Interesting read about Bally Midway sound boards

tec9

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Thought this might be interesting to those who have these machines:
Hey all-
Thought this would be good info to pass along, to see if your machine could use this mod, look at the game list at the bottom. This is a note from Bally Midways service center:


Subjuect:
Premature failing of the 7533 DAC's on the sound boards. During extensive research on the sound boards Digital to Analog converter circuit, we have discovered a transient condition which may result in premature failure of this part.
Cause:
The condition is dus to overstressed metalization related to pin 1 on this chip. The situation occurs on power-down when the Vcc begins to fall and reaches about 1.5 volts. At this time, the voltage on pin 1 exceeds Vcc for a fraction of a second. The DAC is not designed to withstand this condition.
Cure:
Remove the 47uf capacitor connected to pin 1 of this chip. This will eliminate the transient condition on the circuit, and testing indicates that this removal has no negative effect on the overall operation of the circuit.

Board names: Remove Capacitor
1. Sounds Good C122
2-Cheap Squeak Deluxe C122
3-Artificial Artist C122
4-Turbo Cheap Squeak C12
5-T.C.S. Pinball C30
6-Sounds Deluxe Pinball C12

Pinballs List
Black Belt #5
City Slicker #5
Dungeons and Dragons #6
Escape from the Lost World #6
Hardbody #5
Heavy Metal #6
Motordome #5
Party Animal #6
Special Force #6
Strange Science #5

Video Games List
Demo Derby #4
Max RPM #4
Power Drive #1
Rampage #1
Sarge #4
Spy Hunter #2
Spy Hunter 2 #1 +#4
Star Guards #1
Xenophobe #1
Zwackery #3
 
Subject:
Premature failing of the 7533 DAC's on the sound boards. During extensive research on the sound boards Digital to Analog converter circuit, we have discovered a transient condition which may result in premature failure of this part.
Cause:
The condition is dus to overstressed metalization related to pin 1 on this chip. The situation occurs on power-down when the Vcc begins to fall and reaches about 1.5 volts. At this time, the voltage on pin 1 exceeds Vcc for a fraction of a second. The DAC is not designed to withstand this condition.
Cure:
Remove the 47uf capacitor connected to pin 1 of this chip. This will eliminate the transient condition on the circuit, and testing indicates that this removal has no negative effect on the overall operation of the circuit.

That's pretty bizarre.

The AD7533 is supposed to be a R-2R current-mode DAC, but they've got the 2 current steering outputs wired to vdd and ground and are using the vref pin as an output instead of an input to make it a voltage-mode DAC instead of just buffering the current with an amp and converting it to a voltage.

But yeah... the way the 'soft powerup' circuit hanging off pin 1 is done (to prevent a pop on powerup), Pin 1 will drop slower than Vcc (+12V) when the game's turned off, and if it's higher than a diode drop, the transistors in the R2R DAC switches can latch up and melt themselves.

Probably easier to just put a reverse biased schottky between +12 and Pin 1 so Pin 1 never goes more than about 2-300mV above +12 during powerdown which is enough to prevent latchup, but still have the soft-power design work.
 
I responded to a post referring to this post. Here is an ap note that covers the topic. Skip to the last page for the solutions if you want to forego the theory:

http://www.analog.com/media/en/technical-documentation/application-notes/20809090AN109.pdf

None of those 'solutions' are relevant to how midway (mis)designed the circuit. The series diode on vdda could hinder latchup, but it doesn't fix the primary issue of vref>vdda+vbe, and actually makes it worse since it'll drop vdda by a vbe, so if you're going to do that, you at least need to do the same to the +12v to vdda connection.
 
None of those 'solutions' are relevant to how midway (mis)designed the circuit. The series diode on vdda could hinder latchup, but it doesn't fix the primary issue of vref>vdda+vbe, and actually makes it worse since it'll drop vdda by a vbe, so if you're going to do that, you at least need to do the same to the +12v to vdda connection.

So the Midway engineering fix recommended in the service bulletin will fix the issue or won't?
 
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