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. Schematic attached.
Any help would be appreciated.
Headphones hum, but do they pass guitar signal? DO any of the controls affect this hum?
Is there ANY background noise in the speaker or is it utterly silent?
Common problems with small amps are broken solder under the input jack, or broken input jack.
Enzo,
Thanks for the post.
Does not pass the guitar signal. None of the controls effect the hum. Speaker is totally silent. I've now tested the speaker and it's good. As soon as I can I'll take the board out and look for cold solder or broken solder joints.
Let me know if you have any other ideas.
Start at the start. The power supply. The thing runs on +/-27VDC for the amplifier, adn +/-12VDC for the preamp. Look at teh schematic. Do you have something like 25-30VDC both positive and negative at the main filter caps? And ther are two 5 watt resistors of 270 ohms that drop the 27v down to the 12v zeners, so is ther 12 volts of each polarity? Here is a tip - find the two 270 ohm 5 watt resistors. Measure voltage at each end. One end should be 27v and the other end 12v. The 27v wil be approximate, the 12v should be pretty close.
Enzo is right the powersupply is the place to start however, if you have hum at the headphone jack chances are you have power to the TL072 opamps , but no signal , I would puld the board , check for bad solder joints, make sure you check the input jack fender used some really cheap input jacks on this amps. Other than that without more info it's really hard to give any valueable advise on this one.
Rock On