Studiomaster 16-into-4

The Studiomaster 16-4 (officially "16 into 4") is a kind of weired mixing console from the late 70s. It is intended to be connected to a four track tape recorder. It does not really provide a stereo out. Either you use a monitor out, or you use one of the four subgroups as stereo sum. The channel circuit is quite noisy, mostly because it is using carbon resistors in the mic preamp and uA741 all over the rest. But the preamp is nevertheless nice and smooth and the EQ is wonderful (to my opinion). I once owned one, modified it a lot, but finally it sunk in a flood, that soaked our basement, where I had stored the partially modified parts...
 
 
Not my desk. Picture by "equity sounds" on audiofanzine.com; click on the picture to open it in the original size 
My modifications included
  • insert for each channel
  • boosted EQs for each channel
  • aux sends pre/post-switchable
  • extension to four aux busses (each aux could be sent to one out of two busses)
  • mute-switch
  • channel out to stereobus or alternative subgroup
  • subgroupmasters should be converted to four stereo aux returns
  • the subgroup faders should become the stereo master faders and masterfaders for the aux return unit
  • tape ins and detailled monitoring section.
Of course this plan was ridicolous. A used Mackie CR1604 would have provided most of those features, but somehow I wanted to do 'my own unique mixer'. Here are some pictures from the modsi (for all pics: click on it and it will open in the original size):
 
 
Aux sends, pre/post switch (upper) and bus select switch (lower). 
 
The one pole switch is the mute, the two pole switch does the routing to either stereobus or alternative bus 
 
a 19" patchbay at the rear side to provide the sockets for the inserts and stereo Aux Returns 
  
And last but not least here are schematics and a picture of the channels parts side and solder side 
 
click on the picture to open it in the original size 
 
Once again the channel, but this time redrawn in EAGLE, including the aux sends, fader, panpot, and pfl. Click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size 
 
click on the picture to open it in the original size