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A friend has asked me to take a look at his guitar amp: Peavey Valve King VK212.

Problem: suddenly the amp stopped working during operation and the status lamp went off.

First reaction: fuse replacement, the status light came back but zero sound.

This is my starting point and observations (full schematics are easy to google, I have doubts if it would be fine for me to share, though):

  1. Fuse F201 clearly burnt, but continuity tester shows flow on both sides! Obviously short circuit is somewhere around. Resistance measured on bot ends of the fuse = 14 ohms. This confirms the fuse is dead and flow is another way.
  2. Capacitor C201 looks slightly damaged, I bet it is short circuited and blew the fuse.

enter image description here

Now two questions to experienced colleagues:

Q1: I localized at least these two faults. How far do you dig to avoid cooking the parts directly after replacement? I mean is there a rule of thumb to say "ok, I got to the root cause, can do the replacements and switch the thing back on". Ideally I should probably check the whole module but it means soldering quite a few parts out of the circuit to measure them... But maybe at least the diode bridge that follows right after?

Q2: Basically all plugs on the plate are additionally glued. Is this because of the guarantee, or is this to make sure they hold despite vibrations of the amp? Basically I would like to know if I should glue them again after the repair.

Thanks in advance for any hints!

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  • \$\begingroup\$ This may be multiple failures. But assuming just one point of failure, go through the schematic and ask yourself what the impact would be if such a part failed. You'll need to list the ways each part can fail. Once that is in hand, work out which measurements are more useful in dividing the possibilities. Proceed with those measurements. It's EET101 stuff. You've got the schematics. Work it out. This helps avoid cooking further parts, too. \$\endgroup\$ – jonk Sep 23 at 9:35
  • \$\begingroup\$ Organize the schematic into blocks. Regions of high interconnection but low connections to other blocks. That's probably the first step. That way you can quickly isolate the section, which will focus further effort. You also have external observations, so that also helps a great deal. You only need to consider cases where your observations are possible. Sometimes it is also helpful to consider applying a voltage source with a series resistance applied to specific nodes, and remeasure the voltage at that point, when applied. Etc. \$\endgroup\$ – jonk Sep 23 at 9:40
  • \$\begingroup\$ And no. I'm not going to do that work for you. I wish I had the time. But I don't. \$\endgroup\$ – jonk Sep 23 at 9:41
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I got partial answers:

  1. In this module this was C201 shorting the circuit and blowing the fuse.

  2. Plugs and caps are glued because of the vibrations, of course.

Story is not over yet as I have stumbled upon a problem in another module. Will post it separately.

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