Phase Linear 400 Series-2 Amplifier Repair Help

[jeffsplat88]

Hi Folks,
I have a PL 400 S2 amplifier.
While listening to music at a relatively low level, ran for about an hour then I heard a pop-buzz and my left speaker fried.

Yes, I am well aware of PL reputation for flame and no speaker protection. Shame on me I guess for not having a fuse on my speaker! Fortunately, it was just a minimus-7 and I can get replacement woofer for it.

Anyway... The situation:
- Right channel is working fine. LEFT channel now sits at the positive DC rail.
- No fuses blown. No smoke.
- Checked all the output transistors and RCA drivers and NONE were shorted. All TO-3 transistors working fine.
- Main Caps are fine.
- So, I'm assuming something aged and died on the driver board.

Knowing how fragile that driver board is wrt poor quality PCB and traces, I'm not too excited about pulling all the transistors and and checking them. But with the left channel sitting at positive DC, I can't easily check bias points on the system either. I guess my question to the AK community and hopefully some PL400S2 expert is whether there is a known problem point on the driver board I can focus on? Any one of those transistors, diodes, op-amp, or caps a known weak point?

Any pointers on common driver board problems would be very useful just so I can focus on those areas first.
Thanks!!

(PS, still working on my Marantz 2600 and will report on that thread when the parts get installed)

 

[avionic]

WOPL...

http://www.whiteoakaudio.com/

 

[jeffsplat88]

I don't think White Oak makes a driver board for the PL Series-2 (the PL-36)
In any case, the update is that it was the lm356 op-amp that had failed. I replaced that and amp went back to normal.

So as long as I have it apart, I'm replacing the right channel op-amp as well - using the metal can version of the 356 for better heat dissipation (they do get hot).
Also replacing the MPS93 with a higher power MPSU60. Replacing both of the electrolytic caps used in the driver board.

Finally, I'm going to see if there is a better way to do the 15 volt regulators. They just sit there and cook that board. On my D-500 unit, I ended up moving the power soaker resistors to a new heat sink I fabricated just to preserve the what's left of the blackened PCB.

And, since I have a couple of 100v 15000uf can caps that will fit, I may as well do that too... I'll have to install an in-rush suppressor.

And, I think I'll drill some holes in the side and mount a pair of fuse holders for the speaker outputs.

I'm a bit nostalgic for these 400 series-2. When I was a mobile DJ in the 80's, I had a massive JBL sound system that I powered with four of these exact 400s2. Those cylons were mesmerizing!

In the cue: The silver-face Carver C-4000 that came with this amp.
As well as: 400a cube, Marantz 2600, SX-1280, SX-1250, SX-3900, Fisher 800c and 500c, McIntosh MC30 pair, 225 pair, 240, C-20, PL-570 and 6370Q, HPM1500 needs foam, Western electric 15560 tester rebuild and many others waiting for some love.

White Oak! I'd love for you to make a PL-36 driver board for the Series-2, and the driver board for the dual-500, please!