Help needed - troubleshooting sound (Atari & the MB3717 amp)

CoindoorDave

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Help needed - troubleshooting sound (Atari & the MB3717 amp)

OK - should have left well enough alone this weekend, but didn't. Had my Atari Arabian working perfectly except the sound was a bit "tinny". For short-term fix I ran the Speaker +/- lines thru an amp I pulled from a set of PC speakers.

Still sounded "tinny" to me.

Swapped out the AY-3-8910 with a couple known working ones and no diff. Ordered a new MB3713 from Bob and carefully removed the old one and then soldered in the new one..........backward....doh! As a result - there was NO sound.

Removed the MB37137 (assuming its toast, but set it aside) found a donor (off sound board from a Kangaroo set, which was missing the CPU board). Same chip - the MB3713 (not the MB3712, which I also have on another PCB). This time I was careful in making sure it was added correctly. No sound still.

Replaced the electrolytic caps that the MB713 output to (according to the repair guide/manual I found online for the game). Nothing. (and I made sure I replaced the caps with new ones with the same value).

Measured the +12 coming from the PS (it's fine). But haven't checked it on the board (so maybe there's something else friend).

I tried hooking the speaker lines back to the external amp, and also directly to the speakers. I get hum from the speakers (and actually I can hear a faint radio station being picked up).

Any suggestions on what to try next? I really wanna finish this - so close; cabinet's fantastic, got a new repro CPO and new yellow leaf buttons, shined up the shaft on the original red ball joystick - even replaced the badly burned G07 with a newer WG monitor with a fantastic picture and virtually no burn-in.

Thanks!
 
get yourself a set of amplified pc speakers.

connect ground to ground, and the speaker input to pin 6, do you have sound?

work your way backwards ...
 
OK. At pin 6, I get static.

Backing up to the AY-3-8910, pin 38, I get sound. Moving forward, to the volume pot - all three posts...sound! ...

Pin 6 of the MB3713 (a new one I pulled from the donor PCB...although I supposed it could be bad) runs to a .047uf ceramic cap (tan M&M looking job with "473" stamped on it)...it has a noticeable blackish (scorch?) mark whisping up on side. Is there any way to check this cap or should I just replace it?
 

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OK. At pin 6, I get static.

Backing up to the AY-3-8910, pin 38, I get sound. Moving forward, to the volume pot - all three posts...sound! ...

Pin 6 of the MB3713 (a new one I pulled from the donor PCB...although I supposed it could be bad) runs to a .047uf ceramic cap (tan M&M looking job with "473" stamped on it)...it has a noticeable blackish (scorch?) mark whisping up on side. Is there any way to check this cap or should I just replace it?

That is a .047 micro farad cap and they usually don't go bad. They are cheep enough to replace but something next to it probably did the markings on it.

here is some info on the cap markings|

http://www.csgnetwork.com/capcodeinfo.html
 
OK. At pin 6, I get static.

Backing up to the AY-3-8910, pin 38, I get sound. Moving forward, to the volume pot - all three posts...sound! ...

Pin 6 of the MB3713 (a new one I pulled from the donor PCB...although I supposed it could be bad) runs to a .047uf ceramic cap (tan M&M looking job with "473" stamped on it)...it has a noticeable blackish (scorch?) mark whisping up on side. Is there any way to check this cap or should I just replace it?

I'd be more likely to suspect the cap situated directly between the AY and the amp; C2.

Tell you the truth, if I'm having audio issues, I'm apt to often just replace all the electrolytics immediately around the amp, just for good measure (and because they're cheap, and I keep a pretty good supply of them).

Oh, and your amp IC may be OK. Many SIP amps I've read the datasheet for are designed to survive being connected backwards.
 
Those amps are pretty stout and usually another component in the power source will burn open before the amp does. Sounds like this is your case. Follow the 12v in and see if its making it to the amp.
 
Is there any way to check this cap or should I just replace it?

I forgot to answer this question. I'd probably just replace it if I suspected it. However, there are a couple of possible approaches to testing it.

1) The fancy high-tech way: Use a DMM with capacitance. Check the schematic to make sure there are no other capacitances parallel (or desolder one leg if you aren't sure).

2) A quick-n-dirty way: Use a DMM in resistance mode. Measure resistance between pin 6 of the amp and GND (i.e. across the cap). The DMM applys a small voltage to the probe tips to do this. So at first the cap will appear as a short, and the resistance will be quite small. After a short period of time (dependent on capacitance and internal resistance) the cap will charge up and the reading will stabilize. If it stabilizes at close to 560 ohms (the value of the parallel resistor), the cap is not short. If it stays very low, the cap is shorted. Just because it isn't shorted doesn't mean it's "good", but if it is short it's certainly bad.

Looks like that ceramic cap, along with the 10k resistor (R1) form a simple low-pass filter to filter the input to the amp...
 
I *think* I may have found my prob. I tried measuring resistance from pin 6 of the MB3713 and either side of that ceramic cap...nothing...BUT...holding the PCB up to the light, the trace the runs from pin 6 to the one leg of the cap is gone. Burned up? Probably a victim when I replaced the amp...
 

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Went ahead and made a tiny jumper to bridge pin 6 to the cap and YES! Sound! And with the speaker out pins wired straight to the speaker it's loud, which means I can put the original volume pot back in and get rid of the PC speaker amp. THANK YOU to all who replied - you got me to check things I'd overlooked.
 
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