Toshiba 3950 mod evolution
Swenson mod revisited:
The problem we had with the Swenson mod was the very low output level, less than 1% of stock. Hope you have a hot pre-amp!
We suspected that the attenuation was on the DAC chip, which has that capability. However, disabling the clock on the serial control port did nothing. Therefore, Cedric Meza helped by using magnification and extensive expertise to trace out the circuit. The 5.2K resistors were IDd and jumpered. We couldn't find a leg to ground for DC, and we couldn't figure how the series resistor alone could attenuate so much, but bypassing that brings the Swenson mod output up to about 3dB below the stock or Vinnie modded player.
These resistors are approximately the size of an immature mouse's testicle. Teeny tiny wire to cover the surface connection pads after you float them off will work (try a strand of CAT 5 plenum). Test for continuity. The last two pins on the DAC facing you toward the right go to the (-) of caps 910 and 912. These resistors are just to the left of caps 910 and 912 as you view the board in place from the front and above. If you put your probe on those last 2 pins together you should initially get 5.2K to the (-) pins (toward you) of caps 910 and 912, then O after bypassing.
You can pull out caps 910, 912, 918, 920 and 902. Replace the DAC supply caps, 903 and 905, with 12V to 16V @ 10 to 100 uF, of the highest possible quality. These directly supply the DAC, and make a big effect on the sound. Pull out the little yellow film caps by the RCA outputs. These shunt to ground. The inner lead of the left one goes to the hot pin of the left channel output and the inner lead of the right one goes to the right channel hot output lead. Those holes are useful. Place your output caps in the holes left by the negative pins of C910 and 912 and these central holes from the output shunt caps. Left to left and right to right. I'd go with 10V or greater rating for safety, but you can live dangerously with 6.3V Black Gates. 10 uF is a good value. More is good. 1uF is probably OK if you have a high input impedance pre-amp. Non polar is best. You can try the 47 uF NP output caps in the stock player if you want to be really cheap. Fair enough. You'll need a jumper wire for short lead caps.
You can also float off the 6 pin dual transistor by the outputs. It shunts the output to ground when there is no signal. If it were to shunt to ground when there is signal your DAC would be playing into an AC short, unlikely to happen as I understand the machine, but may as well get rid of it. Nothing bad happens.
The outcome of all this is the BEST sounding player yet, and it doesn't reverse absolute phase. Output is 3dB down from stock; no big deal. Use good interconnects. Shorter is better, but mine are 2 meters and I lose a little in the bass.
The rhythm, punch and delicate fine detail are superfluous, or superlative, or something like that. A vale has been lifted. There's more "there-there". Am I ready to rite for Stereofile?
Next? I hear Swenson has reprogrammed the DAC for slow rolloff, which sounds better. The stock is fast rolloff. This is a very fancy mod. It requires actual expertise, not just a devil-may-care attitude, good hands and good eyes. I'll need help again.
John Day