SCSI TERMINATION POWER FUSE

Q) My AKAI gets stuck on "Waiting for HD  SKIP..." msg!

Q) Can I replace the fuse myself?

Q) What does this fuse look like?

Q) Is there a way I can bypass this problem and use the SCSI bus without replacing the fuse?

A) The AKAI's SCSI section, like all host controllers, feeds termination power along the SCSI bus through pin 25 on the DSUB25 pin female connector on the back of your AKAI (which becomes pin 38 on the other side of your half pitch/high density MiniDB 50pin connector cable which more modern SCSI2 controllers use and 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" (infact, you should never hot plug). 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 ofcourse on the seperate SCSI board for users who have the older type models (mounted vertically) and for the newer XL series it is near the back the 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. Ofcourse, there is a solution: the restatable/polyswitch fuse. 

    DISCLAIMER
    Please read this carefully and proceed at your own risk- I will not be held liable of 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 resopnsibility.

    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 it's normal low impedance state after about 15-20seconds 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 1.5Euro each. They take about 5seconds 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). 

    I have replaced one of these fuses on an older s3200 and the user has not had any problems since. He told me that more than once he had to turn off the sampler as the devices could not be used obviously due to some problem on the SCSI line. He then discovered that the problem was a ZIP device connected to a "switchbox" (aargh I hate them) after I suggested disconnecting all devices and double checking cables one by one. When he switched to the ZIP the common ground inside this cheap switch box plus the other connected cables (DB 25 to centronics and DB50) would short-out the termination power line. He also noted a slight dimming of the display due to the brief load when switched which in less than 5 seconds was removed (when the fuse opened). So in effect the new fuse did do it's job rather well.

    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).

    One final note worth mentioning: if you have a blown SCSI fuse but don't have the time to immedtately service your AKAI, you can get it working if you connect a device that feeds termination power to the SCSI bus (and so, to your AKAI). Usually your PC's host controller can accomplish this by merely connecting the SCSI cable from your AKAI to your PC and powering your PC up (you don't even need it to boot, you can pause it) then powering up your AKAI - it will startup normally and you can SKIP the HD search process. If you have an external SCSI HardDisk check it's jumper settings as some (also older) hard disks have the option to "feed term. power to the SCSI bus" by rotating or moving a jumper (not to be confused with the termination jumper, we are talking about termination power not end point termination).

    Having said this, I hope you fix your AKAI; be careful the next time you hotswap or use those damn switch boxes or bad cables.