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Old 6th January 2011, 01:59 PM   #11
BigE is offline BigE  Canada
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Default FIXED!

The DC offsets are now at 75 and 80 mv.

The problem was that a wire was broken between the driver board and the bottom transistor on the second transistor bank from the left (looking into the unit).

Reattaching the wire lowered the DC offset in that channel by 50 mv to 80 mv.

Recapping will be done a bit later -- I am enjoying the sound. It is possible that this wire has been broken for a couple years..... the unit never really sounded "right". It is amazing that it can run without all the transistors working....

Thanks to all who responded.
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Old 6th January 2011, 04:39 PM   #12
llwhtt is offline llwhtt  United States
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The bottom transistors on PL400s are the drivers, so depending on what wire was broken, there are a couple going to the driver, who knows what was happening. Broken wires to the board and to the bias transistors are very common especially if the amp has been worked on. The last one I worked on I just replaced all of the wiring as it wasn't very flexible anymore.

Craig
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Old 6th January 2011, 10:27 PM   #13
BigE is offline BigE  Canada
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The broken wire went to the "middle" of the transistor. The second transistor above was chained to that one with a grey wire.

Anyway, for recapping purposes, it appears that their polarization is marked on the part stuffing diagram in the service manual. That makes it helpful, as I can't obviously see the polarization on the orange caps. (C11, C15/C16)

I'd like to get to that next week. Will report back when that's done.

I may be able to get some MAT12 dual NPN input transistors directly from ADI to replace the input differential pair C1/C2. Apparently, the better the match, the lower the DC offset.

Expensive buggers tho, about $20 each pair.
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Old 6th January 2011, 11:25 PM   #14
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by BigE View Post
The broken wire went to the "middle" of the transistor. The second transistor above was chained to that one with a grey wire.
That would have been "collector". Collector of the negative rail driver would be tied to the output bus. That probably goes to the VI limiters (you sure would have noticed a disconnected feedback signal - the offset would have been more like 80 *volts*).
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Old 6th January 2011, 11:35 PM   #15
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I seriously reccomend getting an upgrade kit.... the ones on Ebay are probably lots better parts then whaty ar now in your amp. It will probably also offer you just about all the upgrade - performance you can possibly do to that amp as it sits... Don't mess around changing this or that... just replace all the caps on the boards!! it's a half hour job at best to do em all...
Good luck with the project! The 400 was the forst high powered amp I owned... when I was a senior in High School!

Mark
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Old 6th January 2011, 11:50 PM   #16
llwhtt is offline llwhtt  United States
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Be wary of what you get off of fleabay. The PL400 I mentioned a couple of posts above came from one of those guys complete with counterfeit Motorolas and questionable workmanship. There is plenty of info out there as to what types of transistors to use, etc. I was able to get everything I needed from Mouser except the bias transistors.

Craig
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Old 7th January 2011, 01:23 AM   #17
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I'm glad you found the problem! That is fairly acceptable dc balance for such an old amp but I think it can still do a little better then that. I can't say how the MAT 12's will work or sound... you'll just have to try them and listen.

@llwht... You'll have a more difficult time finding the 2N1204 and 1305 in the protect circuit!! Mouser of Allied are not going to have those. Either an obsolete semiconductor house and buy large quantities or E-bay. The RCA's were the better ones. They normally fail if you pop an output or two.

Mark
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Old 7th January 2011, 01:31 AM   #18
llwhtt is offline llwhtt  United States
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This was a fully complimentary Series II amp which uses silicon VI xsistors instead of the Series I germaniums. Unfortunately none of the retail suppliers carry any of the oddballs unless you willing to settle for NTE. I don't see why a Series I couldn't be modded to Series II in that regard.

Craig
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Old 7th January 2011, 01:37 AM   #19
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It surely could be done but then it's no longer a real Series 1! Series 1 stuff is classic PL and has that classic PL sound. Be it good or bad... depends on your taste... in reality a decent KSA-50mk 2 klone will clobber a PL400! Series 2 stuff isn't really classic... it is not as good looking nor does it sound all that much better than Series 1. It is probably not a worth the time and $$$ to upgrade a Series 1 to a series 2. Things were also drastically cheapened up in Series 2. I left HiFi repair about a year after Series 2 came out and went into Broadcast TV, I left broadcast TV to repair-install cinema projection and sound systems... now installing-servicing D-Cinema and back at hifi as time permits...perhaps 10 min a day. If you wanted to upgrade a series 1 or 2 you could just screw down one of the digital amp modules from ClassDaudio.com and have an amp that compares with a 1980's Levinson and Krell for very little $$$.

Mark

Last edited by Mark Allen; 7th January 2011 at 01:45 AM.
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Old 7th January 2011, 03:22 PM   #20
BigE is offline BigE  Canada
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When I got the amp, it looked like all the transistors were replaced -- the thermal compound was still wet. It is possible that the contact to the collector was very poor, and just detached when I moved the drive board.

There is no question that reattaching the wire lowered the DC offset.

I did rebias the amp, to 0.435 V each channel -- one was already at that value. The manual says 0.35+/- 0.1, so it is in spec. It runs cool at idle.

The manual also says, don't bias too high, else it will run warm at idle.

What does the voice of experience say is the best bias voltage sonically? As the unit has no power switch, I leave it on 24/7.

thanks!
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