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Marshall dsl401 reverb fault

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  • Marshall dsl401 reverb fault

    Hello.
    I had a dsl401 with a short mains TX and blown tubes come to me . Fixed all that running well. When I put back into the cabinet .
    No reverb.
    Checked the tank ok 208 ohms and 20ohms.
    I swapped for a couple of spares. Still no reverb
    Changed the cable still dead on reverb.
    Just to be sure swapped the connection around just make sure it wasn't connected incorrectly
    At this point I thought bollocks
    I have had no experience on reverb faults that are not the above.
    DL401-60-02 (2002) iss9.pdf
    Now being a novice I'm trying to make head and tail what's going on

    15v +/- all correct
    I connected up the signal gen
    I can see the signal at pin 1/7 on ic3
    I can see a signal on pin 7 / 5 on ic4.
    When using the foot switch. I can switch output pin 7 of ic 4 on or off.
    As I took the board out .I replaced ic4 for a 2121 as it seemed like a replacement. Still the same
    If I rattle the tank I can hear the springs. But zero reverb effect.
    Normally on amps if you turn reverb up it can make the amp hum . No in this case
    If you disconnect the tank. No buzz can be heard if you touch the leads

    I welcome any help.
    Arbutt
    Last edited by Arbutt; 10-15-2018, 02:42 PM.

  • #2
    It appears as if the Reverb Return is not working correctly.

    T8 (J174 fet) is normally closed D to S.
    It needs a gate signal to open it. (G)

    Use a voltmeter and see if you have a voltage at the gate when Reverb is On.
    The fet may be bad if you do have a gate voltage.
    The fet can be removed for testing purposes.
    If it's bad, the reverb return should now work.

    DSL 401 Rvb.pdf
    Last edited by Jazz P Bass; 10-16-2018, 05:27 AM.

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    • #3
      Thanks jazz p bass . This morning I checked to see if I had any j174 . I did. So I cut the old one out . Forgot to check volts on the gate. With jfet out still no reverb.
      . One thing now if I touch the return connector I get a slight buzz. Where I didn't before
      I can hear the noise though the speaker if I tap the tank.
      If I turn reverb pot down it stops.
      Hope this can help
      Mr a

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      • #4
        Fixed it . Found a zener leaky zd5 reverb now working

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        • #5
          That zener seems to be a common thread. Perhaps the wattage rating is too low and it fails prematurely?
          https://music-electronics-forum.com/...=Dsl401+reverb
          When the going gets weird... The weird turn pro!

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          • #6
            Thank you Dr g . I only had 12v 1.3watt so that should be an upgrade. The job was a pain. I got the new tubes that were supposed to fit dsl401. Didn't had to drill out the chassis. Damn thing is done now. Things I do for fun
            Arbutt

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            • #7
              Sorry to bump an old thread, but just wanted to sign up and say thanks Arbutt, was ripping my hair out trying to figure out why I couldn’t get the reverb on a dsl401 I just picked up to work. Replaced ZD5 and reverb is back in business.

              thanks!

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              • #8
                Once again thanks Arbuttfor the heavy lifting here.

                ZD5 measured forward turn on voltage of 0.1v on my fluke and replacement with a 12v 1.3w has worked for me. Other observations on this fault are that the reverb return signal was faintly audible on clean but not OD1/2. Note ZD5 is just to the right of ZD4 but PCB marking is obscured.

                Also watch out for bad solder joints on this model where the solder seems to have softened like putty in my case on all the red poly caps, jacks and tall electrolytics powering the DC heater section. These amps have a reputation for overheating.

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