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Learning to read schematics/Vox AC4TV head issue

Discussion in 'Amp Tech Center' started by Sheaton84, Oct 26, 2020.

  1. Sheaton84

    Sheaton84 TDPRI Member

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    Ok, so I have a Vox AC4TV head that is making a wooshing/crackling/occasional popping sound that increases with increased volume. I’ve tried new tubes and cleaned pots, so that’s all I know to do besides taking the plunge into troubleshooting and replacing components.

    I know this is probably a tough endeavor for my first true electronics repair, but it’s the simplest and least invested item I own so I’m going to go for it.

    Where do I begin to make sense of the schematic vs the component lay out? I don’t care how long it takes for me to learn, just looking for advice on how to begin and what resources to utilize. Thanks for taking the time to help this noob.
    32689734-38EF-4AC3-870A-E0524F4AD823.jpeg 762E7A76-0C9C-443F-960C-59F2EAB2E38E.jpeg B0C3FF45-490C-461D-A3A5-CDE365EBB582.png
     
  2. Whatizitman

    Whatizitman Friend of Leo's

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  3. sluglas

    sluglas Tele-Meister

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    First thing check all points that connect to ground on the chassis.
    Make sure they are clean and tight.
    Check the tube sockets where they are soldered to the board with a powerful magnifying glass for cold solder joints.
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Make sure you know how to safely work on amplifiers. There can be lethal voltages inside.

    The crackling and popping is consistent with bad connections such as poor contact of tube pins in the socket.

    Clean the sockets and tube pins with deoxite. There are *how to* tutorials on youtube. You may want to lightly scrape the tube pins with an x-acto blade to assure good electrical contact.

    *Lightly scrape* means just take the oxidation off. There should be no metal filings left on the ground after this procedure.

    The poor contact may be where the socket is soldered to the PCB. Sometimes the PCB trace lifts/breaks so simply re-flowing solder may not work. Inspect carefully.

    The plate resistors (R9 and R22) may have problems on the PCB similar to the socket trouble. Inspect and re-flow those as well.

    Be very gentle with the PCB. The traces may lift easily. Especially when heated.
     
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  5. dan40

    dan40 Friend of Leo's

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    As Lowerleftcoast mentioned, bad or broken solder joints connecting the sockets to the board can cause issues like this. Be very careful when soldering on these PCB boards as it's easy to damage them if you are not experienced. Having a good soldering station with the correct tips will also make the process much easier.

    The fact that the noise changes with your volume pot settings helps to narrow down the problem to the preamp tube. As mentioned above, start by cleaning and re-inserting the tube. If this does not help I would try replacing v1 with a new tube that you know works properly. If this does not help it will be time to start checking the circuit.
     
  6. Peegoo

    Peegoo Poster Extraordinaire

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    When the noise in the signal changes with the volume control, the cause/source of the noise is almost always in the preamp or before the preamp (the guitar itself, the cable, the jack, volume/tone pots, preamp tubes and related components).

    It looks to me like there's something going on around that V1 tube socket. That discolored crusty caramel-lookin' stuff sure appears suspicious--like maybe a previous owner replaced the socket, or perhaps went in to reflow the connections from the top and cooked the PC board.

    Whatever the cause, it does not look good. Some careful inspection with magnification and bright light is in order!
     
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  7. Sheaton84

    Sheaton84 TDPRI Member

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    Thanks for the info! I initially (prior to this post) cleaned the tube sockets with D5 by spraying tube pins and gently working the pins in/out a few times. Peegoo I thought that Caramel crust and overall brown discoloration around the power tube socket looked suspect as well.

    As far as a proper soldering station, I own a 35 watt iron with a standard tip (seems pretty chunky so I guess maybe I should first try to find a smaller one). I’ve tried three different tube sets (preamp and power) to no success. I guess it’s in the circuit.

    I know the concept of discharging filter caps for safety, but I don’t understand how to get to the cap leads when oriented vertically like that on the PCB. I guess remove the board and get underneath?
     
  8. Sheaton84

    Sheaton84 TDPRI Member

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    What’s the best strategy/technique to make sure those traces don’t get damaged? I assume not hammering it with heat from the iron for too long, meaning to not leave the tip on there any longer than absolutely necessary? Thanks!
     
  9. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    If you are just re-flowing the solder, just be sensible with the heat. If you will be removing a component, the best way is to snip the lead and just remove the small piece left over.
    Do not pull at the component to get it out of the through hole. A solder sucker and/or solder wick with flux can be helpful.
    Don't go poking around in there until the caps are drained.
    Follow the schematic to another lead in the circuit. Pins 1 and 6 on the 12a_7 are in the circuit. They can be used. Preferably you want the dropping resistor junction with the first filter cap though. It looks like that resistor is R16 located between the two radial filter caps. A snuffer stick on either side of that cap will drain the caps.
     
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  10. dan40

    dan40 Friend of Leo's

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    The brown crusty residue you see is just old flux residue left over from the soldering process. It won't cause any harm but it can easily be scraped away or cleaned with alcohol if you like. If the board surface is actually discolored from heat, that might indicate a problem in that area and should be looked at.
     
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  11. dan40

    dan40 Friend of Leo's

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    If you feel comfortable working inside of a live chassis, you could power up the amp and poke around with a wooden chopstick to see if you can recreate the condition. Press against the sockets and pots and all of the other components gently and see if the noise occurs. The tubes must be installed and the speaker plugged in while performing this test.
     
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