Learning to repair my gear...Squier 15G Solid State Help

[Lee W]

Hi everyone, I'm new here. Been lurking for about a month now and have learned a lot already. I've just been intimidated by all of the knowledge that is around, but now I got to the point where I had to register and ask a question. I'm new to this whole thing, but have a bit of electronics theory from some uni physics classes, and a little hands on experience from working on things in the past month. Nothing too serious, but better than no previous knowledge.

Anyways, I'd like to be able to work on my own gear, and I decided to learn how to do so on a little practice amp that died on me a few years ago. This way it won't matter if I screw it up because it's not really needed...but it'd give me a good starting point for the basics.

So my little Squier 15G amp would turn on, the speakers would hum, but no guitar sound. If I turn up the volume knob or either of the gain knobs (there are 2 of them, and it's a single channel amp) the hum gets louder. However, it doesn't even hear the sound of the guitar jack being plugged in/out.

I opened it up and found both filter caps were blown...leaking fluid onto the board. So I took them out, cleaned the board with some isopropanol to get the residue off, and put some new filter caps in (same specs). I also noticed a bit of carbon trace around the diodes which are beside the filter caps, but I tested across this trace and there wasn't a 0ohm reading for resistance, so assuming this won't short circuit the setup. (Please if I'm taking readings wrong or there's something obvious I should do tell me, assume I know nothing). I don't have a schematic for the amp, sorry. But if anyone does please post it!

I plugged it back in, and the exact same problem is still there. I tested the input jack and it is working fine. Any ideas on where to look or what commonly causes this type of problem?

Thanks

*Edit: This will probably seem really stupid, but I assumed the caps were blown because of a yellowish liquid coming out the bottom of them. It was also somewhat charred looking. Would this be glue used to hold the caps to the board, or is this leaked electrolyte? Either way the filter caps are replaced now and fresh new ones are there...so that's one potential problem eliminated.

[Fragger]

The bottom of this page may have your schematic? I see a 15G.

http://www.schematicheaven.com/fender.htm

[Lee W]

The bottom of this page may have your schematic? I see a 15G.

http://www.schematicheaven.com/fender.htm

Mine is the Squier Champ, it's only a single channel and a bit different setup. Although this could give me some insight into the circuitry, thanks.

[Metro]

Yes those caps were blown. That was the electrolyte you cleaned up. There is also usually some hot glue to hold the caps in place on modern amps, but you cant remove that with isopropyl.
You should remove any carbon paths as well, as they may conduct when the circuit is energised and not at the small voltage your DMM is testing at.

[Lee W]

You should remove any carbon paths as well, as they may conduct when the circuit is energised and not at the small voltage your DMM is testing at.

Thanks! I tried getting off the carbon trace with more isopropanol, but wasn't working so well. What is the best way to remove it? I am a chemist (not an electrician), so I will be able to use many various solvents depending on what is recommended

I did wipe off all the excess burnt parts, but the board is still a little discoloured from where this happened. I'm guessing at this point there's nothing any more cleaning will do?

I also noticed a little bit of burnt marks around a post or two of an IC. This could be another place to look...are these easy to find replacements of?

Edit: Just did a random search and found one...like these: http://www.alliedelec.com/Search/Pro...7&DESC=TL072CP

Are these pretty standardized or do I have to worry about the listed voltage/current ratings?

I'm pretty much just trying to look at my circuit board and find anything with charred marks as a hint that the component was acting up...and testing the paths around these areas to make sure there aren't any breaks. At this point I'm going to replace the IC (when a store is open since it's Christmas), and see where that takes me.

As for testing the diodes to see if they were good, I took a resistance reading at a 2kohm setting and found infinity one way, and about 650ish ohms the other way. From what I've read the resistance reading doesn't matter, but as long as it's infinity one way and a reading the other way, all is well. This sound about right? Or should I just replace the diodes anyways?

[bnwitt]

I hate to say this but that amp is probably not worth more than $50 I wouldn't waste too much time on it. Maybe you should build yourself a real tube champ to learn and get some great tone:

http://cgi.ebay.com/Squier-Fender-Ch...lenotsupported

[Fragger]

Is this the critter?

[Lee W]

I hate to say this but that amp is probably not worth more than $50 I wouldn't waste too much time on it. Maybe you should build yourself a real tube champ to learn and get some great tone:

http://cgi.ebay.com/Squier-Fender-Ch...lenotsupported

If you read my initial post it said that I want to learn how to fix my gear, and I consider this amp disposable so a good place to learn.

My main amps include a JCM900 and '77 Super Twin Reverb. I'd rather learn on this solid state amp that is worth nothing, than risk destroying my Marshall or Fender.

[bnwitt]

Lee,
I completely read you intial post. I just hate to see you learning to fix a completely worthless solid state circuit when your desire is for valuable knowledge and experience in the repair of good guitar amplifiers. You might as well work on a ruptured a garden hose. Leaning to fix that amp won't leave you with any valuable knowledge you can carry forward in the world of good guitar amplifiers and their design and repair. You learn to fix this amp and you'll be learning the basics of repairing solid state garbage. Now if that's the kind of equpment you intend to own in the future, then you are going to need that knowledge. If not, I'd suggest you build an amp circuit from at least the 60's or earlier. But, you do what you wanna do. Don't mind my advice. I certainly mean no offense. Maybe your tone desires are different than mine.

[Lee W]

Thanks for that reply, that is a better explanation of your opinion. No harsh words meant.

So in your opinion this works in a very different way from a tube amp and I might as well just try working on my twin, seeing as I am confident enough to start replacing parts such as the filter capacitors?

[bnwitt]

Yes. I think trying to learn how to solder and troubleshoot electronics in amplifiers is going to be very hard on the poor quality PCB that is in that amp. Doing work on low quality pcb boards takes a little more skill than working on eyelet or turret boards from hand wired amps or even on quality pcb construction.

You would be better served doing some research and chosing a circuit you could use to put into that amp after gutting it of its original board. You've already got a cabinet, chassis and speaker to start with. Your JCM900 and 77twin ownership shows you like a broad spectrum of tone. So, I'm not sure what you might want to build. One of my early amp jobs was the gutting of an old Electar 30 amp and conversion to a 5E3 tweed deluxe. I learned alot about proper layout and lead dress building that circuit. I have since built several Marshall 1974 circuits in small solid state 1X12" cabinet bodies. It would seem you have the high power arena covered with your two amps. You might want to build a practice amp like a firefly or some other very low wattage circuit in that old amp.

As far as re-capping your super twin, if you can solder well and put capacitors in with proper polarity, then you should be able to do that job.

[oc disorder]

Welcome to the forum Lee W.
Well I would hate to see the Squire end up as land fill when it could be useful as a practice/tuning amp.Its basically a logical process to repair something
a collection of observations and application of theory.
Guitar Amp. Tiny electrical signal into jack input produces bigger electrical
signal which can push and pull speaker coil on magnet.
Well you seem to have cornered the symptoms .
Now you say it hums well 2 things the speaker is working and power of some type is getting there but turning into hum.
A good starting point is usually the power supply.
If some component has gone faulty its possible its loading (shorting out) the power supply causing it to run hot and possibly supplying a lower voltage
than required. Often the faulty component will be hot itself or a dropping resistor that supplies that component will run hot.
Have to be careful as if it is a serious short the transformer could burn out
trying to supply too much current.
I suspect the mounting glue looked like leakage particularly as its slapped on in production and may have got hot from adjacient components so looks
pretty yukky if you havn't seen it before.
So measuring the supply will give us a few clues. The Squire/Frontman are
similar circuits should have 15 volt rails for the input IC's and a bit more for the output.
As I'm getting sick of typing I suggest that the fault will be the output chip
A TDA2050 or a UPC*118* (cant remember the exact number).
These burn out 98% of the time be carefull of the insulating /heat transfering washer behind the chip and the little insulating washer on the mounting screw/bolt.
Commercially these dont get repaired but as a hobby go for it...!
You can move into valve amps when you have your degreee and can afford the parts !

See this post and see if the chip looks familiar
http://music-electronics-forum.com/s...ad.php?t=10469
EDIT : The voltages are not at a high potential as in a valve amp so not likely to arc across like on a valve socket.
Unless its charred and turned to carbon in which case scrape it out and bridge damaged tracks with wire.
Soldering on these boards requires more precision as tracks can be damaged by too much heat.
You mention some "burnt marks" near an IC these could be production marks check the IC for spots burn marks or fine cracks but unlikely a pre-amp chip would burn like that.
These are readily available ..suggest fitting IC sockets as less likely to damage the chip soldering it in.

[Lee W]

Fragger: Thanks for the schematic, it is VERY similar but not quite it. Again, it is a very good starting point. Just one question, what are the capacitance values with / in them? For exampls C24 is 47/25.

bnwitt: I'd love to build myself a little tube amp, but to be honest I don't think I'm ready for it yet. Eventually, that will be my goal! I do have a lot of reading and learning to do first, but I suppose that'd be a great way to force myself to learn. I realize that this amp really isn't worth it, but I look at this as educational even though it is different from tube amps.

oc disorder: Thank you so much for that detailed post and link. My dad figured the capacitor could just be covered in glue, but it sure was messy! Better safe than sorry I guess when I didn't know what it was. I will get ahold of that IC chip and replace it as you outlined in the post, hopefully all will go well.

[bnwitt]

Don't be seduced by the dark side Luke

I'll check back with you later after you've spent more time than warranted on fixing this $50 amp.

[Lee W]

Don't be seduced by the dark side Luke

I'll check back with you later after you've spent more time than warranted on fixing this $50 amp.

Haha sounds good. I hope it doesn't discourage me if it doesn't work...I'll remember your advice before that happens.

Besides, I have another week or two before next semester starts, so I have nothing to do but bang my head against the table troubleshooting and going snowboarding