I have a buddys jcm 800 at the moment that is giving me problems. He said it was "re-tubed" but I noticed they looked kinda old, (not NOS), so I ran them through the tester... a few new pieces later, and one problem fixed..... The volume on the amp is really low, and seeing as it's the 100 watt model it's defineately not right. Three of the pots were really scratchy (beyond spraying) including the Master Volume, and they were replaced. Now what happens.....
The amp powers up fine, and you can get very loud for a few seconds. Then it loses it's power (the gain seems to stay the same though) Playing at it's new low volume, it will occasionally jump up to higher volume for half of a second, then back down. Then it will occasionally pop a fuse.
I'm not really sure what to check next... aside from the popsicle stick test. Any help trying to de-bug this is greatly appreciated. Thanks!
JCM 800 with problems
Moderators: pompeiisneaks, Colossal
Re: JCM 800 with problems
Power tubes failed ???
When power tubes fail,occasionally redplating internally,due to internal short circuits or similar things;in these conditions the power of the amp go down because the power amp not run properly.This could explain your blowed fuses.
If this is not the problem,could be bad contacts or bad solder in the PCB ,expecially with master volume pot,or the same should be swapped with another.
When power tubes fail,occasionally redplating internally,due to internal short circuits or similar things;in these conditions the power of the amp go down because the power amp not run properly.This could explain your blowed fuses.
If this is not the problem,could be bad contacts or bad solder in the PCB ,expecially with master volume pot,or the same should be swapped with another.
Re: JCM 800 with problems
Two things come to mind. First, check the bias circuit to make sure that it is working properly. if you lose bias, the power tubes won't behave. Second, maybe there is an oscillation that you cannot here (above audible range) that is eating up all of the power. Do you have a scope?
Last edited by sliberty on Fri Dec 28, 2007 6:57 pm, edited 1 time in total.
-
Andy Le Blanc
- Posts: 2582
- Joined: Sat Dec 22, 2007 1:16 am
- Location: central Maine
and you can get very loud for a few seconds. Then it loses i
if you give it a thump and the problem come and\or goes its a mechanical issue or bad solder join....flex the pcb gently and in varios spots and wiggle the wires....if its not tube or mechanical start checking the power supply....and let the amp warm up a bit....heat changes the shape of things and can be the cause ....
lazymaryamps
Re: JCM 800 with problems
Tech to someone who has a scope
It cant be a big problem
Also check all grounds, tighten all pots etc
It cant be a big problem
Also check all grounds, tighten all pots etc