PCX2400 repair

[vincegironda]

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?

 

[vincegironda]

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.

 

[Perry Babin]

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).

 

[vincegironda]

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?

 

[Perry Babin]

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.

 

[vincegironda]

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?

 

[Perry Babin]

Did you have any other components out of the circuit?

 

[vincegironda]

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

 

[vincegironda]

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.

 

[Perry Babin]

Do you have shunts on the 2-pin bias headers?

 

[vincegironda]

There are no shunts on the bias headers

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[Perry Babin]

The shunts defeat the bias circuit when servicing the amp. Try installing shunts to see if the idle current drops.

 

[vincegironda]

So I managed to do some further fault finding, and the rsults are as follows

Removed all output transistors and found two shorted Sanken A1303 output transistors

In the right channel I re-installed 2 pairs of output transistors which had tested ok
Powered the amp up and the current draw with remote applied is 6.5amps
The amp produced clean audio
The two A1303's on the right channel are heating up

the left side HIP4081A Fet Driver ( U9 on the board ) is heating up
R46 has burnt out ( I think this is a result of the failing HIP4081A )

I figure U9 will need replacing and may have been the cause of the left channel problems to begin with?
R46 will need replacing.
All the output transistors will need replacing on both channels

any further suggestions before I order the spares would be appreciated?
thanks

 

[Perry Babin]

Install shunts on the headers.

R46 likely failed because the FET it's driving failed.

If so, The 4081 may be heating because it's driving a shorted FET through R46 or because it failed due to the loading of the failed FET.