Electronics > Repair
roland 808 drum machine u.s power conversion
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cod65:

--- Quote from: Swake on January 08, 2025, 07:09:02 am ---Choosing transformers and fuses are 2 different things. While there will obviously be a relation as both are connected to the same circuit, you will find very quickly that there is not a one rule fits all. The particularities of the circuit to be protected and powered as well as its operating conditions are important to consider when choosing either.

A short insight in how to choose a transformer: https://tameson.com/pages/transformer-calculator.

Fuses: https://www.digikey.com/en/articles/fuse-tutorial

Note that the schematic shows primary side and secondary side fuses. It's is a good thing, actually a must, to have both as these have different goals. Protecting the transformer vs protecting the circuit and of course both are also helping avoid your house burning down which is the often forgotten main reason why they have to be implemented.

The table specifies 250 mA AT fuses. Note the 'T' that comes from 'träge', the German word for 'slow' -> they want you to use a slow blow fuse.

--- End quote ---


thanks again, informative articles.

Regarding in-rush current and this design: seems like it's not a concern of the designers with a .250(EU) or .500(US) ma fuse?  I also note that the N.American fuses of .500 in table make mention of a n SGA designation, of which I have not found any info as of yet. All the examples Ive seen so far seem to be common glass fuses or fusible resitors.

I'm  wondering if the later production numbers(of which this appears to be one) switched to fusible resistors in place of the secondary glass fuses because they were having problems with surge/transients? they are effectively 'slo-bow' by design are they not ?


This guy works on a N. America rebuild that contains the  fusible resistors at F2,F3,F4 (although the original schematic I included from the manual at top of thread notes only F2-F3  ... that makes less sense to me and wonder if its a Roland  typo - why would F4 not also be a fusible resistor like this video ? (also I note that his resistors don't look particularly special - I wouldn't know if these were actually 'fusible resistors' by visual, myself)

https://youtu.be/e6lGCrhv1kE?si=KL_HkdkyXIy_o8fE&t=127


Here's another thing I've noted with this particular 808 power supply - safety capacitor C1 is entirely absent. I read that its purpose is primarily to stop this machine from putting noise back out into the local mains and show up on other devices nearby- that it will function fine without it (but it seems desirable for that reason alone) - also I read that it will help protect against transients,lightning ,etc.  But I read that it actually magnifies inrush current ?   

The particular device is listed as ECQ-UC1A safety capacitor (looks like it says 473mfd, but I'm assuming that is .473mfd because nothing like that seems to exist then or now)

is something like this a modern equivalent, say .68mfd? Is this necessary/advantageous or should I just leave circuit as is,  without ? How will this affect the fuses ability to withstand any surges or spikes?

https://www.digikey.com/en/product-highlight/p/panasonic/ecq-ua-series



Swake:
A fuse before a transformer is nearly always slow blow. 220V x 0.25A is 65W, that is like 5 times more than the summed rating of both transformers. -> They took care of inrush.


The switch to fusible resistors probably was because a pick and place machine could place them automatically, much more difficult with glass fuses and that at the end it was cheaper buying one resistor than a holder + a fuse. If you have normal glass fuses, keep it like that, good enough and easier to replace. + you can very easily measure current with a multimeter in that spot.


While there are some particularly good educational video's out there, always consider that Youtube is a source of entertainment :popcorn: The resistors in your linked video are unlikely to be fusible resistors. That really is not going to help for anything else than heating.


Safety caps. Yeah that's another nerds topic  ;D not a must have to make it work and test the device but a must have to respect the safety standards.
Over simplified: The fuse is about 'current'. The safety cap is about 'voltage' and 'noise on the line'. My personal practice in this specific case would be to ignore it (2 classic transformers are not going to make a lot of noise) or if that instrument is of particular value to add a pre-made inline EMI filter module.

Note that it is written 473MC and 473MF. When there is a number on a cap like this, it is expressed in picofarad. in this case 47000 picofarad or 47 nF. The M probably stands for a certain type (I don't know which one, at that time the currently used classification did not exist yet I believe) and the C and F probably stands for the voltage rating. The modern equivalent of what you need is an X2 (or X1 or Y2 or Y1 as these are all ok in this case) at a voltage rating of lets say 250VAC or higher. in short 47nF/250VAC/X2.

You wrote 473mfd. And that is not the same. mfd markings on older capacitors most of the time mean microfarad, note that it is written in all lower case but is sometimes written in all upper case. The use of the 'm' here is extra confusing of course, therefor I always insist on using only the official SI abbreviations : m = milli ; µ = micro ; n = nano; p = pico.




The other delicate part is that the placement of the dot. for some reason they prefer writing things like .47 over 470n and this is also prone to confusion. Have seen many times the dot was missing.
cod65:
So I got the transformers in there, put on variac and iso tranformer, measured the voltages and put everything back together and its running well. I didn't use slow blow fuses but none of them have popped (yet). I was pricing slowblo at digikey and they were like 7$ each and up. I'm glad I didn't put them in at first because i had mistakenly plugged the 3rd party midi card ribbon cable in wrong and it caused a short and blew F2 a couple times before I figured out what was going on .  Now all is well.

Hers some pics of my layout for any critique or suggestions that might be gleaned from a visual.

I don't like that these transformers have primaries on bottom instead of top,  so close to chassis, but with shrink tubing maybe that is a good thing.
the transformers  have lock washers but i might put a dab of thread lock on the nuts as well
Swake:
Good job. Seems like you could reuse the same holes in the chassis, that is nice and clean.

If you want the heat shrink to fit more tightly around the cable on the plug you could add a short piece of smaller gauge heat shrink first just on the cable to make it 'ticker'. But like it is now it is good enough.

Those stickers on the trans are weird. I wouldn't be able to tell you if it is 36V or 2x36V while in fact it is 2x18V.

For 7$ you can but an assortment of fuses.... here an example; https://www.aliexpress.com/item/1005004114014014.html
Don't let you blow away by those telling things like "you have to buy these fuses from a reputable source" before you have read or watched real live test results.
Haenk:
Nicely done, if it wasn't for the "Made in China" and the date code, the modification would pass as "factory", IMHO.
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