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Thread: replacing a resistor on my ACMP-73

  1. #1
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    replacing a resistor on my ACMP-73

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    hello

    this is the ONLY modification i plan to do to my ACMP73 because it seems to just be soldering. And i can "kinda" do soldering.

    however, i am certainly a newb with it, so if anyone could help with a detailed, "for dummies" explanation of this, i would REALLY appreciate it.

    first off, i need to know EXACTLY what the rating of THIS resistor is:



    isnt there some coding that tells you what it is? like the colour of the stripes?

    basically, here is the context of the repair i need to do, with some pics supplied by ppl with non burnt resistors:

    here is one look:



    closer:





    (mine has wax on the screws that connect it to the board... how do i deal with this?)

    some of the transistor:





    my burnt one:



    some info about it from this thread:

    https://homerecording.com/bbs/showth...80964&page=286

    That's the emitter resistor for the output stage power transistor (2N3055)
    i guess i could decipher it from this schematic but how? -- EDIT: wait, looks like it says 47 Ohms, .5 watts. duh. hehe



    ???

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    I would think that you'll also have to figure out what caused the resistor to burn. Unless the resistor was seriously flawed in manufacturing (or the circuit design is wrong, but the others, pictured haven't burned), something drew too much current through it to cause the burn. The 2N3055 is shorted perhaps?

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    Quote Originally Posted by Whale Bone View Post
    I would think that you'll also have to figure out what caused the resistor to burn. Unless the resistor was seriously flawed in manufacturing (or the circuit design is wrong, but the others, pictured haven't burned), something drew too much current through it to cause the burn. The 2N3055 is shorted perhaps?
    This was uncovered in the group buy thread; an "in between" position on a gain switch causes the bias on that transistor to jump from 2V to 9V, exceeding the resistor's rating by more than 1W.

    OP, cut off the burnt resistor, but leave some of the leads sticking up from the hole. Solder a new 47 ohm 2W resistor in its place by simply laying the resistor leads flat against the old resistor leads. Solder and trim.

    I can send you 2x 100 ohm 1W resistors, which will get you 50 ohm 2W in parallel--you just need to stuff both of them on top of the old leads, and you need to email me your address . . .

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    Quote Originally Posted by mshilarious View Post
    This was uncovered in the group buy thread; an "in between" position on a gain switch causes the bias on that transistor to jump from 2V to 9V, exceeding the resistor's rating by more than 1W.

    OP, cut off the burnt resistor, but leave some of the leads sticking up from the hole. Solder a new 47 ohm 2W resistor in its place by simply laying the resistor leads flat against the old resistor leads. Solder and trim.

    I can send you 2x 100 ohm 1W resistors, which will get you 50 ohm 2W in parallel--you just need to stuff both of them on top of the old leads, and you need to email me your address . . .
    i tried to send you a pm but it doesnt work....

    hmm.

    i was actually thinking of lowering the bias knob a bit and put the original resistor rating in...

    if you had one of those instead (or additionally) that would be cool

    anyway, how much do you want for em?

    my address is

    Jason B. Merrill
    512 Church St. Apt 325
    Honesdale PA 18431
    USA

    (its on my website anyway and anyone can find it so whatever)

    thanks for the offer, and your help. I promise that this is the ONLY work i will do on my preamp. After this, im taking it to an amp guy in state college, PA sometime next month after my valentines day gigs

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    Quote Originally Posted by mshilarious View Post
    OP, cut off the burnt resistor, but leave some of the leads sticking up from the hole. Solder a new 47 ohm 2W resistor in its place by simply laying the resistor leads flat against the old resistor leads. Solder and trim.
    Eh. Why hack it? Judging from my experience desoldering other parts on these boards, a few seconds with a solder wick and the parts literally fall off the backside of the boards. They drilled the holes kind of big.

    http://www.radioshack.com/product/in...ductId=2062744

    Just place a clean section of the wick over one contact and press it down over the contact. Hold it down with screwdriver a half inch away or so. Get a cheap, low wattage soldering iron that comes with a handful of tips. Use the tip with the big dimple in the end of it. Press the hot tip down on the connection and rock it around all four sides of the contact. Then, lift the wick and the iron together as a pair. The wick should lift off the board without any significant effort. If it sticks to the board at all, reheat the wick with the iron to melt the solder, then lift it off.

    Repeat this for the second contact. The part should fall right off the back of the board. If it does not, lay a clean section of wick down on the contacts and press it down onto the board with a back-and-forth motion of the tip to soak up the last bit of solder and press the pin into the board slightly. The part should then fall loose or should pull easily out with pliers.
    Quote Originally Posted by Obi-Wan
    If you mod me down, I will only grow stronger.

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    lol i wish i had 3.99 ... hehe

    i only have an old soldering iron with one tip...

    do you suggest a different type that isnt too expensive?

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    Quote Originally Posted by jasonbmerrill View Post
    i tried to send you a pm but it doesnt work....

    hmm.

    i was actually thinking of lowering the bias knob a bit and put the original resistor rating in...

    if you had one of those instead (or additionally) that would be cool
    I would not change the bias knob, it's not the source of the problem anyway. The regular bias setting is fine, less than 0.1W idle. The problem is that pop on the gain knob sends waaay too much current through that part of the circuit. I do not know if changing the bias knob would affect that issue at all, but it will certainly degrade the performance of the output circuit (lower headroom).

    Resistors are free, I have a bunch laying around I am not using. I do not have the original value though, and I wouldn't use it anyway.

    Eh. Why hack it? Judging from my experience desoldering other parts on these boards, a few seconds with a solder wick and the parts literally fall off the backside of the boards. They drilled the holes kind of big.
    Because I am lazy and didn't want to pull the board. Although if the holes are that big, you could cut out the part, heat the leads, and pull them out from the top. That will probably leave the hole clear enough for the new part . . .

    If you pull the board and desolder, I'd still cut the old part out first, makes desoldering a lot easier, and it's not like this part is reuseable.

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    Quote Originally Posted by mshilarious View Post
    I would not change the bias knob, it's not the source of the problem anyway. The regular bias setting is fine, less than 0.1W idle. The problem is that pop on the gain knob sends waaay too much current through that part of the circuit. I do not know if changing the bias knob would affect that issue at all, but it will certainly degrade the performance of the output circuit (lower headroom).

    Resistors are free, I have a bunch laying around I am not using. I do not have the original value though, and I wouldn't use it anyway.



    Because I am lazy and didn't want to pull the board. Although if the holes are that big, you could cut out the part, heat the leads, and pull them out from the top. That will probably leave the hole clear enough for the new part . . .

    If you pull the board and desolder, I'd still cut the old part out first, makes desoldering a lot easier, and it's not like this part is reuseable.
    my email is jasonbrianmerrill AT gmail.com, ill send you postage via paypal (i have like 1.63 in the account....which is all i got -- i know its basically an insult but its all i gots man hehe... the preamp was my bread and butter for the month)

    its not possible that using a higher rating will blow anything else out? i know nothing about this stuff... lol

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    Don't worry about it, it's not even worth the Paypal fee ($0.42 stamp + $0.06 resistors - $0.29 for PP)

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    Quote Originally Posted by mshilarious View Post
    Don't worry about it, it's not even worth the Paypal fee ($0.42 stamp + $0.06 resistors - $0.29 for PP)
    lol... im such a turd.

    i wish i had something to offer except laughter at my newbness


    very kind of you sir, thank you.

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