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Old 13th August 2009, 03:56 AM   #91
wg_ski is offline wg_ski  United States
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?
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Old 20th August 2009, 02:46 PM   #92
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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?
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