| Electronics > Repair |
| Class D Subwoofer Amplifier Repair |
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| geoffagain:
C209 was crapped out open circuit replaced with a good cap but no joy. I may visit some of the other caps as this one didn't show any visual signs of failure |
| Harry_22:
Anode ZD201 is connected to pin 4 of IC. Yestrday you measured anode D210 to pin 4 and resistance was 0. Ok let us solder ZD201 back and pull D210. After connect the device to mains and check voltage on Zener. |
| geoffagain:
Apologies It's likely I made a mistake between ZD201 and D210 For clarity we have D210 cathode to pin 4 - 69.5Kohm D210 anode to pin 4 - 9.6Kohm ZD201 cathode to pin 4 - 9.6Kohm ZD201 cathode to pin 4 - 0ohm Pulling D210 and checking zener voltage on ZD201 Rises to 14.93v |
| Harry_22:
No need to apologize it's also my mistake. If the resistance was zero then there would be no voltage on Zener. I could notice right away. I forgot to warn you that in non-working SMPS the high tension remains on capacitors for a very long time. In your case two caps are connected in series and loaded with resistors to equalize potentials. Just like in valve amps. It turns out that 195uF is discharged through 360k. Time constant R*C=70s. In a time of 3*70s the caps will be discharged by 95% to a safe value. Please wait at least 3 minutes before touching SMPS ! Now we will work with SMPS disconnected from mains. Let's use your 1k resistor and bench supply. Please connect it via a 1k resistor to 1 pin Vcc of the IC. Ground to pin 4 (pls see the picture). Voltmeter between pins 1 and 4 of the IC. Gradually increase the voltage and monitor following stages: 1. Voltage on Vcc will rise until startup point ( between 10-12V according to datasheet). The current through IC is very small and Vcc is equal to the voltage on the bench supply. 2. Startup point. You will see 1.7V difference between PS and Vcc. The controller begins to draw 1,8mA current which creates a drop across the resistor. 3. At 15,4V Vcc growth will stop as with Zener. Because IC has its own Zener inside. |
| geoffagain:
Good advice on caps - if not for me then for anyone else coming across this thread. Can't get a stable voltage when connected to my bench supply, its not happy in the slightest. I've got current limiter set at 100mA so unless its sinking more than that I have no idea what is happening. Should be plenty of headroom for 20mA (20v @ 1kohm) plus 1.8mA from the controller. Bench supply is probably crappy, its hardly a reference device - not clamping voltage down quickly enough or hitting current limiter or both ..etc Resistance between pin 1 and 4 is 1.12 KOhm |
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