Broke Amp Blues...

[Luke51411]

I built a Nurse Quacky envelope filter on Vero and connected it to jacks and power via breadboard. It worked fine at first although there was some distortion at some settings when strumming all the strings fairly hard (though in hindsight I'm not sure if that was coming from the circuit or the amp). After a while it fizzled a bit and the output volume was quite a bit lower. I didn't have the amp too loud to begin with (it is a fender frontman 25r amp). I unplugged the circuit and went straight in to the amp and now it distorts on the clean channel (and not a pretty distortion) when chords are strummed. Could I have caused this by something being incorrect or shorted in the circuit? I'm thinking most likely its a function of the amp being cheap but I want to make sure I avoid any activity that could damage an amp in the future.  What possible things could  you do wrong in stompbox circuitry that could damage an amp? Sorry if this is a newbish question but I guess sometimes I have to learn by breaking stuff
I haven't done a lot of testing with the amp/other guitars as it was getting late. Maybe its just a loose connection in my guitar as I installed the pickups quite a while ago when I was *inexperienced* at soldering. I've built quite a few stompboxes now with no ill effects to this point so maybe there's something I'm missing. Hopefully it's something I can fix because although I would love to have a new amp... Most of my extra cash is currently going to my wife's therapy so that's out of the question at this point... Plus I've already spent enough on my newfound obsession with building stompboxes as I'm sure you can all relate to

[GibsonGM]

Luke - things like this happen )   Generally not because of anything we build.

1) First, I'd take the Quacky and connect a patch cord to the output (or just use the jacks from inside).  Turn it on, measure for DC at the output. You should get NONE.    Leaking caps can let DC by, which can cause weird problems with amps.  If you measure any, change the effect's output cap.

2) Really, you shouldn't have been able to toast the amp with the pedal.  Could have had a problem before that you made worse, like something wrong withe power supply, a bad resistor/solder joint somewhere causing misbias (what your describe sounds like this).   Something ready to go that just did because you asked the amp to "sprint" a little instead of walk.   

If you DID overdrive an amp way too much, PERHAPS you COULD (theoretically) blow an input cap between the input jack and first gain stage.  Not likely unless you EXCEED the cap's DC rating by having a leaky output cap (or run another AMP into the amp input!).  I'd expect to have to go over 12VDC to do this on anything....or higher. And you'd have no input.   

I really think cheap amp, got pushed a bit and didn't like it, cheap solder connection or something on a bias resistor to a gain stage shook loose, now you are ugly-rectifying your input signal into a razor-like sick sound....gotta look inside if it continues and do some chopsticking.

Hope that helps!

[Luke51411]

Thanks, very helpful. Now I have a few more things to check out. One more thing, I did check the drive channel on the amp and that seemed to sound ok though I never use that channel so I don't have anything to compare it to. If I have to open up the amp is that something I can do safely? I am aware of discharging filter caps. I don't want to get in over my head.
here is the schematic of the amp http://support.fender.com/schematics/guitar_amplifiers/frontman_25r_schematic.pdf

[Luke51411]

It sounds bad in both clean and distorted channels but the headphone out seems ok

[Luke51411]

My bad on the link this one should work. http://support.fender.com/schematics/guitar_amplifiers/Frontman_25R_schematic.pdf
Is there a recommended forum for amp repair?

[GibsonGM]

My bad on the link this one should work. http://support.fender.com/schematics/guitar_amplifiers/Frontman_25R_schematic.pdf
Is there a recommended forum for amp repair?

Hi Luke, you can get more in-depth help at Diyaudio.com...there is a section just for amps.  It's kind of all the same, though...without actually seeing what you're doing, you will have to walk thru on your own - GREAT way to learn more and start  DOING - if you're comfortable with doing so!  You have built some stuff, so you've got SOME experience, but you have to know if you are ok for this or not.    I could fix it, but don't know if YOU can, IOW....yes, you should discharge the "Big Caps" of the power supply, but in this case the voltage is not that high.  Still do it, but don't get frightened by high voltage, ok?  You're under 30V there, and anywhere on the board(s).    At any rate, you might be able to fix it, or you might ruin it - kind of has to be your call.

You need to get your voltages at the last opamp (the TDA), to the left of the headphone out jack (driver).   Just wondering if something is up in that area.  Diode(s)?  Could even be a blown speaker, maybe? <can't hear it to say>

Short of an obvious problem there, I would run from the speaker into the amp, in reverse, with an audio probe, inputting a sine wave at 440Hz or so with the amp on, and listen to the signal.   At some point it will start sounding *crappy*, and then you've found your problem area.   Luckily you GET some signal out, so you can go in there and find the issue.  You have to BE CAREFUL if you have the power on while doing this!!  If you don't know the safety steps of how to do this, please stop right there and write back and I'll walk you thru them.   

If it sounds like poo at the speaker, and at that last opamp, at the transistors Q3 & Q4, and so on, I'd sub in a different speaker...nothing saying you can't sub one in from the start, to be sure!  A different speaker might 'fix' it.   If you still get a bad sound, be sure to poke parts (medium gently, don't pound on them, just a light push) with a non-conductive thing like a chopstick, piece of wooden dowel etc., while the signal is still going.   You are trying to find loose connections.  Don't short out anything!

SOMETHING is out of sorts in there, and your job is to find it!

*****   BE CAREFUL NEAR THE TRANSFORMER, ONE SIDE IS 120V AC DANGEROUS VOLTAGE!!!  Stay outta that stuff!  Make SURE you can't accidentally contact that transformer in any way while working, and unplug amp again after you make a test, each time... ****   Pretend you're measuring radioactive stuff and be all cautious - it's a very good habit to get into )

[Luke51411]

The speaker has quick disconnects and I ran it with a 9v mini amp and it sounded good/ had good output(for a little battery powered amp) Does this tell me anything about the speaker since it's not driving it very hard? I don't have access to any speakers to sub in other than small 3.5inch speakers I got to build cigar box amps with.

[GibsonGM]

That probably suggests the speaker is ok.  The more you pushed it with the small amp, the better.   Now, if you turn on 'broke amp', and even at very low volumes it still does "the crackle" thing, you can probably be sure that it's NOT the speaker.

What you describe really sounds to me like something is physically loose, has maybe 'popped off the circuit board', like one end of a resistor.  As long as you've tested EVERYTHING you can (use guitar and cables on another amp to know it's not them), it seems like time to GO IN THERE!

Have you made/used an audio probe before?  And do you have a decent DMM that you know how to use?

[Luke51411]

Yes and yes, I've tested out all the cables I have and tried my acoustic guitar and all the same deal. I guess its time to break out the screwdriver and DMM! What is the best way to drain the filter caps?

[Gus]

It sounds bad in both clean and distorted channels but the headphone out seems ok

look at the bottom right of the schematic linked in this thread, if the headphones sound good but the speaker bad this could be a dirty connection at J3
There is a connection to the external speaker shown at J3.]

With the amp powered off plug and unplug a plug into the external jack a few times maybe 5 or 10 times this might clean the contact(the up arrow at J3)

It could be other things but this is something simple to try first

[Luke51411]

How do I tell if it's the speaker? If it sounded fine with a ruby amp running at 12v could it still be bad?
It didn't distort with picking but only when strumming though it did seem to get worse the more I played with it

[GibsonGM]

What Gus means is that the jack to plug in an external speaker might be dirty, not 'closing' right. When not in use, it acts like a closed switch to complete the circuit to the internal speaker.  If you plug in, it should cut off the speaker and let you run a cord to a cab.   Amp off, find the jack and just plug and unplug a patch cord into that jack a dozen times to see if that changes anything (or solves problem).    Dirt in there could open the switch a bit, make it "rectify" a little and distort your speaker.  You might have to turn the amp upside down and look in at the chassis to see the jack, I bet.

I saw it, but figured you would be probing in there, anyway...it's a good last test.


For what comes next (after the above is tried), you don't drain the caps.  This isn't a tube amp, and you aren't going to touch anything at ALL in there with your hands!  Just a chopstick or audio probe.   This is your amp, right?  http://static.bax-shop.nl/components/com_virtuemart/shop_image/product/Fender_Frontman_25R_25_Watt_1x10_inch_gitaarversterker_combo_rear.jpg

This type requires you to remove the screws on top of the amp and drop the chassis down and out the back. Disconnect the speaker first.  You'd run jumpers to the speaker for testing.

You sure you're ok to undertake this, Luke?  You have to be *near* 120V mains voltage, where it comes into the power transformer.  From there it drops way down, but you could easily fry a chip or create some other short if you aren't careful.  Or yourself, if you grab hold of a possibly bare wire or switch terminal.   You have any tech-type friends, or know an amp guy?   This repair WILL be simple, but you can smoke these things very easily...even yourself.    It's that tough gray area where you HAVE to start doing things, but maybe you don't have the experience to start on your own, you know?  You learn by doing, but it's GREAT to have someone help who knows what's going on!

This is about what you'll see inside, with a power transformer nearby that those power wires there run to:  http://4.bp.blogspot.com/--96ITqmrN1g/TrR08wiZI9I/AAAAAAAAAOg/fy84glsOpk8/s1600/circuitboard.jpg      It would be up to you to find out how things are connected and what's causing the problem...

[Luke51411]

I opened it up and probed around with the dmm. Couldn't find anything obvious the problem is, the transformer is right on the bottom of the chassis so there is no way to set it flat to probe around with it plugged in and I wouldn't want it unstable. I'm going to think about it for a bit to see if I can come up with a better way. I checked the contacts on the headphone out jack with the dmm and there was no resistance with nothing plugged in. I also played around with my ruby amp and the speaker more just to make sure that is not it. I was able to get it louder than the level where it starts fizzling when pushing it with a booster and it was still good so I'm pretty confident it's not the speaker which is probably a good thing since I don't think it would be worth putting a speaker in the amp that costs almost as much as the amp. I think I'm going to table this for a bit until I get more experience or find someone that knows what they are doing. In the mean time I think I will try and build an amp that can run off of a wall wart that has a bit more output than the ruby. Thinking about the tda2040 power amp on GGG http://www.generalguitargadgets.com/projects/13-amps/59-tda20x0-amp and then one of the preamps from ROG.

[Luke51411]

And yes that is my amp.

[GibsonGM]

That might be a good idea, Luke...just put it aside, maybe make some notes or something so you can go back to them.    I pull the chassis right out of those, and find a piece of wood or something to rest 'the other end' on, with transformer on the ground.  This makes it stable for testing.  I run jumpers to the speaker if the leads aren't long enough. 

You'll have to check out what chips those are, and what their input/outputs are (data sheets).  You audio probe back from the speaker out until you find the place where it does NOT make crap noise...the one before is probably the bad section, or connected to something bad.  This is not a complex circuit, and in time you'll be able to check it out no problem.

Build something first, good idea, and then go back to it!  Fixing little amps like this is a great way to get in the door, as long as you know what to do.    With it off, there's nothing dangerous in there...just the transfomer and switch etc. when it's plugged in.  30V is considered the beginning of the danger zone.    It's bad form to treat ANYTHING that runs on line power with trivia, so we learn to treat them all cautiously unless proven otherwise.

Let us know when you're going back in!!  )

[Luke51411]

I decided to reflow some solder joints and I'm pretty sure I saw a small crack on one of the input jack lugs. I can't be sure since I soldered it pretty quickly and solder plays tricks on my eyes sometimes. Hopefully that will be the fix. Haven't gotten around to putting it back together yet.

[Gus]

Did you check the external speaker jack with a meter?

A ruby amp does not have enough power to tell if a speaker is bad.  Sometimes a speaker will sound OK at low volume but not good at higher volumes. 
The voice coil can rub.
 
Did you do a visual check of the speaker? is the cone ripped? How does the speaker spider look?

[Luke51411]

I'm not sure if there are different versions of the 25R or I have the wrong schematic because mine only has headphone out and not line out but I did check the headphone jack and there was no resistance between the contacts when nothing was plugged in. The speaker looks in really good shape the only thing I thought could be suspicious but I'm doubful, there was a part of the cone edge that wasn't glued down all the way though it looks like it was made that way as I can't see glue remnant or any evidence that it is separating. I'm pretty sure the input jack had a cracked solder joint on the tip albeit a very small crack that was difficult to notice on first glance. I do remember the input jack being a bit squirly before the symptoms presented. If it does happen to be the speaker then I guess I will have a 10" speaker for my ruby then because it sounded pretty good that way!

[GibsonGM]

I'm pretty sure the input jack had a cracked solder joint on the tip albeit a very small crack that was difficult to notice on first glance. I do remember the input jack being a bit squirly before the symptoms presented. If it does happen to be the speaker then I guess I will have a 10" speaker for my ruby then because it sounded pretty good that way!

There you go, Luke....you probably have an issue with our input jack. Remember I talked about a part being loose, vibrating, and otherwise destroying your signal?  ANY kind of bad solder connection can/will make your whole sound go FIZZZZ into the crapper in a very nasty way.    There's really NO reason to suspect that your speaker is toast, man, only that we needed to check before you take things apart because it's easy.   

"I do remember the input jack being a bit squirly"    <<<< YUP, this happens, and I bet your problem lies there.  It's a stress point, jacks lift off the board.  5 minutes with a soldering gun may well solve your problem.  I've seen amps not work at ALL due this.  If you can easily get in there and re-solder it, go for it, just unplug the amp first and don't flex the circuit board.   

I run any cords going to my amp up thru and around the handle, then into the jack.  That may help keep tension off the input jack....

[Luke51411]

Fixed it! That must have been cracked solder and not me just seeing things!  it sounds clean now. Still a bit of distortion when turned up a bit but it's smooth and may just be vibration. I think I need to tighten the speaker down and put  the chassis back in and it should take care of that. Thank you all for your help and encouragement!