fender m80 chorus 212, low end rumble

[gtrdad40]

my fender m80 chorus 212 has developed a nasty low end rumble on the clean side of the amp. if this were your amp where would you look first to try and solve this issue? i thought it may be just a speaker issue, but i replaced the speakers and the noise is still there. i can't tell if it is there on the distorted side, but the clean deff. is not clean any more.

thanks for your help

[tekhed7]

"Nasty lo-end rumble" huh? First, you have to determine if it's in the preamp (or atleast before the stereo EFX jacks) by plugging a shorted 1/4" dummy plug into the "Stereo effects loop" Return jack.
If all signal & hiss goes away, but the rumble is as bad as ever, suspect the "L-PA-In" driver I.C., an MC1436 labelled as U13 (Fender refers to the "clean" amp as L-PA). The nearby protect diodes, CR34 & 35, can also cause this.
Do you have any freeze spray? (Avail at RatShak) If you gingerly zzap the I.C. & diodes w/ the freon, defective silicon usually will either quiet right down or get tons noisier. Then, as the frost vanishes (you can blow a bit to accelerate it), the noise will often dramatically jump in level, 1 way or the other.
But IF the rumble IS muted with a dummy plug stuck in the Stereo EFX return, then I'd suspect the preamp output I.C., U6, a regular 8-pin TL072 [small, square, black plastic] with test points "TP20" & TP21" circled- in ovals nearby- pointing to it's output pins (Pin 7 for Amp-L & pin1 for Amp-R). Are you familiar w/ tracing a circuit using a schematic?
Do you have a scope? I could e-mail you this [19-yr-old] schematic if U need. Just send me an e-address. -M (& don't forget about just bad solder or contacts in/around jacks)

[gtrdad40]

first, thank you for your help.
i may have described the issue i'm haveing wrong i would like to try again.
on the distorted channel there is no issue that i can tell. the prob is all with the clean side. when playing through the clean side with the bass above 4 there seems to be a poping sound mixed with an static sound kind of like an old vinal record only louder. this sound only comes in the lower register strings, like when playing an open g cord i will strike the e string and this is where it is mostly heard. i hope this helps a little better.
also i would love to have that schematic, would you please send it to :
gtrdad40@myactv.net
thanks for you help again. this is the only amp i have and i need to get her sounding right.

[tekhed7]

Ahhh, O.K. I was confused, as this is a stereo amp. So do you mean that when you have the mode in "clean" as opposed to "lead" or "O.D.", this popping static sound is present in BOTH speakers? Sometimes a tear in a speaker cone can be noticeable only when attempting to play clean. Is it still present/ apparent when listening using headphones?
Hard to troubleshoot this type of problem w/o hearing it oneself... preferably while one scopes the output of each amp.
Will send the M-80 Chorus "pdf" along to you. -M.S.

[gtrdad40]

yes, this happens only on the clean channel. i also found that it only happens when pluged into input 1. just found that out last night.

i thought it might be the speakers too, but i replaced them and there's still the same noise. did not think of the head phones not even shure if there is a jack, if there is i am going to try it. i'll let you know what i come up with

[tekhed7]

U changed out the spkrs?! Impressive. Well... I guess that rules That out.
(I would've just donned "cans" & plugged into that jack labelled... 'HEADPHONES' !)

Only when U plug into Input 1, huh? Do you mean the HI sensitivity jack as opposed to the LO one? Only difference is a couple/few dB of sensitivity.

Hope you got the 10-page service lit O.K. I sent it late last night. (I hope it's the right Amp!)

[gtrdad40]

just a quick thanks to you tekhed7 for the schem. it really help in finding the cap's that were loose on the power side. this old girl sounds great again! and with the new celestions i installed, whew! man what a sound.

thanks again sir.

now onto tha darn delay pedal!

[tekhed7]

Not a prob... That wouldn't be the first S.S. Fender with loose filter caps., though usually a deep background hum that crackles/ hum there & gone, is the giveaway.

G'luck w/ the pedal. (Make? model?) Those can be a bitch... sometimes.