Debbuging solid state amplifiers?

[tra]

I bought a Randall Century 170 that powers up, but has no output. I've debugged stompboxes using an audio probe located at http://www.diystompboxes.com/wiki/index.php?title=Debugging, but before I do that I wanted to see if any of you have tips on debugging something plugged in to the wall.

I've built a bunch of gear (including a tube amp) that run on mains power, but never had to debug them...luckily only stompboxes.

Your help is appreciated.

[gbono]

Schematic would be helpful

Check output of the power supply

Isolate the problem - is the input signal getting to the preamp, is the preamp signal getting to the power amp stage, etc etc

[joecool85]

Schematic would be helpful

Check output of the power supply

Isolate the problem - is the input signal getting to the preamp, is the preamp signal getting to the power amp stage, etc etc

The power supply is probably fine if the amp powers up.  As for the no output, I would first check the input jack.  Frequently they will flake out due to a bad solder or worn out contacts.  Get back to us at that point and we can walk you through some more tests.

[gbono]

The power supply is probably fine if the amp powers up.

I have worked on several Sunn Betas that power up but don't work because the =-15 V supplies aren't functioning. Just one example I can think of. My reply was a general one since there isn't much to work with.

[hamnstar]

Hey, new to the forum here, but my experiences say check the easiest things first! switches between channels, speaker continuity, and generally things that can be checked with the power disconnected are always a good start, as they are quick, easy to do, and usually dont require too much disassembly!  Make sure to jiggle mechanical parts and connections while testing continuity too, as this can reveal intermittent problems.

[tra]

Alright guys. I finally got the amp in my possession, and it actually does pass signal.  :tu:

The problem is that you need to have the volume on 10 to get a very low signal. The preamp appears to be functioning fine because the distortion works, EQ works, reverb works....just not loud.

Here is the schematic that Randall sent me:

http://dl.dropbox.com/u/8772719/C500%2C%20C1000%2C%20C170%20-%20Schematic.pdf

I'm not sure how to troubleshoot where in the power amp things aren't right.

[teemuk]

Passing signal is one thing, passing signal without significant attenuation another...

Have you tested that you get flawless signal output from preamp using the effects loop?

If preamp works correctly that only leaves the power amp. Since it passes signal it most likely works ok, which leaves the master volume control: Is it worn, have the solder joints failed?

[tra]

Passing signal is one thing, passing signal without significant attenuation another...

Have you tested that you get flawless signal output from preamp using the effects loop?

If preamp works correctly that only leaves the power amp. Since it passes signal it most likely works ok, which leaves the master volume control: Is it worn, have the solder joints failed?

I'll check this evening. Thank you very much for the help!

[Enzo]

We don;t know the preamp works.  All we know is that some preamp controls affect the signal.  The distortion controls may work, but that doesnl;t tell uss that the outp-ut IC from the preamp is OK.

You have FX loop jacks, use them.  Plug the guitar into the FX return.  Is that strong and clear?  Or weak and muffled?   And likewise, send the signal from the FX send to some other amp and speaker for a listen.  Same Question, loud and clear or weak and muffled?

Turn the reverb up at least midway, then rock the amp to crash the reverb springs.  Does that come out loud or does that sound weak?

And if it is the power amp, have you tried more than one speaker cab?  There is a 0.27 ohm resistor between the sleeve of the speaker jack and ground.  If that is open you get very low output - check that resistor and its connections.  And for that reason, check the speaker jack itself and its solder.

[Jack1962]

Enzo is right use the effect loop to check for quality of signal if it is ok there the go on to the master volume and power amp section of the amp.

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