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PCX2400 repair
PCX2400 repair
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Old 29th July 2017, 02:19 PM   #1
vincegironda is offline vincegironda
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Default PCX2400 repair

I have a PCX2400 showing dead short between all three legs of a bank of A1303's, I pulled one A1303 from the board and the 3 terminals on the board still show a dead short.

Q23,Q240,Q20,Q36,Q27 are all showing dead short.

I checked the power supply transistors for a dead short and they all appear fine.

Any thoughts on what's causing the dead short?
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Old 29th July 2017, 02:25 PM   #2
vincegironda is offline vincegironda
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The amp had powered up previously, but was warming up on the defective channel so I shut it down. Now it wont power up without blowing the inline fuse.
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Old 29th July 2017, 03:43 PM   #3
Perry Babin is offline Perry Babin  United States
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Without base resistors, the pads of all of the parallel transistors are going to read the same (except for the small resistance from the emitter resistors).
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Old 29th July 2017, 04:22 PM   #4
vincegironda is offline vincegironda
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Can the power supply transistors in the defective channel be tested in situ or should I pull them to see if the amp powers on?
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Old 29th July 2017, 05:24 PM   #5
Perry Babin is offline Perry Babin  United States
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With the transistors clamped down, place the meter probes on the emitter resistors of the transistors that show short and power up the amp just long enough to get a voltage reading. Power up through a limiter.

One output will have a higher voltage across the emitter resistor. That
's the shorted output.
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Old 30th July 2017, 01:44 PM   #6
vincegironda is offline vincegironda
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So I ended up pulling all the power supply transistors and no change, soo I pulled all the output transistors and it turned out Q25 was the only output shirting across all 3 legs.
So I powered it back up and r42 and r43 went up on smoke?
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Old 30th July 2017, 09:59 PM   #7
Perry Babin is offline Perry Babin  United States
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Did you have any other components out of the circuit?
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Old 30th July 2017, 10:18 PM   #8
vincegironda is offline vincegironda
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Yes a couple of diodes, I had lifted a couple of legs as they were showing dead short, I resoldered them and the resistors were ok
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Old 31st July 2017, 08:26 AM   #9
vincegironda is offline vincegironda
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The amp is pulling around 6.5 amps at idle, the bank of output devices opposite the blown A1303's on the other side is warming up quickly and I noticed the amp pushes the speaker cone up even with no audio?

The amp has been repaired previously, looks like whoever repaired it only replaced a couple of output devices and power supply transistors on both channels.
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Old 31st July 2017, 06:59 PM   #10
Perry Babin is offline Perry Babin  United States
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Do you have shunts on the 2-pin bias headers?
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