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| Akai Problems with Blown Fuses |
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Important:
This article discusses the known issue that some of the older Akai samplers have with their
SCSI fuse and how a blown fuse will affect the use of certain SCSI drives. Note that we have
encountered the problem of a blown SCSI fuse only on the MPC2000 Classic and very occasionally
on the MPC3000. This article mentions that it can also happen on the older S series samplers
as well but we have not have yet encountered that with any of our customers who own these samplers.
It does not happen with the MPC2000XL nor the S5000/S6000 models.
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The AKAI's SCSI section, like all host controllers, feeds termination power
along the SCSI bus through pin 25 on the DB-25 pin female connector on the back
of your AKAI (which becomes pin 38 on the other side of your half pitch/high
density 50-pin connector cable used by the newer MPC versions). You may not be aware
of it, but pin 25 is one of the
closest pins to the DB connector shield. This means unfortunately that you can
easily short pin 25 out with the shield if you are connecting / hotplugging a SCSI
device "blindly" (in fact, you should never hotplug). This short
creates a great power drain and if the SCSI fuse were not in place and ready to
"open" you would quite certainly "fry" your AKAI's
termination power regulator. Once the fuse has "popped" you have to
replace it. The question is, can you do it yourself? If you are handy in
electronics and feel comfortable with a soldering iron then you probably can do
it without any difficulty. If you never have used a soldering iron, or the one
you have has a "point" which is bigger than a 1/4" screwdriver
then I suggest you get some help or have it serviced.
Now to the fuse. It is a pcb type (not SMD)
mount soldered in place vertically usually near the SCSI connector or term.
power regulator area. "Where is this?" you might ask. Well it is
of course on the seperate SCSI board for users who have the older type models
(mounted vertically) and for the newer series with built-in SCSI interface, it is
near the back left (looking at the front of your sampler), where the DB25 connector is located. The
regulator is a black rectangle about 3-4mms thick and is contained in a TO-220
type package. The fuse usually is located between the output of this regulator
and the SCSI connector. This is what the fuse mounted in AKAIs looks like - it
is much scaled up here - it is smaller in real life: about 3mms wide, 2 deep and
5 tall. It is very similar to a transistor in a TO-92 type package except it's
square and has only 2 legs. The markings on the fuse indicate it's ratings
according to a standard which takes the value indicated in milliamps and
multiplies it by 40. This means that "N25" indicates that it is a
"Normal" (not Fast) 1Amp fuse (25*40=1000mA) usually rated at 50V.
These fuses are available from various online sources like RS components or
Farnell (they have different names but they are identical, just don't buy the
Fast ones).
All this is good, but what happens if you short it again? What if you have a problem
on your SCSI cables and you don't find out until you have replaced it 2 or 3 times?
Not a good perspective of tomorrow. Of course, there is a solution:
the restatable/polyswitch fuse.
DISCLAIMER
Please read this carefully and proceed at your own risk - we will not be held liable
for damage to your AKAI equipment if you fail to observe precautions or something bad
happens! If you decide to replace the standard AKAI filament-fuse with the polyswitch
type you are doing so at your own risk and responsibility.
Instead of buying the same "filament" type ordinary fuse, you may replace this fuse with a
"current induced high impedance switchable" one. If you absorb too
much current for a very brief time like a temporary short or glitch, it won't
open (like the original N type fuse which is slightly retarded) - but if this
short prolongues for a few seconds it will behave like a breaker. However, when
the load has been removed or the power turned off, it will return to its normal
low impedance state after about 15-20 seconds, depending on the previous load. The
intervention time of these fuses is rather longer than the filament ones (some
seconds respect to a few milliseconds) so they may only be used if the ciruit is
designed to support such a load during the intervention time; usually this
polyswitch doesn't do any harm to the regulator for such a short period of time
as the regulator can support very high pulsed currents. Furthermore, they are
widely used on everyday's SCSI host controllers (Adaptec, LSI/Symbios Logic,
etc) even if the +5V is supplied by your PC's PSU which can handle much over
30Amps on that Line. These polyswitch look like round or rectangular yellow
ceramic capacitors and are often mistaken as them. They are usually labeled RUE
or RXE (2 series) and can be found again quite easily. One of the most used is
the RUE 1.1A RS code 183-9590 and they cost around 1.5 Euro each. They take about
5 seconds to "open" at 5 times the ordinary rated current. This means
that if you have a slightly higher load than 1Amp (which can be possible on long
SCSI chains with passive terminator) it won't "break" immediately and
will give you more room for error but will serve as a warning. The regulator
also won't suffer as they can handle currents 5 times greater than 1 Amp if
dissipated correctly for normal usage, but for just 2-5 seconds won't even start
heating up enough to breakdown. Just to throw you some figures, the termination
power line is a +5V and it is rated a max of around 1 Amp. This yields 5Watts of
power Max dissipation which isn't a lot (you can put your finger on it). If you
short the term power line you can get a rather high current (it can reach 10Amps
depending on the PSU) and this means that it will open up in less than half the
intervention time (1-2seconds depending on the prior temperature).
As I mentioned before, if you want to
be on the safe side, then go and get the original 1Amp rated pcb mounted fuse
and simply replace that. I have inserted this "polyswitch" section
more for your information only and for those who like to "experiment and customize".
I have not tested this widely and cannot give any guarantee. Furthermore, the
fact that you "self repair" your AKAI voids warranty, let alone
replace an original component design. If it is new and under warranty, have it
serviced. If it is used and you have just bought it from e-bay or whatever and
rush to connect it and find it is "stuck the waiting for HD message"
and feel that you have been ripped off somehow, well this page is for you! You
can decide now what to do and most important, know that you have not been fooled
(or nearly - you don't know the real reason why it was sold in the 1st place -
but better for you now that you can repair it with little or no cost).
Thank you to the Akai SCSI Faq website for this information. Visit their site
here for lots more helpful Akai
SCSI information. | |
