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Old 30th August 2010, 04:55 PM   #1
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Default RF 200a4

Hello,
First time post. I recently came across a 200a4 with the top upper 2 left mosfets blown all other components (visually) look fine. I found suitable replacements but the thing still will not turn on. Should I replace the other 6 fets on that side (Left side of pic)? Any help or tips would be appreciated.
Thanks
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Old 30th August 2010, 08:18 PM   #2
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You should replace the two on the upper right. The others would not need to be replaced unless defective.

Is the remote terminal loose in the board?
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Old 30th August 2010, 08:26 PM   #3
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The remote turn on terminal is fine. I will give the upper 2 on the right side a try as well. I have the schematic for this particular amp I was going to look at it when I have the board in front of me.
thanks
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Old 30th August 2010, 09:39 PM   #4
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Post the DC voltage on all 16 pins of the TL494. Place the black meter probe on the amplifier's ground terminal. Place the red meter probe on the point where you need to measure the voltage.

IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
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Old 31st August 2010, 10:00 PM   #5
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Here they are as best as I could measure:
pin 1 = .05
pin 2 = 4.51
3 = .07
4 = .1
5 = 1.51
6 = 3.37
7 = 0
8 = 11.68
9 = 11.09
10 = 7.03
11 = 11.68
12 = 11.68
13 = 4.93
14 = 4.93
15 = 4.93
16 = 0
I believe I started at the correct pin there was no notch on my chip to designate the top. If these voltages don't make sense let me know.
thanks
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Old 1st September 2010, 12:18 AM   #6
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I used part number STP40NF10 N-channel mosfet to replace those 2 blown fets. I believe the specs are similar. I have seen people mention th irfz44n but have you heard of using the fet I used? Thanks again.
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Old 1st September 2010, 03:50 AM   #7
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I've never seen that FET used in the power supply. It will probably work but there are better options.

Did you measure the voltage on the IC with the FETs in or out of the circuit?
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Old 1st September 2010, 09:52 AM   #8
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I took the measurements with the two blown fets replaced (Q19, Q20). Q9 and Q10 are still the orig BUZ103's.
thanks
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Old 1st September 2010, 03:54 PM   #9
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With only remote and ground connected (no B+), measure the DC voltage on ins 9 and 10 again.
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