AC Power Fuse Reference For Roland Synths/Samplers

Every synth and sampler DIY'er needs to keep a couple of extra fuses handy for the times a screw is accidentally dropped inside the case or when a Floppy Disk Drive is plugged in backwards. Poooof! There goes the fuse and there are none in your spare parts kit. This is a useful list of the most common fuses used on a few Roland synths and samplers for 117V systems only. All fuses are glass cartridge, measure 5 mm x 20 mm and look similar to the fuses shown below
  SYNTH or SAMPLER            
300mA 250V Fast Blow     F1 (Fuse Board)     BelFuse P/N:   5MF 300-R
    JX-10 1.25A 125V Slow Blow     F1 (Power Supply Board)     BelFuse P/N:   5ST 1.25-R
    MKS-50 (Two Power Fuses!)   200mA
Slow Blow
Fast Blow
    F1 (Fuse Board)
    F1 (Main Board)
    BelFuse P/N:   5ST 200-R
    BelFuse P/N:   5SF 1-R
   MKS-70     S-50
630mA 250V Slow Blow     F1 (Filter Board)     BelFuse P/N:   5ST 630-R
    S-550 (Two Power Fuses!) 5A
125V Fast Blow
Slow Blow
    FUSE 1 (Power SW Board)  
    F3 (Power Supply Board)
    BelFuse P/N:   5MF 5-R
    BelFuse P/N:   5TT 4-R
1.25A 125V Fast Blow     F1 (TDK PSU Board)     BelFuse P/N:   5SF 1.25-R
    S-760 (Two Power Fuses!)
    Note: 1.5A fuse is for the SCSI port  
Fast Blow
Fast Blow
    F1 (TDK PSU Board)
    F1 (Main Board)
    BelFuse P/N:   5SF 1.25-R
    BelFuse P/N:   5MF 1.5-R
  SYNTH or SAMPLER            
    I have opened up all six of these synths and have been unable to find a fuse anywhere inside!!!
    Also, there are no fuses to be found on the schematics for these six synths
    I'm just guessing a fuse is sealed inside the power transformer making this part a "throw‑away" item

     You can always replace a 125V fuse with a 250V fuse of equal Amperage as long as the application voltage is 250V or less
     However, the reverse is not true. You can not replace a 250V fuse with a 125V fuse of equal amperage if the application voltage exceeds the 125V

S-50_S-550_ZONE Extra info about fuse types, case markings and symbols for various fuses at this link and this link

Miscellaneous Fuses
PICO® Fuse Resistor
  • S‑550 / S‑330 / S‑750 / S‑770
    • In addition to AC Power fuses, the Roland S‑550, S‑330, S‑750 and S‑770 have a small PICO® fuse which protects the sampler from any short circuits or current overloads on the EXT CTRL 9‑pin port. This port is used to connect a Mouse or an RC‑100 Controller. If you have a non‑working Mouse or RC‑100 Controller, see the PICO® fuse replacement info at this link for some repair tips
  • S‑760
    • This sampler also has a small PICO® fuse but it is installed on the Video Board and is there to protect any short circuits or current overloads on the digital RGB connector. It's connected in‑between Pin‑1 of the DIN‑8 connector and Vcc. The silkscreen location is FUSE1 on the lower OP‑760‑1 Video Board. This PICO® fuse is identical to the one used on the S‑550, S‑330, S‑750 and S‑770. However, this fuse is used for a different purpose. To my knowledge, there is no fuse protection used on the S‑760's EXT CTRL 9‑pin port
  • S‑50
    • I see no fuses of any kind on the S‑50 schematics for protection to the EXT CTRL 9‑pin port or the DIN‑8 digital RGB connector sooooooo... tread lightly when plugging/unplugging devices to the these ports SMILEY_FACE

Fusible Resistor
Roland synths such as the JV‑880, U‑220 and others have a fusible resistor connected to the LCD circuit. If you monkey around with changing out the LCD, a crossed wire will blow this component. A fusible resistor serves dual functions. When the power isn't exceeded, it serves as a current limiting resistor. When the power rating is exceeded, it functions as a fuse, burns up, and becomes an open in the circuit to protect components from excess current. I've blown‑up a few of these while trying to retrofit new LCD's on my synths when I crossed some wires. Replacement is easy but you will need to solder these in place very carefully. FUSIBLE_RESISTOR Unlike a normal resistor, these are easily destroyed if you keep the soldering tip on it for more than 3 seconds. This fusible resistor is at silkscreen location R116 on the JV‑880 Main PCB and at silkscreen location R35 on the U‑220 Main PCB. This special replacement part is the same for both synths. It's a 1/2 Watt, 8.2 Ohm fusible resistor (Vishay P/N: NFR25H0008208JA500) and is available at mouser.com

If you replace this component, take extra note and observe how Roland installed it! This one sits far above the PCB, about 25mm. This is on purpose so that if this component does blow, it won't burn anything else on the PCB from the excess heat it generates during failure

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Of all the broken gear I've found on eBay, I'm able to fix about 90% of everything I find. It's usually a simple fix. If you have a synth with no power or intermittent problems after it warms up, it's probably due to the fact that back in the 1980's Roland and other synth makers used sub‑par solder and/or not enough solder to hold components in place. After 30+ years, the solder begins to break down and hairline fractures appear. This occurs at a higher rate for components which generate a lot of heat like Bridge Rectifiers, Power Transistors, large Electrolytic Capacitors, power input jacks and audio sockets which get a lot of use. The best approach is to use a magnifying glass and carefully inspect the backside of the circuit board. If you spot any suspect areas, re‑flow a hefty amount of NEW solder

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Safety Precautions and Disclaimer
Modifications made to any factory stock equipment will always pose an element of risk. Sometimes mistakes are made which are irreversible. Improper soldering and handling of electricity can cause serious injury and damage the synthesizer. Use caution when handling static sensitive devices and the PCB. Make sure you are properly grounded, working on a static‑free workbench or table and wearing eye protection during any soldering tasks. The author is not responsible for any damage or injury resulting from this DIY info. Use this DIY information at your own risk. And, I can't stress enough, the importance of wearing eye protection while soldering. That stuff flies everywhere sometimes!

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