DSL401 .....help my sons amp wont work

Discussion in 'The Workbench' started by Leewill, Jan 29, 2021.

  1. Leewill

    Leewill New Member

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    Hi,
    I posting for my son who has a DSL401. We need help please.

    He plugged in the other day and nothing ! The amp has been fine and given no indication of faults. Its not left the bedroom in over a year. Powers up but no sound on any chanel.

    Im no electritian and dont want to electricute myself but can do the odd repir. Replaced the pickups and switches in his strat. So basic stuff is ok. Dont have a multi meter.

    I checked the 2 fuses in the back of the amp and they were fine. Removed the amp from its housing and there was a 3amp fuse on the pcb that had blown. Replaced that and refitted to case. Still nothing. Just the red power light on. Checked power lead and guitar and guitar leads.

    Im now stuck. Cant get it looked at as we are shielding due to covid so any sugestions would be apreciated.
    Many thanks,
    Lee.
     
  2. Max Gahne

    Max Gahne Active Member

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    Hello and welcome to the forum :dude: Here is a link to a schematic for a DSL401 which may or may not apply to your amp. I don't see a 3A fuse on this schematic, you didn't by chance get the fuse number if it was printed on the pcb? But regardless, based on what you described as your background and looking at this schematic I'm afraid the best advice is to get it to a shop. I don't have a 401 but it sounds like they're a good amp and worth fixing.

    https://el34world.com/charts/Schematics/files/Marshall/Marshall_jcm2000_40w_dsl401.pdf
     
  3. Leewill

    Leewill New Member

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    Hi,
    Thanks for the welcome and reply. I agree a shop would be best but they are all closed and we arnt going out because of covid .
    Yes, most definatly a fuse. I think its a slow burn type, has a small blob in the centre, glass type. Took it out myself and it was blown. Rating on pcb where it came from and was correct.

    I would leave it until lockdown is eased and we have had a vaccine but he wants to use it for a virtual univercity interview. He may just have to use another .

    Thanks again. Lee.
     
  4. RLW59

    RLW59 Well-Known Member

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    That schematic (page 2 of 2) only shows the preamp circuit. Presumably the power amp is page 1 of 2.

    The most likely reason for the internal fuse to blow is a power tube fault.

    Check to see if the replacement fuse blew when you powered on the amp. If so, replace the fuse again, but remove the power tubes before powering on. If the fuse holds with the tubes removed, one of the tubes has shorted. Get new power tubes, set the bias, and it should be good to go.

    If the fuse blows even with the power tubes replaced (or if the replacement fuse didn't blow), then there's a more complicated issue.
     
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  5. Leewill

    Leewill New Member

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    Hi,
    Thanks for that. I will give it a go.
    Is there a time frame for leaving after unplugged ? Ive heard they store residule voltage and can shock .
    Cheers,
    Lee
     
  6. Leewill

    Leewill New Member

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    Also should have said that when i turned it back on, the valves did not get warm or glow...any of them. Not sure if thats of consequence.
     
  7. Old Punker

    Old Punker Well-Known Member

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    ^^ +1
     
  8. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    As others said I’d start by replacing the power tubes. A failed tube can often blow a fuse. When you say the tubes don’t glow are you including the preamp tubes or just the power tubes? If none of the tubes glow that could be a heater voltage issue coming off the power transformer. One thing at a time though.

    The filter capacitors (the big ones near your power transformer) can hold lethal voltages for a long time so you need to drain them before working inside the chassis. After draining you need to confirm by checking the voltages with a DMM - which you don’t have so I suggest getting one. You’re going to need one to bias the amp after you replace the power tubes anyhow. If possible get one that can handle 1000v. Many less expensive units only handle 500-600v. Doesn’t mean they are bad or “cheap” but it’s good to have the extra headroom when checking tube amp voltages.

    To drain the caps you’ll want to get an Alligator clip lead and on one end connect a minimum 100 ohm resistor. Clip one end to ground and the other touch the + side of the filter cap and hold it there for a few seconds. Confirm voltage is drained with your DMM. The reason for the resistor is to prevent sparks and possibly messing something up when draining.

    If some or all of what I provided above seems the least bit overwhelming then you should have it professionally repaired.

    Let us know how it goes.
     
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  9. Chris-in-LA

    Chris-in-LA Well-Known Member

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    If all the fuses are good and the new tubes still aren’t heating this might not be an easy fix.
     
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  10. JohnH

    JohnH Well-Known Member

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    im not a tech, but ive had this amp for 15 years, so ive tried to learn about it. It has a few classic problems mainly due to heat from the tubes which weaken certain solder joints (or more). The most common is the heater supply failing due to the bridge rectifier coming disconnected or failing. This makes the tubes not glow. But this bridge rectifier (about a 20mm black square with a chamfered corner) only feeds the first three preamp tubes. The power tubes and last preamp tube get ac supply. But they all come from a transformer tap, and there may be a fuse involved. so, something in this system failed, maybe it took out a fuse for that. Are you very sure that no tubes are glowing?

    But this amp is a real PITA to work on and get to the board under. There's much to learn and things to go wrong, so be careful. It sounds like a tech job...

    If it was my son, id get him a small thing like a Microcube to practice on, and get the amp fixed later when you can
     
  11. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    You should check fuses with a DMM for continuity. Sometimes they look good, not burnt, etc, but the wire is disconnected from the end and you can’t see it.
     
  12. ricksdisconnected

    ricksdisconnected Well-Known Member

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    they can do more than shock you Lee. serious talk.
     
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  13. Leewill

    Leewill New Member

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    Hi and thanks for all the replies.
    He is very lucky that he has other amps to use, probably go with his Blackstar HT20, mainly because im a soft touch and love to support him in his music.
    Im going to ride out the next month or so and get it to a tech. Having worked within the motorcycle industry for 20 years here in the uk, I have seen first hand what a bloke with a tool box and not a lot up top can do .
    Dont fancy frying myself or his amp!
    But again, thanks for the warm welcome, we will keep using the site and i will post when its repaired, you never know it might help another player.
    Cheers guys,thanks,
    Lee.
     
  14. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

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    This would be greatly appreciated. I can’t tell you how many times I’ve searched an issue online, finding the same symptoms I’m troubleshooting, and then then thread dies with no final update.

    Good luck and great job supporting your sons music. My parents were the same when I was growing up. I didn’t realize how fortunate I was until I was older so if he doesn’t show it now, he will later on.
     
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