Phase Linear 400 blowing rail fuses - diyAudio
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Old 5th August 2009, 05:11 PM   #1
AD5DG is offline AD5DG  United States
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Default Phase Linear 400 blowing rail fuses

Hi to all,
I'm new to this great forum, and was brought here looking for help in restoring service to my ancient PL400. Well, I'm feeling ancient these days so this amp that I bought in 74 must be as well.
The 400 worked well in home application and even did a night or two in a PA sound reinforcement gig. 10 years ago we went sailing and the unit (along with a bunch of other ancient stuff) was placed into storage. Well, we are back to being homeowners and one of the first things I wanted to hook up was the stereo.
Upon power-up I had very low, distorted output on the left channel, and zip on the right.
Pulled the rail fuses and both were blown! I hyphenated.
So, I checked cabling- the obvious stuff and still no joy.
Opened the box and did some poking around looking for obviously smoked components. All the emitter resistors are ok, no leaking electrolytic caps and the caps test at about 6400 mfd on the DMM. When I discharged the caps before sticking my fingers in there, I noticed that the bottom cap had a significant charge while the upper seemed to have little to none.

After all this where would be a good place to start troubleshooting? I'd just like to get this flying again- kinda sentimental and my Infinity monitors seemed to like it also. I miss the DC thump at shutdown.

All the best,
Kev
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Old 5th August 2009, 06:30 PM   #2
wg_ski is offline wg_ski  United States
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Fuses can go bad just sitting in storage for 10 years. If the fuse element has any corrosion the first time you put any current through them at all... poof!

Low distorted output with no or blown rail fuses is actually a sign that the channel is working properly. Raise the load impedance to 1k ohms and it would run undistorted. The other (dead) channel would be a concern - it may be the reason the fuses blew if it wasn't just due to age or corrosion.

Check for DC offset, shorted output, driver, or predriver transistors, a leaky bootstrap capacitor, or an open Vbe multiplier in the dead channel. Once obvious reasons for overcurrent are eliminated, put in new fuses and fire the thing up through a light bulb. Then you can further troubleshoot.
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Old 8th August 2009, 09:35 AM   #3
AD5DG is offline AD5DG  United States
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Default PL400 DCM cap replacement?

Didn't get to the amp today, but I had replaced the 5AGX rail fuses and they blew again.
I believe that I located the problem with the channel that had no output- bad speaker connection.
Would bad rail caps cause this problem? These guys are 35 years old! On other posts, I had read about bringing power up slowly with a variac for amps that have been stored awhile- (reforming the electrolytic?)I don't have access to a variac, so is this procedure required/recommended? Without a variac, is there an alt. procedure?
As far as new caps are concerned, this pl400 has SANGAMD,5700 mfd, 85vdc DCM caps. A quick search of digikey turns up nothing in this size. I'll check Mouser.
Would someone be kind enough to recommend an alternate replacement cap?
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Old 8th August 2009, 09:35 AM   #4
AD5DG is offline AD5DG  United States
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Default Mallory 6000 mfd, 100V

Well, Mouser has a Mallory 6000 mfd, 100V computer grade that has about the right dimensions.
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Old 8th August 2009, 09:43 AM   #5
Netlist is offline Netlist  Belgium
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If the caps are the cause, you should be able to measure them dead short with a DMM.
I'd recommend you first check if the output transistors insulation is not playing tricks. Easy to do with a DMM, too, measure between the TO3 cans and chassis.
The bridge rectifier should be checked as well. Make sure to unload the caps before scarifying your measuring gear.

/Hugo
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