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Thread: Problems with Fender "Custom Vibrolux Reverb" Reissue (voltages included)

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    Problems with Fender "Custom Vibrolux Reverb" Reissue (voltages included)

    Somethings very wrong, and I'm trying to pin it down.

    I get a weird, very unmusical distortion even at low volumes, and quite a bit of hum/hiss.

    I measured voltages at several points to compare with reference voltages on the schematic.

    I highlighted the ones that are off by more than 20% in red. I've never changed the tubes, so that's on the list. But does it look like something else is at play here?

    Voltage measurements:

    https://i.imgur.com/GuFmevI.png

    Thanks!

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    Supporting Member Randall's Avatar
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    I will be watching this, as one of these beat me a few years ago. So far it sounds like the same problem.

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    It's weird, because it WAS working fine.....

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    Lifetime Member Enzo's Avatar
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    Schematic?

    Look for ripple on all B+ and bias supplies.

    Play through a different speaker, just to be sure.

    Hum AND hiss? Isolate the problem... Do ANY of the controls affect the amount or sound of the noises in ANY way?

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    Don't forget the joker g1's Avatar
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    schematic attached.
    Tp17 is a typo, should be about 77VDC there which is what you have.
    Your bias is pretty hot, (TP32), make sure tremolo is off for that measurement, if it is actually -39V instead of -45 I think that is a problem.
    The AC voltages do not mean anything unless you are injecting signal under conditions specified in the notes.
    Attached Files Attached Files

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    Lifetime Member Enzo's Avatar
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    OK... thanks.

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    Bent Member Chuck H's Avatar
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    And much of what is noted as "off" is the AC (signal) voltage. The AC input voltage for test that dictates the AC through every other TP point must be 81mV @ 1k and tested in process for accuracy. Is this the actual testing circumstances that determined the incorrect voltage readings.?.

    Also, it's a real PITA to go back and forth between the voltage chart and the schematic to locate TP points. Much more interested in idle DC measurements and a description of what's wrong with the sounds.

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    The voltage measurement chart from the link in the first post:

    Click image for larger version. 

Name:	GuFmevI.png 
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    Senior Member Pedro Vecino's Avatar
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    Quote Originally Posted by ianmgull View Post
    I get a weird, very unmusical distortion even at low volumes, and quite a bit of hum/hiss.
    It could come from an inactive power tube. Turn it on, wait five minutes and with your hand check the temperature of the power tubes.
    Two hot? One hot and one only slightly warm?

    Another possibility is a problem in the phase splitter (open plate resistor). Check the voltages on pins 1 and 6.

    And another one: check in the filter capacitors that there are no traces of substance in the output of the positive terminal (degradation of its operation).

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    Quote Originally Posted by Pedro Vecino View Post
    It could come from an inactive power tube. Turn it on, wait five minutes and with your hand check the temperature of the power tubes.
    Two hot? One hot and one only slightly warm?

    Another possibility is a problem in the phase splitter (open plate resistor). Check the voltages on pins 1 and 6.

    And another one: check in the filter capacitors that there are no traces of substance in the output of the positive terminal (degradation of its operation).
    When I first read his description, I thought the same thing.

    The symptoms you describe in your first post are a lot like what happens when only one output tube (or sections of output tubes) is conducting in a push-pull amp.
    Just like Pedro says, the two most common reasons for this are bad tubes, and/or an open plate resistor in the phase inverter. Happens all the time.
    One other very common reason why a particular output tube won't conduct is when a screen resistor fails open.
    Do you own an oscilloscope? This is easy to identify monitoring the output waveform. If not, test the bias current for each tube. This will also tell you if one tube is not conducting. If the bias current looks good on both tubes, I would the check the voltages.... scratch that, looks like your getting appropriate plate voltages on your phase inverter.
    Fender includes nothing in the schematic test points/flow-chart to indicate whether or not the output tubes are functioning. A reading a bias voltage at a single point is only important to insure that you have a bias voltage, but you're probably going to want to make sure that the bias voltage is making it to each of the tubes.
    In this amp, I don't see an adjustment to control the bias, but I think G1 is right in that -39V could be a problem. Definitely check the bias current in each tube. If the bias is out of spec, use another pair of bench tubes and check the bias on those. If those indicate over-current at idle as well, it might be worth altering the divider in the bias supply to increase you -voltage.
    Happy Hunting.

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