The Aussie Pinball Arcade
Aussie Pinball Forums => Technical Matters => Circuit Board Repair Guides => Topic started by: Pinball Fixers on December 18, 2008, 10:50:03 PM
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If you own an old Bally or Stern pinball, then having working displays is a must...
Every now and then, one of the digits (numbers) on a display will disappear completely. Thankfully, it is usually a really easy fix...
Most of the time, it is a resistor that has gone open circuit. There are 6 (or 7 in the case of a 7 digit display) 100k ohm, 1/4 Watt resistors that are part of the power circuit for each digit.
These resistors are getting quite old now, so they should be replaced to ensure the display continues to work.
Once they are any more than 10% out of spec (110k ohm or above), they are on their way to failure...
When replacing these resistors, use 100k ohm, 1/2 Watt resistors. Or better yet, 100k ohm, 1 Watt Metal film resistors. Metal Film resistors are often around half the physical size of a standard carbon film resistor, so they can handle more heat in a smaller package.
(http://files.pinball-fixers.com/aussie_pinball/bally/bally6digit.jpg)
The resistors marked above are as follows (6 Digit Display):
R1 - 1's Digit
R3 - 10's Digit
R5 - 100's Digit
R7 - 1000's Digit
R9 - 10,000's Digit
R11 - 100,000's Digit
On the 7 Digit Display, R56 is used for the 1,000,000's digit - 100k ohm resistor also.
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Thanks for that mate. Once again, a valuable guide for our members.
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R34 is also a 100K Ohm resistor, and I change these as well. They can eradicate the "flicker" that sometimes affects these displays..
The changing of the digit resistors should be manditory - as Owen has pointed out.
Reflowing the headers pins abd the display pins solves most of the problems with these displays..
My apologies for preempting anything points you wish to make, Owen - I actually enjoy repairing displays.. I used to buy them faulty off eBay just for the fun of repairing them !!
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My apologies for preempting anything points you wish to make, Owen - I actually enjoy repairing displays.. I used to buy them faulty off eBay just for the fun of repairing them !!
No probs Nino. I was hoping you (and others) would chime in with other fixes for these displays... I do have some more points to make on them though.
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R34 is also a 100K Ohm resistor, and I change these as well. They can eradicate the "flicker" that sometimes affects these displays..
I've updated the picture above to show where the R34 resistor is.
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Although it is more common for the resistors to cause the digits to stop working, the drive transistors can also fail, giving the same fault.
If replacing the resistors doesn't fix the non-working digit, then the following transistors will need to be replaced:
2N5401 - Digit Drive Transistors
Q7 - 1's Digit
Q8 - 10's Digit
Q9 - 100's Digit
Q10 - 1000's Digit
Q11 - 10,000's Digit
Q12 - 100,000's Digit
Q21 - 1,000,000's Digit (7 digit display only)
MPS-A42 - Level Shift Transistors
Q1 - 1's Digit
Q2 - 10's Digit
Q3 - 100's Digit
Q4 - 1000's Digit
Q5 - 10,000's Digit
Q6 - 100,000's Digit
Q20 - 1,000,000's Digit (7 digit display only)
Either the 2N5401 or MPS-A42 transistors can cause a non-working digit, so best to replace both for each non-working digit.
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I actually enjoy repairing displays.. I used to buy them faulty off eBay just for the fun of repairing them !!
Sheeet - me too! Bit like solving a jigsaw puzzle. Reflow the headers, replace the 100k resistors and then take it from there...
cheers
Ian
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I actually enjoy repairing displays.. I used to buy them faulty off eBay just for the fun of repairing them !!
Sheeet - me too! Bit like solving a jigsaw puzzle. Reflow the headers, replace the 100k resistors and then take it from there...
cheers
Ian
That's where you start.. and in 75% of displays, thats where you finish !
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I actually enjoy repairing displays.. I used to buy them faulty off eBay just for the fun of repairing them !!
Sheeet - me too! Bit like solving a jigsaw puzzle. Reflow the headers, replace the 100k resistors and then take it from there...
cheers
Ian
That's where you start.. and in 75% of displays, thats where you finish !
Maybe I should have started this thread with re-flowing the pin header and display glass before replacing the resistors...
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When a display lands on my workbench, I do exactly as you recommend - especially if the resistors are the older type - Remove and replace. While the iron is hot - reflow the header pins and display pins...
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If you find the display is missing segments, then it will usually be either an MPS-A42 transistor or the 4543 CMOS IC.
This only applies if you are missing the same segment on all digits. If you are only missing one segment on one digit, then it will usually be the display glass that is faulty.
MPS-A42 transistors:
a segment (top) - Q13
b segment (top right) - Q14
c segment (bottom right) - Q15
d segment (bottom) - Q16
e segment (bottom left) - Q17
f segment (top left) - Q18
g segment (centre) - Q19