Troubleshooting my 30W solid state amp

[slimjohnny]

I am having a big issue with the solid state guitar amp i built. The pre amplifier is from the dean markley k-20x and the power amp is the TDA2040 30W amplifier. I am reading 15.5 to 16 volts dc on the positive speaker output and -15.5 to -16 volts dc on the negative speaker output (those are my power supply voltages).
I have desoldered the jumper wire that connects the pre amplifier and power amplifier so i know the issue is there. I have also checked every single resistor and capacitor in the power amplifier and checked my board design again.
I noticed only the positive side TDA2040 appears to short and heat up and when i switched it for a new one and turned the amp on i was able to see the voltage quickly rise to those 15.5 volts on the oscilloscope(then it just stays on that value). Also when i desolder the shorted TDA only pins 4 and 5 (output and V+) check for continuity(these pins measure about 30M ohm on a brand new chip), and the rest of the pins seem okay. The negative side TDA2040 seems okay too.

I attached the power amp schematic below.

Id love to hear your opinion on this. This is for a final year project and im getting desperate.
I start to regret not having done a little arduino toy like the rest of the class instead of a real eletronics project.

P.S- I did not put any isolation between the chips and the heatsinks because I didnt think that would be a problem. Please tell me if it is. The two heatsinks are separate from each other and arent touching anything else.

Double P.S- Im using a 12-0-12V 80VA transformer with a BR356L rectifier and two 2200uF/25V capacitors as my power supply

[antonis]

I don't think heatsink isolation could be your issue..

Chip tab is connected to pin 3 (-Vs) so you could even place them on the same heatsink feeding it with -Vs and leave their respective pins "open"..

Some pics of your layout should be helpful..

[Rob Strand]

I'd set-up a way to test it like this,
- remove the speaker to prevent damage to the speaker and ampliers
- lift one leg of R7, that way both amps are pretty much operating independently and identically
-  place 5W resistors in series with the power rails (22 ohms to 47 ohms); one positive and one negative.
   If possible use separate resistors to each amplifier.
   These limit the current.

Check the DC voltage at the supply pins and at the output.

[iainpunk]

are you aware that the heat sink tab is also connected to the V- pin? you shouldn't connect the heatsink to ground

[anotherjim]

It's as though C4 & C5 are short-circuited.
If C4 was bad and pin2 pulled toward 0v, the "+" amp output will hit the +supply.
If C5 was also bad, the feed via R7 will pull the "-" amp pin2 toward +supply and drive the output to the -supply.
"Bad" may just be that the caps are fitted reverse polarity or have an insufficient voltage rating. I would fit 50v or 63v caps.

[antonis]

I'm with Jim..

Although it should be weird enough for both at once C4 & C5 been shorted, a layout clearance issue might be the cause..

[Rob Strand]

Although it should be weird enough for both at once C4 & C5 been shorted, a layout clearance issue might be the cause..

That's what I thought as well but the OP states:

I have also checked every single resistor and capacitor in the power amplifier and checked my board design again.

The thing is, this is a bipolar supply amp.  Even if the caps were shorted the DC voltage at the output shouldn't exceed the DC input offset x gain.     That works out to be less than 1.5V when the caps are shorted.

I suspect the chips are fried (or are fakes.  Fakes don't explain the opposite polarity.)   Isolating the amps and measuring the DC voltages is the only way to show they are fried.

Not connecting all the pins, like the supply pins, would also cause trouble.

[anotherjim]

Ok, what if the circuit inputs were referenced to -supply instead of 0v?

[Rob Strand]

Ok, what if the circuit inputs were referenced to -supply instead of 0v?

If it was done consistently on both channels you would expect the output of both amps to swing in the same direction.  However,  the > 18V supply exceeds the maximum input voltage of +/-15V.  If that took out the input stage then the amps could do anything.

Nothing quite adds-up unless the  IC is wired incorrectly or the chip is fried- then it could do anything.   Flipping the inverting/non-inverting inputs might cause a Schmitt latch-up; perhaps with the help of some imbalance at power-up the two amps could go to different states.

[antonis]

I'm not aware of TDA2040 internal circuitry but maybe an exaggereted combination of what Iain (grounded heatsinks w no -Vs isolation) and Jim (shorted C4 & C5 caps) said could be the cause of the problem..

[slimjohnny]

Thank you so much for all the answers. I have been very busy with school and I didnt test C4 and C5 for leakage yet( I had already tested the values and they were  up to spec)

The positive TDA is indeed shorted out (pins 4 and 5 check for continuity, and they give about 30M ohm on a good chip). I had switched that TDA once, knowing that it would probably short again, and I was able to see the voltage rise on the oscilloscope as the chip heated up. I just wanted to know whats causing this. The negative side TDA seems okay though and doesnt heat up.

I'll test C4 and C5 today, and i'll probably just replace them for good measure, even if theyre good.

Jim, the inputs are referenced to 0V and Rob, I did buy them on ebay for cheap but I cant be sure if thats affecting it. And honestly I wasnt aware that could be an issue.

PS. I'll isolate the chips from the heatsinks (which arent connected to ground or anything else) as well and use a lightbulb connected to the transformer to test it because I really cant fry any more chips. Thank you so much for all the help

[antonis]

I really cant fry any more chips.

                  G                          E
But, "These chips are made for fryin'
               G                              E
        And that's just what they'll do..", or not..??    

[slimjohnny]

hahasjhah, i guess so

[Rob Strand]

The positive TDA is indeed shorted out (pins 4 and 5 check for continuity, and they give about 30M ohm on a good chip). I had switched that TDA once, knowing that it would probably short again, and I was able to see the voltage rise on the oscilloscope as the chip heated up. I just wanted to know whats causing this. The negative side TDA seems okay though and doesnt heat up.

First off, if you haven't replace the negative amp you should.   That might be the thing that triggers the  positive side to blow.

Sometimes with these cheap ebay chips they don't meet full specs.   If at power-up the negative speaker output is stuck negative it could cause the positive output to be pulled low.   That could increase voltage across the positive amp's positive side output transistor (between pins 4 and 5).   If the output device isn't up to spec it could fry the transistor.   The TDA2040 supposed to have "safe operating area" protection but if the output devices are weak that won't save them.

One point to note about output transistors is they can fry at quite low currents when they are operating near their voltage limits.   They don't need a shorts or heavy currents.

What you should do is lift R7 so you can test the two amps separately.  If you can get the positive side amp working on its own, ie. with the negative side speaker (or dummy load) connected to 0V.  Leaving R7 open, see if you can get the negative amp working as a single amplifier; as an identical configuration.   You will need to add a cap and feed a signal into the + input, just like the positive side.   Make sure the DC voltage at the outputs is pretty much 0V.  The whole purpose of this to prove the amplifiers are working.   If you get that far and you can drive the amps to full output you know the chips *can* at least work.   If you don't get that far you have bigger problems, like the chips could be really dodgy and there's no point continuing with them.

The problem then is to work out why the devices fry when you use the final bridge connection.   Perhaps the chips are in fact weak and won't handle the bridge connection.   The only work around I see here is to try reducing the supply voltage.   One way to do that would be to add say three power diodes in series between the rectifiers and the filter caps on *both* positive and negative rails.    What that does is lessen the voltage seen by the output transistors.   It also reduces power but with weak devices you have no choice.    If that works you then have the dilemma that you know the devices are weak and will probably fry some time in the future.

It's definitely worth checking the electrolytic caps.   Bad caps won't help.

[slimjohnny]

Forgot to keep you guys posted, it was the TDAs, they were fake. I bought new ones from a trustable source and as soon as i replaced them the amp started working.
Im really happy with the finished product and hope the professors will be too. The evaluation is in a few days.

Thank you all.

Heres some pics

[Rob Strand]

Im really happy with the finished product and hope the professors will be too. The evaluation is in a few days.

Good luck.   It looks pretty good.

[antonis]

Hint (for evaluation purpose only):
Place bridge rectifier on any metal part making firm contact with chassis..