Star Castle Sound Board Repair
Well, I finally decided to tackle repairing one of the two Star Castle sound boards that I have. This, of course, after a lengthly ordeal with trying to fix my Star Castle mainboard. I decided it was time to put that project down (and most likely have Mark Shastok of Cinelabs fix it) and start the nagging sound board. One of the sound boards I fried because I accidentally plugged the power/speaker cable in upside down. The other board I acquired through eBay a few years ago, and never had a chance to troubleshoot it.
I started with the board I recently fried. The Star Castle tech manual has an excellent section on troubleshooting the sound board (page 6-9). An oscilloscope is an absolute must for troubleshooting this component, as this is where all the analog signals are generated. A logic probe isn’t going to be much help here! I used an oscilloscope to look at the final amp outputs and inputs. The display on the ‘scope looked like someone was shouting into a microphone at a subway station. No matter where I probed in the final amp stages, it was not a pretty sight on the ‘scope. I decided to put that one aside and check out the sound board from eBay.
I noticed that two of the resistors were burned at locations R122 and R124. These are 150 Ohm 1/4 Watt resistors. I replaced them, hooked up the board, and powered on the game. The resistors promply burned black as soon as I coined up the game. I powered the game off, and started troubleshooting. I first checked the volume pot located on the coin door with a multimeter set to Ohms. The pot checked out OK. I then check the speaker with a multimeter set to Ohms. If the speaker shows some resistance, then it is ok. In my case, the speaker would not read anything on the multimeter. I checked for continuity, and the speaker was “open.” I had blown the speaker! (On another note, if the speaker had no resistance and the curcuit was “closed”, this kind of short can also cause damage to the final output circuitry on the sound board.)
Since the volume pot was ok, and the speaker was open (as opposed to shorted) I replaced the following ICs responsible for the final amp stage (I ordered these from Mouser Electronics):
Q17: 2N6292
Q18: 2N6107
IC25: TL081
I also replaced the resistors R122 and R124 that I had burned up in the last power on. After replacing those components, I powered everything on, and none of the resistors burned! This was promising.
Now I went to the final amp circuitry on the sound board with my ‘scope. I first checked pin 3 on IC26, which is the input to the final amp circuit. On the scope, (instead of lots of noise like with the previous sound board) it looked like this:
Which is good! I then checked pin 3 on IC25; this is the signal return from the volume pot on the coin door. I didn’t see anything on the ‘scope. Oops! I forgot I had turned the volume down when I was checking the pot with a multimeter before. I turned the volume up, and I saw the same waveform on the ‘scope as when I was checking pin 3 on IC26. I then probed the junction of the emitter and collector on Q17 and Q18, which goes to the speaker. I then saw a nice waveform on the ‘scope. I hooked up a good speaker, and voila! Star Castle sound effects. 
To replace the speaker I blew, I had a heck of a time trying to find a replacement. I was looking for a full range 8 Ohm 8-Inch speaker. Not a low frequency range woofer, but one that can handle a range near 60-20,000Hz. To my disappointment, Radio Shack doesn’t sell replacement speakers any more! After some searching, I finally found a supplier at Parts Express in their menu: Speakers -> Pro Sound Drivers -> Woofers Up to 8″. They had the QSL QS08R10 8″ Driver for $9!
Read my next post on the subject, where I discover (and repair) some other problems with the sound board.