Quoted from Flynnyfalcon:Hi all, given myself a project today of repairing a QS display. It appears to fire up, there’s an orange glow in the bottom corner, TP1 (180v) & TP2 (4.6v) appear to be within spec but no segments work. I thought it must be the decoder chip, so I replaced that with what I believe is an equivalent chip (CD4543BE) and no change. I’ve inspected the board, reflowed anything looking remotely suspect and nothing. Problem follows the display when I change it to a different position. Where can I check next? Everything I’ve searched for is for repairs when individual segments are out, not entire display.
That is the correct decoder chip. Could just be a bad glass.
From pinwiki:
5.12.2.2 A single display is blank
If you have one display that is blank, it could be the same problems as mentioned above (lack of 170VDC), something wrong on the circuit board, or something with the glass display itself. If you see any light glowing at all on the display, then the glass is most likely good and your problem is elsewhere.
The main cause of a single display being blank is a bad glass. If the glass is good, then your problem is most likely a bad 4543 decoder IC. The decoder on the display driver board is used to decode the digit data from the MPU into signals that light the proper displays. If the decoder is blown, then this could cause the display to be blank. A quick check with a logic probe will help you decide if the decoder is function properly.
Sometimes the solder joints on the display glass needs are cracked and need to be reflowed. Depending on which solder joints are bad, this could cause the display to be blank.
If you can verify there is 170VDc, and the decoder is working, then it may be just a bad display. If there is some light anywhere, then it's probably good. If not, it may or may not be good. A totally blank display is the hardest thing to fix. If everything looks good, but the display is completely blank, then you may just have to chalk it up as a bad display glass. Unfortunately they can not be repaired.