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March 29, 2024, 05:47:54 AM

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Troubleshooting a 30W guitar amp i built

Started by slimjohnny, April 15, 2020, 05:54:55 AM

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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

phatt

Sorry to hear but you should read the data specs before building these things. 8|
https://www.st.com/resource/en/datasheet/cd00000130.pdf

Pin 3 is the Neg supply pin,, it is also internally connected to the heat tab.
So unless you isolated the heat sink from ground,,, then yep you have a dead short from neg rail to ground.

YES you need to isolate the chip tab with a mica washer.

If you run the chip on *Single supply rail* then Pin 3 is ground and no need for the isolation but with split supply you certainly do need isolation from heat sink.
Phil.

slimjohnny

Thank you Phill, it does have two supply rails and I will isolate it.
But do you think that could be causing this problem?

phatt

Well isolate and check.
The chips are most likely already blown so readings maybe way off even when you fix the dead short.
I'd remove the supply from circuit and check your supply is reading to specs then isolate and Check with your DMM that you don't have a dead short between heat sink and Chip Tab. If all is well,,reconnect circuit and if readings are wrong then I'd say you need new chips. You could get lucky. xP
Phil.