Bally/Stern unable to adjust display voltage

I have replaced all 3 tranistors R51 and 56, the pot, dioide, and zener diode still no luck. There are no broken traces, all resistors are good, diode and zener diode are good. I have not replaced the larger capacitors, would this amke a difference.

This is the second board that I repair. The first I replaced the same as above plus the 2 large capacitors, this one is working. Can't get the second working.

Any ideas or should i replace the 2 large capacitors? Aside from the high voltage this board works great.

#2 1 year ago

Tuukka

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Lempäälä

5y 78,350 1,762 16 18

If the capacitors are original, they should be replaced anyway. But they do not cause your HV adjustment problem. What is the HV output now - too high or too low? Did you use any substitute parts for Q21-Q23?

#3 1 year ago

???

JethroP

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6y 101K 3,135 9 14 1

Are you measuring DC volts at TP2 and not TP4?

#4 1 year ago

ACE

Nick_C

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Voltage at TP4 and TP2 are 230VDC. I did use substitutes for both boards, the identical parts. 1 board I can dial down the voltage to 175VDC and the other makes no difference.

This is what I used
2N3584 Q21
2N3439 Q22 and Q23

#5 1 year ago

JMS

swampfire

Pinside+ Addict

12y 168K 13,602 48

I have 2 boards with the same problem. I wish there was a Plug-and-Play fix for this. I went with an Alltek solenoid driver board.

#6 1 year ago

???

JethroP

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6y 101K 3,135 9 14 1

Can you post high res pictures of the front and back of the board?

#7 1 year ago

ACE

Nick_C

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20230329_141415 (resized).jpg

20230329_141424 (resized).jpg

20230329_141438 (resized).jpg

20230329_141444 (resized).jpg

#8 1 year ago

???

JethroP

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6y 101K 3,135 9 14 1

Are you sure the variable pot is connected correctly?
Also, would clean up the solder joints and make sure there is no shorting between traces.

#9 1 year ago

ACE

Nick_C

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The pot is connected the same way as the functioning board. This board had a bigger pot connected to the lower holes.

After Q23, I have no voltage. I tested C27 and C28 (out of circuit) they are slightly lower than should be, .001uf compared to .01uf

20230329_152612 (resized).jpg

#10 1 year ago

JWK

sparky672

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Manhattan, IL

11y 55,050 1,730 1

OP
Quoted from Nick_C:
they are slightly lower than should be, .001uf compared to .01uf

10x is significantly lower.

#11 1 year ago

ACE

Nick_C

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Both C27 and C28 have same reading. Would this be my problem? Should I replace?

#12 1 year ago

WFC

Billc479

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Pleasant Plains, IL

9y 79,800 3,139 15

R54 looks bad. Also, did you test those caps out of circuit? I would also check Q22.

#13 1 year ago

JWK

sparky672

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Manhattan, IL

11y 55,050 1,730 1

OP
Quoted from Nick_C:
Both C27 and C28 have same reading. Would this be my problem? Should I replace?

If a cap is supposed to be 0.01 uF and it's 10x smaller, I would change it.

That being said, I'm not a Bally/Stern guy... maybe there was a reason for this... were there any service bulletins about changing these caps to 0.001 uF? Are there any recommended upgrades that involve these cap values?

#14 1 year ago

???

DigeratiOhm

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1y 1,900 3

Just curious if you checked continuity to ground from Q23 emitter, and R55, R56?

#15 1 year ago

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Nick_C

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Quoted from Billc479:
R54 looks bad. Also, did you test those caps out of circuit? I would also check Q22.

All caps were tested out of circuit. I also tested R54.

Quoted from DigeratiOhm:
Just curious if you checked continuity to ground from Q23 emitter, and R55, R56?

I will check that tomorrow.

#16 1 year ago

Tuukka

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5y 78,350 1,762 16 18

Long collector lead of Q23 almost touches the base foil.

#17 1 year ago

ACE

Nick_C

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Quoted from DigeratiOhm:
Just curious if you checked continuity to ground from Q23 emitter, and R55, R56?

Continuity to ground checks out good. I replaced Q21, Q22, and R56 again just to make sure. Still at 230VDC at TP2

#18 1 year ago

PIP

Quench

Pinside member

OP
Quoted from Nick_C:
Continuity to ground checks out good. I replaced Q21, Q22, and R56 again just to make sure. Still at 230VDC at TP2

What voltages are you seeing on both sides of the zener diode at VR1?

#19 1 year ago

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Nick_C

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In the attached image. I have 230VDC at the green circle and 0VDC at the red circle. I would think that the capacitor would cause that. continuity checks out.

20230329_152612 (resized).jpg

#20 1 year ago

PIP

Quench

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OP
Quoted from Nick_C:
In the attached image. I have 230VDC at the green circle and 0VDC at the red circle.

So Q23 isn't turning on and you need to work out why. The schematic shows you should have 140VDC on the other side of the VR1 zener diode - what are you measuring there?

#21 1 year ago

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Nick_C

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I get 0 V on either side of the zener diode

#22 1 year ago

YCV

Arcane

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Raleigh, NC

7y 54,550 890 6

OP
Quoted from Nick_C:
I get 0 V on either side of the zener diode

You should have 140 VDC at the pins of the Zener diode. I would replace it.

Yves

#23 1 year ago

ACE

Nick_C

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Zener diode replaced. No change.

#24 1 year ago

PIP

Quench

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OP
Quoted from Nick_C:
I get 0 V on either side of the zener diode

That means you have an open circuit somewhere in the red circled area below because the zener diode is not getting any voltage from that resistor divider circuit made up of R58, RT1 and R56. Guesstimate the pot is faulty.
Recheck the soldering on R58 and the RT1 pot. Measure the resistance of both and importantly the resistance between the pots middle leg and outer legs. Both measurements of the middle leg to the outer legs should sum the total pots resistance of 25k ohms.

SDB_HV_Section2.jpg

#25 1 year ago

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Nick_C

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The pot works. R58 is at 9.93 Mohm. I will replace it

#26 1 year ago

PIP

Quench

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OP
Quoted from Nick_C:
R58 is at 9.93 Mohm. I will replace it

On closer inspection of your pictures you can see it's burnt.

#27 1 year ago

ACE

Nick_C

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I saw that while checking it.

#28 1 year ago

PIP

Quench

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OP
Quoted from Nick_C:
I saw that while checking it.

Billc479 picked it back in post #12.

Quoted from Billc479:
R54 looks bad.

2 weeks later

#29 1 year ago

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Nick_C

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