Fender Frontman212R distortion problems.

[xh4p4]

I have a fender frontman 212r which was working just fine until the sound started to distort like if the overdrive was on. I am not using a switch pedal or anything, just the guitar to the amp. I checked and i think the problem is in the board not the speakers.
I will look for cold solder joints, and a friend of mine told me to check the resistors too. Is there a way to spot a faulty resistor at plain sight? and if not, do i need to desolder every resistor to run a multimeter test?

[The Dude]

The most common cause of this problem in this amp is defective Q12 and/or Q13. They will check good using diode check function, but fail after warmup. Of course, every amp is different and your problem could be completely different. Check transistor voltages when the amp is distorted and see if they are correct. You can also wait until the amp is in failure mode and give each transistor a shot of cold spray. If the amp starts working correctly, you'll know that a transistor is the problem.

[J M Fahey]

a friend of mine told me to check the resistors too. Is there a way to spot a faulty resistor at plain sight? and if not, do i need to desolder every resistor to run a multimeter test?

You need to do proper troubleshooting, meaning feeding signal into it, following it along (Fender offers Test Points and what you should find at each one) whi9ch will help you find the area where the problem is.

Just pulling 200 resistors to measure them means little to nothing, there are other 100 components more likely to fail, you can also have wiring, connector,pot,switch problems.

You need a systematic approach or you´ll go crazy soon.

A *functional* test is way faster and more focused than a painstaking part by part one.

The schematic already suggests what to do.

[Enzo]

And you could test or replace EVERY part in the thing, and if the problem is actually a cracked copper trace on the board, it still won;t be fixed.

[xh4p4]

And you could test or replace EVERY part in the thing, and if the problem is actually a cracked copper trace on the board, it still won;t be fixed.

I dont see any broken traces, but i do see weird looking solder joints, i guess i will start from there and if the problem is still present, then will track the signal as J M Fahey said.

[Enzo]

I just don't want to see you throwing parts at it.

[xh4p4]

I just don't want to see you throwing parts at it.

Thanks for the advice anyways, you made me double check, and now i know thats def not the problem. I will update as i check the amp thoroughly. This is my first time doing this, and i am liking it, even though i must admit it is quite a job for my first time, nonetheless i like to learn about this kind of stuff. Thanks all of you for the comments.

[Mick Bailey]

To narrow things down try running the 'preamp out' into another amplifier and see if that distorts. You can also try plugging your guitar directly into the 'power amp in' - you won't have any control over the volume, which will be fairly low, but it should help to narrow down whether the problem lies in the preamp or power amp section.

You don't need to remove resistors to check them. Resistors nearly always fail open-circuit so I go on 'near enough is good enough'. There are numerous faults that cause distortion, and any component can be responsible for this, but I would narrow things down first.

Bad solder joints very often present as a circular crack around a component leg.

[Enzo]

Resistors don't go down in value, so if that 100k resistor measures 22k, it isn;t the resistor, there are parallel circuits.

[Mick Bailey]

Mostly components fail without any visible evidence (when there is it can be a bonus in diagnosing a problem) A charred resistor would be evidence of a fault, though the resistor suffers because of higher current draw by a component downstream, such as a shorted capacitor. Replacing just the resistor doesn't solve the problem and you have to fix the circuit, not the component. Having the schematic handy when doing in-circuit resistor checking is helpful - that way you can see if there are parallel circuits as Enzo describes if you get a lower reading than expected. The aim at first is to narrow down the fault; if you don't, you end up checking parts in a section that is good, which is a waste of time. My first step always is to decide whether the fault lies in the preamp, power amp, speaker or power supply. Do the easy stuff first - a lot of information can be got before any dismantling is done.

[Enzo]

The key to it all is to ISOLATE THE PROBLEM.

[xh4p4]

UPDATE.
I have been trying to isolate the problem, and i think it can be in the power amp section. The distortion is still present when i plug the guitar in "power amp in" input. So if i am gettng this right, it means that the circuit fails when trying to follow the path "power amp to speakers", right?
I checked the speakers and they are fine, also the board seems fine too, there are some weird looking solder joints, but using a multimeter tells me there is continuity in all of them.

Now that i might know the section that is causing the problem, i need to check the components in said section, right?

[Mick Bailey]

I always start by measuring the DC voltages (+/- 42v) and seeing whether these hold up when the amp is operating with a test signal into a load.

Do you have a 'scope? If so, scoping the output can tell you quite a lot. You could even be missing one half of the signal, have asymmetric clipping, severe crossover distortion, or any other numerous symptoms that cause distortion.

Checking all those components individually isn't necessarily going to get you anywhere - a component that checks out OK with a bench test may be failing under load. I also wouldn't rely on a DMM to tell you if a solder joint is OK - test voltages and currents are a fraction of what the amp uses in operation. If in doubt, re-solder any suspect joints.