I understand that K403A MOH cards are extremely rare and unobtainable.

Does anyone have a schematic? I can clone one.

The 403 used 1-stage unity gain amplifiers for each output (6/card) to isolate the audio between outputs. You could use just capacitors for electrical isolation, but then those on hold could chat over the MOH!

Im thinking of building a complete new K403A, I have the passive Transformer/capacitor/resistor circuit used in the 451B Music on hold board but not the 403 MOH board. I could etch a new plugcard but for just a prototype, id build it on perfboard and use a plug connector. There are tons of digital music sources on ebay for $25-ish to drive it with.

Arthur would have one if anyone would. Have you tried this site:

The TCI Library

I'll look, but I don't remember seeing one...

I have to get out to the barn and dig. I asked my dad today if we still had one of these cards and we both know we do. We have tons of 400 cards, ring gens, systems, phones, ITT cabinets. My dad retired from Southern Bell in 84. He is 80 now and it would be a great project for us to bring some of this stuff back to the shop, test it and get it to you guys.

I have looked at TCI Professor Shadow but no luck.

CNetworx I'd appreciate you looking, if you find one do PM me with a price, even a broken one should be repairable.