I have the 4401 with the stock SQ and CD-4 options. I have had the main unit rebuilt with new caps and 1% resistors and better op amps. But the CD-4 board still has 4558 ICs and the TDM-19 board. Looks like it would be a job to rebuild the TDM-19 board as it is very compact. Did you ever find any info on that board or try to update it? There are lots of pots on it and I don't know about adjusting it? I'd love to have schematic and service info on it.Hi friends
Has anybody a (hopefully good) picture of the rare BOSE 4401 CD-4 demodulator? I would like to see this board, if its worth searching or rebuilding. Of course, I would like to buy one, if somebody likes to sell (I offer 1k $ or € ;-)
I am restoring currently my Bose 4401; and have already added in the Involve SQ and QS (compatible) Decoders, made from their evaluation kits. I have added a separate power supply and shielding, since the built in power circuit is not capable to supply these boards with their hungry TI signal processor. The Involve boards are slightly modfied (that means, several parts ripped of) and mounted on two carrier boards with suitable Molex connectors. The Molex are still under manufacturing; even with gold plated springs inside.
Beside the usual replacing of the caps, resistors (the old ones drift over the time), chips sockets, RCA sockets and semiconductors, I changed the gain of the last stage. It is by far to high (in my opinion); with my setup I could barely use the first 90° of the 270° volume poti. And with full volume (at 270°), there is a very noticeable noise and hum in the loudspeakers. So lowering the gain by changing the feedback resistors in the last stage is the first step. Lowering the noise will be another goal. Replacing the volume poti will be the last goal.
The opamps are already replaced with NE5532D - still the best. OPA 2134 work as well, but OPA2604 or exotic Oraclell-02 do not work well. And there is no audible difference between old and new RC4136.
I add some pictures, maybe somebody has fun.
The CD-4 is working (red pilot lt on) but my motto so far has been "ain't broke, don't fix" I did drill a hole in the lid so I can see if the light on the decoder is on.
My current problem is the main balance controls on the preamp don't seem to be working right? Only thing I can figure is bad pots and I don't know where to get dual concentric replacements.