Bring-up issue after updating PL400 Series II

spmac

New Around These Parts
Joined
Jul 21, 2018
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12
Location
Minnesota
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#1
I recently did a very successful White Oak upgrade on a nice PL400 Series II, no problems. That is a very awesome amp now. This left me with a good condition PL-36 board. So, I used this board to fix up a different PL400 Series II that was in pretty sorry shape.

I replaced all caps on the good PL-36 board and put it into the sorry PL400II. I also put new main caps in this amp, 10,000uF 100V. Also on the back panel, 4 2n4004 diodes that looked toasty In addition, I put in a speaker protection board that I built, detailed here:

http://audiokarma.org/forums/index....protection-board-for-phase-linear-400.835770/

For the purpose of bring-up, I jumpered across the relays in the speaker protection board, and applied 8-ohm loads on each channel. I fed in a 1kHz Sine wave, starting at 100mV and increasing. The amp did not clip as expected, it behaved very strangely and glitchy as seen in this video, and drew more input current compared to my other PL400II:

https://youtu.be/cYIUkyhmPms

Outputs are 10V/div, input is 200mV/div.

In hindsight, I wouldn't have changed so many things at once, but that is where I am at unfortunately. Has anyone seen this kind of behavior? The PL-36 was known good before I recapped it. This amp had 1 channel much quieter than the other and popped very loudly when shut off. One of the Zobel networks was 500 ohms (!), the other was 11 ohms. I tested all of the output transistors with a diode tester and they all tested with almost identical voltages, none open or shorted. All 16 of the transistors are MJ15024's, so quasi-complementary. When putting the transistors back in, I used mica pads and thermal paste. Based on the transistors looking good, I figured most of the problems were in the PL-36 board. I have not adjusted the bias pots on this board, I just left them where they were at, about in the center position.
 

spmac

New Around These Parts
Joined
Jul 21, 2018
Messages
12
Location
Minnesota
Tagline
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#3
Boy do I feel stupid, I was not supposed to have loads connected yet. The amp was able to go up to 110VAC input no problem with no load and 1kHz input. At 20VAC input, I increased input signal until clipping, and everything was symmetrical. Then I attached the loads, no input, dialed it up to 110V AC, slowly increased the 1kHz input signal until I got up to 2A AC input. Everything appears to work great! New UAA170s and all. Sorry for the false alarm!
 
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