motorboating JCM800...

hey guys - i posted this over on harmony central the other day - i was just hoping somebody around here might be able to help me out a little more specifically, as i still don't really know what to do :[

i've got a JCM800 2204 thats motorboating. i've discharged the caps and checked everything with a DMM. none of the caps appear visually to be leaking or exploded or anything, and thats what i keep hearing to look for.

according to the tom mitchell book, you can test the caps by hooking up a 9V battery to them, checking the voltage, disconnecting the battery, and seeing how long they hold their charge. i'm not really sure how long they should though, so i don't know if they're functioning properly or not. also, i think i'm going to have to remove the circuit board to test some of the remaining caps.

according to another site i found, motorboating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section need replacing. then, a tech on HC told me that is unlikely because they aren't electrolytic. so, i would just like to know which ones to examine, or start off replacing. or if i should just check them all. if anybody could help me out i'd really appreciate it.

thanks very much...

-mike

 

just wanted to say though...

i actually sprayed the power tube pins with some cleaner, worked em around in the sockets for a little, cleaned em off, and repeated a couple times...no more noise.

i think i will be getting one of those cap testers though. any suggestions?

 

hah, i never knew that.

thanks :]