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

The PCB should slot into the front panel.

Plug the connectors.

That green cable is the ground.

hOPEFULLY  it works

Roland Links

+TB 303
       + Intro & Setting up
       + Pattern write
       + Track Write

+ TR 707 Drum
          + Roland Tr 707 - Drum Mapping & Midi triggering
           + Roland Tr 707 Drum - Basic programming
           + Pioneer Squid - Midi Mapping a Roland TR 707

+ TR 808 Drum
           + Repairing a Roland TR-808 - Part 1 - Opening the Machine

+ TR 909 Drum
        + Roland TR 909 Programming Tutorial - getting started
        + Roland TR 909 - How to Program
        + Roland Tr 909 - Drum Mapping & Midi triggering

+ SE-02 (Studio Electronics / Roland)
       + Transposing sequences with the keyboard 
       + Filter/Amp envelope

Repairing a Roland TR-808 - Part 1 - Opening the Machine

I've owned this particular 808 for I think 15 years and in all that time I never opened it up.
This is how I purchased it. ... The knobs are wrong, and the step switches have certainly been replaced.... with the wrong colours too. But I never really cared as it just sounded amazing.

Who knows how many hands it has passed through since the 1980s.
And which technicians have worked on it ??
Lately it has started to play up ...mainly the step switches 3 & 9 have stopped working.
Everything else is OK.

Maybe some dust has got in under the switch contacts.
The choice is to replace the individual switches or the whole switch board.

I've decided to try the easy option first and just replace the board.
If I discovered more problems, Id send it to my technician.
Anyway, I purchased the switch circuit board and some knobs from Sound To Parts in France.
https://www.soundtoparts.com/en/3-roland

They have parts for all the major brands.

Be careful when you open up the 808.
Screws can go missing. And different types of screws are used in different parts of the machine.
So put them in safe places.

Long silver headed screws are mainly used on the underside, whereas black ones are mostly on the face.
... shorter ones on the front metal cover and the front. These hold the circuit board to the metal panel.

The sides of the 808 are plastic... the black screws are longer and designed to screw into plastic.

Enough jabbering. Lets get to it and find out what we have.

i DECIDED  to take the knobs of first.

 This volume knob is certainly wrong.

  I started removing the 6 black screws on the front & rear.
These 6 screws are what hold the front panel to the base.

(The base secures the power supply. The actual electronic circuit board it secured to the front panel).

jUST the 3 on the bottom of the rear

And the 3 at the bottom edge on the front.

Next, remove the side plastic panels.

They are held in with 6 screws.
2 on the top & 4 on the bottom

On my machine, the top screws were black, and the 4 lower were silver.



 You can remove both side panels, but it's not really necessary.
I just removed the left plastic panel.

Once you remove these 6 screws, it just slides out.























You should be able to now lift the top half from the base. The bottom half is the PSU .. consisting of the transformer, a power supply circuit board & the battery compartment.

A close up. Don't touch this unless you are a qualified techno. It can kill you. The whole body and case of the 808 is metal, so any shorts or loose wires from your mains or PSU board could conduct electricity to the front panel and case.  (plastic cases like the ones used by Roland in the TB303 or TR606 don't have this problem).

You can't separate the top and bottom completely as the the main on/off switch is part of the top panel
and the PSU is fixed to the base. This is all hardwired.

The green wire connects to ground.

To Be continued....
Roland Tr 808 repair - Part 2

Links
Transistor at the heart of the 808

Midi Moog - repairs Elk Elektronics

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I've had some very nice instruments come through the workshop lately. This one was a stand out for me. The @...

Posted by Elk Elektronik on Sunday, August 9, 2020

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Repairing a Doepfer A-140 ADSR

I stupidly plugged in the module backwards.

Shit happens .
Remember, Red Stripe Down. !!!!

 If this happens to you , don't panic.
There are only a few sensitive components.
Start by swapping out the ICs.
Thankfully they are socketed.

Just 2 of them:

This is a TL084.
Its a op-amp

This is a CD4001 CMOS chip
They are sensitive to static electricity, and is the most likely culpruit.
It's a quad 2-in NOR gate

I swapped it with one of these and the module worked again

You can :-)

Links
+ CMOS
+ Integrated circuits - packaging 
+ Opamps - basic circuits
+ Synthesizer Logic Modules - The 7 gates