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Kustom double barrel guitar amp help

banjomaniac

Mar 3, 2013
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This is a solid state amp and is blowing the fuse at the power plug a couple seconds after turning it on, I hear a buzz sound and then it blows. So I pulled the 2 bigger caps and they are fine. I thought maybe the transformer might be bad and not sure the easiest way to test it? Can I disconnect the output and then try power to it and see if it still blows the fuse? Is the a way to know what the impedance is on the primary and secondary to test it?20838.jpeg 20840.jpeg
 

duke37

Jan 9, 2011
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The classic way to investivate this type of fault is to use a lamp limiter. A 40W bulb in series with the mains limits the current and so protects the fuses and allows measurements to be made. What is the rated power consumption and the fuse rating?
The transformer should run OK with no load connected.
How did you determine that the reservoir capacitors are fine?

1. Disconnect the secondary of the transformer (it looks to be plug in) and see if the fault has cleared.
2. Check the diodes in the bridge rectifier for shorts.(My favorite).
3. Check the voltage on the big capacitors.They should be equal but of opposite polarity. I would guess 20V or so.
4. Check the voltage on the output of the +15V and -15V stabilisers.

With a lamp limiter you could jump straight to test 3.
 

banjomaniac

Mar 3, 2013
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I have a capacitor tester and pulled them and they are fine. I have pulled the transformer out and was going to see if I could test the impedance at lunch today but not sure what it should be? The primary is about 10 ohms and the secondary is less than 1 ohm. So, going by the formula I looked up online by dividing amps and voltage I see the resistance looks WAY off. I think this transformer is bad. It's a 2 amp fuse and 120vac in and 39.6 vac out at 1.1 amp.
Funny you mention the light bulbs current limiter as I watched a YT video on that yesterday lol.
 
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duke37

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'How did you determine that the reservoir capacitors are fine?'
Did you measure capacitance, ESR and leakage at working voltage?

I have never looked at the impedance of the transformer but if the lamp limiter lights with no load, then there is a shorted turn. You can get plug in power meters.
'What is the rated power consumption and the fuse rating?'
 

Martaine2005

May 12, 2015
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I agree with @duke37 regarding lamp limiter, checking bridge rectifier and powering the transformer with secondaries disconnected.
If everything checks out thus far, check the 7815 and 7915 regulators for a short. And duke37 said that too (stabilizers).

Martin
 

duke37

Jan 9, 2011
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The fuse in the transformer is in the secondary and you said that the fuse in the power plug was failing. Which is it? I am not familiar with US mains supply fusing.
I have used the wrong term before, I should have used regulators, not stabilisers.:)

Nice picture.
 
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Martaine2005

May 12, 2015
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The thermal fuse in the secondary is 130 degree C. I would think the winding insulation needs removing to replace it. I don’t think he’s done that as I would hope he would have mentioned that.
The transformer is more than likely ok.

Martin
 

banjomaniac

Mar 3, 2013
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Hmm, the fuse is right where the power plug goes into the amp so it is primary or at the point where 120v enters the amp...
 

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Martaine2005

May 12, 2015
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Ok, that is the supply fuse.
Connect an incandescent lamp across it’s connections (remove fuse). Power up the transformer with secondaries disconnected.
If you can’t do the lamp, try it with another fuse. Worst that will happen is it’ll blow again. Verifying a faulty transformer.

Martin
 

banjomaniac

Mar 3, 2013
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I'm going to build the light bulb limiter and try it and see what it does without the transformer secondary connected. If it doesn't light up bright I'm guessing transformer is good?
 

73's de Edd

Aug 21, 2015
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Sires . . . .

Initially . . . .

Sir Martaine . . . . Martaine . . . . . Martaine . . . . My . . .MY . . . .MY! . . . that creation is being of such ELLE-GUNT construction ! *** . . . . .
I "consume" that the main power switch is being the larger top rocker and that the secondary, lower one . . .kicks another lamp in parallell with the first one.
Only procedurally topping that, would then be going for the whole enchilada in using a rotary switch for the bottom one and making a complete option of switching
1 . . .Two lamps in series for lowest power
2 . . . A sole lamp for medium power
3 . . . Two lamps in parallel for highest power

HEY ! . . . . . Meester Banjo Man

With the HUMMMING of the transformer . . . that is signifying that any possible inclusion of a thermal fuse being crammed on down within the transformer windings is STILL good.

MY PRIME FAULT SUSPICIONS WOULD BE . . . .
  • 1 . . . . .
Is being that one or more of the 2 or 4 discrete power recifier diodes that are configured into either a full wave or full wave bridge rectifier, have shorted its /their junction (s) .
Your pic will only let us see one of their marked designator I.D.'s of its diode . . . as being D?
That one is located just to the right of the top RED wire coming into RED J9 push on connection.
Then there is R21 located between it and THEN a like second companion diode .

If 4 diodes are incorporated (FWB) , it would be likely for the other 2 to be located below that set and on down towards the lower YELLOW C52 ? poly blockcap.
Two of those diodes +/striped / banded leads should end up at the two + ends of the LARGE black E-caps.

Either 1N4007 diodes with their HIGH PIV / breakdown voltage ratings would be better replacements.
Or the optimum would be 1N5007 diodes with both aditional voltage and current cushioning being offered.
( The latter series of physically larger diameter leads, requre a slight reaming out of the pcb holes to accomidate their hefter diameter leads.)

  • 2 . . . . .

The other LIKELY fault possibility, if this is being an older amp that uses discrete power output transistors . . . .is that circuitry is being direct DC coupled for all but the frontal / initial preamp stage. Therefore there is a good likelyhood of one ? + of the power output transistors crunching from Collector to Emitter and then putting a dead short across your main DC supply. . .
Creating what you are now experiencing. ( Power T- former then say . . . .HUMMMMMMMMMMMM . . . as also, would shorted diodes)

Lastly, if the unit is being new enough, it may be using a "STK" potted power module, that has the same fault inside of it.
If you find that to be, give us its part number for lookup.

**** Except for the visible boo-boo at the very top side of the bottom rocker switch.
Now . . . . ." I got's to knows" . . .a la Dirty Harry . . . . . is the white frontal panel being HARD (Sheet metal) or a softer materiel ?

Thaaaaaaaaaaaassssit . . . . .

73's de Edd . . . . .


I have sexdaily . . . . . . . . no . . .NO . . .NO! . . . I mean dyslexia !


.
 

duke37

Jan 9, 2011
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Since there is a 7815 (positive) and 7915 (negative) regulator, there will be a full wave bridge rectifier. It will take only a few minutes to check for a shorted diode. This can be done without desoldering.
Logically work from input to output.
 

banjomaniac

Mar 3, 2013
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Thanks Duke, I'll check the diodes. Thanks 73's de Edd and very interesting too.
 

Martaine2005

May 12, 2015
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main power switch is being the larger top rocker and that the secondary, lower one . . .kicks another lamp in parallell with the first one.
Yes, exactly that simple. No rotary additions as changing lamps suffices.

The ‘boo boo’ is not pressing the rocker switches ‘home’ yet. The front panel was a retro 80s 4 channel disco light and still required cleaning of pencil marks. The case was an 80s CB radio power supply. I like to use recycle what I can, plus, the panel already had holes for the lamps.
I never had to buy anything.. bonus!.

Martin
 

banjomaniac

Mar 3, 2013
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Since there is a 7815 (positive) and 7915 (negative) regulator, there will be a full wave bridge rectifier. It will take only a few minutes to check for a shorted diode. This can be done without desoldering.
Logically work from input to output.
Not sure how to do that, can you elaborate a little? I see that a lot of them are connected so I'm getting readings in a line from diode to diode. I'll shoot a short video of my stupid attempt lol.
 
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