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Post subject: Help diagnosing FM25DSP
Posted: Mon Jul 12, 2010 9:26 am
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Wife bought me a DOA FM25DSP for $50. I live 70 miles from a good amp tech and I don't think it's worth spending amp tech money on, but I would like to fix it for fun. I have the schematic and some good basic tools temp controlled sodering iron, soder sucker, etc... Would one of you gurus be able to guide me where to start. Amp has power light, voltages right coming from transformer but no signal to speaker. Have hum to headphones when attached.

Any help would be appreciated.


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Post subject: FM 25 DSP
Posted: Mon Jul 12, 2010 12:51 pm
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Hey rksmith357,

If I read your post correctly, it sounds like an internal problem. Look for a "factory reset" button. I'm not familiar with the DSP, but some similar amps have a reset button. If the power light comes on, that would indicate that the fuse is good. If you have no sound out of the (known good) headphones, while strumming a plugged in guitar with all of the knobs turned up, then it may be a problem with the electronic circuit inside. Make sure to unplug the amp from the electrical outlet, and take the cover off and do a visual inspection of the amp internal parts to see if there is any obvious damage like a burnt component or broken wire or bad solder joints (use a lighted magnifier). Smell can tell you if anything might be burnt before you actually see it. The schematics are available from the Fender website, but if you can't see anything obviously wrong, this amp can be difficult to troubleshoot because it is all solid state. There is another fuse internally, check that also (total of two fuses).

http://support.fender.com/schematics/guitar_amplifiers/FM25_DSP.pdf

You might also want to check the speaker just to be sure. Unplug one of the speaker wires from the speaker terminal in the back (doesn't matter which one). Connect an ohm meter across both speaker terminals and check the speaker resistance. If you get a reading of 4 to 8 ohms resistance, then the speaker is good. If you read zero, very high, or infinite resistance then the speaker is blown. Let me know what you find.


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Post subject:
Posted: Mon Jul 12, 2010 1:19 pm
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Shimmilou,

Already checked the speaker, tested good. Checked the fuse, it's good. No reset switch. Next thing is to flip the board and look for cold welds. I'm hoping it's something like that as the amp was returned DOA out of the box and a bad solder joint would be an easy fix. Yeah downloaded the schematics first thing.

Let me know if you think of anything else.


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Posted: Mon Jul 12, 2010 4:02 pm
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This amp have 2 fuses, F2 before the transformer and F1 inside the amp.

It could be a semiconductor chip is defect or transistor.


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Post subject:
Posted: Mon Jul 12, 2010 5:01 pm
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stratele52,

I do not find a F1 fuse on the PCB or in line on P3 as on the schematic. But I've got ~11.5 volts at C38 which is good for the coupled P2 & P3 voltages.


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Posted: Mon Jul 12, 2010 8:55 pm
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If you have a DVM, you can start checking the outputs of the TL072's and 4560's. (Pins 1 and 7) You should get a bump in ac voltage at the outputs with your DVM when you strum the guitar.


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Post subject: Re: Help diagnosing FM25DSP
Posted: Tue Feb 07, 2017 9:32 pm
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The Fender FM25DSP has two common failures at least. One is the failure of the fuse position F1, not the larger F2. It is not a full sized glass tube fuse. On the circuit board it is labeled F1. Do a continuity test on it. If it's bad (open), solder a couple of wires and a full size fuse holder to the fuse leads, hot glue the holder to the circuit board and insert a 1 amp fuse, not the 750 ma fuse specified. The other common problem is the IC U8, a 5 volt power supply voltage regulator that supplies power to a few components, including the FM25DSP small circuit board. Test the output lead, pin 3 of the U8 and see if there is at least 5 volts DC. If not, replace it. 
That's all I got...Dave madasills@infowest.com


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Post subject: Re: Help diagnosing FM25DSP
Posted: Sun Feb 12, 2017 3:20 am
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Greepea,

It is a 2010 post :lol:


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Post subject: Re: Help diagnosing FM25DSP
Posted: Sun Feb 12, 2017 3:27 am
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stratele52 wrote:
Greepea,

It is a 2010 post :lol:


+1

I'm sure that the OP's amp has long since taken up residence in a local landfill.

:mrgreen:

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Help diagnosing FM25DSP
Posted: Sun Feb 12, 2017 4:10 am
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Retroverbial wrote:
stratele52 wrote:
Greepea,

It is a 2010 post :lol:


+1

I'm sure that the OP's amp has long since taken up residence in a local landfill.

:mrgreen:



Arjay


+1000 :lol:


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