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Phase Linear 400 Series 1 Repair Help

Sterling245

Sterling245

AK Subscriber
Subscriber
Hi all,

Yes, I know what most of you are thinking, another flame linear and I should probably just go ahead and White Oak modify it, but I am really interested in getting this working in stock form. Mine has the PCB14 driver board.

The problem:
One channel meter is pegged all the time, and has 65v at the speaker output. R24 and R28 get very hot and burn themselves up. Occasionally when powered up the meter won't peg and I will get very distorted output. Other channel works fine. I have only powered this up on a dim bulb tester, never given it full voltage.

What I have done:
Tested outputs on the faulty channel. Found 2 bad ones and changed all output transistors. Tested Q8 and Q9 and found Q8 to be bad. Replaced both. Checked all other transistors and diodes on the board with my meter and everything looked good. Checked the bias transistor as well. Tested filter caps at full working voltage and they are okay. Replaced electrolytics on driver board as many of them were bad.

I am not sure what the next step here is... everything seems to check as it should but I am still getting this issue. I appreciate any help!
 
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Sterling245 said:
In case anyone was curious, the problem turned out to be a bad output transistor socket connection. After pulling and re-seating the transistors, it works as it should.
Click to expand...
And the best part is you didn't create a bigger problem since it was only a socket. Good job- the supposedly easy ones can be the tricky ones.
 
Thanks all! It took me hours of thinking and checking everything multiple times to find it. Definitely a failure I will keep in mind for future work on Phase Linear gear.
 
Did it happen to be one of the driver transistors? I wouldn't expect a single output being disconnected to cause basically rail voltage to appear at the speaker terminal but an open on the driver sure would.

anyway related to those sockets and all that it might be worth doing the power sharing test to confirm everything else is happy. Basically run it into a dummy load and confirm the voltage across each emitter resistor is within whatever % they say in the service manual. That verifies the transistors, but also the connections.
 
gadget73 said:
Did it happen to be one of the driver transistors? I wouldn't expect a single output being disconnected to cause basically rail voltage to appear at the speaker terminal but an open on the driver sure would.

anyway related to those sockets and all that it might be worth doing the power sharing test to confirm everything else is happy. Basically run it into a dummy load and confirm the voltage across each emitter resistor is within whatever % they say in the service manual. That verifies the transistors, but also the connections.
Click to expand...
Could've very well been one of the drivers. I re-seated all of them at once so not sure exactly which one was causing the problem. Will definitely do that test as well.

Still have a few slightly out of spec resistors I want to swap on the driver board as well as some known problematic transistors, and of course it needs new lamps! Definitely not done with this one yet, but very happy to have it working.
 
Sterling245 said:
Hi all,

Yes, I know what most of you are thinking, another flame linear and I should probably just go ahead and White Oak modify it, but I am really interested in getting this working in stock form. Mine has the PCB14 driver board.

The problem:
One channel meter is pegged all the time, and has 65v at the speaker output. R24 and R28 get very hot and burn themselves up. Occasionally when powered up the meter won't peg and I will get very distorted output. Other channel works fine. I have only powered this up on a dim bulb tester, never given it full voltage.

What I have done:
Tested outputs on the faulty channel. Found 2 bad ones and changed all output transistors. Tested Q8 and Q9 and found Q8 to be bad. Replaced both. Checked all other transistors and diodes on the board with my meter and everything looked good. Checked the bias transistor as well. Tested filter caps at full working voltage and they are okay. Replaced electrolytics on driver board as many of them were bad.

I am not sure what the next step here is... everything seems to check as it should but I am still getting this issue. I appreciate any help!
Click to expand...

The fellows at this site are the Phase Linear rebuild experts. Suggest you join there and let them guide you. They helped me rebuild a trashed PL 700B many years ago.

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