Newbie needs help with class D amp (Blaupunkt THA475) repair

[Mote]

I accidentally bought three of these Blaupunkt THA 475 amps. I did not notice that they are class D amps. If I had noticed it I would not have bought them because I have no knowledge or experience repairing class D amps. I will try to make the most of it and hopefully learn something from this.
Anyway the amp looks like this:


I did some reading on the internet and tried to guess the functions of the components on the board. My conclusions are on the next two photos:



I guess the power supply functions in a similar way to class AB? amplifiers, which I have been trying to repair so far.
It uses SG2525AP chip which is pulse width modulator, similar to TL494.
Power supply has 4 transistors: IRF3205, 4 rectifiers: F12C20A.
I found also voltage regulators from the board, have not noticed those in other amps. They replace zeners?

Big rectangular thing must be transformer, although I have never seen one like this.

Audio filters section looks similar to any other amp. But on this one they use surface mount components. That is to keep the noises to minimal possible level?

Then there is a chip which datasheet states it is a oscillator and binary counter/divider. I assume that chip converts audio signal to pulses.

Those pulses are then amplified by two big chips, no idea what they are, no info on them. After that the amplified pulses go through LP filter, which filter audio signal from the pulses. LP filter consists of capacitors and inductors.

This is my understanding of class D amplifier, correct me if I am wrong.

On to repairing the amp: One was in working order, nothing wrong with it.
Second one is damaged beyond repair I believe, board looks like this:


It should look like this:


So I will only try to repair this third amp.
Symptoms are:
Does not properly power up. After applying power, protection light comes on for a few seconds as it is supposed to and goes off. Power LED does not light up entirely. It only flashes and amp makes screeching noises at the same speed as the LED flashes.
I did not find shorted power supply transistors. Voltages on power supply transistors are 1: 0V, 2: 11,78V, 3: 0V. 1st leg should have 3,7 or something like that?
Voltages on power supply chip SG2525AP:
01: 1,21
02: 5,14
03: 0,02
04: 0,64
05: 2,03
06: 3,63
07: 1,98
08: 4,82
09: 6,02
10: 0,97
11: 0,01
12: 0
13: 11,78
14: 0,01
15: 10,38
16: 5,15
When writing this I realised that I had written the numbers on the paper in incorrect order. 01-08 were actually legs 16-09. I correct my mistake but now I dont know if 01-08 are actually 01-08 or 08-01.
Legs 11 and 14 are outputs a and b and they both have 0,01V whereas they should have approximately 3,7V?
1. and 2. legs are for error amps, that is shutting the chip down?
2. leg 5,14V is shutting it down?

On the other side of the board there is a capacitor and text on the board says:"remove to disable autorem". What does that mean?

[jol50]

I can't help you much but look at the transformer closely many class D have a secondary power supply of a higher voltage to control the outputs, those are usually a pair of mosfets/etc. on the sink and could be what you are seeing. If you test a working one they would have voltage higher than the rails, and the components (rectifiers/etc) will be smaller because the current is low.

I had thought these were tripath amps, run with tripath ICs, but I don't know anything about them never worked on one.

[Perry Babin]

On the other side of the board there is a capacitor and text on the board says:"remove to disable autorem". What does that mean?

That's a PTC thermistor. When it's removed from the circuit, the sense circuit on the speaker level input is disabled.

The gates won't have any voltage if the amp is in protect mode.

The driver ICs are TDA8924s.

[Mote]

I guess its impossible to do much on this one without a scope?
Can I remove recrifiers to see whether it will power up without them?

Perry, does your book cover class D amps as well?
How much does shipping to Estonia cost?

[Perry Babin]

It depends on the fault/symptoms.

Yes.

Email me if you have questions about the tutorial.
babin_perry@yahoo.com

[jeanious2011]

I highly recommend Perry's tutorial. Also he is a great help (dont think I need to state that since everyone seems to know). But he helped me fix a few of my amps.

Clean the board first before judging, all that black sludge can probably be cleaned off easily and then you can see if the board has any damage to it.

[luka]

audio amp ic's are tda8924

[Mote]

That board is toast. There is pretty much a hole in the board in the center of the burnt place.

Perry has been a big time help to me, his patience to ask some of the things three times before I understand what I have to do is amazing. I try to post photos and information about which transistors are used so the threads could be useful to other people as well in the future.

On Estonian forums most people dont bother to reply at all and if they do reply then they something like this: "You are too stupid to repair that, bring it to my shop"
Once I requested schematic for something and a a**hole drew me a picture how the device was repaired by a qualified repairsman (himself).

[luka]

that board damage doesn't look to bad, not a lot of traces burned, and there still is no hole from one side to other... I think you can fix it, probably by removing amps (if both are dead) and other damaged components... then fix supply, if it needs any fixing and then so on, board fix should be one of the first things to do

Amp looks really good, for how much did you get it?

[Mote]

I got a very good deal and paid about 20€ per piece because they were part of a lot. And yes, they look very good.