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Post subject: The Twin : noise problem.
Posted: Wed Aug 17, 2011 9:03 am
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Hi to Everyone, first post.

I have a newish "The Twin".
When the power switch is on low, 25w, and some reverb is dialled in, if the cabinet is disturbed or I thump it I get pops and crackles. It sounds as if there is a dry joint or loose connection. But, if the power switch is on full power, the fault disappears.
I have had the chassis out on the bench, plugged up and switched on looking dry joints using what I know in England as Vibration Testing, ie tapping all components with an insulated minature hammer, or in my case a rubberised ball point pen.I cannot find the problem. All valves have been reseated and leads and sockets checked.
All I have found is that C45 is microphonic (do I need to change this?).
Just to confirm which Fender model this is, the service diagram number is 57014000, rev F.
Could C45 be the prob even though the fault only appears when its on low power?
Cheers from England,
Mel


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Post subject: Re: The Twin : noise problem.
Posted: Wed Aug 17, 2011 9:48 am
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What is a newish The Twin or do you mean The Twin Amp?


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Post subject: Re: The Twin : noise problem.
Posted: Thu Aug 18, 2011 12:42 am
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Hi,
yes a nearly new "Twin Amp" . Manufactured october 2010 i think.
I mentioned it was nearly new as there appear to have been several versions of this amp. The Service Diagram is up to rev F.
Earlier versions seem to have had a balanced out jack to send a feed to the desk, mine doesnt have this, just a preamp out which is not buffered.
Cheers
mel


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Post subject: Re: The Twin : noise problem.
Posted: Thu Aug 18, 2011 2:12 am
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Hi,

Welcome on the forum. First clean up the valve sockets with an appropriate cleaner (ask to a local amp tech). Put a mark on each valve in order to not mix them. Second test with reputed good valves one by one excepted the quartet. Let us know.

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Post subject: Re: The Twin : noise problem.
Posted: Thu Aug 18, 2011 9:04 am
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Check the Johnson rod...

Man, you guys are so over my head on this stuff. Must be nice to be able to work on your own stuff.


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Post subject: Re: The Twin : noise problem.
Posted: Thu Aug 18, 2011 9:59 am
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Hi dawntreader,

It appears that you have "The Twin Pro Tube". I couldn't find C45 on the schematic (rev B), where is it located? But, likely a tube is microphonic near the capacitor, not the capacitor itself. Do you have the noise with the reverb off? If no noise with reverb off, then possibly a bad reverb tube, V7 (12AT7) or V5 (12AX7). :idea:

As Tis-san suggested, try swapping some tubes with a known good one and see if the noise changes.

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Post subject: Re: The Twin : noise problem.
Posted: Fri Aug 19, 2011 3:24 am
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Hi,

my schematic is revision F, the capacitor is a coupler between the output of the clean channel vol and V1b. My friend says change this one first and then see what happens. Its mounted on the sub board with all the pots so unlikely its a valve, and I have been swapping valves around to improve the tone, and its made no difference to the fault. Discovered a well balanced replacement for the PI has improved the tone quite a bit but thats another story :)
To recap : the fault is NOT present when the amp is on full power, I can use the reverb no probs, and thump hell out of the cabinet and no noise apart from the echos from the reverb. The fault is apparent when on low power AND some reverb is used. If I thump it then I get loud bangs and crackles that can last for 1-2 secs after the disturbance.
I will change C45 and then let you know the outcome.
Thanks to all for the input.
Cheers
Mel


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Post subject: Re: The Twin : noise problem.
Posted: Wed Aug 24, 2011 1:01 am
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Hi,

I have changed C45, it fell apart as I removed it from the PCB so it was obviously just hanging in there waiting to fail altogether. It seems like I have slightly more volume and bite on the clean channel now. Its hard to say for definate as position in the room can change the overall tone anyways. Also, I think there is less background hum and hiss so a good result.
The noise prob is still there tho as stated above on low power. I can live with this as I very rarely use the low power setting. Gut feeling is still that its contact bounce on the high/low power switch from the vibrations.
Thanks again everyone :)
mel


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Post subject: Re: The Twin : noise problem.
Posted: Fri Aug 26, 2011 2:09 am
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Ok, I finally got to find the fault.
The wiring from the output socket to the spring assembly in the reverb tank is wrapped around a little tag to secure it. This tag has very sharp edges and was intermittantly making contact with the signal wire, thus earthing it .
Can someone please explain why it was only a problem on the low power setting, no noise at all when on full power. I can't see the reason, its not logical captain :)
Cheers,
mel


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Post subject: Re: The Twin : noise problem.
Posted: Fri Aug 26, 2011 8:06 am
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It doesn't really matter why, it is logical as evidenced by the fact that it did cause the noises. What is even more logical is that you only had the problem when using the reverb, which indicates a problem in the reverb circuit. Makes perfect sense to me. Good job finding the problem. Hope that you can now enjoy the amp without the noises. :)

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Post subject: Re: The Twin : noise problem.
Posted: Tue Aug 30, 2011 12:55 am
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Thanks shimmilou, I just have a curious disposition :)
I am really enjoying this amp, its my first all valve. I play mainly clean and as I mentioned earlier, it is worth trying different tubes in the preamp. I now have a slightly richer tone , not so sharp, that breaks up a little later, which is more to my liking and fits in with the rest of the band better .
Cheers,
Mel


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Post subject: Re: The Twin : noise problem.
Posted: Tue Aug 30, 2011 4:16 am
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Me too, I'm curious and I still don't understand !

What was touching the the tap ? (shield or wire?)

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Post subject: Re: The Twin : noise problem.
Posted: Tue Aug 30, 2011 4:46 am
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tis-san
If you look inside the reverb tank you will see that the internal wiring from the socket to the little coil actually on the spring assembley is wrapped around a thin metal tag . this tag is part of the input or output socket. it has sharp edges. the wiring is just a twisted pair although not very well twisted. i believe as the wire was wrapped tightly around the tag, the sharp edge had cut thru the insulation and was making intermittant contact when subject to vibration. i freed the wiring from this tag and have cleared the fault. I dont know if the tag is at earth potential, or if it was making contact with both wires.
mel


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Post subject: Re: The Twin : noise problem.
Posted: Tue Aug 30, 2011 7:15 am
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You were very fortunate to isolate such a weird and obscure problem -- I wonder how many more of these amps are out there with that same malfunction.

Rawk on

8)

Arjay

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