Phase Linear 400 Series II rebuild

B

BobKandel

New Around These Parts
#1
Joined
Mar 9, 2025
Messages
5
#1
I had a working PL400ii until I tried to upgrade the LF356T (round can) to the LF351N (8-pin dip). I purchased this unit about 30 years ago with 1 channel not working. At that time I replaced the LF356T with a NTE937 (round can). The amplifier worked for the next 30 years. The amp always had a turn-off thump. I read that if I replaced the IC's with the LF351N's it would correct this problem. I also changed the 2-1.8Kohm to 7.5Kohm as stated in the manual. I ended up with one channel completely blown including the 6-FPL909 outputs and several transistors on the PL36 board. I replaced the bad components, upgraded the 2 large caps in the power supply with 10,000uF, 100Volt caps, put 150pF silver mica caps across Q101 and Q105 base to collector. Now I get no signal through the LF351's and I get around 65 volts DC on the speaker outputs. I changed the blown outputs with MJ15024G's. I'm ready to give up on repairing the PL36 board and replace it with the White Oaks PL14-20 if I can get one.
Any thoughts on this project.
I
 
Hexis22

Hexis22

Chief Journeyman
#3
Joined
Dec 28, 2022
Messages
792
Location
Wisconsin
#3
Thoughts on your project highly depend on your goals and what you want to achieve with this amp.

For example,
- What other components do you plan to use around this amp (Preamp, source, speakers)
- What budget you feel comfortable spending (this can be a deep rabbit hole)
- Willingness to continually problem solve 40 year old component issues
- Willingness\ability to justify the higher upfront cost to perform a full WO rebuild
- Is this amp intended for your main audio system, or limited use secondary system

From personal experience, the desire to have a high performance trouble free amp in any of my audio setups is paramount and far outweighs the modest upfront costs of a full rebuild.

Just my $.02, but my situation could be far from your outlook.
 
B

BobKandel

New Around These Parts
#4
Joined
Mar 9, 2025
Messages
5
#4
I did contact White Oak to find the cost and availability of a complete PL14-20 board. Joe said he would put me in touch with a tech does the board assembly. I'm waiting for a reply. This amp was used to drive a pair of Bose Acoustimass sspeaker attached to a computer. I want to drive a pair of Woofers (IMF RSPM Mk !V w/ new Falcon B139's). Some other components in that system: Sequerra Met 7 Mk.II, Accrosound Ultra-Linear-2, Linn-Sondek LP-12, Hana ML, Pro-Ject Tube Box 2 (battery), home-made 4S Universal Preamp. All equipment is upgraded with some mods.
 
Gibsonian

Gibsonian

Chief Journeyman
#5
Joined
Aug 3, 2011
Messages
869
#5
I have a board, I think one rev down from latest version if interested. Was in my 400 and replaced with latest rev from Joe
 
wattsabundant

wattsabundant

Chief Journeyman
#6
Joined
Apr 26, 2011
Messages
958
Location
Central Ohio
#6
If you read the troubleshooting section off the service manual, and do exactly what it says, you'll likely find the trouble. Note that the manual was written before we had digital meters. You can invest in an analog meter like a Simpson 260 and follow the manual verbatim.
 
B

BobKandel

New Around These Parts
#7
Joined
Mar 9, 2025
Messages
5
#7
That's funny you mentioned the Simpson 260. I owned a company that sold and calibrated electronic test equipment and we were a distributor for Simpson. Of course I have a Simpson 260. I'll try it.
 
laatsch55

laatsch55

Administrator,
Staff member
#11
Joined
Jan 14, 2011
Messages
75,126
Location
Gillette, Wyo.
Tagline
Halfbiass...Electron Herder and Backass Woof
#11
BobKandel said:
That's funny you mentioned the Simpson 260. I owned a company that sold and calibrated electronic test equipment and we were a distributor for Simpson. Of course I have a Simpson 260. I'll try it.
Click to expand...

Of course you have a Simpson 260...........Ser 1 thru 8 I imagine...
 
You must log in or register to reply here.
Top