My New PL400

Z

Zach C.

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Southwest Indiana
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I picked up a PL400 Friday evening from a gentleman who is more or less local to me. It's had a rough way to go, but the hot stamping on the face plate is in good shape, and I can clean up the rest with a little elbow grease.

The gentleman I bought it from thought the right channel had a blown output transistor since it was pegging out the meter and was slamming against the negative rail anytime it was powered up. When I asked him if it was blowing fuses, he said nope, just the stuff I already mentioned. He said he hadn't been using that channel when it went out- only the left, and had the input shorted when he noticed the meter pegged out one day, so he took it out of service.

Turns out, the body count is:
C6- shorted
R25 Open
Q5 Shorted C-E

It was Q5 that was making it latch up, but I'm not sure I understand why.

A few caps, resistor and a small signal transistor later and it's up and running. It's gonna need a lot more loving before it's up to snuff, but it's working. Yay!

Am I the only one who feels the need to proclaim, "IT'S ALIVE!" Frankenstein style when they get a busted piece going? The rest of the refurb is fun too, but it's the initial resurrection that I really enjoy.

Zach


Zach'sPL400_Front.jpgZach'sPL400_Rear.jpg
 
Zach C. said:
I picked up a PL400 Friday evening from a gentleman who is more or less local to me. It's had a rough way to go, but the hot stamping on the face plate is in good shape, and I can clean up the rest with a little elbow grease.

The gentleman I bought it from thought the right channel had a blown output transistor since it was pegging out the meter and was slamming against the negative rail anytime it was powered up. When I asked him if it was blowing fuses, he said nope, just the stuff I already mentioned. He said he hadn't been using that channel when it went out- only the left, and had the input shorted when he noticed the meter pegged out one day, so he took it out of service.

Turns out, the body count is:
C6- shorted
R25 Open
Q5 Shorted C-E

It was Q5 that was making it latch up, but I'm not sure I understand why.

A few caps, resistor and a small signal transistor later and it's up and running. It's gonna need a lot more loving before it's up to snuff, but it's working. Yay!

Am I the only one who feels the need to proclaim, "IT'S ALIVE!" Frankenstein style when they get a busted piece going? The rest of the refurb is fun too, but it's the initial resurrection that I really enjoy.

Zach


View attachment 11473View attachment 11474
Click to expand...

Glad to see another nice amp back in comission- kudos!
 
Congratulations, Zach. It looks like an early model PL 400 (I learned that from Joe) similar to the PL 400 I had on loan from Doug. I always like to see these beasts repaired rather than parted out.

Nando.
 
Zach---"It's Alive" is a pretty standard exclamation around here. To keep it from latching up again, put the Gepetto mod on the leads of C6 and change out C6 to 470@25 or 50 volt.
 
Love how the power strip controls the test equipment!! NICE!!
 
Elite-ist said:
Congratulations, Zach. It looks like an early model PL 400 (I learned that from Joe) similar to the PL 400 I had on loan from Doug. I always like to see these beasts repaired rather than parted out.

Nando.
Click to expand...

Thanks,

Serial #2334

I couldn't really make out what that meant time-wise.

Zach
 
laatsch55 said:
Zach---"It's Alive" is a pretty standard exclamation around here. To keep it from latching up again, put the Gepetto mod on the leads of C6 and change out C6 to 470@25 or 50 volt.
Click to expand...

Thanks,

I had that cap hanging out from my 700B repair. I stocked up before I got it up and running since all of the board parts are cheap.

I did the Gepetto mod on the right channel as part of diagnosis, but to no avail. Both sides will get it tidied up later.

Zach
 
laatsch55 said:
Love how the power strip controls the test equipment!! NICE!!
Click to expand...

$5 at the goodwill. Those things are everywhere if you look.

It really is pretty convenient. Keeps me from reaching over projects to get to my DBT, which I stashed at the back of the bench to keep it safe.

Zach
 
Zach C. said:
Thanks,

Serial #2334

I couldn't really make out what that meant time-wise.

Zach
Click to expand...

Without even looking at the serial #, it was the 4-fin heat sink that proved it was an early model run. My mind is like a steel trap for such PL trivia.

Nando
 
Is that like mine Nando, mangles everything that trips it??
 
Zach C. said:
I picked up a PL400 Friday evening from a gentleman who is more or less local to me. It's had a rough way to go, but the hot stamping on the face plate is in good shape, and I can clean up the rest with a little elbow grease.

The gentleman I bought it from thought the right channel had a blown output transistor since it was pegging out the meter and was slamming against the negative rail anytime it was powered up. When I asked him if it was blowing fuses, he said nope, just the stuff I already mentioned. He said he hadn't been using that channel when it went out- only the left, and had the input shorted when he noticed the meter pegged out one day, so he took it out of service.

Turns out, the body count is:
C6- shorted
R25 Open
Q5 Shorted C-E

It was Q5 that was making it latch up, but I'm not sure I understand why.

A few caps, resistor and a small signal transistor later and it's up and running. It's gonna need a lot more loving before it's up to snuff, but it's working. Yay!

Am I the only one who feels the need to proclaim, "IT'S ALIVE!" Frankenstein style when they get a busted piece going? The rest of the refurb is fun too, but it's the initial resurrection that I really enjoy.

Zach


View attachment 11473View attachment 11474
Click to expand...

Q5 is the main Class A amplifier that drives the final output stages. If it shorts, it will pull the output hard to the negative rail and pin the meter (which is only driven on the negative half cycle of the output signal on the PL normal light board).
 
Which begs the question......what happens on the Rev "B" board??
 
laatsch55 said:
Which begs the question......what happens on the Rev "B" board??
Click to expand...

If you mean the Rev B light board with all the components populated, then the meter will peg on either a positive or negative latch up. At least you would have a meter indication that it was frying your speakers if a lead on Q6 broke :-)
 
BTW, Zach, I did not mean that the Gepetto mod would prevent a latchup should Q5 short, it will latch up as Joe has said, the Gepetto mod prevents those unexplained (actually Joe has explained this, check the PLWO archives) to the random latch-up.
 
laatsch55 said:
BTW, Zach, I did not mean that the Gepetto mod would prevent a latchup should Q5 short, it will latch up as Joe has said, the Gepetto mod prevents those unexplained (actually Joe has explained this, check the PLWO archives) to the random latch-up.
Click to expand...

Zach
Those are the latch ups when nothing is broken, the amp just happens to power up the wrong way due to slightly different power sequencing. It was a design problem that was not recognized and fixed.
 
Elite-ist said:
Without even looking at the serial #, it was the 4-fin heat sink that proved it was an early model run. My mind is like a steel trap for such PL trivia.

Nando
Click to expand...

Right on.

I was thinking that the serial # suggested later production.

Guess I didn't quite understand what that doc says.

I'll have to look again.

Zach
 
Gepetto said:
Q5 is the main Class A amplifier that drives the final output stages. If it shorts, it will pull the output hard to the negative rail and pin the meter (which is only driven on the negative half cycle of the output signal on the PL normal light board).
Click to expand...

Thanks for the explanation. I'll have to look at the schematic again to try to see the current flow that's happening when Q5 shorts.

Zach
 
laatsch55 said:
BTW, Zach, I did not mean that the Gepetto mod would prevent a latchup should Q5 short, it will latch up as Joe has said, the Gepetto mod prevents those unexplained (actually Joe has explained this, check the PLWO archives) to the random latch-up.
Click to expand...

I knew that, but it was just a hail Mary kinda thing before I found the real problem. I had the diodes on hand and it's dead simple, so I tried it.

Pretty sure my 700B did the latch up thing one day while I was at work. I cam home and the little lady said it it wasn't working. I replaced a fuse, and she was up and running again. Some day, I'll actually put the white oak board in the damned thing. It's all stuffed and waiting. (Yes, I am retarded, thanks for asking!) I just wanted to listen a bit before I swapped out the boards to see if I could hear a big difference. Now, I'm having a little trouble convincing the wife that the 700B is better. She really likes the looks of the 3300II/ 300II combo that's in there now. I love that girl, but looks over power? Women.

I accused her of blowing the fuse with the volume control. She just might have. :-)

Zach
 
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