Repair and re-use PC speaker amp circuit?

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This circuit is not working, uses 120 V AC but was probably connected to 220 VAC which caused a minor overload.;)

Troubleshooting and fixing should be fun, any suggestions? First get the chip specs and see the input voltage, then connect a 9V batt and see where the power comes up to and where it stops.

Hope the chip is not blown.:bawling:


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I tried out the following:

Cut the AC input wires from the transformer and connected a 9V battery to the terminals.

No output. Some crackling noises when the battery terminals were touched with the wires.

No contact across switch. Removed switch and shorted wires. removed input wires - they looked burned out and shorted. Connected input wires to the bass and treble output. No sound yet.

Maybe it is the diode bridge? Will find output from diode bridge and connect the 9V to that, but with the correct polarity.

Went through this:

Troubleshooting and Repair of Consumer Electronic Equipment

Apparently no output points to a power problem and is relatively easy to fix.
I connected a 9 volt battery to the pins 1 and 12, not thinking about polarity, maybe that put paid to whatever was left of the chip, however I could not get any sound except a crackle when attaching the speakers. As far as I know the input signal was OK.

From the circuit board it looks like the power supply goes directly to the pins, so that would make sense, so a 220 volt supply converted to 24 V ac instead of 12 V AC would have gone directly to the chip and shorted it out.

I will be abandoning this project and looking for a different amplifier board. I plan to take apart this board and re-use the components after checking each one, should be fun.

Maybe I will buy a BA 5406 chip, its cheap.

BA5406 Audio Amplifier IC Free Shipping | eBay
Photo of the circuit board and connections. Some of the labels are visible, some can be deduced from the original wiring.

Test :

1) Connect 9V to the AC in wiring and check if Pin 1 on the BA 5406 receives any power, ie check voltage between pin 1 and 12.

2) If this is OK, then connect an audio source to input and check if there is an output signal on each channel.

"To find out if current is flowing though a circuit or a low-value resistor, turn the project ON and measure the voltage either across the component or the voltage on one end then the other."



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Connected a 9V battery to the purple and blue wires which were originally attached to the switch. Why 9 V battery? Because I do not want to risk burning out any power supplies by shorting the wires. Seems a safe way to test.

Connecting was a bit of a hassle, but 9V showed up all the way up to the Diodes. Voltage across pin 1 and pin 12 of the BA was around 8 volts, so it is receiving some power.

Next step: input / out output signal. Of course this was the original problem.

Will connecting a speaker directly to the output pins cause any problems for the chip?
Some info here:

BA5406 Stereo Amplifier Circuit | audio wiring diagram

C3 and R2 forms a Zobel network for the left output while C6 & R3 forms the same for the correct channel. Purpose of the Zobel network is to cut back oscillations and improve high frequency stability of the amplifier. Potentiometers R5 and R6 serves as the volume control for the left and right channels. CapacitorsC4 and C8 couple the outputs of the amplifier to the speakers. C9 and C10 are noise filtering capacitors. C1 and C5 are bootstrap capacitors for the left and right channels.
OK, more testing and thinking about this:

Testing: Connected a 3.5mm jack to my phone, and connected that to the input wires. Continuity was tested of the 3.5 mm on one channel to the input points the board and was OK. Next connected the output wires to a speaker - nothing, some crackling on one side.

Voltage testing of audio output: With a different chip amp, tested the voltage readings with full volume. Only slight variations on the needle were noted, and seemed to have nothing to do with the music with the speakers connected or without. However there was some signal detected. Testing for output voltages will be difficult then, I suppose. The voltages inside the circuit may be higher though at some points. Further testing confirmed this.

Power on voltage testing: voltages across some of the capacitors was indicated, differing from left and right in one case, where the capacitor was loose.

Resistance testing: different resistances were detected.

Before going any further, what are my realistic options? I do not have much troubleshooting experience, only have an analog VOM and no oscilloscope. However this is interesting:

Option 1:
Draw a circuit diagram and note resistances and voltages across each component, to detect faulty capacitors, etc. Also test continuity across connections to check for bad solder joints.

Option 2: Test the BA using a VOM is this possible?

Option 3: Remove the chip, all components and re-assemble on bread board or similar using the circuits given here:

At least I will have a circuit diagram and components connected in a clear pattern.

Any suggestions? I am determined to fix this even if it takes a month.

Another circuit here:

The objective of this exercise is to learn about electronics, troubleshooting, and to re-use electronic components to prevent e-waste from reaching the landfills.
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Kudos on being determined! This circuit is fairly simple to follow and the components are priced reasonably low. If you let the smoke out of any, you won't cry much.. ;)

Check the values of the components on the board to the values in the circuits you have provided. I would believe they should be close, if not exact..

Option 3 sounds good. It would be easier to follow the circuit and connections. Following traces can be fun if you're not experienced..

Good Luck and keep us updated..
ref Option 3
Removing the chip without damage from excessive heat will be hard (or impossible).
Why not leave the chip on the pcb but connect to breadboard with wires?

Good tip. Just checked the temp of solder:

Most solder melts around 180 to 190 degrees Celsius


That's way above the storage temp of the chip
OK so more testing, this time resistances in circuit

There are about 18 capacitors here in the circuit. I started testing the resistances across the capacitors to check for short circuits or open circuits. These are all in -circuit so I was not likely to get correct values, however I did some checking of left and right channels to see if there was any variation between channels. There was no variation on the two capacitors I tested marked C8 and C1 both showing 100 Ohms. I gave up this line of testing because it was just too difficult to locate the pins corresponding to the capacitors.

I wonder what kind of innovator decided to build the current circuit board layout, with components on one side, labels hidden by components, and connections on the other side. In software engineering this is like having to look at the back of your monitor after running your code. Software troubleshooting is more difficult though, because of the components - custom made by yourself and non- standard, usually. Still, this circuit board thing is not a good user interface. Why not have the components and labels on one side, with the circuit clearly marked? Why use solder at all?

The next step was to test the resistance across the BA5406 IC pins, one probe on the ground and the other one on each pin.

From what I had learned in Physics at college level, the VOM will draw very little current, so it will not short the circuit and cause damage. This line of investigation generated more interesting results.

Pin 12 Ground, resistance when connected to:

Pin 12 Connected to Pin 1 25 Ohms
Pin 12 Connected to Pin 2 15ch
Pin 12 Connected to Pin 3 22
Pin 12 Connected to Pin 4 24
Pin 12 Connected to Pin 5 32
Pin 12 Connected to Pin 6 25
Pin 12 Connected to Pin 7 51
Pin 12 Connected to Pin 8 200
Pin 12 Connected to Pin 9 200
Pin 12 Connected to Pin 10 50
Pin 12 Connected to Pin 11 18

What does this mean? With the symmetry of the circuit, it is unlikely that there should be two 200 Ohm resistances across the pins, one
of which is the IN signal, pin 5 and pin 8 are where the signal goes into the chip. Earlier testing showed that one channel was completely
quiet but the other channel showed some crackling at its output. At least one channel should work, then. And maybe one channel is burned out.

The next step will be to connect everything up and providing an input signal. But what input signal to provide? Ordinary music
barely moves the pointer, even at the amplifier outputs, as tested with another amplifier. I also needed to see if the input
signal could be detected on the meter. To this end I used a 3.5 mm jack with wires soldered to it, and checked the output
from an old mobile phone. Music ( Bee Gees - Staying alive) barely caused any movement, so I decided to try out pink noise
as a source, as there are no quiet moments in pink noise. This worked wonders as I immediately obtained noticeable swings of the needle
from the phone 3.5mm output, at full volume, and with the needle swinging into negative territory. So looks like we have our input signal.

Question is, will any of this damage the circuit or the chip? I plan to probe the input signal terminals and points along the way
for voltage values. I can test the input and output points on the chip and on the board, but not sure what to do in between.

Maybe trace the circuit for voltage values, but I need to identify which components are on the circuit diagram. Testing the output signal
from the amp chip will be able to tell me if the chip is working ok. Again, will this damage the chip?
Your very first sentence made me believe it was the chip.. ;)

"This circuit is not working, uses 120 V AC but was probably connected to 220 VAC which caused a minor overload.;)"

A microsecond of overvoltage will destroy a semiconductor and no fuse can open fast enough to protect one. Perhaps a cap or two was damaged also..?

Remove the chip and plan to replace it. Test all caps for shorts or low resistance. The one I would focus on is the filter cap in the power supply on the board. If no obvious damage is noticed, (burnt up stuff) and no shorted caps, install the new chip and be sure you connect the power to the board in the proper polarity..

Fire it up and listen. If it sounds funky, assume bad caps and replace them all..
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