JonDent - Exploring Electronic Music: Roland Tr 808 repair - Part 2

Friday 4 September 2020

Roland Tr 808 repair - Part 2 - replacing the switches

How to open up the TR-808 .....continued from the last post:
Repairing a Roland TR-808 - Part 1

Now that you have opened up the beast you'll need to unplug the board from its various connectors.

You can just make out 4 of them here.
The connectors are 40 years old & probably brittle ... take care.

The other 2 are tucked at the rear ... They plug into the PCB connecting the inputs & outputs.

Next you need to separate the main PCB from the front panel.
It's held in place with the remaining screws (11) & the nuts/washers for the pots & rotary switches.

(Mine was missing lots of nuts/washers.... I reckon they would have been once present).

These screws circled in green are the ones that are left
Remove the central screw last, and the PCB should just pop out.

If you look at the PCB carefully, you can see it's actually two boards joined just right to the last of 5 central switches

The Main board apart from housing things like the cPU & RAM, also has some of the voice circuits:
Snare, Claps, Maracas, Hi Tom/Conga, Mid Tom/Conga, Lo Tom/Conga, Bass Drum, Claves/Rimshot.

The Voicing Board has the circuits for the Cowbell, Cymbal, Open Hat, Closed Hat,
Mixer, & master Volume.

Now it's time to remove the step switch board

That volume pot (extreme right) is definitely not original.
I'll replace it one day! At the moment, it's working fine ... I prefer to let sleeping dogs lie

iT'S held in place with 7 screws. They sit on standoffs.

Careful you don't break the ribbon cable

The FFC - flat flexible cable.

Admire those ICs

The 4011 is a 2 input NAND gate.
The 4051 is a multiplexer -- a digitally controlled switch

The 4 ICs on the top (D444C) are the memory.
I think they are 4K SRAM chips ???

This D650C is the CPU. It's the brain of the 808
You can also find a similar CPU the TR 606 (the 606 uses a D650C 128)
They are super rare . It's a 4 bit microcontroller
The UPD650C-085 is a special custom Roland (version 0.85) chip. It manages 11 analog channels with individual gain mix.
The voice board generates 16 analog percussions which 8 of them are circuit-shared.

The felt shades help to keep dust out.
There were originally shades on the toggle switches, rotary switches and all the pots I think ,
but mine are mostly gone ;-(
Next time I open this I must remember to put new ones in.

Notice The JRC 4558 (Japan Radio Corporation) in the bottom of the pic -- they are dual Op-amps.
These are everywhere on this board

The HD 14584 B is a Hex Schmitt trigger

The AN6912 is a quad comparator

4013B - Dual type D flip/flop
4001 - Quad 2 input NOR gate
14584BP - Hex Schmitt trigger

The new board is on top.
Its a Technology Transplant board.

The colours, esp the white buttons are slightly different.
Maybe the passing of time has faded the white switches

Screw in your new step board & reassemble the machine