Finally tested my Midibox FM

[napierzaza]

I finally got time again to work on my Midibox FM. Last time I left off I had a switching power supply that I have now done away with. I'm now using a 9v AC supply going through the onboard 7805 and I have also setup a 12v linear circuit with another transformer. I loaded the test tone application and connected it to a cheap mixer I have and I got a VERY loud signal. Seems very maxed out but every channel works fine.

HOWEVER a burning smell began emmenating from my MIDIBOX while I was testing which made me immediately turn off and unplug absolutely everything. I opened the case and inspected, nothing visibly burnt but the top-right amp was particularly hot! Nothing else in the case was significantly hot and there is nothing burning.

Is this smell dangerous or common? COuld it be how my MIDIBOX is hooked up to the mixer (ie the amp is so high it's feeding back)? What else should I inspect?

Considering the tone was continuous the entire time I assume the burning was NOT the system falling apart in any appreciable way.

Please let me know, as I am unsure how to proceed!

[/tilted/]

Are your 0v lines connected? ie, by using two seperate psus, you may have two seperate 0v lines, one for 5v, a second for +/- 12v. You'll need these to be connected, otherwise your rails will be out.

[napierzaza]

Ov lines? As in the middle connection in a transformer? I don't think my 5 v has one, but the 12v does. I'm not connecting it to anything. What should I connection the middle wire of my transformer to?

[napierzaza]

Oh wait, you mean the grounds. On closer inspection I may not have! This is due to having a different setup with my original PSU and changing it with this one. I'll solder on the interconnection tonight. This was bad for which parts? I don't think I broke anything but the smell is rather suspicious.

[napierzaza]

Okay, added the wire from the 12v supply to GND of the Core board. No smell and the volume seems fine. However, after a few minutes running the LCD started flickering. It had just formatted my banksticks. I turned off the unit and all the regulators were very hot (12- 12+ 5+) which is strange. I hadn't played a note at all but the 12v regs were hot!

I have a 9vac adaptor (1000mA) and a 300mA for the 12 supply. Is this not enough?

PS: The actual transformer for the 12v supply is hot too. Very strange

EDIT: Actually even though the tone program is gone, it was still making the tone! What's the deal with that? My LCD say's Rhodes piano and I assume it wouldn't even make noise unless I asked it to...

[/tilted/]

It sounds as though you have a short or low ohm somewhere after your regulators. The supplies you specified should be adequate to your needs (although the 12v might be close), so I would suggest you re-check your voltages.

Disconnect everything from your core (except the PSU), and check for the correct +5v.

Then re-connect one module at a time, and re-check your voltage each time.

If your voltages suddenly drop with the addition of a particular module, then check and re-check that module, then move on once you have restored the correct voltage.

I'm also curious, you mention a 300mA 12v supply... Is that regulated? And if so, are you trying to regulate an already regulated supply? that would cause some heating, as the reg struggles if it doesn't have a little surplus...

[napierzaza]

It's not a 12v supply. It's a center tapped 12v supply that I'm only using the outside contacts in a regulation circuit. Somewhere in this forum it was suggested so I bought a Cana-kit for a 12 regulated supply and got the transformer too. I'll look for shorts. Tricky, hope it's not in my FM module as it's not connectorized.

[NorthernLightX]

It's not a 12v supply. It's a center tapped 12v supply that I'm only using the outside contacts in a regulation circuit. Somewhere in this forum it was suggested so I bought a Cana-kit for a 12 regulated supply and got the transformer too. I'll look for shorts. Tricky, hope it's not in my FM module as it's not connectorized.

AFAIK you need a center tapped 24v supply to get +12v GND and -12v. Or a 2x12v and join 2 wires to create a center tap. Sounds like you're running of a -6 +6 PSU, don't know if that's even possible, but can't be a good thing.

[napierzaza]

This is the kit I bought.

As you can see it's two wire input and one transformer. I can't explain how this works but there it is.

http://www.canakit.net/Default.asp?Contents=/Include/Template/MenuCat.asp&Main=http://www.canakit.net/Contents/Items/CK342V12.asp

[bugfight]

This is the kit I bought.

As you can see it's two wire input and one transformer. I can't explain how this works but there it is.

http://www.canakit.net/Default.asp?Contents=/Include/Template/MenuCat.asp&Main=http://www.canakit.net/Contents/Items/CK342V12.asp

2 things:

1) this is a half wave psu (this is why center tap is left unconnected), so make sure you are not drawing more than rated current.

2) common gnd should be connected to the 0 terminal, not - (maybe you already did this)

hope this helps...

[napierzaza]

2 things:

1) this is a half wave psu (this is why center tap is left unconnected), so make sure you are not drawing more than rated current.

2) common gnd should be connected to the 0 terminal, not - (maybe you already did this)

1) Half wave PSU? What does that really mean? Is there a way to make it work full wave? Why do I have to make sure I'm not drawing more current than I would already? And how?

2) Yes, don't worry about that. ;-)

[fluke]

1) Half wave PSU? What does that really mean? Is there a way to make it work full wave? Why do I have to make sure I'm not drawing more current than I would already? And how?

Half wave means that only 1 half of the AC waveform is being used.  See http://en.wikipedia.org/wiki/Rectifier#Half-wave_rectification for a diagram and more details.  You've got a "dual wave" power supply, which is 2 half wave power supplies, one taking the positive half of the wave and one taking the negative half.  Each supply can only draw half the rated current of the transformer.  It's also good to make sure that each half is drawing approximately the same current.

[napierzaza]

Oh, so it's dual wave than?

How would I go about measuring this?

I'm starting to think this isn't going to work for some time...

[AndrewMartens]

Yeah, I actually built my Canakit with the recommended transformer... and...

Well, the +12V worked fine.  -12V?  Nope, nothing.  Build an identical circuit with some larger caps and a fresh 7912... still nothing on the -12V.  The transformer seems to be sending an appropriate voltage, but it's just not working right for me.  Tried a different rectification scheme, and that didn't do the trick either.  I experimented quite a bit and never managed to get it working.

Then I got laid off, went on vacation, found a new job, and have been busy working on things around the house since then.  My "AC power supply test board" is still sitting on the shelf.  On the plus side, if I ever get it working, I have a very nice switch with a built-in LED that will look snazzy on a 1U FM box :-)

Let me know when yours is working!

[napierzaza]

Oh, I'm guessing I got this kit based on your recommendation!

ARgh! I was hoping to solve my problems by buying this kit. But it looks like I just wasted my time/money.

[/tilted/]

Wait. Don't panic. You can still save things here.

Option 1: You can attach heatsinks to the regulators. (This might be a good idea anyway)

Option 2: You can modify the circuit you have, to be full wave rectified, BUT... You may need a new transformer.

If you have a 12v Centre Tap, you need to get a new one. If you have a 24v Centre Tap, all is well.

Then you pull out the diodes, and use a bridge rectifier component. (there is another slightly messy option)

Then connect the AC inputs of your bridge rectifier, to the outermost taps of the transformer (ie, not the centre tap)

Then connect the rectified outputs (+ and -) of the bridge, to the board you have, at the approriate locations.

Also connect the centre tap of your transformer to the (A2) point on the board shown.

Please excuse my awful MSPaint work!!

[TheAncientOne]

I think this may need a 12 - 0 - 12 or 15 - 0 - 15 transformer, with the centre tap connected to the 0V terminal.

Without that, it may well behave as a single ended 12v.

Never test a supply on a MIDIbox - put a resistors, say 100/120 Ohm, 2 watt rated, between each rail and 0V and measure test before risking you precious MIDIbox.

[rutgerv]

I think you (and other people that gave their advice on the transformer) should take a look again at the link to the PSU kit. I noticed that the website mentions:

Supply voltage: 18V AC / 200mA

It also mentions the use 7812 and 7912 regulators. These regulators need at least a bit more (!) than 12V to do their job (that is: regulating the DC power to keep it around +12V and -12V).

I think that using a 2x12V (this is the same as a 24V center tapped) transformer will not give you enough potential for a stable PSU. I'd rather go for a 2x15V (30V center tapped). If you have a hard time finding this one you could also use a 2x18V transformer but this will result in much more heat on your regulators.

I would also recommend to buy a transformer that can deliver a bit more current than you actually need. This improves the performance of the transfomer.

Greetings,

Rutger

ps. I found this site to be very usefull in PSU design issues:

http://sound.westhost.com/power-supplies.htm

[napierzaza]

Wait, are you guys implying I fried my Midibox. *Sigh*

I'm tearing out the PSU now and will check and see if I can determine the transformers voltage and what I can do with it in its present state.

[napierzaza]

Tilted. it looks like you actually haven't modifed that picture, and it's super small. Could you post a more clear one? I screwed up with my PSU because I just ordered what was recommended. Looks like it's just a 12v center tap. I'm trying to find a new transformer with a center tap but it's not easy.

Okay, well I most certainly destroyed the op-amps. Is there anything else that would have been affected by this? I suppose it could have burned out the op-amp and then burned out the DACs ?

I am trying to measure the voltage for the op-amp supply lines. I put my multimeter on the - and + and I get no reading at all. Is there any reason for this?

[NorthernLightX]

I think you (and other people that gave their advice on the transformer) should take a look again at the link to the PSU kit. I noticed that the website mentions:

Supply voltage: 18V AC / 200mA

It also mentions the use 7812 and 7912 regulators. These regulators need at least a bit more (!) than 12V to do their job (that is: regulating the DC power to keep it around +12V and -12V).

I think that using a 2x12V (this is the same as a 24V center tapped) transformer will not give you enough potential for a stable PSU. I'd rather go for a 2x15V (30V center tapped). If you have a hard time finding this one you could also use a 2x18V transformer but this will result in much more heat on your regulators.

I would also recommend to buy a transformer that can deliver a bit more current than you actually need. This improves the performance of the transfomer.

Greetings,

Rutger

ps. I found this site to be very usefull in PSU design issues:

http://sound.westhost.com/power-supplies.htm

12vac transformers deliver more than enough to drive 7812's, because the voltage gets multiplied by 1,4 when rectified. I made a +12 / -12 regulated PSU for midibox, just look it up in my WIKI, any and all comments are welcome (in the thread linked in the wiki article please, let's keep the topics clean :) ).

[/tilted/]

Tilted. it looks like you actually haven't modifed that picture, and it's super small.

I did modify it, but the mod itself is minimal.

"all" you need to do is remove the diodes, then connect your bridge rectifier where the diodes were (looking at the picture close up, you connect to the right-most diode pins, which connect to the inputs of the regulators, not the left-most pins, which connect to each other!!)

yes. The picture is very small. It seems the "free" image host re-sized it for me...

[stryd_one]

No need to host elsewhere ... You can attach images to your post, or if it is something which may need to be kept for documentation, I will be happy to upload it to the wiki for you :)

[napierzaza]

Okay, well considering I can't really seem to test the voltages I'll just pop some new amps in and see if they fizz.