| KUSTOM K150-6 Head not behaving nicely [message #29689] |
Fri, 15 August 2025 14:40  |
Voxman2027
Messages: 34
Registered: November 2024
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Member |
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Hi ALL,
Just started on a Model K150-6 amp head. Fixed the "dead" channel easily as it was a broken input jack.
Testing further with sine and scope showed all looked good. That is until I plugged speaker in then things went south.
Although amp seemed to be working, it now had a weird noise going on which prompted me to check filter caps again. They tested good but under scope I could see pretty much a normal sign wave. Except for rhythmic noise and fact that there shouldn't be anything like a normal sine wave, the amp still amplified normally inn the preamp stages. It appears to be 60hz. No DC on output with no load.
If I can post a video or photo to show I will.
The bridge rectifier II think might be an older type as it is the first type I've seen in the 5-6 Kustom amps I've serviced.
The thing is, I have schematic s for the power amp PCB and the 2 preamp PCBs.... but cannot find any schematic that shows the actual main caps, transformers and rectifier. I've looked all over but it's not to be found.
If anyone can direct me to the proper power supply schematic I'd be appreciative. The other odd thing is the introduction of a 2nd smaller transformer that is directly connected to rectifier and then to main filter caps. Very strange. Main transformer connected to bridge rectifier then output of rectifier goes to 2nd transformer and the to main filter caps. Isn't that odd?
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| Re: KUSTOM K150-6 Head not behaving nicely [message #29691 is a reply to message #29689] |
Sat, 16 August 2025 07:59  |
stevem
Messages: 4863
Registered: June 2004
Location: NY
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Senior Member |
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The schematic you need is in our sites tech section .
Go to the listing for amps .
Next scroll down to the heading for K150-6 or k150 -1. Or -2 In the piggy back section well below the listing for the K150-6 .
Click on the main listing of either one and you will then get the power supply schematic and a block listing of the rest of the circuits in the amp.
That listing has the whole power supply section .
That second smaller Transformer is a choke.
This greatly reduces current changes in the amps power supply which allows the amp to stay very punchy when played up loud.
In other words this choke is stabilizing the positive and negative power supply rails off of the two supply filter cans.
This choke also reduces any residual 120 hz ripple that might be left after the main two filters.
This choke is also why even a K150 metal face model with no effects tips the scales more then a K250 model with all the effects in it!
In terms of your 60 hz hum in the audio stream ( and if the amp does not have reverb,
The only way that happen is if one of the diodes is bad in the rectifier bridge.
This condition will also show up as either the needed 39 volt negative or positive supply rail being low.
Any other ac ripple that you might find showing up on the speaker output could only be 120hz .
I would order up a modern type “ fast recovery rectifier bridge “ of a bolt down type like the original .
I believe that due to your description of this issue that this is the root causes of your amps problem.
Note that you do not have to have a load on the speaker output of the amp.
For confirming that the amps output is ok I just hook up my volt meter set for ac volts to see what’s going on .
A properly functioning K150 amp with a choke will at idle with all the controls down read no more than .012 volts of ac ripple on its speaker output jack.
But of course if your ears can’t make out the difference between 60 or 120 hz hum then the use of your scope will tell you that.
Note that a bad rectifier bridge can put a big load on the power transformer so test it and change it out as soon as you can.
The new bridge does not need to be rated for more the like 15 or 20 amps ( even 30 is fine, more mass = faster heat dissipation ) but it should be rated for at least 400 volts which is very very safe .
Post back with the results of what you find / do.
[Updated on: Sat, 16 August 2025 08:23] Report message to a moderator |
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| Re: KUSTOM K150-6 Head not behaving nicely [message #29696 is a reply to message #29693] |
Mon, 18 August 2025 20:38 |
Voxman2027
Messages: 34
Registered: November 2024
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Member |
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Steve,
I've said it before but this website is so odd. I wrote a big reply to you and was coming back to tell you I fixed the amp but my big long message is nowhere to be found ... and I'm not rewriting it !! It's a quirky website.
I will mention I couldn't find the power supply schematic you mentioned. I looked at every PC number I thought it might be but nada. There were quite a few "file can't be found " when I clicked on some of the PC numbers. I tried using my PC instead of my iPad but same problem.
A lot of the schematics weren't close as they had op amps in the circuit.... this amp doesn't. It's just a little frustrating to navigate this website... at least for me it is.
The bridge rectifier was bad. I now see very little ripple and no DC on output.... but I'm still cautious about connecting a load just yet. The bridge is just installed temporarily until I get the proper connectors. And I'm waiting on the new 1/4" shorting jack too to replace the one that self destructed.
Thanks for your help as it is always most appreciated.
Gary
I hope this reaches you! Who knows?!?
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