PC Engine GT weird Sound Problem

[RyuX]

Hey Guys, my name is Patrick and I am from Austria.
I love the PC Engine and used a PC Engine DUO-R for a while now.

I recently bought a broken PC Engine GT and started replacing all of the caps and resoldering the whole board.
Now everything works as it should except for the Speaker.
The sound in the speaker is low and distorted - I already tried another speaker with the same result.

The OP-AMP NJM2073 gets the proper voltage and outputs the sound into the capacitors (which I replaced) and all of the traces are fine.
The Audio Socket bridges that mute the speaker when a headphone is plugged in are also ok.
The Headphone sounds very clear and loud and it seems that there is no separate circuity for the speaker as it only takes one of the channels from the headphone socket and routes it to the controller board.

I would need a reference from a known good/perfect PC Engine GT to evaluate if mine is too low on volume or not.
I tried a separate PAM8302 Amplifier board but because of the way the PC Engine handles its audio the PAM8302 will only work with an external power source that does not share the same ground (never had this problem before with any other handheld but on the PC Engine GT I can't get it to work as it will pull down the 5V rail to 2,2V even though the PAM8302 only draws around 0,05A in standby when measured on my lab supply).

So if the headphone sound is perfect - all of the traces and connections to the speaker are as well and even the speaker got replaced - could it be that this is it ? I mean by design its not supposed to be any louder ?
Because of the way the NJM2073 works it is also hard to find a replacement without making your own OP-AMP PCB but maybe I will end up making one.

Does anybody also know how the Sound Input (coming from the sound chip) into the OP-AMP is specified (how many volts amplitude etc.) ?
Would be really helpful. 

[Keith Courage]

Usually it's a bad trace causing the problems you are speaking about. I'd check all the via holes near the headphone port to make sure there isn't a short.

However, what's the condition of the headphone port? Does it have burn spots on the back of it from someone bumping with a soldering iron when changing the caps? The headphone port itself does the switching of the audio amp for louder volume when no plug is inserted. If the port gets heated up too much the internal switch can fail and then the speaker has weak volume.

[RyuX]

Hey

The speaker will behave the same even when hooked up directly to the OP-Amp so its no bad trace.
Also the Headphone output (which is the same as there is no separate circuitry for the speaker) sounds very good.

I checked continuity from the opamp all the way to the capacitors - audio jack - flat cable connector for the controller board up to the speaker connector which is all good.
I think the NJM2073 is just too weak to really drive a speaker properly and maybe my PC Engine GT is outputting normal speaker volume and I was just expecting more which isn't really realistic with just one low power op-amp.

[RyuX]

So today I added a PAM8610 Amplifier and now I get all the volume I want on my speaker.
I also replaced the speaker with and now everything is just working perfectly.
It just seems the OP-Amp is too weak to really drive a speaker 

[pipi]

Hello there!

Sorry for reviving this post but I have a very similar problem and I am going mad.

Maybe adding an amp could solve my headache, can you please explain where did you connect the amp's + and - for power and + an - for left speaker?

Thank!

[lamb2k]

^Bump

Would be great to know what you did for wiring.

Thanks,

Steven