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Chafoweeth

Chafoweeth

New Around These Parts
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Jun 25, 2018
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Kenmore, WA
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I have an older PL400 (4 fins) in which I have installed the capacitor upgrade, LEDs, and the WattsAbundant protection circuit. I did not intend to do a full back-plate, but I just ordered a new control board so that I could get some experience with that while I search for a 700b to go all in on.

I got the email over the weekend that my control board was shipping. However, that night, my dad and a friend ran the amp hard and one of the channels went full tilt. The protection board worked flawlessly, but now I have a more complicated project ahead of me.

I have not had a chance to open it up and see what I can see, but I get the sense that this is not an uncommon failure mode for these amps. What should I be looking for? At this point, I would still like to avoid doing a whole back plate and save that for the future 700, but I realize it is an option...
 

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Check fuses, if it was a soft blow with very.little discoloring on the glass, replace it and try again. If the fuse went nuclear you may have fried an output. Are the outputs original?
 
Thanks, I like how the case turned out, but the design included some vents in the back that we never got around to. Up until this weekend, I have had the amp warm, but never *hot*. It was definitely hot when I got over to check on it. I really didn't think they would be able to blow it up, or I probably would have taken a few more precautions.

As far as the fuse, it would be great if that was the issue. I tried plugging the amp back in the next morning to see if it would sort itself out after cooling off, but the needle on the left gauge still slams to full immediately. Naively, I would think that a blown fuse would leave the channel looking inactive.
 
Especially with a four-finner, I would have had a fan blowing on the back full-time.
 
Here is the 400 manual. Read the troubleshooting section, especially page 20, testing transistors. Then read it again. Then do exactly what it says to find the shorted outputs.
 

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Well, the fuse was certainly blown. Not sure if this counts as nuclear, but this is what I found. It also looks like its a 5A fuse. I am having a little trouble finding a consensus on what the fuses should be, but no where have I seen a 5A being the right value for a 120VAC unit. It should be an 8A, right?

Thanks for all the support with this!
 

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AGX 5 amp, and that is a soft blow, just got tired...
 
An 8 amp is only used for sinewave testing.....

5 amp are also used in the rails of the 700. This is not consensus, that is fact....
 
There is a mains fuse, in case of a dual voltage transformer, there is validity in different fuses for different voltages, but the rail voltages don't change with mains voltage changes...
 
It looks like it was just the fuse. Its back up and running!

I did notice something while testing it though. One of the meters has always been a little slow, but it does its thing and I've never been too worried about it. I noticed that it didn't move at all while I was sending a steady sine wave into the amp. The other meter moved up and held its position like I would expect, but the weird one stayed completely dead. Once I played something more dynamic, it kicked back to life, slow, and a bit weaker like it had always been.

I had figured before that the one meter was just a little messed up/needed adjustment, but now I am thinking maybe the meter is fine and there is a capacitor somewhere that is not doing its job. Has anyone seen something like this?
 
Try a little Dawn soap on your finger on the meter's plexi cover then clean it off, could just be static keeping the meter from working correctly
 
Or some manner of debris Inside. That was so with a meter on my 700B. Opened up the meter and a subdued jet of compressed air fixed it.
 
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