Robotron 2084 – Audio crackling SOLVED!!

I mentioned in my last post that there is a constant background noise, a hissing or crackling sound.  It’s substantially quieter now that the soundboard has been rebuilt, however, it is still there and quite annoying.  Here’s how that one got solved:

First start with the easiest to check, the speaker….

Here is a pic of the original 4 ohm 3 watt power house:

Unsoldered that one and wired up a test but known good speaker in its place:

Same results…crackling, hissing…

So this time I decided to check the power source, make sure that there isn’t any AC ripple or interference causing the issue from the linear power supply (this part would be easier if I had a O-Scope).  So I disconnected the power lines and wired up my test power supply directly to the board:

Exactly the same….hissing, crackling…

Next up, test the audio amplifier on the soundboard.  For this one, I had to get out my old-school Panasonic Boombox (what can I say….) and use its amplifier or line-input to check the audio before it gets amplified on the soundboard. 

Next I plugged in one end of the RCA cable to the line-in on the boombox.  Then, stripped the other end of the cable (since the soundboard doesn’t have a line out I had to get creative).

Used some aligator clips, one connected to ground.  The pic above shows the aligators on the wires which I nested in the wire loom just to keep from touching each other.   The positive connection I used a nail clipped into the aligator clip so I could touch pin 1 of the TDA 2002 audio amplifier chip on the soundboard (pin 1 is input for the amp chip)

Its a little hard to see what I’m doing here, but when I touched the nail to PIN 1 the sound came through my boombox PERFECTLY CLEAR.  No static.  That narrowed it down to basically the audio amp chip, or 3 caps or a bad ground…I had already replaced 2 of the caps and the 3rd is a ceramic and what are the odds there. 

Digging around the bone-yard I found a dead Williams Arch Rivals sound board that had 2 of the TDA 2002 chips, I was able to salvage 1 of them and replaced it on the Robotron and now the sound is PERFECT.  Back to gaming!!!

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