SWR IOD still broken

Discussion in 'Amps and Cabs [BG]' started by jasonbraatz, Mar 20, 2002.

  1. jasonbraatz


    Oct 18, 2000
    Oakland, CA
    a little while ago my tubes went microphonic and sounded really distorted, so i got a set of JJ's, biased them, and it still sounded distorted. so on someones advice i ordered a set of TL072s, because that's what was bad when i took it into service a month ago.

    well...i swapped out all of the TL072's, and it still sounds like poop. (a real high end/farty distortion that's very different from the tube distortion)

    so i experimented with some settings and found out that the DI output sounds normal, and if i plug my bass into the effect return it sounds normal as well. i ran a cable connecting the two, and lo and behold - it sounds fine. so the problem is somewhere within the tube stage (after the DI and before the EQ, including the blend assembly)

    also, i plugged a speaker into the speaker out of the preamp, and it would fart out on the low E and B even with the drive all the way off. could it be that i just have the bias up too high? (although that would be wierd seeing as how the trebly distortion doesn't really happen when the blend is all the way to "line", only when it has some "direct" in there."

    the blend pot has been doing some wierd stuff too, maybe related - it makes kind of a "thump" when it's moved from it's full line postion (all the way clockwise) and the volume of that setting is very low)

    one other very wierd thing i saw while i was1 inside that i hadn't seen before is that on the second or third TL072 from the left, there are two leads going to the same pin - one to a resistor, and one to a capacitor that goes thru the same resistor, and the wire that goes right to the resistor is scratched out (very deeply) - on the schematic it shows that that's correct; that there's no connection between the two, but i'm wondering if any other IOD owners have the same thing.

    any ideas? i really don't want to drive to milwaukee to have this thing fixed, but if i can't figure it out by friday i'll probably have to.

    thanks, that was quite the voluminous tome
  2. mthoople


    Nov 1, 2001
    Denver, CO

    Sorry that the op-amps didn't solve your problem(s).

    Check your op-amp voltage (+15V at pin 8 and -15 at pin 4).

    Check your heater voltage (6.3V across pins 4 and 5 on the EL84s).

    Check the resistance of the Blend pot between the center pin and one of the end pins with the power off. If the resistance doesn't vary smoothly, the pot is dirty or bad.

    You replaced the preamp tube as well?

    It doesn't fart with the blend set to Line?

    Mine has a similar trace cut at that resistor. However, on mine the two resistor leads and capacitor lead are all soldered together above the board.