Centipede Speaker Hum

ThomasM

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Hi all - I've scoured the forum/internet for speaker hum troubleshooting - and looking for any tips any of you might suggest.

It's an original upright Centipede. First, when I turned the sound pot I was getting some feedback and some different levels of crackling and hum levels. So I replaced the sound pot - and now the speaker hum is just consistent - no variability or static, etc.

It was then suggested to swap out the AR board, which I did - a restored board by someone on this forum. Still no luck.

I'm now wondering if it's a ground issue or perhaps a bad speaker?

Just wanted to see if anyone had any good tips from past experience.

Thanks in advance...
 
It could be the same possible fix as the Atari asteroids :



Well today I finally got rid of my HUM and I have plenty of volume.

Ok lets recap my problem. My Asteroids had a bad hum in the speaker and low volume with the volume control turned all the way up.

- I replaced the Big Blue and no change
- I checked all my grounds including the main board edge connector. All grounds checked out good.
- Rebuilt Audio regulator board with no change but I noticed my A/R board was really the board for Asteroids Deluxe.
- Found the correct A/R board for Asteroids, rebuilt it and now I had more volume but the hum was also louder.
- Thought the 2 transistors on this board (2n3904) were pre amps but I was wrong. Turns out they are part of the audio disable circuit,like a mute switch, which is not even used in Asteroids.
- Checked the audio inputs 1 and 2 at the board edge connector and found one input was amplifying audio fine but the other was barely amplifying anything so I figured one of the Amps must be bad.
- Replaced both of the TDA 2002 audio amps and I now have a loud volume with almost no hum to speak of.
- Turns out the 2 TDA 2002 amps are in a push pull configuration and with one of the amps bad it was causing the output to be unbalanced which was causing the hum.

So my fix was to replace the bad audio amp and my hum problem was solved and I have plenty of volume.
 
If the AR didn't do it, then the next place to look is the audio section of the game board.

Atari boards use an LM324 as the final output amp on the game board, which sends the signal to the AR for amplification to drive the speakers. This LM324 can go bad and cause hum, and I always socket these when replacing them, so if they go bad again, you can easily pop a new one in.

The other thing to check are the components around the LM324. The LM324 is found at location K10. Make sure none of the caps or resistors around it are cracked or missing.

If you replace the LM324, the only other thing left is the Pokey, which is the big chip at C3, which generates much of the sounds. I've never seen one generate hum, but I suppose it's not impossible. You can swap it with one from any other Atari board (if you have another), as almost all Atari boards have at least one.
 
Thanks for the quick replies!

Sounds like replacing LM324 on the board is over my head - Andrewb is this something you would be up for repairing? If not, any other sources that could look at it?

thanks
 
One more thing to try:

Power the cabinet on with the game board unplugged. Do you still get the hum?

If it goes away, then that further suggests it's the game board. However, if it still hums with the board unplugged, then something else weird is going on.


(And yes, if it is the board, I can fix it for you. I feel bad that you bought the AR which turned out to not fix the problem, so I feel like I owe you, and can work something out for the repair).
 
Sounds like replacing LM324 on the board is over my head - Andrewb is this something you would be up for repairing? If not, any other sources that could look at it?

You could always drop $3 on the IC, desolder it, put in a socket, and a new LM324, and just shotgun it?

http://www.ebay.com/itm/10-x-LM324N...mp-DIP-USA-Seller-Free-Shipping-/222094215931

Or watch that youtube video, troubleshoot it, purchase tons of fun equipment and tools, or just give someone $100-200 to repair the pcb for ya. Or... just buy a new working pcb, SO many options! :)
 
Ok - cool. I will try powering it up with the board disconnected tomorrow and let you know - appreciate the help! And yes @thegleek there are options - maybe some board soldering is in my future :)
 
gamefixer - no, it hasn't - at least not in the time I've owned the game, which hasn't been that long. Is that an easy/cheap replacement?
 
Big blue installed - blown fuse?

So I ordered a replacement Big Blue for the Centipede, swapped it out tonight, but when I turned it on, I got nothing - game didn't fire up - let it run for a few minutes - then opened it up, and saw one of the fuses was blown - see attached.

Any ideas why?

Also in the pic, you see the pos/neg wires - all look good, right?

Any help appreciated...
 

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Pic looks right. All orange wires go to +, and all purple goes to -. Also, make sure the + terminal of the big blue goes to the + terminal of the bridge rectifier.

Was the fuse the correct amperage and type? You want it to be slow-blow. If it was fast-blow, I suppose there's a possibility the fuse blew from the inrush of current from the fresh cap initially charging.
 
Well I think I pulled an idiot move and didn't pull up the big blue capacitor high enough and it likely shorted on the bottom plate as it may have been touching.

So I'll course adjust correctly and get a new fuse in there and lets hope that solves the hum!
 
Well I finally got around to replacing the fuse that blew and now with that and the new 'big blue' cap I've had no luck - still go the sound hum.

@andrewb, what do you think - would you be up for having a go at the board?
 
Well I finally got around to replacing the fuse that blew and now with that and the new 'big blue' cap I've had no luck - still go the sound hum.

@andrewb, what do you think - would you be up for having a go at the board?

Hi Thomas,

Did you find a solution to your Centipede cabinet hum/buzz? I think I'm having the same problem.
 
Nope, I haven't. I replaced the A/R board, big blue, etc to no avail. I now think (and research suggests) that it may be something on the PCB board. So until I can find someone to help troubleshoot/fix it (at reasonable cost) or I can pick up a confirmed working pcb without hum, that's my only option at the moment.

Would appreciate any intel if you fix yours! Good luck!
 
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