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Kustom Defender V5 Combo Troubleshooting


no_quarter

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Hey all, I've got an issue with my Kustom Defender V5 combo and I've finally decided to dig it back out and see if I can get it up and running again. I was jamming with a friend a while back, and he was using my Fender Greta. He thought it would be a "brilliant" idea to run a cable from the line out on the Greta into the speaker input on the Defender. I'm no electronics whiz, but when I noticed what he was trying, it was too late and the Defender started smoking, before I could pull the power cord from the wall. Upon inspection, I saw that a resistor (r14 according to the board) was fried. I ordered a replacement, put it in and replaced the fuse, but it still wouldn't power on. I looked a little closer and saw that the brown lead from what I assume to be the output transformer (J9 on the board) was slightly burnt. Am I correct to assume that the output tranny has blown? I can provide pics if it helps and thanks in advance for everyone's help.

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Its a small single ended amp. That connection most likely did take out the output transformer but the damage could have gone much farther once it did.

 

You could roll the dice and stick a new output transformer in there but unless you have the electronic skill to troubleshoot the rest of the amp you have no idea whether the shorted output gutted the unit taking out the tubes, power caps and power transformer as well.

 

It did get past the output and took out a resistor. You cant know just how much more damage was done without testing.

 

I believe this is the schematic. Its not marked however so I cant be completely sure. This doesn't show or lists the types of transformers used I suspect any single ended amp transformer types using these tubes could be used a s a substitute if the numbers on the transformers you have don't Google you up any results.

 

fetch?filedataid=118081

 

 

 

 

 

 

 

 

 

 

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I doubt very much that the output transformer is toast.

 

Was the Kustom Defender powered up at the time?

 

When you say R14 was "fried" do you mean it was blown to pieces, black or simply not working?

 

Can you be more specific about the "speaker input on the Defender" ? The amplifier only has one input jack and three speaker output jacks.

 

 

 

 

 

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[uSER=111570]DeepEnd[/uSER], it is being a little nitpicky. I did mean output, and yes, it was my mistake writing "input," especially given the technical nature of the post. However, if I may "nitpick," saying "your friend f#cked up and you've got at least a $200 problem on your hands" does nothing to help the situation. I'm already quite aware of that but I'm trying to get it working again as I really liked the amp. Sorry if I'm coming across as a dick, but I posted on here as an effort to get some help figuring out a solution to the problem, not receive a reply reaffirming what I already know.

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Quite often, when a resistor ends up black and crumpled it is because there has been a short circuit in a power tube. The high voltage from the power supply then "electrocutes" the resistor and blows the fuse.

 

Taking that into consideration, along with the fact that the tubes are the weakest part of a tube amp and the easiest thing to troubleshoot, I would first remove the EL84 and attempt to power up the amp with a new fuse. If that works then I would replace the EL84 and try again.

 

Only after examining the possibility of a bad tube would I consider the transformer.

 

more questions…

Does the fuse blow with every attempt to power the amp?

​Is R14 physically bigger than most resistors on the circuit board?

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I'll have to double check this evening, but I believe the new fuse didn't blow the last time I powered it up. R14 was the same size as the others on the board. Thanks for your help onelife, I actually hadn't considered the tube to be the source of the problem. Would that prevent the whole amp from powering up? I get no sound, no indicator light, or anything when I flip the switch

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If you have no indicator light and no tube filaments glowing you have no power supply.

I agree with one life, a shorted power tube may have caused the resistor to blow but it looks to me like you have more going on there.

 

This is probably not a single part fix. When the power fuse goes out the first time its because the power supply saw a short in either the high voltage or filament circuit. If you have no filaments lighting and the fuse isn't blowing now, its likely the power transformer blew.

 

I cant tell you if the indicator light is connected to AC primary or the AC secondary. Older tube amps used the filament circuit to power a small 12V incadecent lamp. Newer amps may have a 110Vac neon lamp on the primary side of the transformer.

 

If the lamp is on the primary side and both the fuse and switch are good, and the lamp fails to light its an indication, the power transformer's primary winding blew. If the lamp is on the secondary - and your tubes fail to light up, it's and indication the power transformers secondary blew.

 

In some instances you may get a switch go bad too so you have to check it for continuity with an ohm meter.

 

You can also check the transformer primary by setting a meter for 120 AC volts (200V AC range)

 

With the switch off and the unit plugged in, you should read about 120V across the switch. When the switch is turned on the voltage should drop to zero.

 

If you have the voltage, it means the power transformer primary is good. If it doesn't go to zero when you turn the switch on, it means the switch isn't working.

 

Allot of times a switch can get its contacts charred by high current. If its a plastic toggle switch you can sometimes wedge them apart and sand the contacts smooth. Be careful. There are usually Jesus springs inside there which have a habit of shooting across the room when you open a toggle switch. When you launch them accidentally you will likely realize why its called a that.

 

If you find the primary side good, then you still have to check the output. The easiest way is to check the filament supply.

 

Since the diagram has no filament wiring shown, you need to check and see of the filaments are wired in series or parallel.

If they are in series like Christmas lights that's good news, because the one power tube blowing out may have simply removes the heater voltage from the other tube.

 

The heater pins are 4&5. If you remove the suspected bad power tube and test pins 4 & 5 with a meter you should read about 12V with the amp on.

 

 

Again I cant be sure the heater voltage is AC or DC because the schematic doesn't show it, but start with the AC setting first. Set the meter for the 20V AC scale - You should read about 12V with the amp on.

 

If you get no reading, try the 20V DC scale. Some amps convert the filament supply to DC to reduce hum.

 

If you get no AC or DC voltage its likely the secondary of the transformer is blown.

 

If you get filament voltage then you have to fix your HV Bias circuit to see if it is working.

You can make tests on that voltage too. Both the 12AX7 and the EL84 can run with plate voltages between 200~300V dc.

When you measure the supply voltage be sure your meter is set on the 500V range. Be careful too. That voltage gives you a nasty bite.

 

You can probably test the Bias voltage between Frame ground, the chassis and pin 7 on the EL 84 tube. Anything over 200V DC means the HV secondary of the power transformer is working.

 

If you have filament voltage but no bias voltage it likely means you have blown components in the DC power supply circuit.

There's usually 4 diodes that convert AC to DC and filter caps to smooth the ripple.

 

In the schematic they use an R3000 diode. This rectifier is commonly used as a protection diode in tube amps. It is installed with the cathode connected to the plate of the power tube and the anode connected to ground. There should be one diode per push-pull half. This diode keeps unwanted negative voltage spikes from damaging the power tubes or output transformer. This diode can be found in the Fender Hot Rod Series as well as many other amps.

 

This part may have done what it should have and blown to protect your output transformer and power tube.

They are only 25 cents so its wise to change this part along with the tube and resistor.

 

https://www.tubedepot.com/products/r3000-25a-3000v-high-voltage-protection-diode

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Since it's not helpful I've deleted my post. You used the term "speaker input" twice, which indicates, to me, that you might not know as much as you think. In any event, I'm done. Good luck and yes, you're coming across as a dick.

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[uSER=103821]WRGKMC[/uSER], thanks a lot for the info. I put it all back together, following some photos I took when it was intact, powered it up, and as soon as I flipped the switch, the speaker buzzed and the lead from the output transformer to the tube board started smoking. Needless to say, I was unable to test anything. I've attached a couple of photos in case that helps

c870c20b31ca18962a229d0c277a77db.jpg.7ce50ac33f41760de4e53c5e4f0672f8.jpg

ed11a69e009d3fabe51e5a70e13342e0.thumb.jpg.adad7386db215026b5a3c30960792b1d.jpg

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