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Author Topic: Fixing oscillating sound in Johnson warrior 15 standard guitar amp  (Read 345 times)

thejedi

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Hello! I'm new here... sorry for being ignorant and newbie

I have an old Johnson Warrior 15 guitar amp that does not output my guitar sound. Instead, when I turn it on, it makes ever lasting oscillations (boop boop boop boop boop...). I heard the right term is "motor-boating" but I'm not sure. Anyways, it makes no other sound but the (boop boop..) when I plug in my guitar.
It's a solid state amp, 15w. I tried looking for schematics online but couldn't find anything. I tested the speaker itself and it works just fine. Another peculiar thing is that if I plug headphones to the headphones jack, I can here my guitar in my headphones. Another thing is that when I bypassed the transformer and connected DC current, it still occured, though the sound changed a bit.
 
Any ideas?
I'll provide more information if needed

phatt

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Re: Fixing oscillating sound in Johnson warrior 15 standard guitar amp
« Reply #1 on: August 24, 2019, 10:03:38 AM »
If you bypassed the Mains transformer and you are STILL ALIVE to talk about it then consider yourself very VERY lucky.
NEVER bypass the transformer of an amplifier. :trouble :trouble :trouble :trouble

I doubt the amp has been as lucky as you,, as the much higher voltage would have most likely destroyed the circuit and likely not worth fixing. :'(

I'd advise you to get help from a qualified tech shop to check out if it's worth fixing.
Phil.
« Last Edit: August 24, 2019, 10:05:41 AM by phatt »

g1

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Re: Fixing oscillating sound in Johnson warrior 15 standard guitar amp
« Reply #2 on: August 26, 2019, 11:41:02 AM »
I'm hoping he meant that he left the amp's AC cord disconnected, and used a DC power supply for the amp circuitry.
Without a schematic, it is difficult to suggest what the problem might be.
It sounds ok through the headphones? 

thejedi

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Re: Fixing oscillating sound in Johnson warrior 15 standard guitar amp
« Reply #3 on: August 30, 2019, 03:47:14 AM »
It does sound okay through the headphones, my guess is that it is somehow related that the headphones impedance is much higher. I didn't endanger myself, I was under the supervision of a certified electrician who is not so knowledgeable about guitar amps, but it was sorta safe, lol.
I left the AC disconnected, and connected a lower voltage dc transformer directly to the circuit  (that's what the built in transformer is supposed to do isn't it?).
I couldn't find any schematics. But I did find a striking similarity to a different amp, it's name is even on the circuit board, AXL-15 (AXL AA-G15). But I haven't found any schematics for that amp either.
Thank you both for your response! :)

phatt

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Re: Fixing oscillating sound in Johnson warrior 15 standard guitar amp
« Reply #4 on: August 30, 2019, 10:08:54 AM »
OK good to hear you are ok with it.  :tu: sorry i missed the point but I did have one young chap who almost blew himself up assuming the circuit did not need a transformer and wanted to plug it directly to mains. xP

The Headphones might be powered from the preamp hence it works ok to that point.
Sounds like a power stage problem.
Things to try;
number one, with meter set to DC Volts check if there is any DC voltage on th output,, report the result,, it should be very close to Zero DC.
Look for hairline cracks in the solder pads, use a magnifying glass in sunlight to find them.

It may have a Zobel network on the output (google it if not sure),, normally a cap and resistor across the output.

check for any overheating caps or resistors on the power amp as these parts can drift after years of service.

pock around with a wooden chop stick and gently wiggle parts and see if you can get the sound to change.
That may give clues as to what parts are failing.
Sorry We can't be much help.
Maybe take pics of the guts and post ,, that may give clues for folks here to help.
Phil.

 

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