The Rebel
Journeyman
- Joined
- Aug 31, 2018
- Messages
- 62
- Tagline
- ---
Howdy from the deep south, US of A. I'm new to the board, so take it easy on me until I start to really screw things up. 
Here's the front story. I just picked up a fairly nice early PL 400 for a song. Guy says one channel didn't work and one meter didn't work. He was scared of it, and obviously needed some cash, so I plucked that, and a nice 1.5t off of him for less than 2 C notes.
I went over to look at the Carver, and found the 400 in his "junk" room. We already had cranked the Carver for 15 minutes before I even thought about looking through all his other stuff and found the PL 400, so I knew the 1.5t was still in good shape. But enough about that, let's talk about this brushed aluminum faced beast with the huge VU meters.
First thing I noticed was the two output fuse caps were gone. I asked if he had them laying around his bench somewhere, and he's like, nah, that's how it was when I got it. I just laughed quietly to myself and handed him a small wad of cash as I walked to my car with the Carver, and him carrying the 400. I didn't want to act too excited about getting it, just act as if it was a pretty boat anchor or a door stop..
So after getting it home, I popped the covers and gave it a good once over and saw nothing burned or out of place so I proceeded. After locating and installing two fuse caps with a couple AGX5 fuses in them, I brought it up using my Variac until I was at 100%. I knew it powered up when I heard the transformer humming. Hummph, I've read about this humming, but never got to the end of what is causing it. That's when I checked DC offset. one channel was -5.9mV, and the other was a bit less, around -9.4mV, can't remember exactly. So I decided to take a chance on hooking it to some test speakers on my bench. After I got it hooked to my C-1 preamp, and the speakers, I didn't hear the hum as loud so I turned the volume up slowly. SOUND came out. Of one speaker. No movement at the meters. So I turned down the volume, played with the RCAs and tried again. Sound out of both speakers now. Great! One RCA input is sloppy, plan to replace those anyway with DCP board in near future. Still no meters, and no lights. But sound in both channels now, both distorted, so I turned it all off and put it back on the bench to dig into it deeper.
At this point I was pretty happy with it so far so I pulled the front, and both covers to get a better look. I found no meter lights, or even wiring for the lights, they were desoldered and left out. From looking at the wiring going to the meters, and from pics of other PL 400 meter wiring, I believe these were wired 180 out, because the wires were actually crossed from the meter board to the meters. I may be wrong about that, all this is new to me with the PL. This will be a good candidate for the LED board upgrade.
I do know it has the factory style aluminum case filter caps still in it with the mounts that screw to the side of the case. This is a 4 fin amp. I popped the covers off of the outputs/drivers and all 16 are Motorola MJ15022 transistors. Is that right? I was expecting to see 14 15024s and 2 of another # here for drivers. They appear to have been replaced from the word MEXICO stamped on them. lol
Now that I'm asking for technical advice on this amp, can I get an idea of why the transformer has a loud hum to it when not hooked to a source, what I can do to settle that, and possibly if I need to check any or all of the transistors before I replace the large caps? What caps are easiest to replace into the factory mounts? I do plan on doing the DCP board and try to listen to this amp before I decide to go all out and start a WO conversion on it. Can I get a little help on what the minimum I should do to give it a few good runs before I make a decision? Thanks for allowing a greenie into your group of excellent techs. I'm not a tech, but I have a few tools, a decent multimeter, a variac, a Weller soldering station, and a decent desoldering gun similar to the Hakko FR300 that works well for me so far. I can solder and pull and replace parts without burning boards(yet). I have the service manual and have gone over the bulletins. I think of myself taking instruction fairly well, but I can prolly be as hard headed as the next guy at times. Thanks again.
Here's the front story. I just picked up a fairly nice early PL 400 for a song. Guy says one channel didn't work and one meter didn't work. He was scared of it, and obviously needed some cash, so I plucked that, and a nice 1.5t off of him for less than 2 C notes.
I went over to look at the Carver, and found the 400 in his "junk" room. We already had cranked the Carver for 15 minutes before I even thought about looking through all his other stuff and found the PL 400, so I knew the 1.5t was still in good shape. But enough about that, let's talk about this brushed aluminum faced beast with the huge VU meters.First thing I noticed was the two output fuse caps were gone. I asked if he had them laying around his bench somewhere, and he's like, nah, that's how it was when I got it. I just laughed quietly to myself and handed him a small wad of cash as I walked to my car with the Carver, and him carrying the 400. I didn't want to act too excited about getting it, just act as if it was a pretty boat anchor or a door stop..
So after getting it home, I popped the covers and gave it a good once over and saw nothing burned or out of place so I proceeded. After locating and installing two fuse caps with a couple AGX5 fuses in them, I brought it up using my Variac until I was at 100%. I knew it powered up when I heard the transformer humming. Hummph, I've read about this humming, but never got to the end of what is causing it. That's when I checked DC offset. one channel was -5.9mV, and the other was a bit less, around -9.4mV, can't remember exactly. So I decided to take a chance on hooking it to some test speakers on my bench. After I got it hooked to my C-1 preamp, and the speakers, I didn't hear the hum as loud so I turned the volume up slowly. SOUND came out. Of one speaker. No movement at the meters. So I turned down the volume, played with the RCAs and tried again. Sound out of both speakers now. Great! One RCA input is sloppy, plan to replace those anyway with DCP board in near future. Still no meters, and no lights. But sound in both channels now, both distorted, so I turned it all off and put it back on the bench to dig into it deeper.
At this point I was pretty happy with it so far so I pulled the front, and both covers to get a better look. I found no meter lights, or even wiring for the lights, they were desoldered and left out. From looking at the wiring going to the meters, and from pics of other PL 400 meter wiring, I believe these were wired 180 out, because the wires were actually crossed from the meter board to the meters. I may be wrong about that, all this is new to me with the PL. This will be a good candidate for the LED board upgrade.
I do know it has the factory style aluminum case filter caps still in it with the mounts that screw to the side of the case. This is a 4 fin amp. I popped the covers off of the outputs/drivers and all 16 are Motorola MJ15022 transistors. Is that right? I was expecting to see 14 15024s and 2 of another # here for drivers. They appear to have been replaced from the word MEXICO stamped on them. lol
Now that I'm asking for technical advice on this amp, can I get an idea of why the transformer has a loud hum to it when not hooked to a source, what I can do to settle that, and possibly if I need to check any or all of the transistors before I replace the large caps? What caps are easiest to replace into the factory mounts? I do plan on doing the DCP board and try to listen to this amp before I decide to go all out and start a WO conversion on it. Can I get a little help on what the minimum I should do to give it a few good runs before I make a decision? Thanks for allowing a greenie into your group of excellent techs. I'm not a tech, but I have a few tools, a decent multimeter, a variac, a Weller soldering station, and a decent desoldering gun similar to the Hakko FR300 that works well for me so far. I can solder and pull and replace parts without burning boards(yet). I have the service manual and have gone over the bulletins. I think of myself taking instruction fairly well, but I can prolly be as hard headed as the next guy at times. Thanks again.
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