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Thread: Fender hot rod deluxe volume loss issue

  1. #1
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    Question Fender hot rod deluxe volume loss issue

    Hello everyone.

    I have already read all the posts about this problem and I have changed out all the "usual" suspect parts. The problem is I am still having the same problem. After downloading the schematic and tracing the power I ended up figuring that off my phase inverter pin 2 on the opposite side of the .022uf 400v cap I have 0V. So the first thing I did was lift the side of the cap that had 0V and to my surprise I had power on the other side of the cap. After that I tested the R40 and R38 resistors. The resistor values were correct so I landed the .022uf 400v cap back on the board then lifted the ribbon cable that feeds the preamp board. I am now getting power through that cable that should be feeding the preamp board. So the next thing I did was remove the C52(.0068uf 100v) cap. I have power to this point and that's where I am at to this point. Can someone please tell me if the problem could be the U2 IC chip? I have no control at the Master volume and no power at all in the power amp section because of this. It seems like a short somewhere thats dropping my voltage to 0V. Any help would be greatly appreciated. This repair is driving me crazy and I want to make sure I order the right parts so this repair is fixed.

    Thanks
    Colin

  2. #2
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    Hi and welcome to the forum.

    U2 and C52 are related to the reverb circuit. I would ignore that side of things for the time being.

    A check you can make is to plug your guitar directly into the 'Power Amp In' socket. Do you get a sound? You won't have any amp controls, but this will verify that the PI/power section is working.
    52 Bill likes this.

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    Hi thanks for the response.
    I just tried plugging into the "power amp in" and I do have a clean signal coming through but its not very loud. But there is a signal. So does that rule out any problems in the power section. When my buddy asked me to try and fix this amp for him I replaced the bias circuit caps and the 470 ohm 5 watt resistors. I didn't touch any diodes in the circuit. Should I have replaced the zener diodes (D13 and D14)? i was getting +/- 16V so I thought I was fine but any other pointers would be greatly appreciated.

    thanks
    Colin

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    Senior Member Enzo's Avatar
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    To be honest, you should be replacing that which is defective, not just things that seem like they might be involved. If the zeners are doing their jobs, why oh why would we replace them?

    If a signal into the power amp in jack seems to come out clean and reasonable level, then yes, the power amp is probably OK.

    What version do you have? Look on the main board near the input jacks, does it say 1996, 2001, something else?
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    Education is what you're left with after you have forgotten what you have learned.

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    Hi Enzo
    The suspect parts I changed were more than 5% out of tolerance so I just changed them. This is the first amp that has solid state parts in it that I've tried repairing. Im use to vintage amps or home built amps that are less parts and point to point. Im not an expert so Im curious and asking all the questions I can.
    The power amp is not a reasonable level its really quiet. I had to put my head close to the speaker about a 2 or 3 feet away to hear it. My guitar was louder acoustically that the amp it was plugged into. Im hoping that its still means the power amp is good.

    The board is a 2002. Its the special edition version which to my knowledge has the same internals as the regular hot rod deluxe just a different speaker.

    Again I appreciate you helping me out and taking the time to share your knowledge

    Colin

  6. #6
    Senior Member Enzo's Avatar
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    Well then, basics. Is the speaker plugged into the correct jack? The main speaker jack has a shorting contact, so you MUST use it first. If you plug your speaker into the extension jack instead, you get seriously diminished output due to the shorting jack.
    Education is what you're left with after you have forgotten what you have learned.

  7. #7
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    I will recommend that you not pull parts or lift one end for tests unless you are certain that there is a problem. These boards can be damaged from too much heating and you can end up spending a lot of time repairing things that didn't need to be fixed in the first place.

    In general, this amp uses a few solid state devices, but unless you are using the FX loop, there are no solid state parts in the normal signal path. The reverb is all solid state and the switching is all solid state, but the audio does not go through any of the opamps except for the FX loop buffers.

    The schematic has dc voltages listed at different test points, how do your voltages compare to those on the schematic?

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    Tolerance changes in parts rarely stop an amp from working. Wire-wound resistors are about the most stable component there is and if anything they will just fail open. Capacitors often read quite a way over their nominal value.

    The way to troubleshoot an amp is to "divide and conquer", to quote Enzo. So the approach is to determine whether your power amp or preamp is faulty, then break that down into sections. Your Fender has two ready-made points that divide your amp up nicely; the 'preamp out' can be run into another amp to check everything in that section works, and as already mentioned, a signal can be injected into the 'power amp in' socket.

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    Hi guys thanks for the tips.
    I am in the correct speaker jack. I removed the wires and solder from the shorting jack and cleaned solder points then used new solder to reconnect. But still have the problem with low noise. The reason I removed one leg of the C24 cap is because I was getting 0V at TP23. That is where I believe I'm losing the signal. Maybe Im wrong and looking at the wrong place.

    my voltages for all my tubes are as follows:

    V1- pin 1 - 168V (when touching this pin the power tubes are flickering and making loud noise)
    pin 2 - 0V
    pin 3 - 2.5V
    pin 6 - 173V
    pin 7 - 0V
    pin 8 - 2.7V (when touching this pin it does the same as pin 1.)

    V2- pin 1 - 219V
    pin 2 - 0V
    pin 3 - 1.7V
    pin 6 - 217V
    pin 7 - 0V
    pin 8 - 1.7V

    V3- pin 1 - 243V
    pin 2 - 23.58V
    pin 3 - 38.5V
    pin 6 - 241V
    pin 7 - 24.8V
    pin 8 - 38.5V

  10. #10
    g1
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    Quote Originally Posted by ColinAAE View Post
    V1- pin 1 - 168V (when touching this pin the power tubes are flickering and making loud noise)
    How loud? Better than when you plugged your guitar into the 'pwr amp in' ?
    The loud noise you hear implies the power amp is ok, unless it's not very loud.

    Any chance your guitar or patch cord is bad?
    52 Bill likes this.
    Quote Originally Posted by Justin Thomas View Post
    In my ideal world, I'm not too loud - your room is too small!

  11. #11
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    Quote Originally Posted by g1 View Post
    Any chance your guitar or patch cord is bad?
    Or a bad input jack or broken solder on the jack?

    All of the dc voltages seem reasonable.

  12. #12
    Supporting Member Chuck H's Avatar
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    I'll speculate that when ColinAAE says the tubes are flickering and making loud noise that he's seeing blue flashes and hearing mechanical chatter. That seems like the power tubes are not connected to the load properly. I would re check the output jack wiring and speaker connection.
    Last edited by Chuck H; 05-11-2017 at 08:09 PM.
    "I've heard magic defined as "a technology you don't understand". By that aphorism, the folks in this forum are practicing wizards, able to summon AND control the lightning demon, and make charms to allow others to use the demon in certain ways." R.G.

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