Wednesday, 28 January 2015

TKB Build - Part 3A - Main Board

TKB - Touch Keyboard Sequencer - Build Part 3 - Main Board. (Resistors, Caps, etc).

*** Click here for the TKB Build Index ***

Before we begin there are some preliminary pics of the PCB


I'll start of with least temperature sensitive components such as the IC header sockets, resistors & diodes.



Mouser #: 571-5103308-3
Mfr. #: 5103308-3
Manufacturer: TE Connectivity
Desc.: Headers & Wire Housings 16P 2 ROW HEADER LP
These modern headers connect the Main PCB to the rest of the keyboard. In vintage TKBs, the headers used were slightly different:


Notice the silver arrow which is used to check the header orientation.
Vintage TKB headers.

Back to the Modern PCB: 

Looks like the four 22pf caps on the left need to be swapped for 100pf.
According to one of my tech friends, you can experiment a bit here. Serge used 82-100pF,
but you could even go as high as 220pF. Apparently the caps are filtering some unwanted noise from the clock driving the keys, and 100pF suppresses it OK. ....but even higher values might do a better job. The only problem is - go too high and it might start to slew your signal… ????

I'll def do some experimenting with this. (Many thanks to my friend for this advice).

Looks like the surface mount 100n caps need to be soldered to the underside of the PCB across pins 5/4 & 11/10.  of the  LM 324s --- SEE PIC BELOW

The LM 324 is a low power quad Op-Amp


There are 5 surface mount 0.1uF / 100nF caps to solder to the underside of the PCB.
The soldering points are a bit tricky to see at first. They lie across two of the LM 324s -  pins 5/4 & 11/10.
and underneath the LM324 just below the MM74C150


The SMD caps lie under the three LM 324s on the top right

Almost finished the easy part ... soldering the resistors, diodes, caps & headers..
I think its a good time to stop and compare this board to a vintage TKB main board. This one is a unrestored TKB -- also from the late 1970s.

Notice the bud box below is self powered.... in fact it has two PSUs, and was once part of a much larger system.

 Top pics of the main TKB PCBs (vintage on left)

The underside of the PCBs.


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*** Click here for the TKB Build Index ***
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