Help Novice with understanding guitar amp transformer replacement please

Discussion in 'General Electronics Chat' started by ColoradoRobert, Jun 6, 2016.

  1. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Can I replace a guitar amplifiers power transformer from 120vac input power/output power 10.5v not sure if it's ac or dc.... to using a 220vac input power to dual 12vac output power transformer? BTW I still plan on plugging this into standard wall outlet 115/120vac.
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
    759
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    First, diagnose if you need to replace the power transformer.
    If the replacement transformer has two primaries, it is useable as replacement wiring both primaries in proper phase to mains.
    A dual 12VAC secondary can also be used if the windings are phased properly in parallel to replace a near enough 10.5VAC original winding.

    The replacement transformer should be of similar power capability. Similar size helps if no data.
     
  3. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    First, Thank you for the reply. What I've done so far: I unsoldered the 3 legs off the pcb then I checked all of them for voltage both ac setting and dc setting and got nothing from every combination I could think of including grounding to the frame. A bit scary. Next I cut the on off switch out of the equation and wired the cord directly into the transformer which had 2 wires that are larger on the one side (1 white wire = I would think common and 1 black wire I would think positive). Again I checked all 3 legs every which way I could think of and got nothing as far as voltage even on the lowest settings.

    Now the truth about this amp = it is a throw away according to everything written on internet about it.... a Fender Squier SP10. A working unit sells sometimes for only 20 dollars and there is the rub = if I bought an exact match the transformer would cost more than the thing is worth but if I substitute this different transformer (if it would work, hence my question here) from China it costs only $6.00 and would make it worth it to replace and then sell.
     
  4. Externet

    AAC Fanatic!

    Nov 29, 2005
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    The 3 leads desoldered from the PCB sounds like correspond to the secondary winding. Measurements there should be for AC voltage.

    Check for continuity on the primary winding; should be the black and white wires.
    -If there is no continuity, winding is open (unrepairable) or a thermal fuse is blown under the transformer paperish winding cover (somewhat repairable).
    -If there is continuity, secondary winding is open. (unlikely)

    If you can rescue from behind spider webs, any wall adapter with 12V DC output, try to rig it in as replacement for proof of functionality by connecting its output to the filter capacitor on the amplifier supply section, with proper polarity as shown on the capacitor.
     
  5. dl324

    Distinguished Member

    Mar 30, 2015
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    619
    It depends.

    A transformer can step AC voltage on it's primary up or down, depending on the ratio of the turns of wire on the primary and secondary.

    The transformer in question is approximately a 10:1 step down (120VAC in, 10.5VAC out). The voltage is RMS.

    A 220VAC transformer with a 12VAC secondary is stepping down about 20X (it's actually 18.333, but I'm using round numbers to make things simpler). If you connect 120VAC to the primary, it will be stepped down by 20X to 6VAC. This won't provide the same peak voltage as the original transformer.

    If the two 12VAC secondaries are independent, you could connect them in series. That would give you 12VAC. If the circuit connected to the secondary can tolerate the higher voltage, the substitution could work.

    Some 220VAC transformers have two primary windings. The windings are connected in series for 220VAC and parallel for 120VAC. This produces the same voltage on the secondaries at both input voltages.
     
  6. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
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    Hey Externet, I tried the continuity test (that you mention above), on what I believe to be
    the primary (the red and white wire at the bottom of this transformer) and my multimeter read
    nothing on those wires. The torn paper on the exterior was my poking around to see if there
    was a fuse in there. I also took it apart from it's bolt-down frame and I saw nothing that
    looks like a fuse could be hiding anywhere in the winding area.

    Next I tried the 12vdc power adapter and soldered it directly to the number 1 cap. When I
    plugged it into my wall outlet I got a squelch out of the speaker. Much like what you would
    hear in the days of walkie talkies and CB radios. I also tried it with the overdrive button
    depressed and then out. I tried it with a guitar plugged in and with out. I tried it with
    volume all the way up and down. Moving the treble and bass and nothing changed the squelch sound. So the biggest question at this point would be = can a cheap 12vdc power adapter make a guitar amp work like it would if it was working off of its own original equipment transformer. Because if it should then there is another problem that I have not uncovered yet with this amp. [​IMG]
     
    Last edited: Jun 7, 2016
  7. Externet

    AAC Fanatic!

    Nov 29, 2005
    759
    57
    Try peeling very carefully without breaking wires the other paperish winding cover near red and white side looking for a bulge that can be a thermal fuse.

    The 'squelch' noise tells something is alive in the amplifier. At the same time, something is bad in it.
    A proper 12V DC wall adapter transformer should be able to substitute the original transformer and put the amplifier to work if healthy.
    Do your tinkering with low volume.

    Yes, you may have another uncovered problem in the amplifier circuitry that could had caused the failure of the transformer.
     
  8. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
    17
    0
    Something that I think is curious but maybe happens all the time is that the soldered onto the board
    fuse (which I cannot read it's rating) is not blown but the transformer burns up.
     
  9. Externet

    AAC Fanatic!

    Nov 29, 2005
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    Murphy's law clearly states :
    "Expensive electronic equipment blows itself up to protect its fuse "
     
  10. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
    17
    0
    Murphy strikes again!
     
  11. ColoradoRobert

    Thread Starter New Member

    Jan 22, 2016
    17
    0
    I can't afford a high dollar Fluke meter and I only own a very cheap needle type display multimeter (from Radio Shack),
    so anybody have good suggestions or point me to a consumer report, for a mid-dollar meter
    that does testing of many things (diodes,caps,transistors,ac, dc, ohms etc.)?
     
  12. Externet

    AAC Fanatic!

    Nov 29, 2005
    759
    57
    ..."(diodes,caps,transistors,ac, dc, ohms etc.)?"...
    Those can be reasonably well tested using your RadioShack. You do not need a high dollar Fluke as a beginner.

    Very low dollar :
    ----> https://duckduckgo.com/?q=harbor+freight+coupon+multimeter&t=canonical&iax=1&ia=images
    And they are surprisingly accurate. Just wait for the free offer to appear. It is far more important knowing how to use a multimeter than having an expensive one and crying if you goof misusing it.

    Murphy:
    "If any multimeter can be used wrong, it will"

    Edited. added:
    Or, ----> http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html
     
    Last edited: Jun 8, 2016
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