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(Topic ID: 215046)

Dr. Who Exploding Sound Card!


By PinPilot

2 years ago



Topic Stats

  • 19 posts
  • 7 Pinsiders participating
  • Latest reply 2 years ago by PinPilot
  • Topic is favorited by 1 Pinsider

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#1 2 years ago

Before I send the board off to Clive at Coin-Op Cauldron, I thought I would "ask the forum".

Customer brought in an auction-buy Dr. Who, which had been on route for many years based on permit stickers and overall condition. Asked me to "get it playing".

Customer told me the game had been working, but judging by the coils and dis-assembled parts hanging under the playfield, missings pieces, etc, I wasn't so sure.

Power supply seemed to be outputting the correct voltages, so I plugged it in with the customer still present. I'm glad I did, because when I switched on the power, its as if a mini-flash-bang went off on the sound board!

After the smoke cleared, I determined that C28 went all 'splodey on us. What's strange is even though the cap was blown clean in two, I STILL GET CONTINUITY across the legs of the cap! I'm wondering if the traces somehow got shorted in the "explosion"? Or maybe the short CAUSED the explosion.

Another question is, IS THIS EVEN THE RIGHT BOARD? Dr. Who manual calls for a 12738-20006 sound board, but the board in the game is a 12742-20006. Are these interchangeable?

Any and all thoughts are welcome.

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#2 2 years ago
Quoted from PinPilot:

Before I send the board off to Clive at Coin-Op Cauldron, I thought I would "ask the forum".
Customer brought in an auction-buy Dr. Who, which had been on route for many years based on permit stickers and overall condition. Asked me to "get it playing".
Customer told me the game had been working, but judging by the coils and dis-assembled parts hanging under the playfield, missings pieces, etc, I wasn't so sure.
Power supply seemed to be outputting the correct voltages, so I plugged it in with the customer still present. I'm glad I did, because when I switched on the power, its as if a mini-flash-bang went off on the sound board!
After the smoke cleared, I determined that C28 went all 'splodey on us. What's strange is even though the cap was blown clean in two, I STILL GET CONTINUITY across the legs of the cap! I'm wondering if the traces somehow got shorted in the "explosion"? Or maybe the short CAUSED the explosion.
Another question is, IS THIS EVEN THE RIGHT BOARD? Dr. Who manual calls for a 12738-20006 sound board, but the board in the game is a 12742-20006. Are these interchangeable?
Any and all thoughts are welcome.

Common for tant caps to esplode like that when they're older. The continuity, I can't comment at the moment, i don't have schematics handy.
For the part number - the 12738 is the pre-DCS sound board. 12742 is the MPU. No, you can't interchange them.

#3 2 years ago

Thanks for the reply Coyote. As you can see in the photos I added, the sound board has 12742-20006 on it. Its the white sticker in the middle of the board.

Didn't know tant caps liked to blow like that. Quite exciting when they do!

#4 2 years ago

It's the correct sound board. That's a WPC pre-DCS sound board. I know because I blew one up as well. Not once. But twice.

It's likely continuity is present due to the carbon all over the place. Carbon conducts. You need clean all the debris and check the pads and traces.

Double check the AC voltage coming in to the sound board (J501). Make sure it's somewhere around 18VAC across GRY and GRY-WHT as well as GRY-WHT and GRY-GRN. It should be 36VAC across GRY and GRY-GRN.

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#5 2 years ago
Quoted from DumbAss:

It's the correct sound board. That's a WPC pre-DCS sound board. I know because I blew one up as well. Not once. But twice.
It's likely continuity is present due to the carbon all over the place. Carbon conducts. You need clean all the debris and check the pads and traces.
Double check the AC voltage coming in to the sound board (J501). Make sure it's somewhere around 18VAC across GRY and GRY-WHT as well as GRY-WHT and GRY-GRN. It should be 36VAC across GRY and GRY-GRN.

Dumbass, its members like you that keep me coming back to Pinside! Thanks for adding to the discussion. I will do as you suggest.

I put the board back into the game to test voltages AND THE DAMN THING STILL WORKS! There is a distinct "hiss" in the background, but the board still works 100%! Obviously I won't leave it this way, but I found it shocking (no pun intended) that it still functions!

#6 2 years ago

This was a very solid board. I'm still sad that there isn't a newer honest replacement available.

#7 2 years ago

This is one reason why use of compressed air on boards is required to remove carbon buildup, debris, or even wire whiskers, especially around capacitors. It can act as a conductor, develop a a short circuit, and blow up. Every routed game should have this done before servicing the boards. However, do not sustain the air on the board, unless using a electric blower, as the cold compressent can damage electronic components.

The other most common reason is reversing the capacitor lead terminals when rebuilding a board. It has the same result.

It is generally wise to do voltage regulation testing before plugging in primary controller PCBs. This did not occur.

Also, always recheck and inspect work before repowering as well.

#8 2 years ago
Quoted from xTheBlackKnightx:

This is one reason why use of compressed air on boards is required to remove carbon buildup, debris, or even wire whiskers, especially around capacitors. It can act as a conductor, develop a a short circuit, and blow up. Every routed game should have this done before servicing the boards.
The other most common reason is reversing the capacitor lead terminals when rebuilding a board. It has the same result.
It is generally wise to do voltage regulation testing before plugging in primary controller PCBs. This did not occur.
Also, always recheck and inspect work before repowering as well.

BK, great advice. I've done plenty of minor board repair, but I never thought about this before. Moving forward, procedures will be different. Seems like we have to learn the hard way sometimes!

#9 2 years ago

Same thing happened on my DW, it was C26 though.
https://pinside.com/pinball/forum/topic/dr-who-audio-board-cap-exploded
I replaced all the tantalum caps on the board, and all has been well.

-Brian

#10 2 years ago

Thanks Brian. It's always good to hear from someone who has been through the exact same issue! I have a large supply of "normal" capacitors, I'm tempted to use them, albeit with a higher rating, instead of ordering the tantalum caps.

#11 2 years ago

Ok, now I'm a bit confused. It appears that audio board connector J501 brings in 20VAC through pin 1, with a trace that leads directly to the tantalum cap that exploded. My confusion is on the fact that it is my understanding that you never use polarized caps in an AC circuit, only "NP" or non-polarized. But the specs on the tant cap in the manual doesn't have an NP in the specs, making me think its just a normal polarized cap.

Looking at Dumbass's picture, it appears there is a small "+" sign on the right side of the blown cap. This would indicate polarized.

Brian, what exact cap did you buy to do the repair? Can you post a link?

#12 2 years ago

Alright, I looked a the remains of the blown cap and it is polarized. Positive toward the input from J501. I've also come to understand that two polarized caps, fitted in parallel with opposite polarity, creates a "non-polarized" cap. That may be what's happening here.

#13 2 years ago
Quoted from PinPilot:

Ok, now I'm a bit confused. It appears that audio board connector J501 brings in 20VAC through pin 1, with a trace that leads directly to the tantalum cap that exploded. My confusion is on the fact that it is my understanding that you never use polarized caps in an AC circuit, only "NP" or non-polarized. But the specs on the tant cap in the manual doesn't have an NP in the specs, making me think its just a normal polarized cap.
Looking at Dumbass's picture, it appears there is a small "+" sign on the right side of the blown cap. This would indicate polarized.
Brian, what exact cap did you buy to do the repair? Can you post a link?

I will try to remember to check exactly what I ordered when I get home from work and let you know. I am pretty sure I ordered tantalum caps from either Mouser or Digi-Key of the specs as the original or slightly upgraded.

-Brian

#15 2 years ago
Quoted from PinPilot:

Ok, now I'm a bit confused. It appears that audio board connector J501 brings in 20VAC through pin 1, with a trace that leads directly to the tantalum cap that exploded. My confusion is on the fact that it is my understanding that you never use polarized caps in an AC circuit, only "NP" or non-polarized. But the specs on the tant cap in the manual doesn't have an NP in the specs, making me think its just a normal polarized cap.

Just to clarify -
It IS a polarized cap, and it's NOT in an 'AC' circuit. It is AFTER the fuses and bridge rectifier. These caps - C26, C27 are for filtering noise/ripple from the circuit before and after the +12v regulator. (And C28, C29 are used for the same purpose for the -12v regulator.) You'll see that in the -12v (-20v) feed, the + side of the tant caps are on the GND plane, as opposed to on the -12V feed.

#16 2 years ago

I'd use a 50v tantalum since the originals shorted out.

2.2uF 50v electrolytics would probably be fine too. They don't burst into a fireball when they fail too.

#17 2 years ago
Quoted from PinPilot:

Dumbass, its members like you that keep me coming back to Pinside! Thanks for adding to the discussion. I will do as you suggest.

LMAO for a second there I thought you were flaming the guy...then I saw his username actually IS DumbAss!

#18 2 years ago

Okay, it looks like I used 25V tantalum caps from Mouser. I would probably go with higher voltage if I did it again, but these are working okay.

-Brian

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#19 2 years ago

Thanks for all the support guys! I replaced the tant cap with an oversized regular cap, and all is well! Sound card works 100%, and the background "hiss" is gone. Just nice, clean crisp sounds emanating from the speakers!

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