I'm working on a bally display. Only one digit out. The 100's digit. All 100k resistors check out fine. The MC1454 digit controller is fine as well. Swapped that out with
a known good one. I grounded Q3 collector and the 100 digit lit up. I didn't have an MPSA42 at hand but had an MPSA43. I swapped it in and still no 100's digit.
Anyone experience this? Maybe the MPSA43 can't be swapped in? Looks like it's a 200v transistor but the MPSA42 is 300v. I'm not sure that matters much.

#2 4 years ago

INK

Inkochnito

Pinside member

Piershil

12y 91,650 3,169 12

Did you check the 100K resistors?

Never mind, I see you did....

The MPSA43 should work, but the voltage is low like you said.
If the power supply goes bad, it will send 240V to the displays.
The MPSA43 transistor can't handle this.
It should work however.
https://pdf1.alldatasheet.com/datasheet-pdf/view/5143/MOTOROLA/MPSA42.html

Did you check the print headers?

#3 4 years ago

JHG

Barakawins1

Inactive member

Woodcliff Lake, NJ

9y 69,300 2,113 3

When you say print headers? I'm not following that, can you explain? Do you mean header pins? Yes I reflowed them.

#4 4 years ago

EJS

Eric_S

Pinside+ Frequent

Oshkosh, WI

11y 131K 1,720

Pretty generic advice, but did you reflow the solder on the components for circuit that drive that digit? I've had similar issues where one digit just doesn't work regardless of component replacement. I assumed that there was a wire lead break internal to the display glass. Maybe swap that display to the credit/ball display as a cheap hack?

#5 4 years ago

???

KenLayton

In Memoriam

Olympia, WA

12y 83,400 5,965

I wouldn't advise using the MPSA43. It's specs are too low for this application.

An NTE 287 is a good cross to the MPSA42.

#6 4 years ago

JHG

Barakawins1

Inactive member

Woodcliff Lake, NJ

9y 69,300 2,113 3

Ok so here's a first.. I took out my logic probe. I touched resistor 5 (100k). The missing 100 digit came back. And that's the fix. Go figure. Just touching the
100k resistor with a little electricity to it brought it back.

#7 4 years ago

???

KenLayton

In Memoriam

Olympia, WA

12y 83,400 5,965

Replace that R5 resistor.

#8 4 years ago

PIP

Quench

Pinside member

8y 114K 8,041

OP
Quoted from Barakawins1:
I took out my logic probe. I touched resistor 5 (100k).

Oh dear, your logic probe probing 190 volts wouldn't have been happy.

#9 4 years ago

JHG

Barakawins1

Inactive member

Woodcliff Lake, NJ

9y 69,300 2,113 3

I'm sure.. But it is now fixed.. I'll replace all of the 100k resistors as soon as they arrive.

#10 4 years ago

AMS

barakandl

nvram.weebly.com

12y 132K 10,420

OP
Quoted from Barakawins1:
I'm sure.. But it is now fixed.. I'll replace all of the 100k resistors as soon as they arrive.

If you replace that resistor and then the digit seems to lock on bright the level shifter is bad.

I think when a digit locks on from a bad level shifter or MPU issue is when those 100k resistors get burnt and go open circuit.

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