Star Castle sound repair log

DogP

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Hey,

I just repaired the sound on my Star Castle, just figured I'd post in case anyone else has similar problems. The symptom was no sound. I checked the speaker, it was open (bad), and when I turned the game on, I got 30VDC across the speaker (hmm, wonder why the speaker blew :p). I looked at the sound board, and both Q17 and Q18 were dead, and a trace going to Q17 was burnt.

So, I replaced the transistors, fixed the trace, and it was still dead (I was just watching the output with a multimeter rather than trying another speaker, which would just blow). I had 15VDC now, so it was getting better. Looking at the schematics, I saw IC25 (TL081) drives the transistors that were bad, and a quick check with the multimeter showed a short from the output to the VCC-. So, I replaced that, but still no go.

I decided to replace the speaker so I could hear something if it was going to play, but rather than just hook it up and blow the speaker, I added a 470uF cap bipolar cap in series (DC block/high pass crossover at ~40Hz). Why they didn't have this to begin with, I'm not sure. After that, I noticed if I put my finger across the inputs of IC25, I got a hum out of the speaker, which meant I had something good from there on out. So, I grabbed my oscilloscope and started probing the outputs of each stage behind there. I saw IC27 had nothing coming out, but IC26 did, and it was getting to the input of IC27, so I swapped IC27 and it all came together.

Anyway, I hope this helps someone now, or in the future. Thanks to: http://www.outerworldarcade.com/arcade/cinematronics/star_castle_sound.html for a lot of good info as well.

DogP
 
Thanks for the series cap to speaker tip. I am modding my test rig to prevent speaker blowout. I will suggest this mod to cinematronics owners as I encounter them. I have seen a few blown speakers.

Bill
 
Yeah, it seems crazy that there's so many components that can blow on the sound board (LM081's and output transistors) that'll kill the speaker by sending DC through it, and there's no DC block capacitor (~$20 to replace the speaker each time, or ~$2 to prevent it from blowing). Here's what I used: http://search.digikey.com/scripts/D..._go&lang=en&site=us&keywords=P1287-ND&x=0&y=0 .

With an 8 ohm speaker, that's a high pass at ~42Hz, which is well below the range of a typical 8" full range speaker.

DogP
 
And for those of you who do not know better (like me), according to my web searching that bipolar cap should be hooked up to the + terminal on the speaker.

Thanks for the tip DogP!
 
Actually, it shouldn't matter whether it's on the + or -, since the voltage swings positive and negative on both sides of the speaker, and the capacitor is bipolar (so it can handle positive and negative voltages). The real purpose of it is to just block DC current, and if DC is blocked anywhere in the speaker path, it can't flow through the speaker. But I'm glad the tip helped, and hopefully it'll save a speaker or two down the line.

DogP
 
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