SammichSID screen backlight faliure

[mop]

So, I finished my sammichSID last night!! And I was just thinking the screen was a little dim - I had the brightness trimpot at about 12 o'clock - and so I turned it up a little. I turned it to about 3 o'clock and it worked well for about 5 seconds, then i heard a small quite pop and saw some smoke - so I turned it off straight away! I opened it up to look for an signs of damage and saw none, so foolishly I turned it on again (bad idea) and it worked ok for a short time and then the screen backlight went out. so now I am sad.

I assume there is something wrong with some capacitors that do something for the backlight power circuit, but I am a bit out of my depth. The kit was entirely finished except no SIDs were installed. Any idea where to start looking for the problem? the screen I have is the (high power?) green backlit screen (the default kit option) and I am using a 12V 1000mA power supply.

Edited August 2, 2010 by mop

[Wilba]

To start with, an unregulated 12V 1000mA supply is going to output way too much voltage when the current load is low. So check what the actual supply is... is it regulated or unregulated 12V DC? Or is it AC?

It's important to know so we can work out what happened. I strongly recommend you do not use this supply again unless you can confirm it is outputting regulated 12V. Test voltage on the supply's plug, if you can.

Where did you see the smoke? Left side, right side, front side?

My best guess is you burned out the backlight LEDs by supplying too much current, which can happen when supplying with an unregulated power supply that has a high current rating (1000mA), using the "high-power" backlight option (which supplies the backlight directly with unregulated power), and cranking up the brightness.... essentially supplying 15V+ to a circuit that expects 10.5V and 12V maximum.

However, it's also possible that the pop/smoke could be a blown capacitor, perhaps the big blue 2200uF electrolytic, which has a 16V rating.

Get yourself a new power supply. 12V AC 500mA or 12V DC 500mA... or ideally, REGULATED 12V DC (500mA+).

And don't worry... if it still powered on, and LCD shows text, the voltage regulators are still good, it might only require a new LCD and at worst a new 2200uF cap.

[mop]

Thanks for the help. I thought the power supply I was using was regulated and that it would be fine to give that much current, but that may not be the case. I will probably go and buy a different power supply.

I really have no idea where the damage is. I saw a very small amount of smoke from under the control panel board. I didn't realize that the screen could be overloaded, I'll have to watch out for that. I noticed during testing that the areas that should have been at 12V were at ~14V, but I thought that was because there was no load over the circuit.

[nILS]

I noticed during testing that the areas that should have been at 12V were at ~14V, but I thought that was because there was no load over the circuit.

...and that is because your PSU is not regulated. If it were, it would _always_ output 12V, not some other voltage depending on the load.

[coryjrogers]

So I had the same experience as MOP. Installed everything. Voltage checks across the base PCB all fine and stable 12.3V for the JBL right pin. I then put the JBL shunt in for 12V and the J4 assuming I had a high-powered LCD screen. My PSU is labeled 12VDC 1.25A Regulated. But like MOP I turned the brightness clockwise and heard a small pop and saw smoke.

I tried taking the J4 shunt out and tried again, more smoke. I moved JPL to 5v, screen lights up, contrast works but brigthness doesn't.

I think the smoke and pop came from the left side of the board.

Is the 1.25A of my PSU too high?

Cory

[technobreath]

Just some general stuff:

Is the 1.25A of my PSU too high?

No. Current is not something that you squeeze into the circuit. you can use a 100 amp psu if u want to, the circuit won't demand more than it wants anyway... So says Ohm.

I then put the JBL shunt in for 12V and the J4 assuming I had a high-powered LCD screen.

Are u sure it's 12V?

I tried taking the J4 shunt out and tried again, more smoke.

Not surprising if u had magic smoke release in the first place :D.

I moved JPL to 5v, screen lights up, contrast works but brigthness doesn't.

Again, one would assume that u broke something when there has been magic smoke release...

OK, seriously, I don't have very much spesific knowledge about the circuits of the sammichSID. But the thing is, if you wire everything correctly in the first place - and make sure the voltages are right - not assuming, but making sure, it will work without magic smoke.

If voltages are too high at some component in there, it will damage it, and if u attemt to test more without correcting the first error, it will most likely cause more errors.

So back to the first question, if the powersupply is too powerfull. Now that magic smoke release is a fact, simply swapping the powersupply won't fix the circuits that are welded or burned off... - as I said, swapping powersupply to another amp rating is not gonna do nothing in the first place at all, as long as the amp rating actually is high enough - if it is lower than required, regulation will fail or become unstable. Not enough current supplied = not high enough voltage.

This is not magic :) but very logical, and if you study ohm's law, you will see why. As I said before, if u make sure 100% that everything is correctly set up in the first place u save a lot of time - if something fails, it will be 10 times the job to locate and correct the error. :)

[nILS]

Magic smoke is what makes the components work. If you let it out the components are dead. Some components on your sammichSID are now dead. Congratulations.

Do not power it up again until you know what's wrong.

Remove all ICs (SIDs, PIC, opto coupler, eeproms).

Remove the CS PCB.

Look for any traces of broken components (which left, btw? left as in west with the connectors facing north? oh and on which board? CS or vase?) or smoke residue. Find anything? Replace the part.

When all that is done and looks good, check ALL voltages again as described in the build guide. All 5V good? Voltage for the SIDs good?

Check the components around the backlight circuit next, make sure you got the right values in the right places (R3, R4, R4A, P1, T1).

Report back.

And remember: If there's smoke, don't start guessing and power things up again. There's been smoke for a reason.

[Gilesjuk]

Sounds like the golden rule for fuses. If a fuse pops, don't replace it until you know why it popped.

[coryjrogers]

When all that is done and looks good, check ALL voltages again as described in the build guide. All 5V good? Voltage for the SIDs good?

Check the components around the backlight circuit next, make sure you got the right values in the right places (R3, R4, R4A, P1, T1).

All Voltages on Base PCB fine.

What should I be measuring at T1 and P1? Voltage or Resistance?

I got a new PSU. The 12VDC 1000mA Regulated one Wilba links to and I'm waiting on a new LCD screen.