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Post subject: frontman 25r - drive channel always on
Posted: Mon Apr 01, 2013 3:36 pm
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Hi

I have a problem with my frontman 25R.

The amp doesn't switch between the normal and drive channels. Whatever position the selector switch is in, the amp responds to the gain/drive volume controls, but in the clean position the volume control has no effect. The selector switch seems to be ok, as the LED switches on and off.

It has a 5 year warranty, but unfortunately I bought it 5 and a half years ago ... :s So I'd like to repair it myself if I can. I've done a bit of electronics, know how to solder, and have access to oscilloscopes, voltmeters etc.

Thanks


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Post subject: Re: frontman 25r - drive channel always on
Posted: Mon Apr 01, 2013 6:55 pm
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Hi scalpas,

Sounds like Q1 and Q2 transistors aren't switching off when you switch channels. Since the LED switches on and off, I would expect to see the +13.9 VDC when drive selected, and -13.9 VDC when clean is selected, out of pin 7 of the op-amp U5. That voltage is fed to R54, dropped and then to the cathodes of D5 and D6 which feed the transistors used to switch channels. Do you have the correct voltages at pin 7 of the op-amp? If so, check to see if you get a small voltage change at the gates of the transistors (middle pin).

http://support.fender.com/schematics/guitar_amplifiers/Frontman_25R_schematic.pdf

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Post subject: Re: frontman 25r - drive channel always on
Posted: Mon Apr 01, 2013 9:18 pm
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Hi Shimmilou. Thanks for the answer. I'll try what you suggested.
When taking the circuit board out, are there any precautions I should be taking other than unplugging the power supply ?


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Post subject: Re: frontman 25r - drive channel always on
Posted: Mon Apr 01, 2013 9:38 pm
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Of course you'll need to do the testing before removing the board, which will require the amp to be on, so take the normal precautions when working on a live electrical circuit. Be sure that the caps are discharged before removing the board.

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Post subject: Re: frontman 25r - drive channel always on
Posted: Tue Apr 02, 2013 6:11 am
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Good point ! :)
What are the normal precautions, apart from not touching the circuit with your fingers ? Until now I've only done 5V analogue circuits ...
To discharge the caps I just need to leave the amp turned off and plugged in for a few minutes right ?


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Post subject: Re: frontman 25r - drive channel always on
Posted: Tue Apr 02, 2013 6:22 am
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scalpas wrote:
...What are the normal precautions, apart from not touching the circuit with your fingers?.....
To discharge the caps I just need to leave the amp turned off and plugged in for a few minutes right ?


Right, don't contact the live circuit with any part of your body. Always check the caps with a volt meter before proceeding with dis-assembly.

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