Classic 50/212 Mod Trouble

[jwmallett]

Recently changed my tubes. Decided to do a mod on blueguitar.org. Basically I know how to solder, drain caps, check resistance ect. but not an electronic guru by any means. Anyway, Pulled boards out and did the mods to the eq caps and preamp board as per the orig50mod.pdf on Steves site. Put the amp back together and it didnt sound very loud and had a hum, as i took it back apart i noticed a red two prong plug not connected properly to the preamp board, only one prong was connected, i plugged it in properly and restarted the amp, the amp sounded great. I played it for a while then noticed if i pushed it harder, like a switch but never in the same spot on the voume, it would have more low mid response. if i turn it down it would either go away(be more tinnier) or stay the same, if it did go away and i turn the volume back up it would reappear(never in the exact same spot on the volume meters). In certain spots it would actually appear then disappear(again like a switch). Anyway, suspecting a bad solder joint i took it apart and removed the caps and desoldered everything and resoldered and put everything back together. that one wire i talked about earlier( the red two prong plug) was marked but i thought maybe i marked it wrong and that was the problem, since i didnt notice anything wrong with circuits prior to and after removal/installation.. i hooked that plug up backwards and started the amp, there was a buzz noise(not real loud) and the guitar was not very loud. So i swapped the wire back around(with the amp off of course) I restarted the amp and it sounded good, i could hear a little snapping noise, i noticed if i touched a pot and push it against the chassis it would make this snap noise. so i turned off the amp and tightened the connections to the main board so there would not be a ground problem, when i turned the amp back on, i smelled magic smoke Open the amp up and the IC marked U1 had a crack in it and was smoking. What may I be looking at here. I am worried if i replace the IC the problem may be lying somewhere else. I dont want to further hurt the amp but really do not want to have to take it somewhere to get worked on. Any suggestions are greatly appreciated. -Jeff

[jwmallett]

Found two resistors that looked cooked, but testing them, they are fine. Going to change them anyway. Waiting on IC to come in. The two resistors were r61 and r62 located between two vertical elytic caps.

I realize this could go all kinds of ways but, maybe im overlooking something that is typically common sense. Anyway I am going to install the resistors and hook it all back up and try the new IC and see if it is ok. But i suspect something else. Dont have the time right now to check every component. If the IC ends up burning again, is there a possibly narrow list of things to check? I have looked over the boards several times nitpicking on solder spots and cleaning small debris which none of appeared to be any real find. I suppose maybe a whole cap job is in order if the ic burns again, or maybe some rogue resistor. Also, I read somewhere if you burn an IC change them all because they will fail soon after, does this apply to the transistor on the board as well.

Sorry for the runons and the many off the wall questions and generally sounding like im talking to myself! LOL

[tubeswell]

Found two resistors that looked cooked, but testing them, they are fine. Going to change them anyway. Waiting on IC to come in. The two resistors were r61 and r62 located between two vertical elytic caps.

If the resistors test okay there may be no point in replacing them

I realize this could go all kinds of ways but, maybe im overlooking something that is typically common sense.

If the amp worked 'okay' before the mod, then it was usually something you did that made it not work okay

Anyway I am going to install the resistors and hook it all back up and try the new IC and see if it is ok. But i suspect something else. Dont have the time right now to check every component. If the IC ends up burning again, is there a possibly narrow list of things to check? I have looked over the boards several times nitpicking on solder spots and cleaning small debris which none of appeared to be any real find. I suppose maybe a whole cap job is in order if the ic burns again, or maybe some rogue resistor.

Well it seems the IC is obviously buggered. But in addition to that, I think you might possibly've stressed something when you were moving the board around, as well as when you tightened the nuts on the pots? Those board traces and pads are prone to damage from being over-strained. Also the inter-board jumper wires are prone to movement fatigue when you are fiddling around with the boards. I would re-trace your steps and maybe re-flow the solder over all the joints you did, and take some solder off joints that look too full and check the trace pads around the pots for damage (- you might have to 'repair' one or more of the pads), and check everything for connectivity with your r-meter, and checking resistor and cap values.

I know i sound like im talking to myself! LOL

I do that quite a lot too. Helps to figure things out.

[jwmallett]

thank you! Ill check those wire things again they appeared to be ok. but i guess you really wont know unless you unsolder them and solder them back if one wire is cracked in the pcb materials tiny hole.

can the pots inerds being stretched by overtightening the nuts and cause an open circuit lead to this type of failure? I did not notice the prong on the top of the pots being bent upward as if they did get stretched.

[tubeswell]

can the pots inerds being stretched by overtightening the nuts and cause an open circuit lead to this type of failure?

Probably more likely that the pot's legs could cause damage to the trace pads on the boards from twisting as you tighten the pot nut. They do tend to strengthen these somewhat by using more solder for these connections, but I guess that doesn't make them bullet-proof.

[jwmallett]

Is it possible the pots not being screwed in well and creating a intermittent ground cause the ic to burn up? either way im going to put in a new ic and put it all back together carefully as i dont see anything wrong, ill report back. Hopefully this theory is correct or it will just work anyway.

[jwmallett]

ok installed new chip, reinstalled all boards wires ect. cranked it and it sounds good, there is now a barely audible high pitch noise that i cant make go away, it seemed to come out of nowhere after playing for a minute. tried to play with settings and couldnt make it go away, unless i turned the tone pot on guitar all the way down. also if i walked closer to the amp there was a sweet spot where the noise would go away but if i walked any closer it would start to make a hum/buzz sound like from the pickups. that was never there before. turned off amp and came back an hour later, dont hear high pitch noise anymore? too early to tell maybe.

[Enzo]

Unplug the guitar, turn the amp up. Is the high pitched noise present? If not, then your guitar is picking something up.