HELP! PL400 crackling/static

Since I haven't said this in a while, any 50 watt amp can kill you. The 150 volt DC power supply in this amp will stop a heart and not let go of you. The arc flash from shorting out the outputs while measuring bias, plus flying debris can cost an eye. I never work on anything without safety glasses.

The probes on my test leads that I use for checking bias and load sharing has spaghetti tubing on them so that only the tip can make contact. Lesson learned the hard way.

Static and crackling is typical of noisey front end transistors. Most likely Q1 & Q2. The pinouts for the various recommended replacements are different. There is a service bulletin in the 400 manual that addresses it. For $20 on ebay you can get a transistor tester that identifies pinouts . A freind got me one and it never leaves my bench. Several people sell it. Here is a link for one of them. transistor tester
 
wattsabundant said:
Since I haven't said this in a while, any 50 watt amp can kill you. The 150 volt DC power supply in this amp will stop a heart and not let go of you. The arc flash from shorting out the outputs while measuring bias, plus flying debris can cost an eye. I never work on anything without safety glasses.

The probes on my test leads that I use for checking bias and load sharing has spaghetti tubing on them so that only the tip can make contact. Lesson learned the hard way.

Static and crackling is typical of noisey front end transistors. Most likely Q1 & Q2. The pinouts for the various recommended replacements are different. There is a service bulletin in the 400 manual that addresses it. For $20 on ebay you can get a transistor tester that identifies pinouts . A freind got me one and it never leaves my bench. Several people sell it. Here is a link for one of them. transistor tester
Click to expand...

Or the ClassA high voltage amp too, the 40327s encircled below. I have had a couple of these go on the fritz and produce sporadic crackling and noise.
PL400 Snippet.PNG
 
wattsabundant said:
Since I haven't said this in a while, any 50 watt amp can kill you. The 150 volt DC power supply in this amp will stop a heart and not let go of you. The arc flash from shorting out the outputs while measuring bias, plus flying debris can cost an eye. I never work on anything without safety glasses.

The probes on my test leads that I use for checking bias and load sharing has spaghetti tubing on them so that only the tip can make contact. Lesson learned the hard way.

Static and crackling is typical of noisey front end transistors. Most likely Q1 & Q2. The pinouts for the various recommended replacements are different. There is a service bulletin in the 400 manual that addresses it. For $20 on ebay you can get a transistor tester that identifies pinouts . A freind got me one and it never leaves my bench. Several people sell it. Here is a link for one of them. transistor tester
Click to expand...
Great advice, thanks! I have been very nervous/cautious working inside with the unit powered up, but it hadn't occurred to me to wear my goggles. I got the mini grabbers reccommended above, which will be a lot safer than fumbling around with probes. I also just ordered the transistor tester. Thanks for the tip. I might end up just getting the White Oak driver board if that will solve all my problems vs replacing several components on the PL14. This could end up getting expensive, but I'm getting an amp AND an education, so I'm at peace with it.
 
Before installing a White Oak board, if you are NOT going with the WO backplane boards, be sure to cure any problems on the backwall...
 
laatsch55 said:
Hook your meter to the first row 10 ohm bias resistor, turn amp on and adjust thumbwheel pot while observing reading. With those old open pots it can get real erratic..
Click to expand...
Ok, I turned the R20 on the left side to get -375mv on the first row. Now do the same thing with the other R20 for the third row?
 
Yeah, how is your offset? Measure accross the speaker posts for each channel in DC volts and see what ya got.
 
I'd try it, just not full power max out type stuff...
 
If it's still noisy, and you want to try and fix the factory board I have lots of whatever it takes to fix it...
 
You must log in or register to reply here.
Back
Top