Panasonic SA-BTT270 Power Supply Fault

These notes are for qualified and a well-seasoned engineers and definitely not for faint-hearted hobbyists because there are high voltages involved in the switched-mode power supply unit (SMPSU). Even I never work with a powered SMPSU board due to the high voltages involved, and all the component testing is done cold without any power. The trail began with the obvious fuse on the mains power board being blown and requiring replacement. Unfortunately, after replacing the fuse, the unit still did not switch ON. After some RFI filtering coils, the mains AC (black and red wires) goes directly into the bridge rectifier. in the UK, we use 240 V and that is rectified to DC. The large electrolytic capacitor then smoothes the DC voltage. Hence as you can imagine, there are going to be extremely high voltages present in this board and you should send this unit to the service centre for qualified engineers to repair it.

In this SMPSU design, the chopping is done by IC5701 (C5HACYY00007), which now a days, they call switching regulator. As you can see, this component is hidden behind the small heatsink on the RHS. The SMD resistors R5727 and R5726 are connected to the source side and they carry the bulk of the current. As you can see, capacitor C5747 is connected across the drain and source pins. This is a very critical component because it is effectively across the switching power rails; hence it would need to be able to handle high voltages. Consequently, it has a rating of 1000 V. For the replacements, I chose high-voltage blue-coloured ceramic disc with a voltage rating of 3-kV (3000 V).

C5747

Heat marks around C5747 indicated activity in this region.

As you can see in the photograph, the original C5747 had failed completely. It had hollowed away, and there was a tiny hole from the underside developing.

Capacitor C5747 is rated 3300 pF (332/3.3 nF) 1000 V and it had gone open. The service manual indicates the following description.

• C5747 F1B3A332A008 3300 pF -- 1000 V

Turning over the board, I discovered these heat marks.

The heat spots corrospond to burnt resistors.

Here is the heat mark. It is not as bad as it looks though because I was able to completely wipe it away when cleaning.

R5726 and R5727

R5726 and R5727 are current-sense SMD resistors with a rating of 0.15 Ω and 1-watt. Although the service manual shows both resistors are required, in actuality my unit had only R5726, and R5727 was not present. Obviously, two resistors in parallel allow more power handling capacity, than one alone.

• R5726 ERJ1TRSJR15U 0.15 Ω -- 1 W
• R5727 ERJ1TRSJR15U 0.15 Ω -- 1 W

These are such low values that essentially they behave as slow burning fuses, hence this type of damage may have happened slowly over many months. Usually, amplifiers draw more current at high volumes, and eventually down the chain, the weakest point, where these resistors are, has to give way. Once these resistors go open the unit goes dead as there is no power going into the SMPSU circuitry.

K5720

K5720 is a 0 Ω link, which also had signs of heat damage. According to the service manual the part number and description is following. This was open and therefore needed replacing.

• K5720 ERJ6GEY0R00V 0 Ω 1/8W 0805

R5732

R5732 is a 220 Ω resistor, was open and also needed replacing. According to the service manual, the part number and description is as follows.

• R5732 ERJ6GEYJ221V 220-Ω 1/8W 0805

Conclusion

After checking all the peripheral components for shorts, I did not find anything that would account for those resistors to heat and burn. If there was a short or excessive load to the power rails then it was temporary and no longer present, hence I decided to replace the missing/burnt components to see if it came back to life again. This article continues to the next page called "Repair"...

This Article Continues...