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#11 WPC DCS Sound PCB (Roadshow Revisited)

Discussion in 'The Menace Files' started by Menace, Nov 26, 2014.

  1. Menace

    Menace Well-Known Member
    Staff Member

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    In this installment I will be revisiting the exact same DCS sound PCB that I worked on last year.
    (which you can find here http://www.pinballrevolution.com/viewtopic.php?f=43&t=1040 )

    As you may or may not remember, the previous repair dealt with a serious ground buzz issue in the amplifier section which turned out to be a bad trace on one of the electrolytic caps. This time around, the issue is loud transient peaks and general distortion through the speakers. The problem was so bad I took a short video for everyone to hear for themselves.



    That's some nasty transients!

    First order of business, figure out where the issue actually is originating on the PCB. Where you can hear the sound under the crackling, it's likely in the pre-amp or amp section. I did a bit of reading and the op-amps on this board can and do fail so I wanted to rule all of those out first. Now I could have done this with my scope but I thought I'd attack it slightly different this time as I wanted to actually HEAR the sound in the pre-amp section to make sure it was clean. As I have mentioned in the past, I do some work from time to time as a Dj and sound reinforcement for bars and nightclubs etc. The plan I had come up with was to use a mixer to listen to the audio after each op-amp in the pre-amp section to verify the sound was clean and free of artifacts. So i dug out a really old Dj mixer I had in storage from an old club repo YEARS ago (as you can see in the pic below).

    IMG_20141121_154942 (Large).jpg

    Used my alligator cables to connect the tails of the RCA cable into the pre-amp section after each of the 4 op-amps. Put my headphones on, fired the game up and ran the sound & music tests.

    IMG_20141121_154927 (Large).jpg

    By doing this it verified that all of the audio in both channels was nice and clean. (DCS games have two, one high pass for the backbox speakers and one low pass for the cabinet). I shifted focus to the amplifier section, and I confirmed the transients were present on both the backbox and cabinet speaker outputs so the chances of both TDA amplifiers being bad was un-likely. Looking at the schematics the only other parts in the amp circuit that I felt could be causing this issue was once again the two electrolytics in the power supply section. So I grab my trusty DMM to check for A/C ripple, set the dial to A/C and probe the power lines of each TDA and here's what I found;

    IMG_20141121_161249 (Large).jpg

    This is telling me there is almost 1V of A/C voltage on the DC supply for the TDA amps, which is WAY too high. Just for reference, here is a good A/C ripple reading from a working DCS sound board;

    IMG_20141121_161636 (Large).jpg

    Tossed the board on my bench, and pulled the two caps from the PCB.

    IMG_20141121_164452 (Large).jpg

    Once these are replaced (along with the jumper I installed to fix the broken trace from the previous repair) the owner will once again be able to enjoy all the humor and brutal country and western melody's that we all have come to love/hate in Roadshow.

    D
     
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    Last edited: Mar 18, 2015
  2. jooky

    jooky Member

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    Thanks Doug! Definitely no way I could have figured all that out! You are my go-to guy whenever I'm in over my head for pin repairs.

    I just rebuilt my flippers and put in cool white GI LED lighting throughout the machine. Plays and sounds terrific and feels like a brand new machine!

    Yes Road Show does indeed have some licensed country music (the only pin that does?). Interesting trivia, the voice of Red is Carlene Carter who sings the jackpot tune. She is the daughter of June Carter, who goes on to marry Johnny Cash. She also made a cameo appearance in another movie that was turned into a pin - Maverick! Seriously... only Road Show owners would look this stuff up :p
     
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