TAF Sound Board issue

[good_buddy83]

OK, I am hoping you guys can help me nail down exactly which component failed.

Here's the story:

I was swapping out some LED GI lights, lots of turning the machine off and on to see what looks good. As Murphy's law would have it, I am all finished and power things up to play a game and get a really loud hum out of the woofer. (Note: pinball Pro Speakers and Sub installed.) Powered off and on a few times and always got the same thing. The hum would remain until the sub light began to glow and then when the light was full brightness all sound would stop (I think this is build in overload protection doing it's job on the sub.) Note, I can visibly see the voice coil pull in on power up.

So I tried, the following:
-Left the ribbon cable on the sound board disconnected (no change).
-Swapped for my TZ sound board, powered up (ribbon cable left off) - No hum, problem fixed.

My thinking is the issue is a board component associated with the sound amplification. Question is which component is the culprit? I reflowed solder to usually culprits and checked for bad traces, all of which as no help.

So if it is a cap, which cap do I replace? Is it the amplifier component on the big heat sink? if so, what do I replace it with. Let me know your thoughts. I'll post some pics and maybe a video. Thanks for the help!

 

[good_buddy83]

 

[good_buddy83]

Was fortunate enough to find all the components at Nutech in St. Catharines.

New LM1875 amp.
New C20 10 micro ferrand, 20V tantalum cap
New C46 and C47 caps (1 micro ferrand 35V tantalum)
New C23 0.22 microferrand, 10 V ceramic.

I think that is everything. Decided to take the shotgun approach to have a fresh start on the amp circuit. Wish me luck soldering tomorrow, but I think I have enough pieces to make this sound board great again!

Also picked up new caps for C15, C36 and C38 to clear up sound. I'm fairly certain these are not the culprits, but it can't hurt if I'm in there doing surgery anyway.

 

[good_buddy83]

Fixed!!!!! just in time for the New Years party.

Incase someone is reading this with he same issue, I believe the problem was a failed LM1875 Amp. This is a real pain to swap, as I had to pull the big cap in order to gain access. I also replaced C46 and C47 tantalum caps while I was there.

The symptoms were that there was a really loud hum at power up and a lot of heat damage on the board at the big heat sink area.

Feels good to have that one fixed!

 

[superjackpot]

Thanks for the write up. Always great to see the follow up post to a problem solved.

 

[Fleetwood mIkea]

Agreed- it’s always good to know the end of the story, there are far too many threads that go silent and we never know what actually solved the issue.

“Also picked up new caps for C15, C36 and C38 toclear up sound.”

....does this mean if I replace these on mine it will show an improvement? I don’t really know much about the sound board etc., I have already replaced the speakers and am currently in the process of replacing the 8” sub and adding an L-pad for balance. Sounds way better already and always looking to tweak a bit more.

 

[good_buddy83]

Here is what I was reading from pinwiki:

6.24.5.4 Distorted or slightly "off" sounds


C38 is not stuffed on some pre-DCS sound boards.
The pre-DCS sound board uses 47µF capacitors as filters in the sound circuit; C15, C36, and C38. If one or more of these caps fail, certain sounds may not be as loud or as clear as intended. The game will just sound "off" with perhaps slightly unbalanced sounds. An amazingly high percentage of these caps fail. It is recommended that if your sounds are a bit "off" that you replace all of these caps.

 

[Fleetwood mIkea]

Awesome - I’ll give it a shot. Thanks!