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MB-6582 current draw


jaytee
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Just now, Hawkeye said:

Thanks for measuring! 840ma of 1000ma is too close, as there isn`t even the CS installed... subtract 10% error and it will go into shutdown...

Upgrade the switcher to 1.5 amps... that is also the spec of the 5v rail of the original C64 PSU...

Many greets, Peter

 

Alright, makes sense. What's another $10? ;)

I went back through the Altitude thread and noticed that he's using a 1.5A sVreg in his build (not the Recom part, but an older, obsolete one). I only found one person using the 1A Recom in that thread, and it's not clear how many SIDs they were running or whether their MB SID ever completely worked.

What a power hog!

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Looks good! (Just looked into the datasheet, advertised as a direct 7805 replacement, 1.5 amps, perfect! Also the switching frequency above 300kHz is really good!)

Now you know, why you will always have a special bond with your MB6582! It is just something different than buying brand-name synths from a retailer! :-). I also have to thank you for your inspiration "the 7th sid" as a new songtitle! :cheers:

As a sidenote, i also had power supply worries in my MB6582 build after upgrading to a VFD (which puts more load on the 5V rail), some of my old C64 PSU bricks just would not deliver enough 5V juice, I then built a linear psu with a 78S05 (up to 2 amps on the 5v rail) with a huuuge heatsink. Nowadays, i would also go for a switcher, which is more energy efficient. But, on the other hand, in these cold winter nights, a linear psu as a footwarmer comes in handy! :-)

Hehe!
Many greets,
Peter

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On 1/31/2017 at 0:09 PM, latigid on said:

Also:

If you have the large caps in there, removing them or adding the flyback/Schottky diode could help.

 

:cheers: @latigid on wins the day!

I was reading up on various DC/DC converters for an unrelated project this morning and stumbled on a datasheet that went into more detail about the "max capacitance load" and how it could stop the converter from starting up properly. I recalled your comment, did a little more research, popped C3 off the board and voila! Everything starts up just fine with eight SIDs stuffed. I already ordered that replacement sVreg, and may yet install it, since as Hawkeye points out, I'm already cutting it close without my CS installed, but for now the 1A seems to do the job.

Sooooo, C3. I think all it's doing is filtering noise on the 5V line. With the C64 PSU, it might have provided some protection from voltage spikes, but behind my sVreg, it seems less necessary. Running a quick audio test, it sounds totally silent to my ears. Is there any danger to leaving it out completely if no noise is audible? Analyzing circuits is not my wheelhouse at all, so it'd be good to hear some input on this.

 

(edit: And just to be safe, just went through each stereo output and confirmed that all four cores and all eight SIDs are firing as expected. I'll have to double check the SIDs more closely to make sure there are no blown filters, but the six that didn't come out of my known-working sammichSID don't appear to have ever left Wilba's original packaging, even though I got them elsewhere. Anyway, woohoo! I do believe this counts as a functioning MB-6582!)

Edited by jaytee
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Nice!

To be honest, I don't think large caps do much outside smoothing a rectifier or acting as DC blockers for the audio path. They're a standard addition for any power entry (including things I've designed...) but I recall that their actual effect is negligible. Something to do with a circuit block's reactance? I think the idea is to form a low pass using the power line as a resistor, but if you crunch the numbers they don't add up. Better to implement a series inductor or properly rated resistor at the entry point, and decouple chips locally. The notion of a "reservoir" topping up the supply rail as it drops is nice but I don't think it rings true. 

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Looking over the MB-6582 PCB document and the original MIDIbox schematics, it seems a-ok. The sVreg is wired opposite how the PCB expects it. Because the 5V regulator is hooked into the rest of the circuit correctly via jumper wires, it doesn't really matter, and the only effect is that the input caps are on the output and vice versa. According to the MIDIbox Core schematic, the output of the 5V reg should have a couple smaller caps, while the big cap should go on the input (where there already is a big cap, C1).

I think I'll solder in a smaller cap, like 10uF, which is what the Core schematic indicates. If it works, I'll call it good, and if it doesn't work, I'll take it back out and call it good. (Edit: it works, calling it good.)

Edited by jaytee
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Congrats! :cheers:

You might need the 1.5A switching vreg, when the control surface is installed, if it is quite close to the limits now... There are some LEDs on there, while they are matrix-driven (only a few are "on" at any given point in time), this might add another estimated 50-200mA of current consumption on the 5V rail (depending on the LEDs) ...

Many greets!
Peter

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