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PostPosted: Sat Dec 02, 2017 1:51 pm 
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Bottle Rocket

Joined: Sat Dec 02, 2017 12:27 pm
Posts: 3
I recently got a used JP2C and have had it for a few days, so I'm not very familiar with it. I removed the chassis to swap the V1 tube and when I got it all back together (a bit of a pain), the reverb is behaving strangely. It was working before as expected, but now when I turn up the reverb it is behaving like a gain control. I'm not hearing any reverb, but the volume increases as I turn the reverb up. If I turn the channel gain completely off and the master at reasonable position, the reverb knob will kind of behave like a gain knob increasing the volume. Any ideas on what would cause this or how to fix it?


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PostPosted: Sun Dec 03, 2017 5:47 am 
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Mark II

Joined: Sat Aug 05, 2006 11:15 pm
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Location: San Jose, CA
its probably a hardware mod youll have to reverse

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PostPosted: Sun Dec 03, 2017 1:39 pm 
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Bottle Rocket

Joined: Sat Dec 02, 2017 12:27 pm
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I don't understand how it would be a hardware mod as it was working fine before I removed the chassis and changed some tubes. Now, if I have any gain at all on the channel, adding reverb results in a squeal like there is way too much gain. Any insight would be appreciated appreciated. I'd hate to have to send it into the shop already as I'm really enjoying playing it!


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PostPosted: Sun Dec 03, 2017 7:23 pm 
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Are you certain you did not reverse the reverb connections?

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PostPosted: Sun Dec 03, 2017 8:23 pm 
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Dual Recto

Joined: Fri Feb 22, 2013 7:31 am
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Location: North Carolina
I would double check you reverb connections. Sometimes reverbs tend to do odd things if the connections are reversed. Usually results in some squealing.

Also, next time you need to swap out preamp tubes, there is no need to remove the chassis as the front plate can be removed. You may need to use a long stick to tap on the back side (use a flashlight to see where the cut out is for the preamp tube access area and tap on the wood or black painted plywood that is in that section, you need to loosen or remove the 4 mounting screws first.

for reference, from a rear view perspective (at the power tubes), white cable connects on the left jack closest to the huge power transformer, the black RCA cable connects on the right (near the reverb transformer that is very small compared to the power transformer).

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Current amps: TC-100, TC-50, JP-2C, MK V, Roadster, RA100


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PostPosted: Mon Dec 04, 2017 4:00 am 
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Joined: Sun Apr 14, 2013 5:45 pm
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I had an original C+ that did something very similar. It's like the reverb knob worked as volume one. Except for with the reverb knob on 10 I had no sound at all. But as I turned the reverb knob down towards zero, the volume came up as well as the gain. And when I got the reverb knob to zero it was like volume one was on 10. At no time did either the reverb or volume one knob have any effect on reverb, there was no reverb at all. I brought it to the local Tech who used to be authorized boogie in the 80s. And if I remember right it was either capacitor or a resistor in the preamp section. So he swaped that out and it's worked fine ever since. Hope that helps you.


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PostPosted: Mon Dec 04, 2017 11:01 pm 
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Bottle Rocket

Joined: Sat Dec 02, 2017 12:27 pm
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Thanks, bandit2013, for providing the correct reverb cable connection. I see the gain was a consequence of having them swapped. So now that they are in the correct position, my problem is just no reverb!

I mistakenly thought something was better than nothing, but I suspect there is something wrong in the reverb tank as both cables are connected and have continuity.

Thanks for the help and suggestions. I'll let you know if I get it working.


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PostPosted: Tue Dec 05, 2017 12:37 pm 
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Single Recto
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tonesmyth wrote:
Thanks, bandit2013, for providing the correct reverb cable connection. I see the gain was a consequence of having them swapped. So now that they are in the correct position, my problem is just no reverb!

I mistakenly thought something was better than nothing, but I suspect there is something wrong in the reverb tank as both cables are connected and have continuity.

Thanks for the help and suggestions. I'll let you know if I get it working.


I opened this thread to suggest that the cables were swapped. I bought a DC-3 cheap not knowing that had this issue. I didn't know swapped cables would cause the problem back then.

It's not unusual to have a problem in the reverb tank itself. The tank in my Express 5:50 was replaced under warranty when it stopped functioning. Check the tube, too.

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PostPosted: Mon Jan 29, 2018 7:22 am 
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Dual Recto

Joined: Fri Feb 22, 2013 7:31 am
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Location: North Carolina
I would also check to see if the reverb tank has the cables in the correct position. the white wire connects to the side where it says "IN" . You can see it embossed on the metal tank if you turn the amp over on its side. There could be a number of things at play here, bad reverb tank, loose connection on the tank, no connection on the RCA (clip on the inside may be loose or not making contact with the center post of the plug, or bad solder joint. Tube is probable. You can also see if the reverb switch on the front panel does anything when turned on. V4B is the driver circuit for the reverb, and V3A is the return circuit. Other things that may cause the reverb to stop working other than the reverb tank itself could be related to the JFET used to mute the reverb. I would call Mesa tech service to help you in the right direction.

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