amp help?! found my fried componant

i posted on the bass forum about my fender bassman 100 that wasnt working.

http://acapella.harmony-central.com/showthread.php?t=2412600 figured i would post in the DIY section to.

anyway, i finally might have found what was wrong with it... heres the pics!

i did three different zooms so you can see where it is in the head. obviously what is circled in red is what burned. what caused it? is there supposed to be something be in the blank spot circled in yellow?

PCBs often have missing components due to design changes, different models etc. like the missing C239 below, and the holes usually are solder filled. I am also not sure the resistor you circled in red is burned, I'd get some hook probes at radioshack (~$5) and check to see if its conducting.

+1

I wouldn't worry about the missing components. What exactly is wrong with the amp? Does it power on?

yeah it powers on but it would just hum. even when i plugged in my bass nothing changed. then it smoked a little bit and i could smell burnt electronics. so i opened it up and thats all i could find that looked out of place

I'd replace that resistor just to be safe, but if that did burn, there is something else that caused it. Have a look at all the electrolytics and check for bulging or leaks.

ok... which are the electrolytics?(noob) and how do i know which resistor to buy? i know they are identified by the lines on them but how do i figure out which is what?

FYI SS amps are complex and have a steep learning curve; money for a tech to trouble shoot might be well spent.

Also http://music-electronics-forum.com/ has several helpful techs as regulars who could suggest trouble spots on this amp if $$ is tight.

I would think that that resistor is pretty well cooked. Looks like it scorched the board underneath and just about cooked the paint off the one next to it. BUT - I would not just replace the resistor and power the amp back up. You need some idea of what that resistor does. What components is it tied to? There's a good chance that some other component is actually the primary problem. Check tedmich's link, andor find a schematic and get someone with a little electronics experience to look at the schematic and make a guess about what may have taken the resistor out.

If you have humming you want to be sure the amp hasnt gone DC. Disconnect the speaker and hook up an AC meter. If you read 120VAC then you got bigger issues than you think.

Take it to a qualified tech.

With blown power supply transistors, the hum may be coming from there but as with all things, one problem can lead to another. A pro tech can test the supply via a schmatic and confirm the power amp is in good shape. My guess is you have multiple issues. Also testing all components statically with an ohm meter is the only sure way to be sure you dont have hidden damage.

ok. well im kinda on my own here. i live on guam and ive looked for someone who could find the problem and fix it, but there is nowhere to take it here. the fender dealer here would charge me to ship it to the states, get it fixed, ship it back and then add on their own charge for doing nothing. basically my only options to get it fixed here are either do it myself, or pay way more than the amp even costs. ive had it sitting around for a while so i decided to see if you guys could help me fix it. at this point i have nothing to lose. how do i test each componant? i have a multimeter.

Oh well. Not your nite. I thought I'd maybe do a good turn. Found a Basman 100 schem on the the Fender site, but I forgot to note the component #s. Thought it was R110, or 115. When I went back to check, your pics were down. Anyway, the parts location page on Fender shows 110 and 115 at opposite ends of the board - may not be the same amp?

yeah i found that schematic also... i sat with the head in my lap and looked at the diagrams but nothing matched up to me. im pretty sure it is an older model schematic or a different version of the bassman 100. mine is a '04 combo

I lightly read both threads and although I don't know anything about this amp and everyone gave suggestions on what to check, I will throw in an opinion that despite the possibility of the cascading failure (something must have caused a resistor to burn and maybe fixing the resistor will just let the true failure re-occur or worse), it may be worth replacing any obvious damaged small parts if there is nothing to lose anyway. I had my Triple XXX suddenly lose all volume on me 4 or 5 years ago, I had nothing coming out at full volume so I knew something was wrong. I got the schematic and used it to trace a test sine wave I injected at the input jack through all the stages with an oscilloscope. Eventually I found a tube where the audio was present at the input but not the output so I started looking around all the components on the schematic (I know nothing about tube electronics so I was hoping for a miracle). I found a single burnt resistor that was all melted on the under side (almost didn't notice it) and that's all I saw wrong. I even assumed replacing this will not fix anything, will just all it to fail again, but it did fix it and it's been fine for all these years and counting.

R209 looks cooked, and it's neighbor R203(?) looks funky.

If you can't get a schematic, try "walking" along the traces to and from the burnt out parts, and look for something else discolored.

You can't test a resistor while it's still connected to the board. You have to unsolder or cut the lead on one side. Otherwise, you're testing everything in the path along with the resistor.

Good luck trying to get parts, too!

Best of luck, and keep Google-ing!

OK, I looked again after the pics came back, and then went back and looked at the schematic on the Fender site. Just doesn't seem to be anyway to use the Fender info in relation to this amp. Even if they are electrically related, the board layouts are just too different.

If it were me, and I was in your situation, I would spend the 10 bucks or whatever it is now to get the correct schematic from Fender - make sure you give em model and serial #s, whatever, to get the right one. And then I would look to see if 209 (209 also looks to me like it's cooked too),and 215 have either a common connection, or are mirror image components in a double ended circuit. If they tie together that would give you a pretty good clue as to what other component nearby might be the problem. If they're in the same place in a double end circuit (although from the pics they don't look like the same value - 215 is burned up so bad I can't really tell what it is), then you might look to see if there's a common output in that circuit that might that have been shorted or overloaded. If there's no obvious relationship, I guess you punt. In your situation I would probably go ahead and try a fix on this also - I would just bet on there being some other component that's contributing to the prob.