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]

Originally Posted by Fragger

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]

Originally Posted by Metro

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]

Originally Posted by 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

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
Behringer GM108 problem
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]

Originally Posted by 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.

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

[52 Bill]

For what it's worth...

Some of these amps have the filter caps glued down to the board with what looks like brown contact cement. There is very little liquid in any modern electrolytic capacitor. At least not enough to leak out and drain onto the pc board.

Is the amp humming or buzzing really loud? Does the hum diminish as you turn down the volume control?

From your description of the symptoms, I'd first check the input jack and then the controls. Check to see if the jack has pulled up out of the board, either breaking the solder connections or breaking the traces on the board itself.

If the jack is ok, then check the pots for broken elements. If the pot has been flexed too far or smashed from the front, the resistance element will sometimes crack breaking the signal path. If this has happened, the pot will read open with an ohmmeter and the top section will pull or flop away from the terminals that are soldered to the pc board.

If the pots are all ok, then look for either broken or loose signal caps or a bad first stage IC.

Hope this helps.

[Lee W]

Thanks 52 Bill.

I checked the input jack (as that was my first suspect) by plugging in a cable, and checking the tip/side of the cable to where the jack is soldered to the PCB, measuring 0 ohms. This indicates the jack is good, as far as I can tell. I also tested resistance between the jack and the next components on the PCB, got 0 ohm readings as well, indicating there are no breaks in this part.

I do get a different volume of hum when I change the volume and gain settings, but the same amount of background hum as I recall is normal from this amp when it was working, but this was several years ago. From the symptoms I immediately thought it was either the input jack or a preamp problem, so a faulty preamp chip seems to be the most likely culprit from my limited knowledge, and what everyone is seeming to recommend checking.

I'll first go through all of the pots to make sure they are functioning properly, and none of the traces are broken around them.

This board is so helpful, I can't believe the amount of detailed responses I've received for such a minor amp!

Edit: Ok I checked all of the pots, they all work great in both directions (check between lug 1/2 when counterclockwise, then check between lug 2/3 when clockwise to get a 0 ohm reading. My meter was on 2k setting and got infinity the other way...except for cases where the lugs were on the same trace, then 0ohm obviously ). I also checked all the traces around each of the pots and all was well on the ohmeter. It's got to be the preamp chip at this point....I hope haha

[Lee W]

Hey guys, back after quite a while. Took a little time for the parts to come in...but...IT WORKS!

So I did what you guys said and replaced the preamp chips. There were two TL072CP in there, and I replaced them both. Used solder wick to remove them which worked great, and installed a socket into the PCB which was quite easy. Now if they get wrecked again I can just snap in new ones.

So I was nervous plugging it in the first time, but all was well. And I only electrocuted myself once...

I had it all apart when I turned it on the first time, and touched the back of the on/off switch which was wired into the power cord. Well I know better now haha.

All in all I had to say it was a great experience and I have to thank you guys a lot for taking it back to basics and helping me out. I understand now what was meant by this is very different from a tube amp in troubleshooting. Instead of finding out why preamp tubes aren't getting the right power etc., I just had to replace some IC's. But during the process I learned a lot about troubleshooting, soldering, and safety. I also built myself some little clip and resistor tools for draining filter caps. I guess I thought I was safe from getting zapped because I was so careful with the filter caps that I just touched the switch without thinking

Now my next task will be retensioning the tube sockets in my JCM900 and biasing it. Just got my bias probes in and found out it's running wayyyyyyyyyyy cold. I can't wait to hear what a difference this is going to make!