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?
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.
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.
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.
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.
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?
Not sure.
On a free moment while my coffee brews I may try to guess what those diodes were added for.
Good luck.
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.
Led 1 is the clipping diode.
It will only blink (faintly) when using the amp's built in distortion, in rhythm with the music.