Marshall G80R-CD; Some sound, Speaker is fine - diyAudio
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Old 18th September 2012, 09:37 PM   #1
diyAudio Member
Join Date: May 2012
Default Marshall G80R-CD; Some sound, Speaker is fine

Alrighty, I have a Marshall G80RCD here. Plug a guitar in - no sound. Checked the speaker in another amp, works fine. So I plugged a little tone generator in it and started going at it with my audio probe. There is sound all the way to the end, so I plugged the speaker back in, and i was getting sound. But even with everything turned all the way up, I'm still getting hardly any sound from it.

Any ideas would be greatly appreciated.
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Old 19th September 2012, 05:23 PM   #2
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Join Date: May 2012
Turns out there is sound from a guitar, its just very quiet , like the tone generator.
Also, I believe the problem lies in the main amplification stage, as i get the same amplitude from both channels.
All solder joins seem fine, and I cant find any burned out componenets
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Old 19th September 2012, 10:20 PM   #3
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
You have FX loop jacks. First, plug a spare crod from FX send to FX return. ANy help?

If that restores sounhd, your return jack needs service. If no help, then plug the guitar into the FX return. Got sound now? And the other end test: run a cord from FX send to the input of some other amp. Now plug the guitar in the front as usual and play. Does the signal from the FX send sound OK on the other amp? The FX return is access to the power amp, the FX send is basically the preamp output.

You have a couple other outputs. I see one that appears to be some sort of emulated out, JS8. Does it have good signal on it? I also see JS7, Direct Out, same question?

The red wire from W1 on the board is the speaker hot side, the speaker cold side, black wire, returns to W2 on the board. But that is not ground, and the speaker nbeeds a complete path. So check 0.33 ohm R109, between W2 and ground, for opens or cracked solder.
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Old 20th September 2012, 07:35 PM   #4
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Join Date: May 2012
Thank you very much for the reply.
I plugged the output of the broken amp into the input of my other amp, and heard a lot of noise, as if it were not grounded right. So with this in mind, I checked the schematics, and checked R109 as you suggested. There was no connection at all across the resistor, so I pulled it out and confirmed that there is no signal passing through.
So thank you very much for suggesting that resistor!

Now then, onto buying a replacement. The closest I can find is a .47 ohm resistor. Is this close enough to the .33 ohm or do I need to look harder?

Again, thanks a million!
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Old 20th September 2012, 09:25 PM   #5
diyAudio Member
Join Date: May 2012
I took that resistor out broke it with a pliers. After exposing the wirewound part, I resoldered a leg to it and wrapped a bit of insulation tape around it. After checking the resistance and confirming that it was right, i put it back in. Voila, it works!
Now all that's left is to just buy a new .33 ohm resistor for it.

Thanks for the help!
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