Electronics tech notes, part IV: repairing sound amplifier and TV

Executive summary

Passed week I have been working on repairing eight more electronic devices and generated for business ~5 000 rubles and much more for our clients including securing investments in organic assets which heavily rely on stable work of electronics which had got broken.

This publication shares some interesting things about how to fix your car's sound amplifier and TV at your home.

Cases

Sounds amplifier

How does it work?

By changing amplitude and frequency of an electrical signal transmitter encode a sound into a plain electrical signal which is recognized and decoded back to the sound by a receiver.

What sounds amplifier does is increase a power of this electrical signal. Often transformers are used to increase a power of an electrical signal as it was described here, but they create a lot of noise which drastically impact a sound wave, so something else should be used.

Transistors are used for this aim. They also increase a power of an electricity, but don't have such drawbacks as transformers. This circuit has several powerful ones which are served for this aim and has a lot of smaller ones which intention is not fully clear for me (likely they are used for setting up base voltage for the rest of the circuit).

This sound amplifier supports four audio dynamics, this circuit has many symmetrical elements to do the same work for each of four dynamics.

An instrument to simplify diagnostics

Diagnostics of such circuit could be complicated as there are ~100 elements, most of them could be in one of three states. In this context you have to check abnormal voltage drops on each segment of this circuit.

Fortunately there is another way suggested by my colleague.

The electrical signal in circuit carries a sound. It means connecting something which is able to decode an electrical signal back to the sound will help up distinguish by ear a sound from a noise.

The instrument is old cheap headphones and a probe connected together over capacitors (0.5 mF or smaller) which is served as a buffer for a sound. You can soldering it on your own and even use a jack adapter which gives you all three pins without cutting wires.

The issue and a solution

Client complained a sound on one of four dynamics is disappeared.

Visual overview didn't give us a clue. The next step was checking key elements of the circuit. A couple of transistors shown signal which was different from expected. Replacement of this transistors should fix the issue.

However when new transistors had arrived from China and got replaced, the issue was still there. Moreover they had been overheated and got broken after the first launch. Usually it means the issue is also somewhere else further in the circuit.

That is where our handmade probe was useful. Going through the circuit with it didn't give much results. Origin transistors from a different channel were used after this to check if this segment of the circuit is valid or not.

The replacement shown the sound is here! It means our circuit is fine and the issue is in defective transistors. It is quite surprising, because by working with electronics you are usually faced with broken circuit, sometimes circuit design issues or device firmware, but not with just ordered electronic elements from a factory! Chinese can do quality products, I have a Lenovo laptop which has been working for me 8+ years, but quality control to sort out defective products is a known issue for many purchasers in China. (it is also worth to note Chinese electronic products often lack of some elements defined on PCB, but replaced by jumpers in reality, they might work without it, but becomes less reliable)

Transistors from another much more powerful device were used instead. When they had been put on place, sound was appeared. However a final test with one more sound dynamic shown the device was turned down in some places of music playing. The investigation shown there was not enough power, replacement of wires from a power supply had solved the issue. Another thing to think about when work with electronics.

TV

How does it work?

Design of LED TVs is a much more sophisticated topic to cover. A good general overview could be found here. The issue with a TV could be in the circuit or firmware where second one is usually more difficult to investigate and repair.

Repairing TV is the most lucrative niche among the rest electronic devices and at the same time is considered as an one of the most complex, not only because of complexity of firmware issues, but also fragility of the matrix which is easily can be broken.

I will go in more details about design of TV in future publications which would be focused on specific part related to the concrete issue.

The issue and a solution

Client complained TV started to respond strangely on buttons presses.

It is a common issue for TV. The root cause is buttons itself. Usually producers put the cheapest ones, with time their contact with the rest of the circuit just stops working. The less expensive way to fix it is just replace them.

Meanwhile visual diagnostics shown a couple of capacitors require a replacement. Also the rest of capacitors used as a signal filters were checked by oscilloscope. In the default mode they show a smooth line of voltage. However a few of them shown a sawtooth signal. It means with time they stopped work properly, without proper work of such filter they quality of the image will suffer. If you notice change in the image quality, likely it is a time to replace some of capacitors in your TV.