Phase Linear 400 Series Two Blowing fuses

[Blake Wilson]

Hi guys, this is my first post. I've got some electronics experience (synths, kits, stompboxes) but not a lot with amps.

I've got a PL 400 Series 2 amp whose main fuse blows immediately on powering up (i.e. plugging into the mains). Here's what I've been able to determine:

On the right hand side grouping of 6 output transistors (as you are looking at the unit for repair) all three of the conductors short together, meaning I get continuity between all three. I get no continuity on the left side group. I've also determined that the 40327 and MM4003 tophat style trannies on the right side of the PCB are shorted. I know this isn't much info...or maybe it's enough for some clues? Thanks,
Blake

[Blake Wilson]

Ok, so I've done some tests. 3 of the right channel's output transistors and one driver are blown. So, would this cause the mains fuse to blow? Sorry for the newbie question.

Blake

[kevinkr]

Yes, and it is not unlikely that there is further collateral damage to other parts of the amp. This model is notorious for blowing output devices.. Try to find a Sams for this unit or the original factory manual (eBay maybe?). I'm not sure how hard it is to find replacement output devices, but with three gone I would seriously consider replacing the drivers and all output devices. Given that it is probably 30+ yrs old it may be difficult to find any of the original semiconductors, but I'm not sure. Once you think you have fixed it bring up with a ballast lamp (100W incandescent to start) in series with the AC power in so that in the event you have missed something it doesn't all blow up again.

Depending on your level of experience you might want to find an experienced tech to work on it for you.

[Tekko]

And replace all the electrolytic capacitors. Very important in old amps.

[Blake Wilson]

Thanks for the input fellows. I removed the bad outputs and drivers and it powers up without blowing the fuse, which is good. I checked the 4 electrolytic caps on the PCB and they are good (but will replace anyway). Anything else I might spec at this point? I found a source for outputs and drivers at around $5 each, so I'll probably replace all of them while I'm at it. Thanks again,

Blake

[c2cthomas]

Hello Blake - and Welcome to diyAudio!!!

I found this over at HiFi Engine Phase Linear PL-400 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine

You might need to join up to down load the PDF - it should help

[stratus46]

Originally Posted by Blake Wilson

Ok, so I've done some tests. 3 of the right channel's output transistors and one driver are blown. So, would this cause the mains fuse to blow? Sorry for the newbie question.

Blake

Three transistors failing is rather unusual. Far more typical is 2, 1 in each leg of the amp.

The original PL400 used XPL909 or PL909 transistors but I believe those were actually house numbers on DTS-410 / DTS-411 transistors. I suspect there are some ON semi power transistors that could do the job as well or better.

G²

[Tajzmaj]

Hi
I have some experience with 700b amp......which I have.
So my advice is. Replace all electrolytic caps. Replace idle curent trimmers....and of course replace all output transistore and drivers. MJ15024 devices from ON semi will work fine. Because you have blown predriver replace them all if you want both channels to behave the same. Please use new mica insulators and renew thermal conducting grease. All output transistors should be closely matched.......
All such things are almost must if you want pl400 to work reliable. There is also good idea to install some speaker protection inside.
Of course there you must check amp if it oscillate after repair jobs and you must readjust idle curent......
And some modifications can also be done to improve performance. Wiring is poor and input op amp is old type.......
There is a lot of info here on diy audio but if you need we can start from the zero point again. And of course you can email me......if you want. I even made some pl clones so I know a lot about this amplifiers. I destroyed a lot of transistors....hehe.
Best regards, Taj
P.S.
Predrivers are hard to find and hard to replace. My clone 700 series 2 amp work fine with MJE15030/31 predrivers......

[Blake Wilson]

Fellows: thanks for the help.

I replaced all the MJ15024s and MJ15025s. I also replaced the 40327 and mm4003 on the (what I believe to be) the right channel (as you look at the PCB from the front, I'm assuming that the components on the right apply to the right channel?). The amp powers up fine (and doesn't blow the fuse) on the left side and plays fine. The right side is not operating. On the right side I'm getting a lot of 20 v readings where I think I should be getting 82 volts. Since I've replaced the 40327 and mm4003, any other thoughts about which components to troubleshoot? Again, the left side is fine.

It might be obvious that I am no expert, but I've gotten this far and would appreciate any help.

Blake