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Fender stage 160 amp blowing fuses

Discussion in 'Troubleshooting and Repair' started by Resorb, Mar 25, 2015.

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  1. Resorb

    Resorb

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    Mar 25, 2015
    Hi I am a hobbiest at electronics, I know enough to troubleshoot most minor problems but I have ran into a problem I can't seem to track down. I need help so anyone familiar with guitar amp repairs I would really appreciate some help. So here is a link to my amp

    https://www.google.com/url?sa=t&sou...AQFjAC&usg=AFQjCNGQ6cCxuT1XLGULEnus3axV8HowUA

    I received this amp with a large hum found the high voltage side filter caps were loose I resoldered and it played beautifully for about an hour then pop. I have replaced the transitors 10-13 and also the darlington transitors 15-19 15 and 14 were shorted so I replaced them all, also the filter caps on the high end........ The transistor shows the right voltage when I tested it. Here's what's happening. If I plug it directly into the wall it blows the 4amp fuse, I built a poor man's voltage limiter with a light bulb, when I run it through that the 300 Watt light lights up but I am able to play guitar through the amp with a loud 60hz hum. If I unplug the two red wires for the circuit (hi voltage end) the amp does not hum or pop fuse. Any help anyone could be I'd really appreciate it! Thanks for your time
     
  2. duke37

    duke37

    5,364
    769
    Jan 9, 2011
    I have not played with guitar amps but here are some tips.

    The use of a lamp limiter is good. The 300W bulb should not light up when the amp is not giving output as the quiescent current should be low. 100W bulb should be plenty.
    We do not know what the red wires do.

    If there is a loud hum, then the power supply is not doing its job.

    If the reservoir capacitors have been overheated, they are not likely to be good.
    A shorted diode in the power supply bridge will give massive hum and overheat the reservoir capacitors. Measure the positive and negative voltages, they should be the same.
     
  3. Resorb

    Resorb

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    Mar 25, 2015
    I have checked all the diodes in the rectifier bridges and they are good, the red wires are the high voltage legs of the transformer. I checked the voltages through the caps and yes they are the same
     
  4. davenn

    davenn Moderator

    13,785
    1,936
    Sep 5, 2009
    that doesn't tell you if the cap(s) have failed

    how about some sharp and well lit pics of the PSU section for a start
     
  5. Resorb

    Resorb

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    Mar 25, 2015
    1427313679609.jpg do these work? You can see both sets of rectifier bridges and filter caps
    1427313710725.jpg
     
  6. TenderTendon

    TenderTendon

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    Dec 20, 2014
    Try checking the voltage across the filter caps with your meter set on AC volts.
     
  7. elebish

    elebish

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    Aug 16, 2013

    Keep the light bulb connected (100 watt), power up and check the temperature of the thermal run-away resistors of each output transistor with an 8 ohm load at the speaker outputs. They are the (less than 1 ohm 5 w) sand filled types. If one or more are getting a bit too hot too soon, there might be a bad out put transistor-leaking or shorted. If there are coupling capacitors feeding the speakers, check them. An overloaded PS could cause hum because the filter caps are not being allowed to fully operate and are heating up also. If all seems to be normal in the supply voltages, check with a scope or AC meter at the filter caps to see if an AC component is larger than spec. A large AC component will lower the DC supply voltage (B+). You also might want to check the emitter to base bias on the output transistors and the driver transistors. Make sure that you don't have a hum signal coming from the pre-amp section. Use a dc coupled scope to check the signal path but short amp inputs first. Ed.
     
    Last edited: Apr 7, 2015
  8. jaysgr

    jaysgr

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    Oct 8, 2014
    I had a similar problem with an amp like yours. Didn't blow the fuse but had a loud hum.
    It turned to be a short from the screw of one of the transistors to the heat sink. Check the Q14-Q19 transistors.
    Also check if you have by mistake overheat them when you soldered them.
    jay
     
  9. Petetrucks

    Petetrucks

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    Jan 8, 2016
    Hi
    Can you tell me the colour wires which connect to the terminals P7,P9,P10,P11'P8.
    I have four wires I believe white goes to P8, red and black to P9 there is a spare terminal.
    I have a white and mauve and a black wires to connect.
    Also on the terminals P6 P5 P4 does it make any difference to what blue wire goes where I know brown is P5.
    Lastly I have three wires connected to P14,P13,P12 White to 14,yellow and black to 13 and green and black to 12.
    My son took the amp apart and never photographed the wiring system.
    I searched the web and seen your picture but the P17 to P8 are just out of shot.
    I appreciate your help if you can.
    Thanks
    Pete (UK)
     
  10. Petetrucks

    Petetrucks

    4
    0
    Jan 8, 2016
    Hi when I say white and mauve I mean one wire white with mauve strip and the same with red and black.
    Sorry to sound so stupid but I am a sixty year old novice tiring to sort my sons amp
    Thanks again
    Pete
     
  11. elebish

    elebish

    177
    12
    Aug 16, 2013
    The red wires supply the 65 vac that develops the plus and minus 45vdc for the output transistors, the yellow wire being common to both. The blue wires supply the 17.8 vac that develops the plus and minus 15 vdv that powers the preamps. The brown wire being common to both.
    The red wires are P1 and P3, common yellow is P2. The blue wires are P4 and P6 and the common brown is P5. All the other terminals should be checked against the schematic. Use a scope to check for ripple at the filter caps. Shorted caps will blow fuses and open caps will create 120 hz hum, big time! Ed.
     
  12. elebish

    elebish

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    Aug 16, 2013
    All the wire colors look ok and the blue wires are ac so it does not matter in which order they are connected. Double check the big filter caps. They may open at the rated voltage. I don't suspect the bridge diodes since the caps would burn up and the fuse would blow if the diodes were shorted.
     
  13. Petetrucks

    Petetrucks

    4
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    Jan 8, 2016
    Thanks for that.
    Those wires mentioned were all correct.
    The problem I have is the p,7,8,9,10, and eleven wires.
    I blew the 2.5amp fuse by switching wires about.
    Just one last question there must be a significant reason why P7and P8 are either end of the connections
    They run bottom to top thus
    P7/P9/P10/P11/P8.
     
  14. elebish

    elebish

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    Aug 16, 2013
    I assume your supply is 230 vac being in Canada. P8 connects to P11 and to one end of a .01 cap. P9 and P10 connect together and to the other end of that .01 cap. P7 connects to the 2.5 fuse and the other end of the fuse connects to an MOV. The other end of the MOV connects to P9 and P10. The MOV is supposed to short out and blow the fuse when ac voltage 'spikes' above the rating on the MOV. That may be your problem. I suspect the .01 cap is ok. It is used for noise only. Ed.
     
  15. Petetrucks

    Petetrucks

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    0
    Jan 8, 2016
    Cheers have a beer on me all sorted and working now thanks for all your help
     
  16. kemjet

    kemjet

    6
    0
    Aug 7, 2017
    HI Sir i have same problem with Fender Stage 160 (DSP), Trouble Humming sound, my friend disconnect wires from transformer to main board but he didn't put any markings or photo shot, now i have no idea where is the exact connection for remaining 4 wires cause there is no markings in the main board. pls. help. Thanks.
     
  17. elebish

    elebish

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    Aug 16, 2013
     
  18. elebish

    elebish

    177
    12
    Aug 16, 2013
    The red wires are the higher voltages that feed the output transistors. When disconnected, the hum will disappear but the hum could be coming from the pre-amp section if the big filter caps and bridge diodes are good. Leave red wires disconnected and check lower dc voltages that feed the pre-amp section. This is assuming that the voltages going to the preamp section is separate sourced. A scope is always the best way to find the ac component riding on the dc voltage that should be almost pure dc. If those dc voltages are reading what they should be on a voltmeter, try jumpering the small filter caps in that section of the amp that are used for filtering those small voltages. Watch polarity and discharge substitute caps between use. It also wouldn't hurt to check the "coupling" caps that connect signals from transistor to transistor. These caps block dc but pass the signal, including hum!
     
    Last edited: Aug 7, 2017
  19. kemjet

    kemjet

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    Aug 7, 2017
     
  20. kemjet

    kemjet

    6
    0
    Aug 7, 2017
    To clarify my post here's the pics
     

    Attached Files:

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