Broken vintage TS-808, help?

Discussion in 'Effects, Pedals, Strings & Things' started by WaltC, Jul 12, 2006.

  1. WaltC

    WaltC Member

    Messages:
    2,130
    Joined:
    Feb 21, 2006
    Location:
    Northern CA (Sacramento area)
    So a friend of mine walks into the music store the other day and hands
    me a black painted Ibanez (maxxon mfd.) original model TS-808 tube
    screamer.

    Says " it doesn't work, can you fix it?"

    So I take it apart and here's the deal:

    - passes signal in "bypass" mode with no problem
    - switch to "boost" mode and no sound, nothing,not even a crackle

    Did some checking and as far as I can trace it there is good guitar
    signal up to the last 1uf Cap before the IC (RC4558P FWIW). I can't
    find any signal on the other side of the cap (can, like an electrolytic
    but Non-Polarized).

    Does this mean that the IC is bad and should be replaced? Should I try
    replacing the cap? Look somewhere else? Send it off to AnalogMike or
    Keeley to get fixed?

    I'm having fun screwing with it, but I'm used to 500V DC on the places
    and 100Watts, this 4.5 VDC and 9VDC and miniscule mili-amps of current
    are driving me slightly nuts <G>.

    Any ideas (full or half-baked <G>)? Places to check? Voltages to check
    for?

    I'd appreciate any help you could give me....

    Thanks,
     
  2. dosmun

    dosmun Member

    Messages:
    1,765
    Joined:
    Mar 17, 2005
    Location:
    Midland, MI
    Try replacing that cap and see if that works.
     
  3. theelectic

    theelectic Guest

    I agree. Also check the voltages on the IC, pins 1,2,3 and 5,6,7 should all be within roughly 0.5V of each other. Otherwise it could be the IC.
     
  4. EXP

    EXP Supporting Member

    Messages:
    3,052
    Joined:
    Aug 16, 2004
    send it to Mike at Analogman.
     
  5. AL30

    AL30 Member

    Messages:
    2,987
    Joined:
    Jul 20, 2005
    Build the DIY Audio Probe !!! YES !! It'll take 5 minutes, cost about $2 and save you about a million hours of debug time on stompboxes

    http://www.geofex.com/FX_images/audioprb.gif

    Here's some more debug info

    http://www.diystompboxes.com/pedals/debug.html

    Here's a complete breakdown of the tube screamer circuit

    http://www.geofex.com/Article_Folders/TStech/tsxtech.htm

    But, it sounds like it may be your cap. Is the switch working? Those things go bad all the time. Without more info it'll be hard to diagnose. Exactly what was your friend doing when it stopped working?

    I wouldn't pay to have it fixed. If you're used to fixing amps you can fix this. The audio probe is a gem.

    AL
     
  6. GBlekas

    GBlekas Supporting Member

    Messages:
    530
    Joined:
    Sep 22, 2002
    Location:
    Manorville, NY
    Although I have seen caps gone bad in older pedals the 4558 goes bad much more often.

    I would take out the 1ufnp and test it's value but my gut tells me it's the ic.

    Regards,
    George
     
  7. theelectic

    theelectic Guest

    Yeah but would that explain why there's signal before the cap but not after the cap? Shorted + input maybe?
     
  8. AL30

    AL30 Member

    Messages:
    2,987
    Joined:
    Jul 20, 2005
    We really don't know what cap it is. It could be any number of things. According to RG's article it could also be the DC jack - "DC adapter connector is prone to craciking either the solder or the board where it's soldered on."

    Too many unknowns at this point. Need some more info.


    AL
     
  9. analogmike

    analogmike Gold Supporting Member

    Messages:
    6,242
    Joined:
    Mar 17, 2004
    Location:
    Connecticut
    Here is a simple thing to check.

    Check to see if the pots have rotated, that would cause that problem.
    It's one of the things our KWS option prevents.
    Open the pedal and carefully pull back the circuit board (it's held
    in by 1 screw). Make sure all the pots are straight, with the three
    lugs pointing down towards the battery. Then if any are not nice and tight,
    pull off the knob and tighten the nut under the knob.

    Also check the IC chip voltages as someone recommended above, you should have 9V on one of the pins and six of them should be close to 4.5V, if not the chip is probably shot.

    good luck!
     
  10. GBlekas

    GBlekas Supporting Member

    Messages:
    530
    Joined:
    Sep 22, 2002
    Location:
    Manorville, NY
    Mike makes a great point and that is if any of the pots are hitting the shell then so goes your signal as well. That would be the quickest fix as well!:cool:

    There are two 1 UFNP caps in the circuit and you said the last one in line so this one is off pin 7 of the ic.
    Piin 7 and 6 are the second feedback loop in this ic and they do blow opens. Think of it like a cut guitar cord

    The cap can show the same type of symptom.

    If you have signal on one of the two solder points for the cap then, it's the cap.
    If you have no signal on either of the cap's solder points then, it is the ic.


    Regards,
    George B.

    www.PedalworX.com
     
  11. VaughnC

    VaughnC Supporting Member

    Messages:
    15,474
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    Could be one or more of many things. There's some FET switching going on in that circuit that could be the culprit too. Best thing to do would be to check the audio path with an oscilloscope and the switching logic with a DVM. Otherwise, anyone's guess could be right.
     
  12. WaltC

    WaltC Member

    Messages:
    2,130
    Joined:
    Feb 21, 2006
    Location:
    Northern CA (Sacramento area)
    Thanks Everyone! I appreciate your patience...<g> (Mike I've got one of your TS-9s and an SD-1, George, I've got one of your TS-9 Turbos modded for 808, etc. <G>)

    I try and answer some of your questions:

    - my friend can barely remember his last name, let alone what he was doing when the pedal failed. Odds are it didn't have anything to do with guitar playing!

    - the pedal works in "bypass" mode and the guitar comes through with no problems

    - switched "on" the pedal is completely silent and no sound makes it through

    - tracing the signal from either end (in and out) gives me lots of signal at lots of points, but *no* distortion ever on any point

    - I've got signal to the level control and it will drop the signal to zero even when the pedal is switched "on"

    - no place where I can pick up signal (including *both* 1ufNP caps) is the signal distorted nor will the drive or tone controls make any difference

    - there is 9VDC on one pin of the IC and 4.5 on the others (one has 4.3 actually)

    Is any of this any help? I don't understand the FET switching at all and am a relative novice on solid state stuff anyway. But... I am enjoying the learning if it's not boring you guys too much.

    Thanks again,
     
  13. VaughnC

    VaughnC Supporting Member

    Messages:
    15,474
    Joined:
    Jan 4, 2002
    Location:
    Pennsylvania
    Troubleshooting via keyboard is difficult but, if you have a good signal at the level control and you're not hearing any output, then there is a loss of continuity between the level control and the input to the output buffer amp.

    This loss of continuity could be a bad solder joint, broken PCB trace, broken wire connection between the level control & PCB, or something wrong with the output FET switching. If you've ruled out a bad solder joint or broken trace or wire, then the FET switching would most likely be the cause of continuity loss. The output FET switch resides between the level control and the output buffer stage and, if the FET switch isn't working for some reason, the overdriven signal can't get to the pedals' output buffer stage. The FET itself could be bad, or it may not be getting proper control voltage from the flip-flop circuit, or it may have lost its +4.5v bias voltage. In bypass mode, the voltage between the output FET switch's gate & source should be about -4.5v. In non-bypass mode, the voltage should be near 0v. +4.5v bias should always appear on the FET's source.
     

Share This Page