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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Thu Jan 19, 2017 3:24 pm
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Wildstrat wrote:
I think I didn't understand what you said. Your suggestion was to make a mod on my Twin Amp?


Some mod ? yes and no ; Only If some shield wires may help to quiet the reverb. I will replace some unshielded wires ( if needed and possible with a printed circuit board !!! ) with shielded wires.

A tech should know wich wire act like antenna and make noise when he have a look inside the amp.

With printed circuits it may impossible too :(


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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Fri Jan 20, 2017 9:37 am
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I found this service note in an addendum to the '94Twin service manual.
Changed the Reverb muting circuit to eliminate noise in the reverb recovery circuit caused by poor power line conditions. Change R59 from 100k to 1.8k, add Q2
(JFET) and R155 (1.8k). (See zone 4C on rev C schematic).
Refer to Tech Note TN96-

http://bmamps.com/Schematics/fender/94_ ... Manual.pdf

There are a few other mods suggested there. If your amp hasn't been updated, you should do them. A Fender service center should be able to determine whether they have been done.
The other things to look for are leaky or broken electrolytics. If there is a tan ooze coming out around the positive leads of the caps.
On this model, I have encountered broken leads on the electrolytics in the power supply. Its not the wire that breaks. Its the carbon rod inside the cap. I think the vibration from speakers causes it.
The only way to detect this problem is to tug on the positive lead with needle nose pliers and see if it pulls out.

Another simple issue that occurs in any amp is that the tank is oriented so that the output coil is under the power transformer. Try swapping the tank end for end to see if that doesn't improve it.


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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Fri Jan 20, 2017 9:40 am
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TimAudio you have always the right answer

+100


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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Fri Jan 20, 2017 10:14 am
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Thanks TimsAudio. But mine it's not the 94 model, but the next generation. It's from 2002.


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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Sat Jan 21, 2017 9:39 am
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The reverb on both amps are very similar.The additional JFET is incorporated in this model on the reverb recovery mute circuit.
A bad jfet could cause the hum, but I suspect the big electrolytics have internal breaks. I went round and round with a similar problem described in my very first posting here. in that case, I found bad 20/500 caps on the main board and 10/500 caps on the controls board. They weren't leaky, but broken inside. My cap checker said they were good. Only by pulling on the leads did I find the cause. According to my notes, I also found a noisy reverb select switch.

In any case, I don't think you're going to get it fixed by fiddling with external components.
You need a good tech, some new caps and a good resolder job, given its age.


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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Sat Jan 21, 2017 11:49 am
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Many thanks, to all of you.
I'll save some money and put the amp on repair. I'll pass your notes to the tech guy to speed up the solution.

Best regards.

PS - apart the weight, I like everything on my amp. It's made to meet all my needs. It would have been hard if I had to get rid of it because of this issue.


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Post subject: Re: Noise on Twin Amp with reverb on
Posted: Sat Jan 21, 2017 11:58 am
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TimsAudio wrote:

In any case, I don't think you're going to get it fixed by fiddling with external components.
You need a good tech, some new caps and a good resolder job, given its age.



+1


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