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Post subject: No Sound - Fender Twin Amp
Posted: Sun Jan 04, 2015 7:18 pm
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I need a bit of help identifying an issue with my Fender Twin Amp (2003 Pro Tube model)...

I was at band practice the other day and the amp was working fine. I stopped to chat for a moment between songs and rolled back the volume knob on my guitar. When I went to turn the volume back up to play there was no sound. There was no indication that anything happened to the amp while chatting - No hum, crackle, pop...just silence.

The power indicator light comes on like normal, but there is no sound. The amp doesn't even hum or hiss like normal when turned up. It's totally quiet.

I thought maybe it was something obvious but I checked all the silly stuff (changed guitars, cables, etc.) with no luck. So I've started troubleshooting and still haven't found the culprit. Here's what I've done so far...

Unhooked the speaker cable from the amp and plugged it into another amp. Worked fine. Ok, nothing wrong with the speakers or speaker cable.

Checked fuses. All look good.

Swapped out preamp tubes one by one. Nothing, no changes. Just the same silent amp.

Swapped out power tubes. No luck there.

In addition to the effects loop on this particular amp, there are Pre Amp Out and Power Amp In jacks. I plugged my guitar straight into the Power Amp In jack and got sound. Ok, good to know.

That's all the more I've done so far. Any ideas on how to proceed. I know enough that I'm not going to put myself or the amp in danger, but I'm no well-seasoned amp tech.


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Sun Jan 04, 2015 8:21 pm
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Hi tyler23,

Seems that you've narrowed the problem to the preamp section. Can you use a meter and read schematics?

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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 1:57 am
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Yes, I can use a meter and read schematics (well enough, anyways). This was going to be my next step. I already have the schematics for this amp, as I've had to reference them in the past. Is there anything in particular that I should be looking for?


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 3:01 am
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tyler23 wrote:
I need a bit of help identifying an issue with my Fender Twin Amp (2003 Pro Tube model)...

I was at band practice the other day and the amp was working fine. I stopped to chat for a moment between songs and rolled back the volume knob on my guitar. When I went to turn the volume back up to play there was no sound. There was no indication that anything happened to the amp while chatting - No hum, crackle, pop...just silence.

The power indicator light comes on like normal, but there is no sound. The amp doesn't even hum or hiss like normal when turned up. It's totally quiet.
Power indicator glow = fuse is good

I thought maybe it was something obvious but I checked all the silly stuff (changed guitars, cables, etc.) with no luck. So I've started troubleshooting and still haven't found the culprit. Here's what I've done so far...

Unhooked the speaker cable from the amp and plugged it into another amp. Worked fine. Ok, nothing wrong with the speakers or speaker cable.

Checked fuses. All look good.

Swapped out preamp tubes one by one. Nothing, no changes. Just the same silent amp.
If you don't use know good tube , this test is not reliable . If you work with your tubes , you just need only one bad = no sound

Swapped out power tubes. No luck there.

In addition to the effects loop on this particular amp, there are Pre Amp Out and Power Amp In jacks. I plugged my guitar straight into the Power Amp In jack and got sound. Ok, good to know.

That's all the more I've done so far. Any ideas on how to proceed. I know enough that I'm not going to put myself or the amp in danger, but I'm no well-seasoned amp tech.



Are you sure there in no light hum or hiss coming from your amp ?
Ear close to the speaker and NO guitar plug ?

Did you flip Standby switch to "play position " ?

It look you have the phase inverter tube out of order . It is the one close to the output tubes .
Or you may have few output tubes dead.


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 4:31 am
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tyler23 wrote:
...I plugged my guitar straight into the Power Amp In jack and got sound...


This means that the PI and output tubes are working.

Check all power supply voltages, from sources all the way to tubes (Plate voltages, heaters, etc).

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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 4:34 am
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shimmilou wrote:
tyler23 wrote:
...I plugged my guitar straight into the Power Amp In jack and got sound...


This means that the PI and output tubes are working.

You are right , I did not read all , cause my poor english

Check all power supply voltages, from sources all the way to tubes (Plate voltages, heaters, etc).


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 7:46 am
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All inputs are quiet?

Check to see if the all the small 12AX7's are glowing when you turn the amp on. Should see orange glow at the base of each tube --- even with the Standby still on. If not, check for break in heater line to the sockets.

You have the '57 Twin, correct? If all tubes are heating correctly, check R43. It maybe burnt, broken, way out of spec, or poorly soldered into the circuit. This resistor is in the power supply and drop the voltage to the front end (front end, pre-amp).


http://bmamps.com/Schematics/fender/57_twin_amp.pdf


Good luck! :mrgreen:


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 2:28 pm
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tyler23 wrote:
...Fender Twin Amp (2003 Pro Tube model)......


Not the '57.

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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 2:42 pm
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OK... any schematics of the model in question?


Thanks!


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Mon Jan 05, 2015 2:44 pm
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Pro tube here ;

http://blueguitar.org/schems.htm#Fender


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Tue Jan 06, 2015 12:59 pm
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Thanks for the help, everyone. After a bit of exploration with a meter and the schematic, here's what I found. Test points TP52 and TP53 (electrolytic capacitors) on the schematic appear to be problematic. They are supposed to read +315VDC and +242VDC, respectively. I was getting 2-3VDC on my meter. All other test points I checked seemed to be ok (including TP50, 51, 54-58).

Again, I don't know much about this stuff. Is this definitely what would be causing my issue, or are there additional points on the circuit I should check?


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Tue Jan 06, 2015 1:10 pm
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tyler23 wrote:
T , here's what I found. Test points TP52 and TP53 (electrolytic capacitors) on the schematic appear to be problematic.

A ?



Problem is comming from resistors feeding these TP and capacitors


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Tue Jan 06, 2015 1:16 pm
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If TP51 is good as you say check resistor 205 , 10 K , 2 watts . Check value with desoldering on end , or it is only a broken printed circuit , check this carrefully . Sometime printed circuit burn.


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Post subject: Re: No Sound - Fender Twin Amp
Posted: Tue Jan 06, 2015 5:10 pm
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+1 stratele52

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Post subject: Re: No Sound - Fender Twin Amp
Posted: Wed Jan 07, 2015 12:02 am
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Ok, so I took a look at R205 and found a couple of interesting things. I didn't take any measurements as I was unclear about what exactly I was supposed to measure and decided to play it safe. Perhaps someone could clarify what stratele52 means by "Check value with desoldering on end".

So, I found that not only is there a scratch across the printed circuit, but there also appears to be an odd-looking solder joint on the resistor. I attached some photos so you can see what I am describing.

The solder joint feels solid. I poked at it with my trusty chopstick probe, and nothing felt weak or loose. As for the scratch, I have no idea how or when that happened. Is there anything else I can do or check, or is this the point where I've done all I can and now I take it to an amp tech? Once again, thanks for all the help.


[img][IMG]http://i76.photobucket.com/albums/j7/t_speicher/1_zps99bfcf69.jpg[/img][/img]

[img][IMG]http://i76.photobucket.com/albums/j7/t_speicher/2_zps4b18ac23.jpg[/img][/img]

[img][IMG]http://i76.photobucket.com/albums/j7/t_speicher/3_zps35f8d4a0.jpg[/img][/img]


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