Roland JC 120 Problem

[Dakes2]

I have a 1982 Roland JC 120 that works ok apart from a loud Thump noise when turning on, I was told by a Tech guy that the amp needs a capacitor putting across the on off switch to prevent the power surge when turning it on, my question is, what type and what value capacitor do i need to be able to do this, and how is it connected to the switch, i would also mention that my amp is 240 volts, I have checked on the schematic's and only the 110 volt models have a capacitor on the power switch, the 240 volt model does not have one from what i can see, any helpor advice please.

[Enzo]

Power switches can make it pop, but a thump is more likely to come from the power amp.

[nevetslab]

I can't recall any of the Roland JC120's NOT having some amount of Thump when you turn them on. We have a number of them in our rental dept, and it's typical behavior. There is a 100uF/50V electrolytic cap from the power amp output buss to the voltage divider string from which the upper driver stage is fed you could change. C24 & C63, depending on what vintage your amp is. C29 & C129 in another version of the JC120, with the cap value being 47uF/50V on this version. C27 & C69 on yet a different version. I'd try changing those if the thump is excessive.

[DrGonz78]

Can't we determine if the pop/thump is coming from the switch or the power amp? I recall Enzo giving advice about this situation. Leave power switch on and use the power plug to apply the on/off by pulling out or pushing it in the power socket.

[Dakes2]

Thanks for your replies and help guys, the amp still does the same when turning the amp switch on and then using the plug to turn it on, so im guessing its not the main power switch