mbSID alternative PSU and expert advice needed

[sineSurfer]

Hi everyone, as my limited electronics knowledge comes from the web and basically I'm just learning as I go, some expert advice would be apreciated.

I found in my trasure chest an old Atari 5200 psu labeled as DC 11.5V at 1.5A, the multimeter shows 14V out of it so I figured it could serve as a c64 psu substitute for my mbSid about to be built.

And after some tests and failures following the optimized c64 schematic I got a modified version that outputs 5.35v and 13.65v.

Are this voltages safe for 1 core and 2 sid modules and 1dinx4?

I did the schematic and board on Eagle just to be sure (probably i'll build it on veroboard only), can anyone check if its all right, or any possible shortcomings I may be facing going this way? I would like to be as sure as possible before commiting to this solution.

thanks in advance!

[warland]

The 13.5 Vdc will most likely shorten the life of the SID considerably. Also SID's

generally run quite warm to hot so the higher voltage will not help. See the attached

schematic as to how I would do it. C1 should have a 25v rating. C6,7 = 16v rating.

Keep the capacitors C2,3,4 and 5 as close as possible to the regulators.

Hope this helps

[sineSurfer]

thanks a lot for that schematic warland, I had a talk with smashTv and now i'm not convinced about using the Atari psu after all, but will see when I finish with all the soldering on the modules.

btw. I forgot to tell i'm using smashTv's modules, Core already has 7805 and SID a 7812(for the 6581 sid) so I guess whatever extra volts there will be handled by the regulators on the modules, is this right?

maybe the 13.5v is too low to drive the 7812 on the Sid module?

I'm wondering if leaving duplicated regulators would give me even less voltages:

7812 and 7805 on the input stage plus the 7812 and 7805 on the modules

or should I skip the regulators and leave only the filter caps to filter the noise?

thanks for the help!

[dj3nk]

yes, duplicating your regulator will not make any sence, because your 1st 7805 gives out 5V. The second one need MORE than 5V (at least ~7.5) to work. Then I am not sure if it won't blow your 7805, because you input voltage could be to high. 9V are quite perfect. The more Voltage you drive on it, the hotter it will get until its blown. There are lots of examples in the forum how to build a SID suitable psu. Try to search for them. The cons and pros of some ideas are listed in the threads.

You could try S.TH. LIKE this: 

14V input =======> 6581 pcbs

              \\

                =====> 7809 ===> core input

dont forget the caps and use a GOOD psu. NO SWITCHING PSU FOR AUDIOCIRCUITS !!!

greetz

[sineSurfer]

Hi dj3nk... yes, I been reading thru almost every psu related thread here in the forum.

Well, so far it looks like the atari psu won't do the job for both voltages, it drops to 9v when loaded,  i'll have to use the c64 psu until I can find or build a suitable replacement ;)

[sineSurfer]

Update:

In the end my C64 psu 9AC part died, then I tried to open it, wasy task but biiiig surprise inside the psu enclosure is filled with some hard substance, like epoxy glue, including the broken fussible >:(

So I changed plans once more and bought a transformer labeled as 18v/500mA AC with central tap, multimeter reads 10v AC from 1 of the outer wires + central tap or 20v AC from the 2 outer wires, no central tap.

I went with the first option and the weird thing is after the rectifier the multimeter reads 14.5 DC volts!? which turns out to be enough to feed the mbSID module from smash with the 6581 chip so I just added a 7805 to get the 5v for the core and everything worked fine and without any noticeable noise, of course I added the caps too. so the only noise is from the 6581 chip, but that one goes away with a gate:)

But I'm wondering if I'm not pushing too much the 7805 with the 14v? spec sheet says it will take it... but it gets hot, should I get the biggest heatsink I can find?

is possible for the 7805 to pass the full 14v if it gets damaged?

a fuse between the 5v wires and the core can help to protect the core?

[yogi]

"I went with the first option and the weird thing is after the rectifier the multimeter reads 14.5 DC volts!? " This is 100% norm, you will always have a higher DC then the input AC from a bridge rect. You are converting both the pos going and neg going halfs of the sign wave to a pos DC output.

"But I'm wondering if I'm not pushing too much the 7805 with the 14v?" you will need a heat sink, the 7805 is 'wasting' 10V as heat. A 7805 can handle this, but in the long run heat will kill it so it does need a heat sink; use heat sink grease too.

"is possible for the 7805 to pass the full 14v if it gets damaged?" Yes and No, if the reg 'melts' down it can fail as a short but a dead short will burn its self out to a open in short order. The usual fail mode is an open (no output). The best placement for the fuse is before the reg., it will blow before the rest of the PSU if a short happens.

;-) Yogi

[sineSurfer]

thanks for the so informative answers yogi!!!, next psu will be easier ;D

btw, going this way I got the parts count to be the same as on the optimized c64 psu schematics(only the reg. is different), and it seems like a good replacement for the c64 psu, only time will tell :) i'll post some pictures later.

[bugfight]

...

you will always have a higher DC then the input AC from a bridge rect. You are converting both the pos going and neg going halfs of the sign wave to a pos DC output.

...

that description is not exact, there are no positive and negative halfs.  dc is higher because the transformer is rate at RMS volts (peak * 0.707 for a sine wave), and after rectifying and filtering you wind up very close to peak voltage.  Note that transformers are also rated for voltage at the rated current, with voltage rising as current drops.  So expect that 10vac to drop close to 9vac when current rises to full load.