If you can't find anyone else to work on your TR-909, I can do repair work and pot cleaning.
The TR-909 is difficult to work on, since the component sides of the two main
circuit boards are normally not accessible when they are connected inside the machine. I have
extension cables to enable me to access both sides of the PCBs when
they are connected and operating.
I have only occasionally worked on TR-909s since they came out and
am not aware of any systemic problems with them. As far as I know
all the push-button switches are quite reliable.
The LEDs inside the buttons can be rather dim and uneven, but it would
be a lot of work to replace them. Its not out of the question
that I could do this if someone really wanted them to be more
In two machines I worked on in early 2018, three NPN transistors
(2SC2603-F) were faulty, producing two faults in one machine and one
fault in the other. These machines were made a year or more
apart, so I think this whole set of 2SC2603-F transistors may be
unreliable, rather than just one specific batch. There are about
61 of these transistors in each TR-909. It would be a lot of work
to replace them all. 3 failures over 122 transistors which are
all about 35 years old is higher than the general failure rate for
electronic components. I still think it indicates that other
TR-909s may have the same kind of trouble as the decades parade past
It is important to upgrade the firmware
to version 4.0. This apparently solves a problem with the machine
not recognising the first MIDI In Clock byte following the Start byte,
if this Clock byte follows by less than 1ms. For many sources of
MIDI Sync, it would be the very next byte, which arrives 0.3ms after
the Start byte.
To find which version of the firmware your machine has, turn it on with
the Track 1 button pressed. One of the 16 button LEDs 1 to 4 will
light up. The service manual only mentions versions 1 and
You can buy pre-programed EPROMs with this firmware from:
The above page has a copy of the note from Roland describing the problem:
TR-909 will rarely miss-read a $F8 (MIDI Clock) just after
The interval between Start data and the first Clock of the
MPU-401 is so short (about 1msec).
Due to space constraints the EPROM (28 pin 27C64, 250nsec or less
access time) is soldered to the front panel board. The above
seller provides a socket so the new EPROM can be in the socket, in case
it ever needs to be removed. However, I can't see how any EPROM
could fit with any socket I know of, given the clearance of
I solder the new EPROM to the PCB, rather than use a socket. I do
so in a way that the EPROM is raised 1.5mm or so, in order that only
the thin parts of the pins are in the holes, so it will be easier to
desolder if this is ever necessary. (Normally the thick parts of
the pins would be resting on the tops of the holes, which makes
desoldering mare difficult are more likely to damage the PCB.)
I use a 27C256, with the firmware programmed into the 4000
locations, because the 27C256s are more modern, faster and easier to obtain. This requires that pin 26 be grounded.
Here is a zip file of the firmware in raw binary form, with a second
file ready to be programmed into a 27C256 as just described: TR-909-firmware-V4.zip
. Version 4 is missing one feature: the ability to load the
"factory patterns" into RAM. With version 1 (the only other
firmware version I have tested so far) this is achieved by turning the
machine on with Track 1 and Pattern 1 buttons pressed. (See page
7 of the service manual for full details.)
The patterns and tracks are stored in a TC-5565P-15 static RAM
chip. (150ns access time.) Its pin-out is the same as a
standard 6264 static RAM, and - as far as I can tell - the same as
Toshiba's TC-5564P RAM chips. (I am not sure what the difference
is between these two types of Toshiba chip.) If the RAM chip is
faulty, it can be replaced with any 6264-style 8k x 8 RAM chip, but it
is important to use one which has a standby current of 1uA or so.
The current will change with temperature, and the actual currents of
these chips are far below the maximum current specified in the data
sheet, which might be hundreds of times more than this. You can't
just rely on data sheets to select a chip, since none of them show
currents as low as the chips in practice actually have.
I have replaced these chips with NEC D4364C-15L chips. The LL
series are specified to have lower maximum standby currents than the L
series, so either L or LL would be fine.
When the machine is turned off, the data in this memory chip is
retained by about 3 volts (via a diode, so about 2.5 volts to the RAM
chip) supplied by two AA cells
in the battery compartment. These should be alkaline batteries,
which have longer lives and are less likely to leak than ordinary
I could also install a cylindrical CR1/2AA lithium battery in a little
holder, within the battery compartment. These have a very low
self-discharge rate, and so would probably work for many decades.
They will never leak.
Generally I think it is fine to use fresh Duracell or Energizer
alkaline batteries. When fresh these are typically specified as
having a 10 year shelf life. The current drain of the memory chip
- 0.000001 amps - is very small compared to the capacity of the
batteries themselves, not counting self-discharge, which is about
2.4 amp hours. Without self-discharge, the batteries would last
for 274 years. I suggest installing fresh alkaline batteries
every 5 years, with the machine turned on while you change them.
These batteries fit into a special holder which has terminals like a 9
volt battery, and this clips into a conventional 9 volt battery
snap. The trouble is that if the battery holder goes missing,
then one might be tempted to plug in a 9 volt battery. If this happens,
it is probably a good idea to replace the RAM chip, since it can't be
ruled out that the 8 volts or so might damage the chip in some subtle
way which could cause erratic operation.
It is tricky to find these 2 x AA battery holders with the 9V battery
style clips. In April 2018 I bought some from this eBay seller in
Here is the final battery compartment arrangement with EVA and
polyethylene foam to secure battery holder. Another piece of
polyethylene foam goes on top of the battery holder.
There are a series of updates which need to be done to TR-909s with serial numbers below 426700. From the service manual:
I will usually do all these updates apart from the first, which
involves replacing a rubber battery compartment cover with a metal
one. If the cover is missing, I will be able to create one from
aluminium, but it won't be as attractive as the stainless steel ones
made by Roland.
For overseas customers, the cost of insured shipping in both
directions, plus the cost of getting the package through Australian
Customs (typically AUD$100) is significant, so I suggest finding a
technician closer to home for this work, unless you also want the sound
modifications - in which case it is best if I do all the repair work.
Whenever I work on a TR-909, I clean the the case, buttons and knobs,
test the machine and do any necessary repairs. I also resolder
some joints for rear-panel connectors which might break due to flexing
over the years - past and future - though generally the sockets are
mechanically well anchored and I think this is unlikely to be a problem.
The Volume pot is typically noisy. Whether or not it is noisy, I dismantle and clean it with isopropyl
alcohol. This should keep it noise-free for quite a few years.
Without removing them from the circuit boards, I clean the 27 other
pots with isopropyl alcohol and compressed air. After a few decades they can
become noisy, but this cleaning usually fixes any such problems and
will hopefully have them working properly for another decade or two.
The exact amount of work involved in this or any further repairs can't
be determined beforehand. It is best to allow about 3 hours for
cleaning, including the pots, testing etc.
I don't normally test the tape sync / data save circuitry, but I can if
required. I will only be able to test the cartridge slot if you
supply a suitable memory cartridge.
If you want your TR-909 to run from a different mains voltage, there is
no need to install a new transformer. You can have a technician
alter the internal wiring so it runs from the 100V, 117V, 220V or 240V
tap of the primary winding.
The TR-909 operation manual is:
A good copy of the TR-909 service manual (TR-909 schematics) can be found at:
Here is the BD section of the voicing board schematic:
The schematic of the voicing board is about 20 times as complex as this. Here it is as a 3591 x 2550 image: TR-909-schematic-voicing-board.png
Some TR-909 parts can be obtained from:
This US eBay seller has (April 2018) replica knobs for the TR-909: