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Williams System 4 display problem on a Flash

RichS

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Mar 17, 2022
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Bexhill on Sea
As some of you know, I am investigating a display problem on a System 4 Williams Flash. The "D" segmant was not lighting on Players 3 & 4. Swapping them around proved the displays were good. I did not have a schematic and there was not one for Flash on IPDB. Thank you to you all who pointed me to a "Blackout" that did have a schematic. Looking into it I discovered that "R11" (10K 1/2 watt) on the Master dispay was O/C. Hopefully I replaced it thinking it cannot be that simple, and it was not! It just started burning up again. So onward. Next in line is IC 10 (UDN7180A) the segment driver chip. I have now ordered one but refuse to buy the cheap Chinese "knock-offs that often dont work anyway so have ordered from the UK. My question Is , (and I appreciate how long is a peice of string?) But have any of you had experience of missing segments? Normally it is the display itself. Bearing in mind all the segments of all the othe rdisplays are good it is only "D" in 3 & 4 What is the likelyhood of the issue being further down the signal chain to the MC14543 Segment Latch Decoder/ Drivers or beyond. I know it is a long shot but I am sure someone on here has experienced problems of this kind and comments much appreciated.
 
If the resistor is burning, then the problem is with the display glass, not the driver chip usually.

One of the glasses connected to that output is drawing too much current. You will need to disconnect the D segment pin from one of the displays to find out which one it is.
 
If the resistor is burning, then the problem is with the display glass, not the driver chip usually.

One of the glasses connected to that output is drawing too much current. You will need to disconnect the D segment pin from one of the displays to find out which one it is.
thank you
 
Thinking about this it cannot be the displays themselves as if I swap players 3 & 4 into players 1 & 2 they work perfectly and the ones that were in 1 & 2 into 3 & 4, the fault appears on them so it must be on the main display board. I am very confused!
 
I shall explain further.....

So a 7180 is basically a bunch of high voltage transistors in an IC package. It is used to switch on/off the -100v to the segments in the display.

If it fails, it can fail shorted on, or open circuit.

If it fails open circuit, you get no output, so you get a missing segment on 2 of the displays

If it fails shorted on, you will get a segment that is permanently stuck on.

There are resistors connected to the output of each pin of he 7180.

Normally a resistor is used to limit the amount of current that can pass, but in this instance it looks like Williams designed the values so that the resisters will deliberately burn and act as fuses if the current is exceeded (so they are half watt resistors, but if the output is shorted, the current draw will actually be 1watt, burning the resistor)

The displays themselves, use a tiny amount of current to illuminate a segment, so even if a segment is stuck on forever, the resistor should not burn (so if the 7180 is shorted on, the segment should just be stuck on forever)

When the resistor(s) fail, it usually means that one of the glasses connected to the output was at sometime shorted and the resistor sacrificed itself to protect the 7180

If you have a dead resistor AND the segment still doesn't work, then it unfortunately means the 7180 was also damaged.

Whilst it is possible for the 7180 to fail on its own, if you have a failed resistor and also a failed 7180, then logically there has to have been a short on the glass side of the circuit because a 7180 failing on its own (open or short) does not also cause the resistor to fail.



Hopefully that makes sense. In summary, yes your 7180 could be bad, but if you have burning resistors, you have to first address the short on the output side
 
I shall explain further.....

So a 7180 is basically a bunch of high voltage transistors in an IC package. It is used to switch on/off the -100v to the segments in the display.

If it fails, it can fail shorted on, or open circuit.

If it fails open circuit, you get no output, so you get a missing segment on 2 of the displays

If it fails shorted on, you will get a segment that is permanently stuck on.

There are resistors connected to the output of each pin of he 7180.

Normally a resistor is used to limit the amount of current that can pass, but in this instance it looks like Williams designed the values so that the resisters will deliberately burn and act as fuses if the current is exceeded (so they are half watt resistors, but if the output is shorted, the current draw will actually be 1watt, burning the resistor)

The displays themselves, use a tiny amount of current to illuminate a segment, so even if a segment is stuck on forever, the resistor should not burn (so if the 7180 is shorted on, the segment should just be stuck on forever)

When the resistor(s) fail, it usually means that one of the glasses connected to the output was at sometime shorted and the resistor sacrificed itself to protect the 7180

If you have a dead resistor AND the segment still doesn't work, then it unfortunately means the 7180 was also damaged.

Whilst it is possible for the 7180 to fail on its own, if you have a failed resistor and also a failed 7180, then logically there has to have been a short on the glass side of the circuit because a 7180 failing on its own (open or short) does not also cause the resistor to fail.



Hopefully that makes sense. In summary, yes your 7180 could be bad, but if you have burning resistors, you have to first address the short on the output side
That is very helpful thank you Luckily I have already removed the 7180 and put in a socket. I ordered from Niko Electronics but they buggered me around for 3 weeks. I Think they were "drop shipping" from China snd quadrupling the price. I have no ordered direct from China 7180A's
 
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