Alpha S9000 No Sound

[Justin]

Hello All,

I haven't posted in a while, but I recently got my hands on a few Bally S9000's and V32 that have a few issues. First one is that I am getting no sounds out of the machine. I have verified the 24v supply at the Amp connector and continuity from all speakers back to the RCU backplane. The speaker coils seem to be good ohming at about 3.7 ohms. I pulled the Amp and checked for shorts but could not find any, the only suspicious thing is output A of the secondary amplifier IC was grounded, but I don't think much of it as there are only 3 channels in use, left, right, and sub with each amp IC having two channels.

I have also swapped with the amp out of the other machine I got, but I do not know if that one was functional. Both had the same result.

Any advice, or tips would be appreciated greatly!

[dailey]

Is the cable plugged into the MPU audio out?   Have you checked the setup menu Sound Setup? Volume could be turned down all the way.

[Justin]

Yes sorry, forgot to mention that:

Audio cable is plugged into line out on the MPU, and I have tried the Play Test Sound with volume all the way up in the menu, no difference.

I have also tried connecting the audio cable to my laptop to play sounds to rule out the MPU, still silent.

If I connect an external speaker to the MPU line out, I can hear sound, so the MPU is good.

[Stayouttadabunker]

Say, for instance, you plug in a set of headphones into the audio output of the MPU...does adjusting the volume change in the headphones?
If so, then you've singled out the problem that is something is cooked on the amp or a broken ground wire to the speakers?
I'd be ripping that thing out and inspecting every bit of it looking for a short or zapped IC component or broken wire.

[Justin]

Plugging in headphones works, I can hear the sound. I have already ripped apart the amp, no signs of failure that I can find other than the assumed unused output that was grounded that I mentioned in the first post. I'd hate to buy another amp to find out it wasn't the amp in the first place, but that always seems to be how my luck goes...

[hsjerry]

Plugging in headphones works, I can hear the sound. I have already ripped apart the amp, no signs of failure that I can find other than the assumed unused output that was grounded that I mentioned in the first post. I'd hate to buy another amp to find out it wasn't the amp in the first place, but that always seems to be how my luck goes...

I've worked on S9's for quite some time now and while your wires, connections, and everything look good? Those amplifiers themselves can go bad so you may need a new audio amplifier assembly

[pbwizard25]

Whenever I have had a short in the speaker circuit it takes out a component in the upper backplane. Remove the sound amp and look at the  backplane. It will be pretty obvious if it’s smoked.

[hsjerry]

I’ve seen that happen on an Alpha a few times, that means whole RCU backplane assembly gotta be replaced… yeesh.

[Justin]

I pulled an amp from my v32 and tested it, and the sound does work. I guess that does prove the amp is bad. I just swapped the amp chips (LM4765T) for the new replacement LM4766t, now I heard the speakers pop when power is turned on which is the same behavior as the working amp, but still no sound.

It looks like it may be an issue with the mute circuitry, digging into that now. Hopefully I can find a way to repair it!

I wish there was a schematic for the amp board itself...

[hsjerry]

Download page has S9000 schematics

[Justin]

I didn't see one for the amp board...

[Justin]

Update: I was able to fix both my amps.

I am unsure if it is very useful for anyone since these amps are available and inexpensive to replace, but the LM4765T can be replaced with a LM4766T with a small bodge. I swapped those out, replaced the LF347 preamp and recapped the board, something like 7 capacitors, not bad to replace. And it works again!

If anyone does this, in all likelihood, the LF347 was probably fine and did not need to be replaced. After swapping the amp chips, you just need a 10k resistor between pins 6 and 11 tied together going to pin 4 to unmute the IC.

[Chris-socal]

Justin, was the reason for using a different part because the original is harder to find or you just happened to have the other one?

[Justin]

Original is discontinued. I'm sure you could find them, but it would probably be difficult and more expensive. Everything I used can be bought off Digikey/Mouser