DRRI replacing bias resistor?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by BudLite, Apr 9, 2015.

  1. BudLite

    BudLite Supporting Member

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    Hi, I need to replace one of the bias resistors on my friends DRRI, do you guys remove the board and remove the resistor completely? or do you leave the board in place and clip out the res. leaving some lead to solder in the new res.? Thanks
    Here is the part RES MOX FP 1W 5% 470Ω R62,63 Do you guys think Rat Shack has in stock?
     
  2. Chris Scott

    Chris Scott Silver Supporting Member

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    Yeah, Radio Shack will have them (if they're inventory is up to snuff of course) as it's a fairly common value.

    Always better to install components as they should be, and since it's not a huge deal to swap that one out (free the pots and inputs first) I'd do it right.

    Conversely, if you're good with the iron, done carefully (heat sinks) it will be fine as well.


    EDIT: Wait a minute, just re-read your post - you mean screen grid resistor, right? - in any event, it's the same procedure.
     
  3. Blue Strat

    Blue Strat Member

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    yeah, the 470s would be screen resistors. Curious, why do they need to be replaced? Power tube melt down?

    Are those not on the sockets in those amps? Last time I worked on one (12 years ago) I vowed never to go inside one again. The experience was so bad that I've blocked the visual from my memory. :D
     
  4. BudLite

    BudLite Supporting Member

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    Yes a screen grid resistor. On a real DR they are located on the PT sockets on this toy they are on the circuit board. It was a set of PT's I am replacing he said one was shot and making noise. The amp sounded distorted. I put in a new pair of 6V6's and could only get a bias reading on one of the tubes. I checked the plate voltage and it was 429 volts on 1 tube and about 297v on the other. After inspecting the circuit board I saw a resistor that was broken, you could tell just by looking at it. Turned out to be one of the screen resistors. Do you think I should change both resistors or just the one that failed?
     
  5. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    To quote Col. Walter E. Kurtz: "Horror... Horror has a face... and you must make a friend of horror." So here's a visual dose of that (you'll notice the SG resistors cinched up tight to the main PCB to ensure good combustion):

    [​IMG]
     
  6. BudLite

    BudLite Supporting Member

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    Beautiful isn't it? Of course many folks on this board will tell you circuit boards are the way to go, and they are not any harder to work on:dunno
    A real DR or a Bruno is WAY easier to replace this resistor. This amp is a POS for build quality and what are they 1100.00 new? I offered this guy a Bruno CT 22w for 1500.00 but he said no. Look at the Bruno, much better.
    [​IMG]

     
  7. fjs1962

    fjs1962 Silver Supporting Member

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    That depends on the amp you're talking about. Something with thick double sided plated through boards like a Mesa, Rivera, etc. are just as easy, something like the Fender definitely not.
     
  8. fjs1962

    fjs1962 Silver Supporting Member

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    The right way to do it is to desolder and re-install on the pads. Not terribly difficult on the DRRI board. What I do is pull loose the ribbon cable connectors from the pot/jack board, take out the screws holding the board in place then flip the board up to get to the back side.

    Be very careful with your iron, try not to put heat on them too long and don't pull on any leads if they aren't free. The traces on the Fender boards will lift if you look at them too hard. :)

    Since you have to pull the board anyway I'd replace both screens to be on the safe side.
     
  9. BudLite

    BudLite Supporting Member

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    Thanks for the tips, I will give it a go if RadioShack has the parts.

     
  10. BudLite

    BudLite Supporting Member

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    So are you saying you could change the resistor or whatever part on the top of the board w/o removing it and flipping over?

     
  11. fjs1962

    fjs1962 Silver Supporting Member

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    On something with boards like a Mesa, yes. Not on the Fender. You COULD clip the leads and solder to the old leads on top of the board, but that's just asking for trouble, especially on something carrying high voltage like the screen resistors. Best to do it the right way.
     
  12. pdf64

    pdf64 Member

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    Why not mount them on the power tube sockets?
    Are you sure that Radio Shack carry such a specific resistor type?
     
  13. fjs1962

    fjs1962 Silver Supporting Member

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    You could potentially do that but you'd have to redo the wiring for the screens on the circuit board too. That would make it a more complicated repair.
     
  14. BudLite

    BudLite Supporting Member

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    Went to Rat Shack by my house and,.....It was gone:omg I was just there last month to pick up a tip for my soldering iron. I found another RS not too far away and they where having a close out sale, they did not have the resistors I needed. I will have to order online, anyway here is a pic of the blown resistor, looks like it burned the board a little:worried
    [​IMG]
    [​IMG]
     

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