Fixing and upgrading a vintage PC speaker.

[Frazatto]

I started cleaning and fixing this old SBS38 Sound Blaster speakers and things are going as expected, except that I can't take a single DC measure on this board!
It's just super weird, DC measures just fluctuate but if I switch to AC, the voltages are all exactly what I was expecting.

So first question, does anyone knows what is going on? Why a circuit would use 6v DC just to change it to 6v AC??

If I just wanted it to come back to life, it's done (just had to do some cleaning on the mechanical components), but I would like to upgrade it with a bluetooth receiver.
Originally these speakers were designed to work with fat alkaline batteries and I thought about using the leads for the batteries to power the bluetooth receiver. Each box would have two batteries in series, which gives me the 3v, I'm guessing, the bluetooth circuit will require.

If the previous issue is confirmed, there is no way I can just solder this to the new receiver, right?

Here is a picture of the board, they L and R are identical, the only difference is one have the power jack on the back and the other has the sound jack in it.

[Rasz]

Just out of curiosity what is the opamp? LM386?

[Frazatto]

jrc4465b on one box and jrc4069b on the other.

Can't find any datasheet though.....

[magic]

What voltages are you measuring with respect to what "ground" and how?
There's no AC in those things besides audio signals...

One chip is probably NJM4565 (opamp, but not a usual choice for driving loads), the other I don't know.

[Frazatto]

What voltages are you measuring with respect to what "ground" and how?
There's no AC in those things besides audio signals...

I'm very much aware this is strange.....
If I was making any gross mistakes wile measuring the circuit, the measures would simple make no sense at all. It's just so extra weird that it does make sense in AC.

The design choices are strange all around.
Originally, the setup supported alkaline batteries and I'm trying to make it work with lithium ones.
If I measure the the old battery leads, the left box measure oscillates on DC but gives almost stead 12V AC, but the right one reads stead 6V DC (it should be 3V or the original batteries would blow!).
If I remove the power connector, source and ground are shorted on the left box (how?!?!?!) but not on the right one.

This thing is breaking my legs 
I think they made one box the "power regulator" and the other one as the "sound filter", but both boxes have independent volume adjustment and power switches.

[magic]

Power input jacks often have three pins and the third one gets shorted to the negative when no plug is inserted. This is typically used to switch the negative of the batteries.

Buzz out all power rails, draw a schematic and it will certainly start making sense. It's possible that it uses some sort of virtual ground.

Is the wall wart DC or AC? I see no obvious diodes, so it should be DC

[Frazatto]

Power input jacks often have three pins and the third one gets shorted to the negative when no plug is inserted. This is typically used to switch the negative of the batteries.

Well...I can completely ignore this them.
Considering I want the batteries to be charged when power is connected, I can simple use the "6V leads" on the right box.
Thanks, one less problem to go!!!!