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Kustom KML-80 issues

Started by exnomine, December 27, 2011, 02:07:07 PM

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exnomine

Hello, I'm new to troubleshooting and repairing amps, but have a reasonable knowledge of electronics. I have an old Kustom KML 80 MosFet amp. The amp powers up fine, but the output is weak and distorted, even on the clean channel. Upon examining the power amp, I saw 2 resistors (the big rectangular ones, metal oxide i believe they were) were somewhat burned, and the circuit board under them was discolored from heat as well. I removed and tested them and they were off (684 ohm, theyy were supposed to be 750 +/- 5%) that didnt seem like a big deal, but I replaced them anyways, and tested 2 zener diodes and another resistor that were in line before them and they checked fine. I re-assembled the amp and still have the problem. 
Does anyone have any suggestions on where to begin, or am I going to have to test every single component on the circuitboard?

J M Fahey

Google its schematic or ask for it to Kustom themselves, providing serial number.
Otherwise it´s close to impossible.
Even testing *all* parts, which is impractical, you still might have a connection or soldering problem.
Those resistors you checked probably feed the +/- 15V preamp supplies, it´s normal for them to be very hot.

exnomine

#2
I have a schematic now. The PDF was too big to upload, here's a link: http://www.mediafire.com/download.php?770a8ghrs85sdxf
I was hoping it was a common problem and someone could point me in the right direction as to where to start, based on the symptoms. I guess its much more complicated than an obviously damaged component, so I may just have to take it somewhere for repairs.

exnomine

UPDATE:
When I run a cable out of the preamp output (for the fx loop) into my tube power amp it sounds pretty good. The only things I noticed was the pot for bass was a little loose (i will re-solder) and the pot for the treble was a bit extreme. (anything above 3 or 4 was god-awful ear piercing treble, who knows maybe thats just the amp's design).
When I use the speaker output and connect a speaker, the sound comes out thin and distorted, and even has a slight phaser effect kind of sound. The speaker out jack is lose as well and when wiggled, the sound cuts in and out.
Now I'm gonna re-solder the bass pot and output jack and see where I stand.

J M Fahey

#4
As drawn, your speaker out jack has an extra contact inside which shorts the amp output when no speaker is plugged.
It´s often used in Tube amps to protect the output transformer, but in a SS amp it´s crazy.
I´m referring to contact #4 , clearly grounded, which touches contact #5 (speaker output) with no speaker plugged in.
The contact might be worn or bent, you mentiones the jack was loos, intermittently shorting the output.
Bad idea.
Replace it with a new jack and do not even solder pin #4.
In fact, clean its hole and use a small drill (by hand, no need for machines here) to "eat" some of the copper around the hole, so it *never* can contact the jack leg.

EDIT: I´ve also been looking at the power amp schematic which is unconventional.
It includes a thermal muting circuit, triggered by a PTC (Positive coefficient thermistor) which is a component that looks like a lentil with two legs (don´t know its colour and even less its value :() which is touching the heat sink, between both Mosfet transistors.
When it overheats, it turns the amp off or at least mutes it .
*If* one of its legs cracks , or its solder does, it may permanently mute the amp, even if cold.
Just something to consider.

exnomine

Thanks for the advice. Hopefully I'll have time to work on it this evening. I would like to add that diodes d6 n d7 which were added to the schematic later, are not on my amp. After everything else is fixed, should this be something I should worry about adding?

J M Fahey

Not sure.
On a free moment while my coffee brews I may try to guess what those diodes were added for.
Good luck.

exnomine

I'm going to have to order a new jack to replace the speaker output.  I don't have one lying around. The PTC seems to be fine. Im guessing maybe the jack is the problem. I definitely believe the problem is somewhere in the power amp since i get good signal coming from the preamp out jack. I've noticed one more thing that I definitely forgot to mention: I'm not sure if it matters but LED1 does not light up.

J M Fahey

Led 1 is the clipping diode.
It will only blink (faintly) when using the amp's built in distortion, in rhythm with the music.