From eli@gs160.sp.cs.cmu.edu Thu Nov 14 13:49:53 1996 Comments: Authenticated sender is From: Robin Whittle Date: Thu, 14 Nov 1996 00:31:40 +0000 MIME-Version: 1.0 Content-type: text/plain; charset=US-ASCII Content-transfer-encoding: 7BIT Subject: TR-808: memory mod, pulling apart & wire colours CC: Henning Kristiansen X-mailer: Pegasus Mail for Windows (v2.23) 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 . . . . 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