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'78 Bally Playboy no feature lights


the_tinker

Question

First off thanks for the add. My first post here.

 

Helping a friend fix a 1978 Bally Playboy. Got the machine to wake up with a new rectifier board. Two of the voltage were

half wave rectified. All looks clean now (I have an o-scope).

Everything seems to work except the playfield lights. GI is on, counters work, self test runs trough each step.

If I put +5v on the gate of the SCRs I can light the corresponding feature light so I know the SCRs are ok.

I can't believe all the decoders are bad. I don't see clean, square pulses on the A0-A3 pins. Looks more like a

saw tooth.

 

Anybody every see something similar on these era Ballys? I'm still searching through the threads.

 

Thanks,

HPIM2887a.JPG.698421435283e0716c5114e4f73004e9.JPG

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No "strobe"....J4 pin 13. That line is common to all the U1, U2, U3 and U4 decoder chips on the lamp driver board. No strobe and the chips will not put out any decoded signals to drive the SCRs. I am presuming you have already checked for 5vDC between TP1 and TP2.
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If I put +5v on the gate of the SCRs I can light the corresponding feature light so I know the SCRs are ok.

 

When you want to manually activate the SCRs, jumper the gate leg to test point TP3 on the lamp driver board - that's what TP3 is for.

 

I can't believe all the decoders are bad.

I've repaired boards where one or more 4514 decoders are bad and when it's happened, the output enable pin (pin 23) and/or the strobe pin (pin 1) has internally shorted to a supply pin. If you measure low resistance between pin 23 or pin 1 to GND or VCC on the board, the chip is bad. Note the strobe pins are all connected together on the board so if one chip has a short to ground on this pin, all decoders will be unable to update the SCRs.

Also when bad, they can get hot to touch fairly quickly. They normally run cool being CMOS chips.

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Any battery damage on the mpu board?

 

Minimal. mostly confined to the ground plane area. Don't see any significant corrosion around component leads.

Thanks for you interest. I see your name on a lot of successful repairs in the archives...

 

- - - Updated - - -

 

When you want to manually activate the SCRs, jumper the gate leg to test point TP3 on the lamp driver board - that's what TP3 is for.

 

Yep. I did essentially that. I may have a bad SCR or two but generally I can get each one to light in turn by switching SCR on with jumper.

 

 

I've repaired boards where one or more 4514 decoders are bad and when it's happened, the output enable pin (pin 23) and/or the strobe pin (pin 1) has internally shorted to a supply pin. If you measure low resistance between pin 23 or pin 1 to GND or VCC on the board, the chip is bad. Note the strobe pins are all connected together on the board so if one chip has a short to ground on this pin, all decoders will be unable to update the SCRs.

Also when bad, they can get hot to touch fairly quickly. They normally run cool being CMOS chips.

 

I was wondering that. Could not believe all the decoders were bad but wondered if one bad could pull the other address lines down. My friend says he has access to

another light driver board so we can try that next. Thanks for helping.

 

- - - Updated - - -

 

No "strobe"....J4 pin 13. That line is common to all the U1, U2, U3 and U4 decoder chips on the lamp driver board. No strobe and the chips will not put out any decoded signals to drive the SCRs. I am presuming you have already checked for 5vDC between TP1 and TP2.

 

Yes sir. All voltages are good now since we replaced the rectifier board.

 

- - - Updated - - -

 

Minimal. mostly confined to the ground plane area. Don't see any significant corrosion around component leads.

Thanks for you interest. I see your name on a lot of successful repairs in the archives...

 

- - - Updated - - -

 

Lamp voltage is good. Have voltage at the lamps. When I turn on the SCR by applying a jumper to the gate, the lamps will light. Everything to the right of the decoders on the

schematic seems to be working. I think the issue is with the decoder logic either in the MPU or the driver board. There must be some signal that disables all lights like in a tilt

situation or something that is overriding the feature light logic.

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UPDATE: It was the lamp driver board. Put a replacement one in today and the feature lights work as expected. Must be a bad decoder keeping all the lines from cycling.

Thanks to all who chimed in with suggestions.

 

That's one one to fix a lamp driver board I guess.

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