JTM-60 no preamp tubes lit

jrenna

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I'm new to this forum so let me know if I'm explaining things properly. A friend gave me a JTM-60 which had a pretty loud 120hz hum. So I replaced the filter caps and also the two Power Amp tubes because one was only dimly lit. This reduced the 120hz quite well with only a faint sound of it. I played through the amp for about an hour and thought it was all well. Then I took a few minutes of break time and when I went back I had no sound at all. The amp was still powered on and all the switches and LEDs light when they are suppose to. I looked at the back of the amp and found the Power tubes were still lit but none of the preamp tubes were on. I figured there must be a power supply problem somewhere in the preamp section. I have not opened it back up yet. After doing some searching I found some people saying that the Bridge Rectifier is known to have problems and should probably be replaced with one that handles a larger amount of current. Can you forum people give me some advice as to what else to check in this amp? I know how to check a BR pulling it out. I know the electrolytic caps around it should also be checked. Is there anything else that could cause the preamp tubes not to light up? Sorry, I know this was a long explanation. Thanks for any help you can give.
 

StingRay85

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What makes a tube light up is the heaters that are required to bring to cathode to the correct operating temperature. The heaters are powered by 6.3V low voltage high current supply of your power transformer. The amount of light they give has nothing to do with performance of the tube (they are not light bulbs but vacuum tubes).

How experienced are you? I would recommend you to bring it to a tech, to make sure you don't mess things up (lethal voltages in there too)
 

Jon Snell

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If BR1 has failed, there are usually tell tale skid marks and or burn marks on the board.
If you have no heater voltage (between 5.5 and 7volts DC), replace the bridge fitting the replacement 1/2" off the board to aid cooling and replace C134, 135, 145 & 144 at the same time.
It is advisable not to fit a larger recifier block as it will damage the board.
 

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jrenna

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I have been a tech since 1983 and do know about voltages. I just don't have too much experience with guitar amps because I mostly worked on Broadcast cameras and production switchers and also old Klysrton satelite uplinks (really high voltage there). So I have no problem working on this thing, I just need a good schematic or someone that already knows which parts usually go out.
 

jrenna

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Thanks for the replies. Is there a schematic out there somewhere to point me to test points for voltages?
 

jrenna

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I see your photo of the schematic. I just wondered if there was a full one I could get.
 

jrenna

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Jon Snell -- I think I found the schematic JTM-600 at Dr.tube.com. Is the JTM-600 the same schematic as a JTM-60? The Bridge Rectifier section looks the same as your photo.
 

Jon Snell

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Jon Snell -- I think I found the schematic JTM-600 at Dr.tube.com. Is the JTM-600 the same schematic as a JTM-60? The Bridge Rectifier section looks the same as your photo.
They are one of the same. Unfortunalely for others, I cannot share Marshall Copyright drawing as that is part of the agreement between service agents and the manufacturer.
 

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