TURBO-R poor audio quality ...Why????

Yes you have to connect them to a stereo or a headphone to hear differences...

There is tons of noise on the audio output. I hear hiss. Noises and the nice 50 and 60 herz interrupts and there is some audible ripple there as well. The lack of bass and the horrible filtering doesn't make it better too.

The only solution is to poke around with a scope in the signal path and see where things go wrong. My bet is that it is somewhere near the mixer stage. Perhaps they tone the volume down too much before they amplify it back in the final stages. The volume is also very soft compared to the other MSX models. The soft volume indicates low output amplitude of the signal, a great way to invite even more problems into the audio.

I could check... It happens to be that I have a panasonic WSX laying around here . Ill hook it up and let you know what I hear.

I still remember an old article wrote by Henrik Gilvad about how to reduce the turbo R noise (leds etc) but I don't know where to find it now. Precisely he told that noise level was too high.

PS: BTW It would be interesting to know something about Henrik. He was a VIP in the past for the MSX scene.

I see the internal cause as well.

the final stage uses a summer amplifier to mix all the separate audio outputs. I count like 6 inputs so lets say the attenuation is x6 this signal is then fed into a pretty shitty quality transistor for output amplification. In theory the problem can be solved by doing a few simple modifications to the last part of the circuitry:

replace q13 with a good audio grade opamp and feed the signal into the inverter stage, then amplify the signal to something nice. Decouple the output with a good combination of capacitor/resistor. Lets say 1uF/100K.

Removal of C57 will take out the filter removing the "thin" sound effect with the risk of audible ringing. Replace IC 8 with a modern audio grade opamp with low noise characteristics.

For even more effect use separate audio wire in the scart cable as mentioned above.

R22 is way too big 47 K will do fine the click is nearly inaudible.

in the end R25 can be removed as well if Q13 is to be removed.

Q13 is hooked to FILTER of the S1990 as well. Too bad as this pin is described int the service manual it is in japanese and thus well.... propably this FILTER pin gives the humming and other switching noises. My guess is that removal of Q13 is sufficient to solve all the problems.