SmashTV Core + GM5 = Low Voltage

[MONSTA]

Hey everyone,

I bought an entire Midibox64 kit from smashTV, built it up and tried testing the voltage of the core with a GM5 supplying the power and I'm only getting 1.4v (the GM5 is ouputting about 5v).

With my first kit from Mikes Midi Shop I had exactly the same problem, but could get around this by going straight from the GM5 to J2 on the core. The SmashTV core however doesn't have the J2 pins, anyone have any idea how I can get this all connected up?

Thanks for any help.

Monsta

[Fozzy The Bear]

The SmashTV core however doesn't have the J2 pins, anyone have any idea how I can get this all connected up?

The SmashTV core certainly does have the J2 Pins..... I just had a look at mine... You'll find them just below the small electrolytic capacitor at the left hand side of the board (assuming you have it oriented so that the power input is at the top).

Also... From Smash's Web Site: "J2 +5V output, can be used to supply other core modules in a multiprocessor environment, so that you only have to mount the parts for the power supply (BR1, IC3, C5, C6) on one core module. Restriction: the 7805 gets very hot when it delivers currents above 500 mA, so only core modules without backlit display like MIDIO128, MIDImerger, MIDIfilter should be supplied over this port."

you really should be powering the core via its own rectified power input using an AC or DC PSU running at between 6V and 9V and putting out at least 500ma I run mine on a 1A supply. You really don't want to be powering the core via J2... It's not designed to do that. I'm not sure what effect that may have on the voltage regulator circuit, probably not a good one!

Your best way of connecting this is to take a feed from a separate PSU to the core... Using the power that is coming from the USB connection to the GM5 is a BAD IDEA! a lot of USB connectors on PC's can't handle much above 150ma let alone the levels you're now trying to pull off there (which is why you're getting a huge voltage drop). By the time you start connecting other stuff to your Midibox64 you're going to be pulling way more than you're allowed to.

Hope that helps...

Best Regards,

Julian (Fozzy The Bear)

Edited April 14, 2010 by Fozzy The Bear

[nILS]

The 1.4V you got are easily explained - You're feeding 5V DC into the bridge rectifier of the core module - diodes cause a voltage drop - then the voltage <5V goes into the 7805 which needs ~7-10V to work properly. What you are doing can't work.

J2 can be used to supply power to a core module (if you know the voltage is constantly really nice 5V, polarity is right etc.) that's really all that bad at all - if you know what you're doing (the "no J2" comment makes me kinda doubt that a little bit :whistle: ). But I do agree that powering the core via J2 from USB power from a gm5 is bad juju :) With a gm5 and a core + some peripheral (there's going to be more than just the core, right?) you'll easily get over the max. current a USB port can deliver. And that can do anything from simply not work to blow up your maiboard.

-

[MONSTA]

With a gm5 and a core + some peripheral (there's going to be more than just the core, right?) you'll easily get over the max. current a USB port can deliver. And that can do anything from simply not work to blow up your maiboard.

Hey guys,

Nils, no I don't know what I'm doing :) I've managed to wing most of my electronics projects, but I'm not a circuits genius by any means!

I can't believe you can't power everything off the gm5! I did my research and people were saying that as long as you weren't using LCD's etc it would be fine. Take a look at the attached image to see what I'm building, there is a CORE, 2xDOUT, 2xDIN and 1xAIN in each one + the GM5.

I guess it's not really a massive issue having to use an external power source (there's no room to build one inside), but is this definate that what I'm trying to do with the GM5 is completly misguided and not going to work?

Thanks

Monsta

[nILS]

Well... I see (among other things) 9 * 4 = 36 LEDs. With the default 220 Ohm resistors you're looking at roughly 15mA per LED. 36 * 15 = 540mA. To bring the total current consumption of the the LEDs to a reasonable level of say 72mA you'd need 1.5k resistors, which will dim the LEDs to the point of them barely being visible (if they work at all @ 2mA)...

[Fozzy The Bear]

Well... I see (among other things) 9 * 4 = 36 LEDs. With the default 220 Ohm resistors you're looking at roughly 15mA per LED. 36 * 15 = 540mA.

As nILS said, you're just pulling too much of the "go go juice" Amps.... There are two ways to tackle this problem.

As we both said to you, pulling the power from the USB on the GM5 is risking a lot of things. You could very easily damage your computer motherboard that way.

separate power supply to the core is the answer... or if you really have a very stable clean 5V DC supply then you could also power the DOUT boards directly and sever the power connection between the DOUT and the CORE while leaving the data lines in place. BUT either way, you still need a separate power supply, and the easiest way is to power the core properly via its normal power input and let the voltage regulator circuit on the core take care of the power being clean.

Best Regards,

Julian (Fozzy The Bear)

Edited April 14, 2010 by Fozzy The Bear

[MONSTA]

Ok, thanks guys, I hadn't really anticipated this. I guess I looked at the akai thing and read a few half related posts on here and jumped to conclusions.

So I can test with a wallwart I have now, which i have used for testing a different core in the past. But idealy I'd just want to have an iec/kettle plug port on the back that'll connect straight into a psu then into the j1 etc.

Can I build one up based on that c64 optimised schematic I've seen around?

[Fozzy The Bear]

So I can test with a wallwart I have now, which i have used for testing a different core in the past. But idealy I'd just want to have an iec/kettle plug port on the back that'll connect straight into a psu then into the j1 etc.

You could do what I did, and strip a wallwart down (dump the case and wall pins), connect the Pos, Neutral and Ground of its circuit board to an IEC Chassis socket, and connect the output to the core, and mount the whole lot into your case. That way you get a really small neat PSU inside your case, that can supply more than enough power. Make SURE that that you know what you're doing when it comes to mains voltages though (all at your own risk and choice if you follow advice given) If you're not sure about anything when it comes to mains voltages, find somebody who is to give you a hand with it.

I've not seen the schematic for the Commodore supply on here, so I can't answer that one.

Best Regards,

Julian (Fozzy The Bear)

Edited April 15, 2010 by Fozzy The Bear