Fender Red Knob Twin troubleshoot

Discussion in 'General Electronics Chat' started by guitarlos, Jan 25, 2012.

  1. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    My Fender "Red Knob" Twin guitar amp does not turn on at all. I've changed the fuses and power tubes and it doesn't turn on. Any ideas? I'll take it to a pro to work on it but I would like to know what it might be.

    Thank you.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    Does the POWER light come on? If not, it could be a bad power cord or power switch.

    Is the light in your room on?:)
     
  3. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    The power light does not come on and the power in my room is on. ;) Power switch seems to be heading in the right direction. It worked and sounded great and then one day I turned it on and it didn't work.

    Thanks.
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    Do you have a multimeter? There are some simple tests you can do yourself and just might save you some money.
     
  5. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    Yes I do have a multimeter.
     
  6. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    The first thing to do is set the multimeter to the highest resistance range.
    Turn the "Red Knob" power OFF. Unplug the power cord.
    Measure the resistance across the flat prongs on the AC plug. You should read an open circuit.
    Turn the "Red Knob" power ON. Measure the resistance across the AC plug again. You should read a low resistance (about zero ohms).
    Set the multimeter to the lowest resistance range and measure across the AC plug again. You should read a low resistance (about 10 ohms maybe).
     
  7. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    Okay, I'll do this tonight after work. Will this maybe tell me if it's the on/off switch?

    Thank you.
     
  8. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    Hi,

    I did it but I think my multimeter isn't so good. It's a cen-tech and I had it set at 200k. There didn't seem to be much of a difference with the power switch off then on. Here is the reading (off 11.2) (on it begins to drop and goes to 0). Man I wish I knew more! Thanks for your help.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    How long does it take to drop to 0? This does not make much sense.

    The schematic diagram for the Twin Red Knob shows two fuses on the AC power input.
    Remove the fuse on the back and check the fuse with your ohm-meter.
    If that is ok, you have to find the second fuse. I don't know why they would put two fuses in series.
     
  10. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    The reading falls slowly. Both fuses are good.
     
  11. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    Well I am stumped. Sounds like you have a bad connection the AC mains circuit somewhere.

    You could check for a bad power switch by putting a jumper across the contacts.
    This is a double pole switch and both LIVE and NEUTRAL lines are switched. Do be careful and know exactly what you are doing. If you don't, take it into a reputable repair shop.

    Where was that second fuse?
     
  12. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    Hi
    I checked the power cord with the multimeter and it's good. I checked the on/off switch and it's good. I checked the connection from the switch to the fuse and it's good. Second fuse is inside the chassis next to the fuse the pop's out the back end of the amp. My skills are very limited and I know that one can get hurt with an amp so when I get the money, I'll take it in. I really appreciate your time and effort. Thanks.
     
    Last edited: Jan 27, 2012
  13. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    When you measured the fuses, where they in the circuit or did you take them out?
    If they where still in the circuit and the value is lowering, you could mearure the charging of a capacitor with a defective fuse.

    Bertus
     
  14. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    Here is the AC Power section for the "Red Knob" amp.
    There is no cap on the input section. The Panel Indicator LED should come on unless:

    1. AC power is not reaching the mains transformer T2.
    2. The mains transformer is faulty.
    3. The power LED is bad.
    4. Power is not reaching the LED.

    My suspicion is a bad contact or wiring in the AC power line circuit.

    A resistance check across the AC power plug should show a solid unchanging resistance of about 10 ohms instantly, no slow changes.

    What happens when you short the leads of the testmeter while on resistance range?


    [​IMG]
     
    Last edited: Jan 28, 2012
  15. bertus

    Administrator

    Apr 5, 2008
    15,647
    2,346
    Hello,

    Seeing the schematic it is clear that there is no capacitor.
    How is the resistance when you measuring the resistance on the powerconnector pins, when the switch is closed?
    There could be a resistance in the switch due to dirt / oxidation.

    Bertus
     
  16. MrChips

    Moderator

    Oct 2, 2009
    12,432
    3,360
    I have a Fender Super Champ XD and a Fender R.A.D. Bass and they both read about 10 ohms at the AC plug when the Power Switch is ON.
     
  17. guitarlos

    Thread Starter New Member

    Jan 25, 2012
    8
    0
    I'll check once more before I take it in. I'll report back on the readings. Thanks.
     
    Last edited: Jan 30, 2012
Loading...