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Post subject: Fender Frontman 25r blowing fuses
Posted: Sun Apr 07, 2019 7:12 pm
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Hi there, first time poster. I have a Frontman 25R Series II that blows fuses. You'll need to refer to schematic and and service diagram 356ICT002 but when I disconnect the power from terminal P10 power the fuse does not blow and the amp works. Output voltage from the transformer that would connect to P10 is normal. When I check voltages from TP1 through TP6 I only have voltage at TP1. Does anybody know if this is a capacitor thing or a diode thing or any advise, thoughts would be appreciated. Thank you.


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Tue Apr 09, 2019 2:09 am
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Location: Province de Québec, Canada
You may have more answer if you post link for diagram.

You disconnect power from P10 ( do you mean TP10 ? ) fuse don't blow and amp work !!!!! :shock:

Leave it like that and enjoy the amp :lol:


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Tue Apr 09, 2019 1:27 pm
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Works, but probably doesn't work normally.

Normally, I would say it is an amp that cost about $100 new, so it is not worth fixing.

But that makes it ideal for learning basic troubleshooting, de-soldering, and soldering skills on a possibly fragile circuit board. Would I rather that they would take it to a tech? Yes, if that is an option available to them, and makes economic sense.

My intent is that people come here to get the best advice possible, and secondly, learn something they didn't know before. That probably doesn't align with Fender's reasons for sponsoring the forums, but I don't really give a rat's $@!. They have their reasons. I have mine.
Hope it doesn't get me banned.


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Tue Apr 09, 2019 2:56 pm
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P10 is the ac input to the negative power supply.
On amps like these, the first thing to do is pull the power pins from the output ic. That is pins 4 and 6 on the U6, TDA1514a. You might look for a little crater on the face of the ic. Or there might be a little black soot near the mating edge of the plastic and metal of the ic.
Good hunting


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Tue Apr 09, 2019 3:12 pm
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@stratele52, yes, P10, not TP10.

@vinyl, thank you for your reply. I know it is not an expensive amp, but I'd rather fix this one instead of replacing it and throwing this one away and adding to the landfill. I am technically inclined, can solder and like the challenge of trying to fix this.

@timsaudio, thank you for your reply. I will check out U6 and let you know what I find. If I do not see any soot or cratering, is there a way to test the U6 by measuring ohms across any of the pins?

Thank you in advance for your input.


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Tue Apr 09, 2019 4:40 pm
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Well, you can check with an ohmmeter between those same power supply pins and the speaker output pin. In your case, it sounds like the negative supply is shorted to the speaker terminal. A good one should be high resistance. They are available for a few bucks.


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Mon Apr 15, 2019 6:52 pm
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Hi Tim,

Just as an aside, the power amp is not a TDA1514a. It is a LM3876T. I assume Fender used both? I haven't removed it from the board but I don't see burn, soot or marks. I think I will just replace it since I have to remove it to check it anyway. Make sense to you?

Thank you,

Andrew

Andrew


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Post subject: Re: Fender Frontman 25r blowing fuses
Posted: Tue Apr 16, 2019 7:44 am
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The LM3876 uses pins 1 and 4 for power supply and pin 3 for the output.
You can measure it without pulling it.
If its blowing fuses, it will measure almost a dead short from ps to output.
If its suspicious, you can cut the lead right where it necks down near the case. That way, you can resolder it topside without loosening the solder underneath.


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