HELP! PL400 crackling/static

[laatsch55]

Yep..

 

[wattsabundant]

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

 

[laatsch55]

Thanks Don, wise words there bud..

 

[wattsabundant]

Lee, it's just another day on the job. Don't get hurt.

 

[laatsch55]

Amen!!

 

[Gepetto]

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

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.

 

[Lewcifer]

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

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.

 

[laatsch55]

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...

 

[Lewcifer]

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...

Oh, for sure. I just mean I might go that route if it would be an all-in-one fix.

 

[Lewcifer]

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..

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?

 

[laatsch55]

Yes..

 

[Lewcifer]

Yes..

Seems odd, but these were both maxed out at the end of their adjustment range before I began. I have them set now.

 

[laatsch55]

Still sound bad?

 

[Lewcifer]

Still on my dining table. Should I hook it back up and give it a shot?

 

[laatsch55]

Yeah, how is your offset? Measure accross the speaker posts for each channel in DC volts and see what ya got.

 

[Lewcifer]

Yeah, how is your offset? Measure accross the speaker posts for each channel in DC volts and see what ya got.

Offset is now 0.0mv at the binding posts and .2(left) and 15.2(right) entering the DCP relay, so not much change there.

 

[laatsch55]

I'd try it, just not full power max out type stuff...

 

[Lewcifer]

Will do. Thanks so much for walking me through this!

 

[laatsch55]

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...

 

[Lewcifer]

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...

Awesome! It may come to that. I’m going to hook it up in the morning and see how it goes.