>

Blew MGP 9001 Series Preamp - switch problem/fuse ??? Tech help

Discussion in 'The Workbench' started by CroTone, Apr 10, 2021.

  1. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    Hi guys!

    I would need your help. I just recently got a used MGP 9001 tube Marshall preamp.

    I've put in the screw that was too long that touched the base of the ON/OFF switch on the PCB plate that is connected to the mains cable. I clicked the ON button to turn on the preamp and I saw lights in the house go out and a short moment of electricity burst behind the switch (obviously where now the burnt area is).

    Although I could swear I heard the fuse go off, it does not look damaged to me.
    I have no replacement fuse for this one. It says T100ma (I am using preamp at 240 V, Europe).

    Then I've removed the screw and went on to turn the preamp ON again, nothing happens, and no LED lights turn on. The thing is definitely not ON.
    Did I blow the switch? Or the whole thing (unit)?
    Or should I try getting this fuse and then try to turn it on?

    Looking at the board nothing else seems bad or burnt and there are no odd smells.

    I was really looking forward to this preamp but I messed it up.

    I really appreciate your tech input on this problem. Pic of the culprit area and the damn screw is attached.

    Thank you so much!:bowdown:
    [​IMG]
     
  2. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,239
    Likes Received:
    1,176
    Location:
    flo rida
    looks like it might've sorted one of the smd... diodes? on the board? fun
    take it to a tech
     
    CroTone likes this.
  3. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    Could you please be more precise (in a bit more lay language) for an idiot such as myself? :)
    Which component do you think might have gone bad? This would help to troubleshoot better, I don't have some good techs around myself but will try to get help.
    Thanks!!!
     
  4. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    By that do you maybe mean bridge rectifiers (BR1/BR2)? BR1 is nearby. I managed to pull the board out and I see no burned/black parts from underneath the PCB.
    My father is an electrician so he will know more once I hand take this out to him, however, he is by no means an amplifier/music gear tech.
     
  5. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,239
    Likes Received:
    1,176
    Location:
    flo rida
    I'm only going by odd angle pictures so it's only guessing, but those tiny components right by your screw appear to be diodes. what else could be wrong is difficult to troubleshoot remotely.
    i build and repair amps and i won't even mess around with smd components like that
     
  6. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    Can you please pinpoint on what you mean and where is this tiny diode that you refer to?

    Here, I took better pics...

    [​IMG]

    [​IMG]

    In the picture below, on the back of the PCB plate, I notice that there is a short disconnect regarding the part that has to do with the power source...
    Take a look at the part that is disconnected, right side of the picture, third line looking from right to left. Could this be the problem? It corresponds to 220V mode that I am using and that is connected on the other side of the plate.

    [​IMG]
     
  7. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,239
    Likes Received:
    1,176
    Location:
    flo rida
    that's why it's hard to troubleshoot via the internet: smd diode was an optical illusion. from those angles i can see they're not there
     
  8. Dogs of Doom

    Dogs of Doom Moderator Staff Member

    Joined:
    Aug 2, 2012
    Messages:
    24,320
    Likes Received:
    29,453
    Location:
    Los Angeles
    is that where the screw hit?

    upload_2021-4-10_21-1-8.png
     
  9. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

    Joined:
    Sep 5, 2017
    Messages:
    1,910
    Likes Received:
    2,472
    Location:
    Chicago Suburbs - USA
    Check continuity of the fuse with a meter. Sometimes a fuse can look good but it’s blown on the very end under the cap and you can’t see it.
     
    Dogs of Doom likes this.
  10. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    Upon closer inspection, NO...I can confirm that there is no way that screw could have hit there. That's what I thought at first, but...
    It is too short and when I mount it back, the segment with this disconnect is far away from where the screw could maximally reach. So I guess this discontinuity on the PCB is likely from before or was made that way.
     
  11. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    UPDATE!
    Upon the close inspection of the board, in better light condition, it seems to me that the component known as bridge rectifier is toasted on the board!
    One that is designated as the BR2.
    It is the only spot behind the PCB board that has that "burned" look behind it.
    Could this be the culprit and explain the symptoms?

    Speaking of which...where can I buy a replacement bridge rectifier?
    This one only has BR62 designation on it, I don't have any additional values.

    Regarding the fuse, I am yet to check it with the multimeter when my dad arrives later on.
     
    Last edited: Apr 11, 2021
  12. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

    Joined:
    Sep 5, 2017
    Messages:
    1,910
    Likes Received:
    2,472
    Location:
    Chicago Suburbs - USA
    Rectifiers convert AC voltage to DC so if that’s toast then you are not getting needed DC voltage somewhere in your circuit. Could be B+ that powers your tube plate, the bias circuit, or if it’s a channel switching amp, the low voltage needed for op amps/transistors. Since your unit is a preamp you won’t have a bias circuit so I was just letting you know what they are for.

    I’m not familiar with your amp nor have I seen the schematic so I don’t know which one applies here.

    To test the rectifier you’ll need a multimeter that can test diodes. Then watch some videos on YouTube as you have to connect the + and - leads in a certain way to check it. And it needs to be out of circuit so you’ll need to remove it from the board. If your not familiar with soldering it may be better to have a tech look at it as it’s easy to inadvertently lift the traces off the board with too much heat.
     
    CroTone and thetragichero like this.
  13. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    Dear KraftyBob, many thanks for your detailed answer and for explaining some things to me. In any case, I would not dare to do any soldering since I am not too good at it except for some rudimentary stuff such as jacks and few cables here and there.

    Here is the preamp power/rectifier scheme. BR2 is the top diamond on the left side of the image...also labeled PW02. That is one that burnt.
    From what I found on the internet, that one is a standard 4-pin bridge rectifier with 8A, 200 V specs.

    On a side note - will definitely get the unit to the tech. No question about it. However, none of the techs in my area have experience with preamp/guitar electronics so what I am trying to achieve here is to take some of your advices, get some troubleshooting going so that I can pass that info on to the electronics guys.

    [​IMG]
    [​IMG]
     
    Last edited: Apr 11, 2021
  14. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,239
    Likes Received:
    1,176
    Location:
    flo rida
    i would think br2 would have to be sorted (rather than open) in order for the led not to light up. you'd think that would trip the mains fuse
    heater winding should've been fused

    if it is the rectifier, i would go with something oversized and chassis mount like this: https://www.taydaelectronics.com/single-phase-bridge-rectifier-mic-35a-1000v.html

    it's possible a tube short took out the heater rectifier
     
    CroTone and KraftyBob like this.
  15. KraftyBob

    KraftyBob Well-Known Member Gold Supporting Member

    Joined:
    Sep 5, 2017
    Messages:
    1,910
    Likes Received:
    2,472
    Location:
    Chicago Suburbs - USA
    OK, your amp has DC heaters for the tubes (some amps are AC and others DC). The purpose of heaters is to literally heat up the cathode in the tube to get the electrons flowing. That's the glowing you see inside the tube when it's powered on. Right now you shouldn't be seeing the tubes glow assuming that rectifier is bad. You can confirm by checking for DC voltage where it shows Red and Black on the schematic.

    The next section down "HT to valve board" is for your tube plates. HT = High Tension and that is dangerous/lethal voltage so you need to be really careful when working on tube amps. Only put one hand inside the amp at a time as to not inadvertently compete a circuit through your body. You can also tell by looking at the voltage spec of the filter capacitors 33uf, 450V.

    If you're going to work on the amp, you need to drain the filter capacitors first as they can hold the lethal voltage even when the amp is powered off and unplugged. Lots of YT videos on this as well, but basically take an alligator clip lead (600v rating) and on one end connect a minimum 100 ohm, and ideally minimum 5w resistor. Connect the non-resistor side to ground first, then touch the resistor side to the + terminal of the capacitor for a few seconds (do not touch the resistor lead or metal part of the alligator clip when doing this). The resistor prevents an inrush of electricity which can cause a spark and potentially damage something in the amp. Then check the voltage of the capacitor with your meter to confirm it's drained.

    See if there are any radio techs in your area. Might be a long shot but there's a following for older tube/ham radios and those guys should be able to fix a tube amp with a schematic in hand. Rectifiers are cheap so it's a matter of confirming that's what the actual problem is, and getting it replaced.
     
    CroTone and thetragichero like this.
  16. CroTone

    CroTone Active Member

    Joined:
    Nov 24, 2019
    Messages:
    92
    Likes Received:
    167
    Wow, thanks for the useful info guys!

    One question regarding the bridge rectifier? Do I really need exclusively one rated as it is...this one being 8A 200V (RMS)?

    Is it safe to use a more robust one such as this one, for example? Would that be OK? Asking this so I can order parts.

    https://www.banzaimusic.com/GBPC1506W.html
     
  17. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,239
    Likes Received:
    1,176
    Location:
    flo rida
    you want as robust as possible. that 35a one i linked to is less than a buck
     
    KraftyBob likes this.
  18. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

    Joined:
    Nov 16, 2013
    Messages:
    7,138
    Likes Received:
    9,825
    Location:
    Midwest, USA
    That has to be a problem right? The trace is gone.
     
    Dogs of Doom likes this.
  19. Gunner64

    Gunner64 Emotional Support Animal Gold Supporting Member

    Joined:
    Nov 16, 2013
    Messages:
    7,138
    Likes Received:
    9,825
    Location:
    Midwest, USA
    I would be testing that switch before I went changing anything else. It may be as simple as that. That switch may have fried before the fuse had a chance to.
     
    thetragichero and KraftyBob like this.
  20. thetragichero

    thetragichero Well-Known Member

    Joined:
    Jun 6, 2010
    Messages:
    3,239
    Likes Received:
    1,176
    Location:
    flo rida
    oh definitely, proper troubleshooting beats shotgunning parts any day
     
    KraftyBob and Dogs of Doom like this.

Share This Page