Cool, looking at it now, pretty straight-forward operation.
Which part of the diagram dont you get, when trying to trace the tracks?
IF you buzz the output pins of the LM339 (U16 pins 1,2,13,14) which goto the 74LS240 (U15a pins 8,6,4,2 respectively)
then you've proved that much.
Each of these 4 pins ALSO connect to a 10k resistor, and the other ends of those 4 resistors will (should) be all joined together which is the 5v pullup for them.
Confirm that LM339 pin 12 = GND (ground), and that pin 3 = 5v.
http://pdf1.alldatasheet.com/datasheet-pdf/view/3064/MOTOROLA/LM339.html
Inputs (-ve) to the LM339 are pins 4,6,8,10 and should be connected to 5v.
Inputs (+ve) to the LM339 are pins 7,5,11,9 and should wire to 1K resistors, the other ends of these 4 resistors should connect to diodes D14,13,12,11 respectively, to their 'Anode' ends.
At the join where the resistor connects to the Diode, there will ALSO be a 1.2K resistor connected too.
The other end of this 2nd lot of (1.2k) resistors connect to the 12v supply.
Buzz the diodes with a diode-check function on your meter too.
Assuming everything above checks out, you may have an external wiring error? a shorted/missing diode on your switch somewhere? or possibly lost a connection AFTER the output of your U20 chip.
There's a signal on the 'Direct Switches' connector named 'Enable', buit oi'd say that is ok if all the other direct switches are working?
Otherwise, replace Transistor Q1 (2N3904) and/or confirm U5f (74LS14) also works too.
I'll leave it here for now until you've eliminated all thse, then we'll dig deeper, as there's more chips that control U5f (enable signal).
Good Luck,
MM.
Firstly, Thanks MM for your help...I am more trying to get this working now not to beat me!
I was just having trouble working out what goes to where? I've looked at ones where the tracks are drawn from each pin (numbered) to where it goes...I find this easier..
Outputs of LM339 U16 - :
pin 1 buzzs to 74LS240N U15 to pin 6 (you had 8?)
pin 2 buzzs to 74LS240N U15 to pin 8 (you had 6?)
pin 13 buzzs to 74LS240N U15 to pin 4
pin 14 buzzs to 74LS240N U15 to pin 2
Respective pins buzzs to R35 to 38
Inputs (-ve) to LM339 (U16) 4, 6, 8, 10 5v.
OK. I am guessing that these pins would share the power rail 5v in common. If I buzz from U16 to U17 on pins 4, 8, 10 - it buzzs but NOT pin 6- which is where I was lacking voltage before..
Inputs (+ve) to LM339 (U16) 7, 5, 11, 9
pin 7 buzzs to R?? (this has broken pad)
pin 5 buzzs to R43
pin 11 buzzs to R45
pin 9 buzzs to R46
Diodes D14 to D21 test good.
Sooo...I need to jumper pin 7 to the right resistor and sort out power to pin 6. Can the power for pin 6 be jumpered off pin 6 on U17?
pin 12 = ground (tested ok) pin 3 = is this 5v or 12v.?
Pin 12 U16 - ground
