Help! Hi channel input JCM800 cuts in and out

wolfman922 · Post by **wolfman922** » Wed Nov 02, 2005 10:45 pm

hi guys, 1st post.

as the title says i am having difficulty diagnosing a problem with my 84 JCM800:

the low input works fine with both the master and pre sweeping perfectly . i plug into the hi input and the amp cuts in and out randomly,seems just like a dirty master pot but i don't understand why the low input works fine???...any help appreciated.

Post by **VelvetGeorge** » Wed Nov 02, 2005 10:46 pm

Check to see if the input jacks have loosened up and might have rotated enough to short out on each other.

George

flemingmras · Post by **flemingmras** » Wed Nov 02, 2005 10:48 pm

The low input works fine because the high input routes the signal through an extra stage of gain that it doesn't go through when plugged into the low input.

Try replacing the preamp tube. One side of the first preamp tube might be going bad. Also, do what George mentioned above as well. It's something in the very first gain stage so it shouldn't take much to get the high input rockin' again.

Jon

Flames1950 · Post by **Flames1950** » Wed Nov 02, 2005 11:12 pm

Sometimes simply cleaning the jacks does the trick also........if you don't have a good way to clean it you can try inserting/removing the plug into the jack over and over again (quit smirking Jon) but a cleaner may work better......

flemingmras · Post by **flemingmras** » Thu Nov 03, 2005 12:30 am

Uh...huh...huh...huh...Heh...heh..heh...Uh..huh...huh

Flames1950 · Post by **Flames1950** » Thu Nov 03, 2005 6:37 am

That's what I was afraid of.........

Guest · Post by **Guest** » Thu Nov 03, 2005 3:37 pm

thx much guys.

i have replaced the tube in P1 (tried a few different ones) and no diff. i have tried cleaning the jack (used a liitle roll of sandpaper actually) and the pot...no diff.

may be the input switch itself???
could there be a circut or component prob in the preamp?

still bewildered....

flemingmras · Post by **flemingmras** » Thu Nov 03, 2005 4:06 pm

Try replacing the low input jack. When there's nothing plugged into the low input, the high input gain stage gets coupled to the rest of the amp through the switch contact on the low input making contact with the low input's "tip" connection.

Jon

Guest · Post by **Guest** » Thu Nov 03, 2005 5:22 pm

thx . i'll try that next. may be just replace both input switches. i wonder if i could put a meter on it and chk it somehow...hmmm...

on a side note, what do the JMP / JCMs do when plugged into both inputs?

flemingmras · Post by **flemingmras** » Thu Nov 03, 2005 6:05 pm

If ANYTHING is plugged into the Low input, the High input is disconnected. This is because the extra gain stage on the High input connects to the rest of the amp through the switch contact on the Low input jack. When you plug into the Low input jack, you break the connection between the High input gain stage and the rest of the amp. So it would be the same as connecting into the Low input jack.

Jon

Guest · Post by **Guest** » Thu Nov 03, 2005 6:21 pm

thx very much Jon. appreciate your time and input. will get back to you on this little problem of mine as soon as i figure it out.

kev

Flames1950 · Post by **Flames1950** » Thu Nov 03, 2005 10:46 pm

Je fume deux joints pendant que JF liches mon sac

What the hell is that? Looks like you're trying to say that after smoking two joints JF licks your sack........but the only thing I can say in French is "fuck you" anyway.......won't get me far overseas.
I am far more adept at swearing in German, and not just because of the translation in Zappa's liner notes on the song "Stick It Out."

flemingmras · Post by **flemingmras** » Thu Nov 03, 2005 11:17 pm

Actually, it says "I smoke two joints while JF licks my sack"...LOLOLOL!!!

JF(5150loveeddie) mess with each other all the time. Take a look at his signature!

Jon

Billy Batz · Post by **Billy Batz** » Fri Nov 04, 2005 7:48 am

Andy we were doing that for a while but you took the high road. I didnt do anything that creative tho.

Flames1950 · Post by **Flames1950** » Fri Nov 04, 2005 4:26 pm

Billy Batz wrote:Andy we were doing that for a while but you took the high road. I didnt do anything that creative tho.

But I could read ours without getting a headache......