur.com/u/wacotrumpet)\
**Post date:** [August 3, 2020, 7:46pm UTC](https://forums.syntaur.com/t/sq-80-blank-vfd-timing-crystal-prob/2119/1 "2020-08-03T19:46:28Z")

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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.

QUESTIONS:

(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,  
SB.

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**Author:** ![PPD](https://forums.syntaur.com/letter_avatar_proxy/v4/letter/p/59ef9b/32.png) [@PPD](https://forums.syntaur.com/u/PPD)\
**Post date:** [August 3, 2020, 9:01pm UTC](https://forums.syntaur.com/t/sq-80-blank-vfd-timing-crystal-prob/2119/2 "2020-08-03T21:01:45Z")

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Your diagnosis is most likely correct.  
Crystals are sensitive devices and are susceptible to shock and vibration.  
I would first start by replacing the crystal as it is an easy and cheap fix.  
Good luck!

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**Author:** ![wacotrumpet](https://forums.syntaur.com/user_avatar/forums.syntaur.com/wacotrumpet/32/717_2.png) [@wacotrumpet](https://forums.syntaur.com/u/wacotrumpet)\
**Post date:** [August 5, 2020, 4:24pm UTC](https://forums.syntaur.com/t/sq-80-blank-vfd-timing-crystal-prob/2119/3 "2020-08-05T16:24:36Z")

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Thanks for the response!