TR-77 Hacking!

[Bob Schrum]

     Wow!  You are the first TR-77 owner I've ever known since I bought one 
     new in 1971!  As I recall, the beast cost over 900 pre-JimmyCarter 
     dollars then!! :-o  It's been a nice surprised to hear it on a few 
     records, tho. (The Cars' "Candy-O", and a recent one I can't recall)
     
     If you track down some schems (I've long lost my owner's manual) let 
     me know.  I've oft thought about hacking it myself.
     
     --Bob


______________________________ Reply Separator _________________________________
Subject: TR-77 Hacking!
Author:  jupiter4 <jupiter4 at bt-sys.bt.co.uk> at hcinternet-server
Date:    11/13/96 12:21 PM


Hi all,
     
I had the TR-77 up on the bench last night to have a look at how easy it would 
be to trigger the voices externally. Unfortunately all I have to go on is a v. 
basic block diagram in the TR77 manual, which nevertheless contains a 
suprisingly usefull amount of info. At leat all the boards are labbeled with 
the wiring between them (sometimes) labelled. I just had to do a LOT of scope 
probing....
     
Well the triggers are interesting :
     
The guiro takes a positive level about @10 to 15 v to start it and if you keep 
the voltage on it retriggers at the start of each bar with an accent.
     
Everything else takes negative triggers! Most are @-7 but a couple of the 
others vary between -3 to -10v to give them a bit of accent.
     
Th TR77 has a touch plate start and get this, non of the sounds will trigger 
unless the start switch has been pressed(or you ground the pad on the PCB 
where it goes to on the tone board).
     
Top this with the fact that the tempo is running constantly! you can't turn it 
off!(except unless you did what I did later!!) The start stop switch seems to 
reset the patterns to the start of the bar AND allow all the sounds to 
trigger! I checked on the scope and the triggers are always going and the 
clock is always running. Vary the tempo slider and everything on the scope 
slows down with it.
     
The only way I could get it to sit there and be triggerablefrom the  sequencer 
gate outputs (without the whole drum pattern playing! constantly!) was to 
first throw the seq gate O/ps through the Digisound inverter, then apply the 
pulse to the triggers on the tone board, ground the start pin, and disconnect 
the 22V power supply to the logic control board that does the tempo and some 
of the HARD wired rythms! All with 2 hands!
     
Next thing I did was reconnect the power to the logic board and drive my 
sequencer from the TR-77 clock direct off the board. (bear in mind that the 
TR-77 only has 3 jacks on the back! 2 audio outs and a start/stop jack). What 
I need to be able to do now is find the signal that resyncs the bar to the 
beginning when the start button is touched and I'll be able to drive any of my 
kit from the TR-77.  cooollll. I have a bit of an idea where it does this but 
I really could do with the diags so if anybody has schems for the TR-77 I 
would really appreciate a copy.
     
The next thing i've got to do is build a load of op amp inverters for all the 
triggers. I plan to have Trigger outs for each sound, i.e. when the pattern is 
playing I get triggers in time with the drum machine for all the sounds, so I 
need to get all thes to positive trigs preferably. Same for trig ins, which 
will require a switch to dis the power to the logic control board and also to 
ground the start lead to the tone board.
     
Phew, if I think of any other mods, like separate audio outs I'll shout up, 
I'll have to do some more probing first tho! Unless some kind soul has the 
diags! I hope someone else on the list has a TR-77 or you'll all find this 
pretty boring:)
     
Cheers, Dave ...