M-35/MX-35 design question.

[dino74]

I picked up a MX-35 for cheap but I'm having a hard time understanding the design. I see they use two Sanyo STK4863 amplifiers. Each STK4863 has two amps. There is a switch on the back for 2ch versus 4ch. From the schematic, it seems the switch does two things: Change the taps on the transformer and enable/disable the second set of channels via relay RY101.

The schematic shows the voltages to the STK changes depending on the switch.
4ch -> 2CH
28.8 -> 36.5
30.8 -> 39.11
27.8 -> 35.4
-30.8 -> -39.1

I've seen other comments from people saying the 2ch mode is internally bridged but I cannot see any evidence of it in the schematic. But the stats seems to indicate they are. Can anyone explain to me why the 2ch mode has more power and less distortion than the 4ch mode. Is it just because they are using only one amp in each of the STK chips?

The stats from the Manual (http://sportsbil.com/yamaha/m-35-om.pdf) are:
Min RMS output
2CH( 20Hz-20kHz)

.05% THD 8ohms 40W
.06% THD 6ohms 43W​

4CH( 20Hz-20kHz)

.07% THD 8ohms 20W
.08% THD 6ohms 22W​

Dynamic Power

2CH (1 kHz, 8ohms/6ohms) 57W/70W
4CH (1 kHz, 8ohms/6ohms) 32W/37W​

Total Harmonic Distortion

2CH (1 kHz, 20W/8ohms) 0.008%
4CH (1 kHz, 10W/8ohms) 0.015%​

I reference the M-35 docs because I could find the MX-35 SM online but they appear to be the same. The schematic also includes the STK4863 as well.
https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Yamaha-M-35-Schematic.pdf

 

[Jazzmuso]

Just for the record: my M35 gave some trouble apparently STK 4863 gave up the ghost. Unobtainable so I got a set from a friend which matches pin for pin: STK 4893. Just got it fixed and it sounds great. I'm not sure all is OK to do such a job as the bigger STK has higher power apparently. I am just to glad its working and back in use in 2 channel; mode