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Old 20th April 2006, 06:36 PM   #1
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Default Help! no signal! Marshall JTM60

I have the Marshall JTM60 with the 15". It all started out with a hissing noise that suddenly appeared one night, then a few weeks later it would make popping noises as well, then finally the other night it popped and then had no signal at all.. The amp still powers on, but has no sound. Any ideas?
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Old 20th April 2006, 07:01 PM   #2
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Could be anything really!, what sort of electronics knowledge do you have?, valve amps like these are really pretty simple to mend, but if you don't know what you're doing could KILL you!.
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Old 20th April 2006, 07:02 PM   #3
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As a first check, look at the input, send and return jacks.
Some have contacts inside which are normally closed and if not used start to corrode.
Schematic can be found here:
http://www.drtube.com/marshall.htm#JTM60

/Hugo
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Old 21st April 2006, 04:29 AM   #4
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Thanks guys, as I am newbie to tube amps. I have heard of using contact cleaner in the jacks, and replacing a preamp tube with a good one. anything else I can try? What is the chance of a blown fuse? What else can burn up in the electronics that is fairly common?
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Old 21st April 2006, 07:07 AM   #5
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Be thrifty with contact cleaner. Usually, I spray a little on a male jack and insert it in every chassis jack. A few time back and forth is mostly enough to de-block the 'rusty' contacts.
If any difference in behaviour of the amp is noticed, you know where to spend the afternoon. I wouldn't start modding the preamp or any other part until you found the problem.
Also, check the pots. With no signal applied, give them all a few turns back and forth and listen for cracks in the speaker. A tiny drop of contact cleaner can do wonders. Never spray 'plenty'.
I does more harm than good in the long term.
As for the tubes, gently wiggle them a bit and listen. Bad contacts in the sockets are not uncommon. Be even more careful with spray in the sockets. Rather give the tube pins a little juice and proceed as you would have done with the jacks.
Of course you pull the mains for that.

/Hugo
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Old 21st April 2006, 12:56 PM   #6
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I opened it up to look on the citcuitboard and a small glass fuse looked blown, meaning no filament connecting the 2 metal ends, I am going to go get some fuses today and I'll let you know if that worked. Any good recommendatins where to buy these fuses and the contact cleaner?
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Old 21st April 2006, 08:00 PM   #7
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You should check with a meter to make sure it's blown. Some fuse filaments are so thin you can hardly see them. Well, I need glasses to solder so it could be just me.
As for the spray can, no idea where to buy it in Texas.

/Hugo
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Old 21st April 2006, 08:08 PM   #8
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fuses blow for a reason......
especially since you heard hissing and popping before the safety device worked like it should........


so...if you have to put in a new fuse before the problem is fixed........be ready to shut it off once you've seen/heard what's wrong....

it's generally not a good idea to replace a fuse before the underlying problem has been fixed.......
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Old 22nd April 2006, 11:33 PM   #9
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@rynestyne

Yes before using 'contact' cleaner on tube pins uplug and let her set for awhile and if you have a meter check to make sure 'she' won't bite you, as the filter caps can store a charge and give you one heck (maybe LETHAL) of a shock! You may need to drain the charge on the power supply caps with a resistor, 10K 1W. Clip one end to ground (do first!), the other on the B+ side of the caps PLUS keep the other unused hand in your pocket!
One other caution, some contact cleaning sprays are flammable AND conductive! So be very careful as stated! You've got a metal aerosol can in your hand!
Don't be tempted to use a bigger fuse, like a 12V 20A fuse in place of a 250V 1A etc.! Always use the same type! Fast-blo/Slo-Blo... If you can't solve your proplem with fuses and you are scrathcing your head unsure as to what to do next; GET some help from someone who is experienced in tube circuits!

Fuses and contact cleaner can be obtained at your local Texas RatShack.
Also if the power cord is removable by all means remove it and stash it somewhere, otherwise tape the ends up so no-one will help you by PLUGGING it in for you!

Wayne
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Old 23rd April 2006, 12:34 AM   #10
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Quote:
Also if the power cord is removable by all means remove it and stash it somewhere, otherwise tape the ends up so no-one will help you by PLUGGING it in for you!
the sheer horror.........a mother full with good intentions.....ahw, i know why it isn't working, let's help the poor boy ...... .....mwuhhahahahaha


sorry....
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