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Post subject: Fender Concert problem!
Posted: Fri Aug 06, 2010 9:33 pm
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I have a 1995 Fender concert amp that has a problem. Tonight I turned on the amp with all knobs on 10 (kids!). When I turned the amp on, a loud hum came with it so I put it into stanby in about 3 seconds. Now I have a distorted muddy sound when I play on the clean channel and dirty channel. Do you think it may just be the speaker is blown or something else? All tubes glow when powered up so I dont think I blew a tube....
Please help!


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Post subject: Concert amp hum
Posted: Fri Aug 06, 2010 10:02 pm
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Did you have anything plugged into the input jack when you turned it on and got the hum? The reason that I ask is that just turning it on with all knobs at ten wouldn't cause a loud hum, unless the guitar was plugged in and the volume turned up. Maybe the kids put something into the holes on the amp, input jacks, loops etc. I remember the graham crackers in the VCR when my kids were young. :!: Depending on what type of jacks that you have, something could be in them, or been pushed through to the circuit board inside. Look in all of the jacks with a flashlight to see if there is anything shoved in there. Maybe you could remove the cover and look for damage inside the chassis. Unplug one side of the speaker and check conductivity with an ohm meter to see if it's blown. Looks like it should be about 4 ohms. Make sure you unplug the amp from electrical outlet before poking around.

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Posted: Fri Aug 06, 2010 10:06 pm
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I had my guitar plugged in when I turned the amp on. My kid only had time to turn the knobs up but not enough time for anything else. I am just hoping it was the speaker.


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Posted: Fri Aug 06, 2010 10:16 pm
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Could be the speaker, the voice coil itself or even a torn cone. If you are familiar with an ohm meter, check the speaker continuity with one wire unplugged from the speaker terminal. And check the speaker cone for tears around the edge of the cone. While you are at it, at least open the back and check for any burnt looking components on the circuit board.

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Posted: Fri Aug 06, 2010 10:57 pm
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Gotta' love that kid, my grandson will do the same thing ! I'm guessing a tube. The phase inverter would be my first suspect. If the amp was turned"on" before the standby was swithched"on", then it took a pretty good smack when the standby was switched to"play". Without any level of input, I can't see speaker damage. There are guite a few pieces ahead of those speakers that would see that surge. Art

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Posted: Sat Aug 07, 2010 3:06 am
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+1 Aclempoppi

Not the speaker, but good chance for a bad tube ( microphonic ?)


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Posted: Sat Aug 07, 2010 6:15 pm
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I agree with Art. Sounds like a bad gain stage or PI tube. First, maybe pull and clean them & their sockets. Try again. If still muddy, do you have one good, spare 12AX7 to start rotating through each position? My guess is first gain stage (V1) or the PI.

Note: the reverb driver usually takes a 12AT7. No subbing a 12AX7 here. But, you knew that.

:wink:


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Post subject: Hum problem with Concert Reverb
Posted: Wed Aug 11, 2010 10:56 am
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I saw your thread and thought I might be of help. I incountered the same type of problem in my 1984 concert reverb. It made a loud hum and I couldn't get a good signal through the amp. I thought it was the tubes and took it to my local fender specialist repair shop. He also thought it was the power tubes. It turned out to be a short in one of the reverb tank leads at the chassis.

Try wiggling the wire at the rca plug and see if it stops. My repair shop replaced the leads and the amp was like new again.

worth a shot before replacing $50.00 - $100.00 worth of tubes.


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Post subject: Re: Hum problem with Concert Reverb
Posted: Sat Aug 14, 2010 12:41 pm
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Venturas wrote:
I saw your thread and thought I might be of help. I incountered the same type of problem in my 1984 concert reverb. It made a loud hum and I couldn't get a good signal through the amp. I thought it was the tubes and took it to my local fender specialist repair shop. He also thought it was the power tubes. It turned out to be a short in one of the reverb tank leads at the chassis.

Try wiggling the wire at the rca plug and see if it stops. My repair shop replaced the leads and the amp was like new again.

worth a shot before replacing $50.00 - $100.00 worth of tubes.


+1
What a coincidence. I had this exact same fix when I had this problem. I figured that it was a fluke and one time thing. Guess ya never know.

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Post subject:
Posted: Sat Aug 14, 2010 1:11 pm
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I can't see the volumes being pegged, is the cause of a loose reverb tank wire connection! I would strongly suspect a preamp tube or possibly a power tube, they go microphonic also! The vibes from the amp being pegged, set the microphonics in motion on a borderline microphonic tube!

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