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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

1990 fender twin reverb treble problem

Discussion in 'Amp Tech Center' started by Swampman, Apr 19, 2017.

  1. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Hi all, looking for some direction. Hey . I have a 1990 twin reverb with a problem on the clean channel. Chanel 2 works great. The volume and mid work correctly. The treble has no effect on tone and the bass can only be tolerated set at 1,any higher and the bass goes 110%..any ideas before I lug this thing in for repair? I'm very capable with a meter and soldering iron-just not strong enough to lug this thing across town. I had it apart and used a high quality contact cleaner on the pots, no change. Any ideas? Thanks Wayne
     

  2. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Welcome, Swampman.
    The '65 Twin Reverb RI was introduced in 1992...and both channels are 'clean'. The two channels are virtually the same but for the effects and a gain stage in the Vibrato channel to make up for the losses incurred by the reverb.
    What amp do you have there?
     

  3. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Since the treble pot is putting out signal we know it is getting a signal from at least one of its side terminals (treble on top terminal, bass and mids on the bottom). We know moving the treble wiper doesn't change anything. We know bass isn't getting filtered out properly.

    My guess is there is a break in the tone stack caused by an open cap. The treble and bass caps are where I'd start.

    [​IMG]
     

  4. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Wow, that's more than I was expecting. My amp is between 1990,1992.its known as a red knob (knobs are black with the white snakeskin) white snakeskin amp. It says the twin on the front. It's my favourite amp for sure, no master volume, watts lo, hi switch. Also has parallel input, allowing clean, dirty to be mixed together. And it has many pull pots for more sounds.
    Enough bragging about my baby. I'm going to pull it apart and check the caps. Thanks for the direction.


    BTW. The treble is missing, meaning it's all bass and mid. Mid knob functions well, bass is overpowering even when set on 1.
     

  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    If it's not one of the caps look for a break in the stone stack, broken lead or bad solder joint.
     

  6. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Thanks for that insight, Rob. I have a Pro Sonic here with a bad tone stack. All controls have some effect, a with treble at zero there is no signal..as it should be. The amp has no brightness to it. I suppose it is time to lift the board and replace some components. The treble pot seems to be good. Man, don't we love those handwired boards!!!
     

  7. Silverface

    Silverface Poster Extraordinaire Ad Free Member

    Age:
    65
    Mar 2, 2003
    Lawndale CA
    Do you also understand safety precautions needed when working on an amp (discharging filter caps, using only insulated tools, only using one hand at a time etc)? If not - you're working with deadly voltages, even with the amp unplugged and should take it to a professional.
     

  8. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Im an electrician, so I'll be cautious. That drawing helps alot, as iIcan follow the logic of the circuit, I'm not sure it's exact, but it looks similar to the correct schematic iIfound fn
     

  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Your mid pot may be wired as a variable resistor (input and wiper jumpered) and the component values may be different but that's a pretty standard Fender tone stack.
     

  10. Swampman

    Swampman TDPRI Member

    Age:
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    37
    Apr 19, 2017
    Ont
    Here is a pic. From left to right.
    Volume, treble, mid, bass.
    The 4 pots ohm out correctly,
     

    Attached Files:


  11. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    So.... This might get a little tricky. I am getting capacitance readings, but they don't match with my drawing. I may have the wrong drawing, or I can't get proper capacitance readings without removing 1 leg from the board. Only c2 seems to show no capacitance.
     

  12. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Here is the drawing
     

    Attached Files:


  13. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Swamp, note that the midrange tone cap is a .047mfd while the bass tone cap is a larger value---.1mfd.
    It seems you have 8.1mfd written in for that. I can't really tell if the midrange cap vale you have penned in is in fact .047mfd.
    You may have noted my post above that I am faced with a similar situation with a Pro Sonic. I don't particularly like to work on PCB amps, so here is what I am going to do. The PCB in this amp will have to be lifted, and I will replace all three tone caps while checking all components and connections. I don't want to go through the operation more than once. If I have to lift a cap on one end, I am replacing it.
    Your amp has four caps there that I Would replace....with no hesitation. The amp you have is more challenging IMHO due to the fact that your front panel controls are PCB mounted whereas the Pro Sonic has chassis mounted pots, switches and jacks.
    If these amps were handwired on eyelets, turrets or terminal strips; then I would not shotgun the whole tone stack. Ymmv.....but I have limitations in some areas....patience being one of them, formal training is another.
     

  14. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Iv been trying to find a clearer diagram, iI believe it's the correct diagram, but the values are only partly legible. I am guessing the value, that's not going to get this solved...... There is no chance it's just a bad v8 tube is it?
     

  15. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    You can't measure capacitance with the caps connected to the tone stack circuit because all the caps in circuit will come into play.
     

  16. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Time to fire up the iron
     

  17. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    The values I gave you are correct. .047mfd for the midrange and .1mfd for the bass. tThose can be changed during the process IF a person wants to revoice that tone stack.
     

  18. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Thanks guys, iv confined that my fluke 87 can read the small capacitors. I'm still thinking that I should be able to read some capacitance value while the capacitors are in the circuit. Although the numbers would be wrong, a capacitor can only fail 2 ways, open or short.(right?) this is bed on 39 years on ac circuits, iv never seen a capacitor fail in any other way, open or short...... Most of the capacitors, I am getting a reading, I'm thinking I should focus on the few I can't get a reading on at all.
     

  19. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    Im honestly playing in the dark a little on this. I could just take it in-i actually know a guy who can repair this. I think this is a small component failure, (since all other options function well, even this channel works) that I will enjoy figuring out. With the help from the folks here that have pointed me in the right direction. I'm trying to remove as few points on the board as possible to reduce the chance of me wrecking something else on the board.
     

  20. Swampman

    Swampman TDPRI Member

    Age:
    47
    37
    Apr 19, 2017
    Ont
    So, I used my meter on capacitance. I measured all the capacitors on my circuit from each of the 3 pots. My thinking is that I should get capacitance of some sort from any good capacitor, back to the pot. So far c2, and c3 show no capacitance. All of the others show from each pot to the other side of the capacitor. So, I'm just going to pull the leg off c2, c3. See what I get.
     

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