no sound... at all - Im stumped

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tom_k
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no sound... at all - Im stumped

Postby tom_k » Sat 12/22/18 9:38 pm

I have just complete a Mojotone 18 watt build.
All went well, the amp worked well.
Really sounds great, no hum or other noises or issues,

Next day, turn on and blows a fuse. I replace fuse and it powers up.
But no sound, No hum, nothing. It is like the speaker outputs are not connected.

Could something have happened to the output transformer?
How do I test it?
Any other ideas?
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JMPGuitars
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Re: no sound... at all - Im stumped

Postby JMPGuitars » Sat 12/22/18 11:49 pm

First, I would be concerned why you blew a fuse the next day. My first thought is that maybe you have some shrapnel, or loose wire floating in your chassis that got jarred and caused a short?

Before doing anything else, unplug your amp, turn it upside down, and shake it out. Like you're a cartoon trying to steal its lunch money. If you have a blower, use that too. Then inspect for loose wires, or any frayed wire ends that might be grounding out that shouldn't be.

Make sure all your ground connections are secure, and that none of them have come loose or broken. Make sure the tube sockets don't have any loose or broken wires.

If you have more than one fuse in the amp, check them all.

Make sure everything is connected. Check the speaker connections at the speaker, and at the amp. Test the speaker cable(s) if you have to. Check all the volume knobs, including the guitar.

Once you've done all that, test the amp again. If you still have no sound, try swapping the tubes before you start worrying about the transformers.

If all that fails, it's time to test your voltages and do a highlighter test. Let us know how everything else goes first.

Thanks,
Josh
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tom_k
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Re: no sound... at all - Im stumped

Postby tom_k » Sun 12/23/18 8:50 am

Thanks for the reply

Tubes are good - I do not have a back up rectifier.... so i did not swat that one.

nothing loose and everything looks tight. speakers and cabling all good. I am fairly new to amp builds. this is my third project. It seams odd that there in absolutely no noise or hum at the speakers....

Is there a voltage test chart for this amp? I can check it all, just do not know where or to what values to test...
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Re: no sound... at all - Im stumped

Postby JMPGuitars » Sun 12/23/18 11:18 am

storage/18_watt_voltage_chart_template.doc

storage/Voltage-Chart.htm

Post some current photos also.

Thanks,
Josh
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Re: no sound... at all - Im stumped

Postby tom_k » Sun 12/23/18 5:37 pm

Problem solved! um.... I stupid.....
Looked at pin voltage and have crazy numbers.... looking at the tubes, I noted an EL84 in the rectifier socket. hmmmmm. No I dea how that got there..

Popped in the EZ81 and bingo. all working.

I have attached my pin voltage chart. - Keep in mind this is a TMB Circuit.
I am not sure what they should be... but the amp is quite and working.
You do not have the required permissions to view the files attached to this post.
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Re: no sound... at all - Im stumped

Postby Daviedawg » Mon 12/24/18 3:30 am

The voltages on your rectifier are good. The plate voltage on your EL84s is in the right region at 325 or so for the tone you are looking for. But there should not be 25 or 30 volts on Pin 2. Are you sure you read that correctly?
Your PI (V3) voltages look fine.
I do not have any examples of preamp voltages for the TMB to compare.


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Re: no sound... at all - Im stumped

Postby JMPGuitars » Mon 12/24/18 8:27 am

Dd is correct, did you mean mV for pin 2 of your power tubes?

If you did mean volts V, and not millivolts mV, then your path to ground needs to be resoldered.
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