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Thread: Fender Hot Rod Deville switch chatter no bias reading

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    Unhappy Fender Hot Rod Deville switch chatter no bias reading

    Hello
    I am new to this forum and have to say I love it already!! I currently have a Fender Hot Rod Deville (4x10) that has an issue with the channel switch/relay chattering.. I have no clue as to why it's doing this.. Also, at the bias test point there is no mV reading.. I have tried a new set of tubes and even a second set of tubes.. Still no reading.. I have knowledge of electronics and even have performed some pretty cool MODs to these style amps but this one has me operationally stumped... Please help!!

    Oh and by the way I replaced the power tube sockets as well with new belton brand sockets, the plate resistors have been replaced, electrolytic caps have been replaced, tone stack caps have been replaced, all solder joints on both boards have been reflowed and have been verified to have no cracks under a magnifying glass, relay switch has been replaced with no effect, input jacks have been replaced with switchcraft jacks.....

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    Is the amp working or not? If working, it's probably measurement error with the bias. Are you on DC volts?
    If it's not working, check the cathode resistor R66 on attached schematic. (if this schematic is not the right version, please post correct one)
    For the relay chatter, make sure the low voltage dropping resistors and especially their connections are ok. (R78 & R79 in power supply area)
    Attached Files Attached Files

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    Amp is working. Using my Fluke with a mV setting and unlike all other Hot Rods I have worked on, this one doesn't read.. Pretty familiar on the process to check and adjust bias on these amps.. I will check then resistors you have mentioned and post findings.. As for the schematic I'm just using the one off the fender website.. Thanks for the help!!

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    Do you have any idea how old this amp is? Some of the real early ones had a problem with relays chattering. There was a circuit mod to fix it.

    As for the bias problem, have you tested the cathode resistor?

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    Ok, I have the amp back up on the bench and:
    R66 with one leg lifted from the board reads 1.2 ohms
    R78 & R79 read about 335ohms a piece with one leg lifted off the board.
    The date written on the inside of the chassis is 4-1-97.
    Plugged in, tubes in, board grounded, power and standby ON:
    Bias point reads 12.3 VDC
    Bias test point reads OL in mV setting (using a Fluke 117)
    R78 reads 42.20VDC at bottom leg to ground 16.65 VDC at top leg to ground and 26.50 VDC across both legs
    R79 reads -23.05VDC at bottom leg to ground 15.87 VDC from top leg to ground and 7.16 VDC across both legs

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    Plate resistors R4, R11, R16, R22 read 384VDC
    Plate resistor R58 reads 435VDC on the left and 270VDC on the right (???)
    Plate resistor R57 reads 435VDC on the left and 282VDC on the right (???)

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    Chattering happens when channel select is pushed in or ON and remains while more drive is selected. Though there is a recognizable delay before it starts chattering. Footswitch has the same effect. Clean mode has no chattering. So this tells me the chattering problem is somewhere in the channel switching circuit, but I can't figure it out.. What am I missing?? I also know the voltages on R79 and R79 are way off...

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    Lifetime Member Enzo's Avatar
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    The schematic has a series of test points in the switching circuit, go through them and compare your readings to those specified. The LEDs on the panel and in the circuit are not just pretty lights, the voltage drop across each one matters. Bad LEDs can cause relay buzz.
    Oh and by the way I replaced the power tube sockets as well with new belton brand sockets, the plate resistors have been replaced, electrolytic caps have been replaced, tone stack caps have been replaced, all solder joints on both boards have been reflowed and have been verified to have no cracks under a magnifying glass, relay switch has been replaced with no effect, input jacks have been replaced with switchcraft jacks
    Why did you replace all that stuff?

    Please don't refer to top and bottom of parts unless you specify on the drawing. In the unit, we usually have no idea which way a part is facing. Note that when I work on one of these, I see it with the controls on the top, but look at the layout drawings, they have the controls at the bottom. Ambiguity results. A resistor doesn't have a voltage "on" it. There is a voltage on each end (usually measured to ground), and there is a voltage across the resistor (end to end). When you say a resistor has a voltage on it, that can mean either end, and the two ends usually have different voltages by design.

    R78 reads 42.20VDC at bottom leg to ground 16.65 VDC at top leg to ground and 26.50 VDC across both legs
    R79 reads -23.05VDC at bottom leg to ground 15.87 VDC from top leg to ground and 7.16 VDC across both legs
    Both resistors drop a low voltage supply down to a 15 or16v zener. So i tlooks like the R78 side is working. 16.65 is close enough. Your negative side reads very roughly half what it should. My immediate suspicion is then drawn to a probably loss of filtration. IS C42 open, unsoldered, or bad? Measure ripple on that supply, is it miniscule or 20vAC? Even with that failure, the zener still seems to be close at 15.87v.

    SO how is this scenario? That negative 15v supply is working but with no C42 action there are a ton of pulses in it. The switching circuit relies upon smooth +15 and -15, and so the operation is interfered with by the excess ripple.

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    Plate resistors R4, R11, R16, R22 read 384VDC
    ONE end of each is B+ node "X", wwhich my schematic says is 388 volts, 4 volts away is pretty damn close in my book. The other end of each will read the same with no tubes or substantially lower with tubes.

    Plate resistor R58 reads 435VDC on the left and 270VDC on the right (???)
    Plate resistor R57 reads 435VDC on the left and 282VDC on the right (???)
    The PI plate resistors. OK, one end of each is B+ node Y, specified as 441 volts, darn close. The other end is at the plates of the tube, and sound perfectly reasonable. What were you expecting there?

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    Quote Originally Posted by Tonejunkie View Post
    Bias point reads 12.3 VDC
    Bias test point reads OL in mV setting (using a Fluke 117)
    This is confusing. Is there a difference between "Bias point" and "Bias test point"?
    "OL" on the meter does not mean that there is 0V voltage measured. It's exactly the oposite - the input voltage is to high for selected range (milivolts in your case). Just switch the meter to 200V range and you will see correct voltages.

    Mark

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    Quote Originally Posted by Tonejunkie View Post
    R66 with one leg lifted from the board reads 1.2 ohms
    And with out the one leg lifted? Is there a chance that there is a short across the resistor on the board?

    Agree with Enzo, fix the negative side of the low voltage supply before anything else.

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    Quote Originally Posted by MarkusBass View Post
    This is confusing. Is there a difference between "Bias point" and "Bias test point"?
    "OL" on the meter does not mean that there is 0V voltage measured. It's exactly the oposite - the input voltage is to high for selected range (milivolts in your case). Just switch the meter to 200V range and you will see correct voltages.

    Mark
    Meant bias test point.. Sorry about that.. And yes, I agree with you on selecting a different measure setting on the fluke.. My reason is be because I never get a OL while measuring here in mV.. Again trying to point things out as I'm missing something that perhaps you guys can help me pinpoint.

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    Last edited by Tonejunkie; 12-18-2016 at 05:59 PM.

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    Quote Originally Posted by Enzo View Post
    ONE end of each is B+ node "X", wwhich my schematic says is 388 volts, 4 volts away is pretty damn close in my book. The other end of each will read the same with no tubes or substantially lower with tubes.



    The PI plate resistors. OK, one end of each is B+ node Y, specified as 441 volts, darn close. The other end is at the plates of the tube, and sound perfectly reasonable. What were you expecting there?
    Enzo, I'm just going through the amp getting readings in known areas that have fault In these amps. Trying to understand where or what I may be overlooking.. I agree with you that the plate voltages are pretty darn close to the schemtic and can be ruled out.

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    Quote Originally Posted by 52 Bill View Post
    And with out the one leg lifted? Is there a chance that there is a short across the resistor on the board?

    Agree with Enzo, fix the negative side of the low voltage supply before anything else.

    Not a short, but I have seen many things working on these, as we all have, and with one leg lifted my thought process is that I'm measuring resistance through the one component and not others along the long.. One of those Me'isms..

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    Bit of advice, don't think in terms of common issues, think in terms of solving thios problem. it may or may not have something to do with other amps problems, but we can spend a lot of time going over ground that is irrelevant. For example, teh B+ voltages will tell you nothing about the relay operation.

    Please look into that faulty -15v supply.

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    You said the amp is working. If there is no bias current, I would expect audible crossover distortion on clean settings.
    What is the DC voltage on the power tube grids?

    Edit: re-reading, you are getting more than 600mV (OL for fluke 117) when checking the bias test point on mV range?

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