Two years ago I bought a nice gadget - DAB+, Internet , Wi-Fi, FM radio and media player - all collected together in one device. The radio was sold under the trade mark "Medion". This is a German company selling cheap, but according my opinion, not reliable electronics. (From four their products, which i bought, three of them defected short time after the fabric warranty expiration :-). Who knows - may be this is "planned obsolescence" ). Never mind. The radio was performing well during the last two years. By itself the parameters are not so good - the sound is like coming from old tin box, but I did not use the internal amplifier.Its volume was set to "0". I purchased a TA2020 2x20W amplifier on ebay, I connected it to the Linear audio output of the radio, which I supposed is directly coming from the output of the audio DAC. Having two JBL speakers as load for the amplifier brought me pleasure while listening the DAB+ radio transmissions. But each happiness doesn't exist long. One sunny day, during listening of a regional music station, a loud shoot was heard, and the radio became dead! :-(.
Step 1: Failure Analysis ...
I have opened the radio, and I have found that some damages were happened inside. The SMPS (switched mode power supply) module was defected. Few parts, starting with the SMPS chip, were blown up. On the pictures can be seen the big black hole appeared on the chip. I have searched all the damages, checking all the parts by ohmmeter, and I have found that the number of the burned parts is not so big - they were 3 :
- the SMPS chip
- one inductor (shown on the last picture)
- a SMD resistor which was melted
I was not sure about the functionality of the opto-coupler, and I decided to replace also it.
To check the state of the HF transformer, I have to unsolder the SMPS chip, because it was making shorts between the different transformer windings and also affecting the other surrounding the chip devices.
In this moment my main problem was: "Which is this SMPS chip?". It was impossible to read its mark....There are a lot of SMPS chips, which have absolutely different pining. Mission impossible !!!.
But, in this moment the luck came back to me again. From the radio case, fell down on the table, the blown away plastic piece of the SMPS chip package, on which was written :"PR6203DP". New surprise... Research, continuing hours in internet, did not give me any information who is the producer of the chip, and where its datasheet could be found. Another black hole! I was hopping, that finding the datasheet, I could see the typical chip connection circuit, and what surrounding devices are used. Nope...
I contacted the manufacturer of the radio, and kindly asked them for the schematic.We had long conversation, but they were always refusing to supply the circuit ( For me this was strange - I think that each electronics equipment manufacturer should deliver, together with the product, its electrical circuit. That was normally the case few years ago.. For example, I have the full schematic of my TV "Grundig", which I bought 7-8 years ago.... ) Their explanations were, that the schematics is production secret, and should not be distributed. Very naive... their PCB's are mainly single sided and for experienced electronics engineer, the reverse engineering, could be matter of minutes....
Step 2: What to Do Now?!?
I was quite curious, and I did some additional research in internet. There I have found the following:
The Japanese company "Sanyo" is producing a digital radio model R227, which looks exactly like the "Medion"'s one. (see the pictures). This could be some explanation about the missing circuit diagram and the secrecy. I have contacted "Sanyo" support, asking them for their schematics, having in mind, that practically this is the same product. For me seems unbelievable, that if the external view of a device is fully identical with another one, something internally could be different. May be only the software....The "Sanyo" support, or what is remaining now from it ("Sanyo" was acquired by "Panasonic"), did not answer. I understand them - now they have other problems.
In this complicated situation, I have decided, to go to reverse engineering by myself, and to try to extract the schematic of the radio SMPS.
Step 3: The Reverse Design...
As I said, the reverse engineering was not difficult. The result can be seen on the picture. I suppose that I did some small errors in the part values, or I did not write it ( it is very difficult to define the value of SMD capacitor, for example, without precise laboratory equipment...:-) ). It can be seen that the schematic is simple, with small number of devices. The biggest efforts were given to filter well the output voltage, what is nice approach, and to try to reduce the EMI noise, induced in the AC power network. It can be seen that three different ground planes are used. Full galvanic isolation is implemented by the use of the opto-coupler device. This approach gave me faith, that the main electronics of the radio is not affected by the explosion. But I have to be sure. I wanted to supply the radio with external power supply module, and to be sure, that I am right. Here the main problem was, that I did not know, what is the generated by the SMPS module supply. Because the radio is digital, and it has a lot of microcontroller and other chips, and sometimes, they allow only 3.3V supply and the radio by itself does not have powerful output stage, I thought that the supply should be around 5-6V. I have found the 3.3 LDO, what told me that minimum 4.5 V should be required as input. The way to calculate the correct power supply was to look again at the circuit. There, as feedback error amplifier is used the chip KA431. I have found in internet that it can be programmed to switch at a given voltage, defined by two resistors. In this case R313, R316. The formula is the following:
V= 2.5V* (1+ (5.1+0.12)/4.7), where 2.5 is internally precise voltage generated by the chip.
This calculation gave me supply voltage value of ~ 5.3V.
I have applied 5V supply voltage by external source between the ground plane and the power supply output of the SMPS block and the radio started up. All functions were present.
Now were possible two different ways :
- to supply the radio using external power source and to use it in this way totally discarding the defected SMPS module.
- to try to repair the SMPS module, and in the case of unsuccessful results, to go for the first variant
Step 4: The Repair Works ....
The main task was now to find this exotic chip. In the beginning, I thought that it can be a copy of the most used chips, designed by Motorola, STI or NXP. I did not find any of them having the same pining.
Again the luck came to me. I have found that in the Chinese sites like Aliexpress this chip is offered, but in quite big batches. I have contacted around 20-30 sellers, offering the chip for the datasheet, but none of them was able to find it for me. This was very surprising for me - I was not able to understand how is possible to sell electronic components, without delivering the supporting them documentation. Finally, I decided to risk. I bought a batch of 10 pieces, for the price of about 10 USD.
After two weeks of waiting, the chips arrived home. To make my life easier in the future, I have soldered an IC socket. If the chip explodes some day, it will be easy to insert the new one. I have soldered the 1mH inductance.
Tha main problem was, that I did not know the value of R303. I looked in the internet for similar filters, but I did not find any methodology to calculate the value of the resistor. In the predicting its value, my thoughts were the following:
- Very small value is useless - it will short the 1mH inductance.
-Very high value is also useless - no current will flow through it.
- May be its resistance shall be identical to the impedance of the inductance at the frequency of the SMPS chip switching (not known - no datasheet found...or exist)
Finally I put 1.5K resistance....
Here, I would ask you all guys : if you have at home the same machine, or the "Sanyo" R227, could you check this value, to insert it in the circuit for the further possible use of it for repair works by whole the community of DIY hobists.
Step 5: Final Checks...
After soldering of all replacement parts (4 - additionally, I have replaced also the opto-coupler), I wanted to test the repaired SMPS block. I was not sure, how the block would work, and what the produced voltage should be. It could burn the whole radio. To test the module, I interrupted the current path from the SMPS module to the radio. I have unsoldered one of the filtering beads terminal (L304). To load the module I have soldered 10 Ohm 5W resistor at the output of the module. I have inserted the AC plug and supplied the module. It was quiet.... :-). I have measured the voltage over the load - it was 5.25V. I have recovered the all connections on the PCB and supplied the radio again.
It was functioning as before....
Step 6: The Pleasure of the Music ....
Having DAB+ radio, with nice amplifier and speakers, make the life brighter....