Need help fixing an odd Phase Linear 400 - Page 10

wg_ski · 13th August 2009, 03:56 AM

That could very well be doing it. 200k collector load in the VAS would reduce the current to a fraction of a milliamp - and that's a recipe for oscillation big time. And if there is insufficient current in the Vbe multiplier bias string it won't turn on properly and you get low bias in the outputs/drivers.

And the resistor isn't obviously stressed/discolored?

blue lander · 20th August 2009, 02:46 PM

Nope, there was no discoloration on the resistor or the board. The PCB underneath its brother on the other channel, however, is cooked to a toasty brown.

So I replaced the 2 watt 7.5k carbon composite resistor with four "flame proof" 7.5k 1w metal film (two in series, two in parallel). I then adjusted the bias, and the channel started working without distortion or other weirdness.

Next I hooked up a small pair of insignia sattelite speakers and fired the system up. Both channels worked at lower volumes, but started to distort at louder volumes. I assumed this was because of sagging DC voltage because of the light bulb limiter.

I was feeling adventurous, so I plugged the PL400 directly into the mains and powered it up. Music played fine at all volumes, and I verified with the scope that the outputs were clipping evenly.

Tempting death and dismemberment, I plugged the PL400 into my double Advents and powered it up. Everything worked! The power on and off thumps are terrifying, but other than that it's behaved itself through at least 10 hours of listening. The warmest I've seen the heat sink get is slightly warmer than body temperature.

So congratulations, you guys fixed my PL400! Are the metal film resistors I used "acceptable", or do I need to find carbon composite ones? If they're okay, I'm going to replace the carbon composite 7.5k resistor in the other channel since it reads closer to 10k than 7k, and it looks like it's cooked the PCB like a poptart.

I'm also going to replace all the caps in the unit, do you have any suggestions for "audio" grade caps? Is there anything else I should upgrade while I'm at it? What about the power cord?

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Phase Linear 400, SERIOUS AC!	Planerguy	Solid State	41	29th March 2009 02:41 AM
Phase Linear 200 Series 2	IrishboyM4	Solid State	23	28th May 2008 03:04 AM
Phase Linear Model 700	Gibbi	Solid State	123	9th May 2007 04:25 PM
Phase Linear	lumanauw	Pass Labs	3	17th December 2005 02:38 AM
Phase Linear 200 schematic	coco33	Solid State	2	4th December 2004 10:52 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

13th August 2009, 03:56 AM	#91
wg_ski diyAudio Member Join Date: Oct 2007	That could very well be doing it. 200k collector load in the VAS would reduce the current to a fraction of a milliamp - and that's a recipe for oscillation big time. And if there is insufficient current in the Vbe multiplier bias string it won't turn on properly and you get low bias in the outputs/drivers. And the resistor isn't obviously stressed/discolored?

20th August 2009, 02:46 PM	#92
blue lander diyAudio Member Join Date: Jul 2009	Nope, there was no discoloration on the resistor or the board. The PCB underneath its brother on the other channel, however, is cooked to a toasty brown. So I replaced the 2 watt 7.5k carbon composite resistor with four "flame proof" 7.5k 1w metal film (two in series, two in parallel). I then adjusted the bias, and the channel started working without distortion or other weirdness. Next I hooked up a small pair of insignia sattelite speakers and fired the system up. Both channels worked at lower volumes, but started to distort at louder volumes. I assumed this was because of sagging DC voltage because of the light bulb limiter. I was feeling adventurous, so I plugged the PL400 directly into the mains and powered it up. Music played fine at all volumes, and I verified with the scope that the outputs were clipping evenly. Tempting death and dismemberment, I plugged the PL400 into my double Advents and powered it up. Everything worked! The power on and off thumps are terrifying, but other than that it's behaved itself through at least 10 hours of listening. The warmest I've seen the heat sink get is slightly warmer than body temperature. So congratulations, you guys fixed my PL400! Are the metal film resistors I used "acceptable", or do I need to find carbon composite ones? If they're okay, I'm going to replace the carbon composite 7.5k resistor in the other channel since it reads closer to 10k than 7k, and it looks like it's cooked the PCB like a poptart. I'm also going to replace all the caps in the unit, do you have any suggestions for "audio" grade caps? Is there anything else I should upgrade while I'm at it? What about the power cord?