Valvestate 8080 Issues

Discussion in 'The Workbench' started by Techniciousd, Apr 11, 2019.

  1. Techniciousd

    Techniciousd New Member

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    Hello all,

    I recently acquired a VS8080 that had a terrible hissing problem. It basically sounded like rainfall through out all of the channels. If I plug a source in, i have no change. If I go through the FX Return it is significantly better.

    I then proceeded to test the voltages on the first OP Amp. (TL072cp) I found that pins 1-3 gave voltages all over the place between 2 and 16 VDC, and that pins 4 and 8 read 15 VDC, and pins 5-7 were again all over from .2 - 16 VDC.

    I then proceeded to replace the IC1 op amp.

    This now has made the problem worse, and now i have almost a pure tone going through the amp when nothing is plugged in.

    I installed a socket on the first IC so i could swap and test easier, and i get the same voltages when the OP AMP is not in the socket. Now it gets interesting as pins 1-3 should be near 0 VDC, 4 and 8 should be 15 VDC, and pins 5-7 should be close to 0 as possible.

    Has anyone had this issue, or seen something lie this?

    Thank you all.
     

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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Welcome to the forum Techniciousd. :cheers:

    1. Did you replace the preamp tube?
    2. Did you plug a guitar cable from send to return and then re-test the amp?

    Soldering the board can damage the circuit tracks.
    You may have broken the tracks when you installed the socket.
    Can you post clear pictures of the solder side of the board?
    Don't try to solder the board anymore until you diagnose the problems. Don't change any more parts except the preamp tube.

    8080ps.png
     
  3. Techniciousd

    Techniciousd New Member

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    I tried replacing the tube first, and it had no effecr on the amp, and the problem persists if the tube is in or out.

    I can get a clear picture soon, but upon close inspection the traces look intact.

    If I jump the FX send and returns, i get a signal on dry and wet if the switch is on -20db, but if i switch it to 0db, i get signal only on dry and it cuts on wet.

    I can confirm 15.5 v on both Zeners.
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    It seems like the level switch / effects jacks switches is oxidized.

    Soak the switches, jacks, spray out the pots, drown those puppies.
    Then operate the switch, control, jack, over and over, and spray (drown it) it again.
    Use lubricated non conductive cleaner such as LPS-1, Deoxit D100, G100.
    Cleaner that says: "no residue," is not the correct cleaner.
    The correct cleaner must have permanent lubrication. The cleaner should be specifically for electronic / electrical.

    After cleaning see if the noise is still there.

    The effects jack contains a switch.
    When this switch oxidizes it can break the signal path or become noisy intermittent.
    Bypassing send to return with a cable skips the switch contact.
    This tells you the jack switch is dirty oxidized corroded and cleaning may fix it temporarily.

    A new jack is always better....

    This switch inside the jack must work even if you never use the effects loop.
    the switch is always in the signal path.

    cliffjack.jpg
     
    Last edited: Apr 11, 2019
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  5. Techniciousd

    Techniciousd New Member

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    I have hit them all a few times already. With keep poking around that section.

    This still does not explain the voltage issues on the IC1 though.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    IC1 Pin 7 and Pin 1 should have 0 volts steady DC.

    If the input pins are dancing around, the jack switches can be corroded (re-solder the input jack) or the power supply could be unstable (check zener diodes above).

    Check and see if any IC is getting hot. But be careful, don't burn your fingy.

    [​IMG]
     
  7. Techniciousd

    Techniciousd New Member

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    I'll have to do some more with it soon. I checked the zener diodes in the circuit and they read out at about 15.5v so the 15v rail is getting power. Next ill have to pull the input jack and do a deep clean. Shouldn't the switches in the input jack work as a bypass, or mute of the channel if there isn't anything plugged in? If this is the case, this could be the start of the problem.

    on IC1 without a chip in the voltages read:

    1 -3
    2 -3
    3 -6
    4 -16
    5 -7
    6 -3
    7 -3
    8 15.7
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    After you spray some cleaner in between the switch contacts,
    You can push on the contacts with a chop stick, and see if the noise stops.
    wooden chopsticks can be very useful (besides eating / cooking).
    You can tap on the board, wiggle components in search of a bad solder connection with a wooden chopstick. I guess plastic chopsticks would be OK, but I like wooden ones.

    The -3 volts indicates that there is a bad component, which is causing a DC offset.

    There should be 0 volts DC on that rail to the output jack.
     
    Last edited: Apr 11, 2019
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    If the opamp IC1 is removed how do you get VDC on pins 1 or 7?
     
  10. Techniciousd

    Techniciousd New Member

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    That is the over all reason for the main post. I am currently going along with a magnifier to see if i can see shorts or bad traces. I was wondering is there is anything in the main channel section that would cause the voltage issues on IC1. The rest of the channel sections ic's are fine, except the reverb, which according to the old notes i found in the amp, never worked as well.
     
  11. ampmadscientist

    ampmadscientist Well-Known Member

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    Plug a cable into the speaker jack.
    Set meter for DC volts
    Turn the amp on and measure DC at the end of the speaker cable.
    The speaker cable should have 0 volts DC with no voltage offset.

    So you are fighting DC offset from the output of the IC, or you are fighting DC offset from a bad transistor in the power amp.
    What is the voltage on the speaker cable?
    This gives you a clue.
    If it's higher than -3 volts, it's a transistor. If it's way less than that the offset is coming from the op amp.

    See, the issue may not be the op amp at all.
    You may have a transistor leaking DC into the output.

    So measure that and tell us what you get.
     

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