PL400 question

Billboard

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#1
I was listening to my PL400 at medium to high volume and the Watts Abundant DC protection kicked in.

Brought the unit to the bench and put it on the DBT, sure enough the 75W bulb did not dim, problem with the right channel.

Removed all of the output transistors and found one shorted and one other testing good, but did not match (previously replaced). With the right output transistors removed I tested the unit on the DBT and the bulb dimmed normally and the DCP relay clicked.

I then took the time to test all of the right channel back plane resisters and diodes, all tested good. I tried the unit on the DBT and the bulb would not dim this time, and there was some smoke, shut things down and saw that one of the empty output transistor sockets was arcing and smoking.Two questions, are these sockets still available? any one have a good used one left over from a Whiteoak back plane conversion?

Also what is a matched set of transistors? Are they the same date code? measured in some way?

So I need a socket and at least six matched output transistors.

Socket.jpg Transistor.jpg
 

laatsch55

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#2
A matched set of outputs is where someone measures gain and batches the same gain together. Buy some MJ21196's for that channel. No need to match the new outputs, they are damn close anyway. What's your address, I'll send ya all the sockets you could ever want...
 

laatsch55

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#4
Don't replace just the blown outputs, replace all the outputs, you've alraedy blown one from rompin on it so those old ones are showing their age....
 

hayabusa

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#7
A matched set of outputs is where someone measures gain and batches the same gain together. Buy some MJ21196's for that channel. No need to match the new outputs, they are damn close anyway. What's your address, I'll send ya all the sockets you could ever want...
I'll second that. I just updated my WOPL 400 with MJ21196 outputs and MJ15024 drivers and it worked perfectly,without doing any matching.
 

Billboard

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#8
Received the transistor socket today, installed socket and 12 new output transistors.

Adjusted bias (had to turn pots down and set both sides to 350mv, checked DC on speaker output terminals (17mv on each side).

DCP relay acting normal.

Ran the amp for an hour or so, everything looks good, time to place the amp back into the system.

Thanks for everyone's help.
 

laatsch55

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#9
Oh damn.....dropped that balL......I apologize for that sir.....
 

Billboard

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#10
While I was in there I secured the 2 main capacitors down with a homemade bracket, and also added some RTV to act as a strain relief on the two bias transistor wire sets.

The meter light board was converted to LEDs, but that is kind of a mess and will be soon getting the WhiteOak LED board.
 
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