Home > From eli@gs160.sp.cs.cmu.edu Thu Nov 14 13:49:53 1996

From eli@gs160.sp.cs.cmu.edu Thu Nov 14 13:49:53 1996

From eli@gs160.sp.cs.cmu.edu Thu Nov 14 13:49:53 1996
Comments: Authenticated sender is <firstpr@mail.ozemail.com.au>
From: Robin Whittle <firstpr@ozemail.com.au>
Date: Thu, 14 Nov 1996 00:31:40 +0000
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Subject: TR-808: memory mod, pulling apart & wire colours
CC: Henning Kristiansen <hekr@sn.no>
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Henning Kristiansen asked about TR-808 power lead wire colours and
about opening the machine.

Also, here is a mod to make the TR-808 memory more reliable.


Wire colours
------------


> I just received a TR-808 (without the power contact) and the three
> wires inside the power lead is LIGHT BLUE, DARK BLUE and BROWN...
>
> I have no volt/ohm meter (or whatever i should use) and need to know
> what color that is EARTH (it was received from england)

The international standard since around the early seventies is:

Green with
Yellow stripe: Ground. Safety connection between chassis, and
audio ground and the power outlet earth.

Light Blue: Neutral. Mains supply, usually 0 volts, but
don't rely on it.

Brown: Active Mains supply usually 240 volts or
whatever, but don't rely on it.


It is common for the active and neutral of power outlets to be
reversed, so never assume that switching off the "active" wire makes a
machine safe. For electrical safety reasons, *unplug* the machine -
most power switches on power outlets only switch one wire.

The Green with Yellow stripe is easy to remember as ground. I
remember that Brown is active because that is the colour of the dirt
they bury you in if you touch it.

The old standard was:

Green: Ground

Black: Neutral

Red: Active

However this was changed due to the high incidence of red-green
colourblindness - many males (it is usually blokes) see very little
difference between red and green.

As for the anomalous wiring on this TR-808 (mine has the Green with
Yellow stripe ground wire) - I could hazard a guess, but I would
prefer not to, in case the guess turns into a hazard.

The two alternatives are using a meter to check which is ground, or to
pull the machine apart to see where the wires go inside.



Pulling apart the TR-808
------------------------

Partial disassembly:
--------------------

Remove 3 screws at the back (below the output sockets) and 3 at the
front (on the front edge - not on the front panel near the switches).

Note that those at the back are short self-tappers. Those at the
front are short metal-threads.

Now on the front panel of the machine, remove 2 screws at the far
left. These are long self-tappers.

Remove the rearmost 2 of the three screws on the right. These too are
long self-tappers.

Now you should be able to lift up the TR-808 main panel and see the
rear side of the main board and the smaller board on the right, which
is called (for some obscure reason) the voicing board.

Note the power supply in the middle of the bottom part of the case.

The fuse on the left is mains. It, the nearby connectors and the mains
connector block present a serious shock danger when the machine is
open. If I have a TR-808 open for testing or repair, I always place
some masking tape over these live items - basically over the terminal
block and over the left side of the power supply. I take the tape
off before reassembling the machine, because it is not likely to stay
put.


Complete disassembly:
---------------------

Remove the six knobs.

Remove the nut and lock washer from the mode switch.

Remove 11 screws in total from the front panel: 3 at the rear, 3 at
the front and 5 in the middle. (One is normally hidden by the tempo
knob.)

Now you can pull the two circuit boards out as a unit.

Beware of the little felt things disappearing. They are the staple
food of the vacuum cleaners which are often observed foraging in a
newly opened TR-808.

The board with the 16 tact switches can easily be removed and
unplugged. The switches are ALPS type SKHCAB - similar to those in
the TB-303 but without a stem. The LED, black actuator and coloured
button are generally regarded as a unit, and the LED wires must be
de-soldered before you can remove this unit to get at the switch.

Usually, simply desolder the two LED pads and the four switch pads
and remove the whole kaboodle as one. Then separate the switch from
the rest of the unit.

The LEDs are easy to replace with a standard miniature LED, but the
black and coloured plastic actuator/button units are not ALPS and as
far as I know can only be obtained as spare parts from Roland.

There is also a metal plate below the main board to shield it from
capacitively coupled noise from the power supply.


Memory reliability: The Write Spike problem and how to fix it
-------------------------------------------------------------

Many TR-808s suffer from having their first pattern and/or first
song corrupted, despite the batteries being good. (These seem to be
the usual victims, because the CPU lines to the memory are usually
low when the problem occurs.)

The cause is a spike from the power cable inside the machine -
usually when it is turned off. The spike, which may be a thousand
volts or so (guess) with a very fast rise time, is generated when the
current to the power transformer is turned off.

The two wires leading from the power switch to the power transformer
are in a clear plastic sheath, and are generally pressed up against
the main PCB just behind the mode switch.

The exact placement of these wires probably accounts for the
variability between machines - so if you put it back together with
these wires close to the mode switch you will probably have the
problem. Maybe you don't have it now because the wires are not so
close.


Super-easy fix
--------------

Never leave the machine in pattern play (manual play) or track play,
when you are turning the power on or off. I don't know how helpful
this will be, but it may make a difference.


Easy fix
--------

Just figure out a way to place these wires so they are not pressed up
against the main PCB. I don't do this, and I can't be sure it will
fix the problem, but it might and it is easy to do.


Complex, magic bullet fix
-------------------------

This is what I do as standard procedure on all TR-808s I work on.

You don't need to do a complete disassembly, but you do need to be an
experienced electronic technician and have the TR-808 schematics.

You must take responsibility for everything you do, including your
own safety. See the notes above about the dangerous mains voltages
on exposed terminals inside the TR-808. The TR-808 should be
unplugged from the mains for this entire operation.

Take the proper precautions against static electricity. (If you
don't know what this means, then you shouldn't be opening any
electronic equipment).

Leave the batteries in, but expect some corruption of memory as a
result of performing the modification. This will probably be the
first pattern and/or track.

The write signal from the CPU to the RAM chips takes a long march to
the Mode switch, so that the Mode switch can physically write protect
the RAM by disconnecting the /write signal in two modes.

The first mod is to put a pullup on two positions of the Mode switch
so the write line to the RAMs is pulled up solidly in the two
positions where the /write signal is disconnected from the CPU.

The existing 1 Meg pull-up resistor (R91) to VRAM (VCC of the battery
backed up RAM chips) is evidently to weak to counteract negative
spikes capacitively coupled from the power leads.

The second mod is to put a low pass filter into the write line near
the RAM chips to filter out short spikes, but let the long write
pulses from the CPU through.

At the rear of the Mode switch, are two pins which are currently not
connected to anything. Solder one end of a 33 k resistor to these and
connect the other end to VRAM: IC7 pin 18.

Now cut the track leading to the left of IC7 pin 10 (conveniently
labelled on the PCB). This breaks the active low /write signal from
the Mode switch. Solder a 100 pF capacitor between pins 9 (ground)
and 10 (/WE) of IC 7. Now install a 10 k resistor from IC7 pin 10 to
the track coming from the Mode switch - just to the left of the cut
you made.

Now you have a 10k -> 100 pF first order, low pass filter on the
write signal to the RAM chips.

These mods, as far as I know, completely solve the problem of
intermittent corruption of the TR-808 memory due to the coupling of
spikes into the /write line.


- Robin







. Robin Whittle .
. http://www.ozemail.com.au/~firstpr firstpr@ozemail.com.au .
. 11 Miller St. Heidelberg Heights 3081 Melbourne Australia .
. Ph +61-3-9459-2889 Fax +61-3-9458-1736 .
. Consumer advocacy in telecommunications, especially privacy .
. .
. First Principles - Research and expression - music, .
. music industry, telecommunications .
. human factors in technology adoption.
. .
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