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roland 808 drum machine u.s power conversion

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cod65:
greetings! I have a client who wants to convert his euro 240v import roland 808 drum machine to US 120v. I have a service manual and schematic and looking for appropriate transformer(s) to swap. Based on schematic I am looking for 120 primaries, 10vac and 23-0-23 vac secondaries. What I am confused about is current rating. the schematic seems to suggest 150ma, but im not certain. the euro fuses are rated at 250ma, th US fuses at 500ma, and further the schematic notes suggest later versions of this machine use 1/4 watt 10 ohm fusible resistors  in place of 500 ma fuses on the US version. there are no additional mods that i know of, so how can these ratings be so disparate ? Why wouldn't the euro and US version both have 250 ma fuses ? doesn't a 10 ohm 1/4 watt fusible resistor fed 23v blow at sqrrt(.25/10) = 158ma?
 what is an appropriate amperage based on the schematic  below?

Plse help me to find apropos transformer(s)- they cant be higher than about 43mm, so this, and figuring out amp rating has  proved challenging to me to find



Swake:
My opinion is that if you don't know these basic things yet, then you should not offer 'transformer conversion' services to customers because you clearly cannot assess whether what you're doing is safe or not. Build some knowledge and experience first. Offering such services comes with responsibilities.

If you cannot identify suitably sized transformers, you might want to convert to SMPS power supplies as these are generally much smaller. Or move it to an external box.


themadhippy:

--- Quote ---you might want to convert to SMPS power supplies
--- End quote ---
And possible introduce lots of extra drum beats as the switching frequency is picked up by the rest of the circuit. Me id just use an external  120-240v step up transformer,and a little tip when dealing with musos toys ,whilst not applicable here ,watch out for things that are using the mains frequency as the clock source,they tend to be  out of tune if you try running a 50hz device on 60hz.

Fraser:
I second the recommendation by “themadhippy”. It is desirable to use an external step up transformer that converts 110V to 220V. You then avoid any responsibility for changing the configuration of the musical instrument. Being accused (even if unfairly) of causing a problem with your transformer change would be no fun.

Regarding fuse ratings….. you need to consider the purpose of each fuse and what it is actually intended to protect. On the utility mains supply side of things the current rating of the fuse for a 110V configuration is often around twice that of the 220V configuration. This is simple maths and Power = Current x Volts. Half the Volts means twice the Current for the same Power. Fuse ratings are often not an exact science but sensible values should be used. Fuses take a relatively long time to open in an overcurrent situation so are often intended for protection against catastrophic failures where very high levels of over current occur. The fuse ratings on the secondary side of the transformer will not need to be changed as the secondary winding voltages will remain unchanged. In some products, fusible resistors are used in place of fuses and it would appear that this was introduced in this equipment at some point in its product life. It is not essential to change from fuses to fusible resistors however.

The input fuse “F1” is stated as being 0.5A on the 110V version of the equipment. This appears sensible. Changing the secondary fuses from 0.25A to 0.5A,as shown in the provided schematic, makes no sense at all, except maybe for the convenience of using a single fuse rating during manufacturing !

Fraser

CaptDon:
First off, the 240 volt unit only draws half as much line current as the 120 volt model. Basic first day Power/Ohms law knowledge that is why the fuses are different. Second, there is no doubt the Roland 808 DID NOT use an 'off-the-shelf' transformer. It is certainly a custom designed part. IF indeed they used resistors instead of fuses then they MUST be flameproof!!! Buy or build an external 120vac in 240vac out 'transformer in a box' for your friend. That Roland unit is around 50 watts (actually less) and one solution may be to find a 50va rated transformer with dual primaries (120 / 120). The secondary windings are typically 12 / 12. Ignore the secondaries, we won't use those. Wire the primaries in series observing the proper phase relationship. Feed 120 from one end and the center point and get 240 across the far ends of the two primary windings in series. This may be way beyond the safe area of your current skill set?

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