| K250-1 Popping [message #9916] |
Sun, 16 November 2008 23:11  |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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This is a "from the dead" project. I got a basket case K250-1 that was supposed to be for parts. As usual, I started to put the thing back together. I got it to fire up and the upper channel works great. The lower channel has issues. I had to wire a new set of jacks (copied how the upper channel was done). If I turn the volume control up, I will get almost a radio like static, and then it will start a steady popping, almost motorboating kind of noise. I can't remember if I have the bright control out or in, but that will kill the popping noise. I'm still left with faint output and lots of hiss. The only thing done to this channel was repairing a gummed up treble pot. I've pulled and tested every capacitor on the preamp board.
The power switches have been worked over a bit. I used a push button LED setup from Mouser. I have the polarity switch installed and it's LED wired up (2k resistor on the power LED to cut the voltage down). I have the other switch for the power button on backorder. So, I have things hardwired and the power lead to that side has just been taped off.
[Updated on: Sun, 16 November 2008 23:15] Report message to a moderator |
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| Re: K250-1 Popping [message #9919 is a reply to message #9917] |
Mon, 17 November 2008 14:44  |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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Okay, here's the full story with the pot. I had bought a K250-4 last year that needed some work. My repair guy said he needed a new treble pot. I searched high and low, but could not find an exact replacement. I bought this dead K250-1 on eBay so we would have a pot and a better looking cabinet. He got the K250-4 working perfect and gave me back the parts K250-1. It was all there, outside of having a loose "junk" treble pot kicking around there from the K250-4 (pots are the same between the two models). I tore this pot apart, cleaned it, and reassmebled it. I threw a meter on it and it worked perfect. So back in the board it went and here we are.
The traces appear to be in good shape. I really took my time with this board and I'd like to think it shows. I'll try putting a meter on this evening to be 100% sure. Also, it is the bottom board (non-selectone) that is giving the trouble. So it has the transistors on it.
I made an mp3 of what's going on. The conditions are that the top board has the volume fully down. The bottom board (the problem one ) has all the EQ controls tuned wide open and the bright switch turned off. What I did was slowly turned up the volume and you will here the startic roll on. I went up maybe 1/3 volume. If I had a guitar plugged in, you would be able to just barely hear. I then turned the volume back down, flicked the bright switch on and began to raise the volume again. I didn't go very loud because the annoying popping noise. Lastly, the first little "thump" you here is the amp powering up. That should give some idea of volume levels here. Thanks
http://www.250r.us/misc/k250.mp3
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| Re: K250-1 Popping [message #9921 is a reply to message #9919] |
Mon, 17 November 2008 16:51 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The symptom that you describe leads me to think that there is a break in the signal path somewhere. The signal that is getting through is via capacitance coupling.
I'd check for open or loose coupling caps. Or as Steve mentioned, a broken board trace.
If you listen to the hiss, does it's character change with the tone or volume controls? This will lead you to where the signal dies.
Is the fuzz board jumper (J1) in place?
Inject a signal (this could just be your finger tip) to the top of the volume control. Do you hear a good buzzing sound? How about if you touch the top of the treble pot?
Let us know what you find out.
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| Re: K250-1 Popping [message #9923 is a reply to message #9921] |
Mon, 17 November 2008 19:34 |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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Got it going! It was that missing jumper. I just soldered the "in" and "out" leads together on the back of the back of board. Man, this things sounds fantastic though. I've got it running against a 4x10" A model cab with Weber California's in it.
[Updated on: Tue, 18 November 2008 00:43] Report message to a moderator |
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| Re: K250-1 Popping [message #9931 is a reply to message #9925] |
Tue, 18 November 2008 12:04 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Glad to hear it's up and running again. There's still a lot of life left in these old workhorses (myself included)!
Steve is right, as I wouldn't have even thought to mention the fuzz jumper if you hadn't told us the entire story of how the board had been removed and used for parts. But like I said, it sounded like a break in the audio path.
Again, glad that it's working again.
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| Re: K250-1 Popping [message #9942 is a reply to message #9940] |
Wed, 19 November 2008 23:33 |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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Okay, here's the latest. I pulled the two transistors on the board and tested them, they are fine. This lead me to rob the lower pre-amp board out of my K250-4 and try things with it installed. This gave no change at all. So, I think we can rule out the pre-amp section of the amp.
What sets this off are bright tones. The telecaster really drives it wild. You have to have the amp's volume up somewhat for the trouble to start and it will be even worse the louder you get. If you roll the tone control back on the guitar, or the EQ on the amp, the problem will stop. I have the amp on the floor, so I don't believe it to by a vibration issue.
The power caps are tight and I tested them out as per the last time you guys you helped me. I'm getting 40v DC and 150~mv AC and rating in the high 5,000uf range.
I need to be more sure on this, but the problem seems to get worse after the amp has been on a few minutes. I was checking voltages and forgot I still had a load hooked and when I hit my probe on the case of the transistor in the heat sink on the far left, I get some static out of the speakers. The other three are silent. I'm about half tempted to drop the power amp board from my 250-4 in there and see what she does.
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| Re: K250-1 Popping [message #9944 is a reply to message #9943] |
Thu, 20 November 2008 11:52 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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I really don't like the idea of board swaps for troubleshooting, unless you're pulling boards out of dead amps.
I'd be afraid that you'll end up doing some damage to the working board, just for the sake of proving that the problem lies in the power amp, which you have already done.
My guess is that it's some sort of parasitic oscillation in the power amp that's causing your problem. Check all of the caps on the board, like you did on the pre-amp. Also check for any loose connections, bad grounds, etc. Something else to consider is lead dress of all the connecting wire bundles. Kustom did a very good job of routing wires in their amps, so make sure that you don't have input lines too close to outputs, etc.
When you remove the board from the amp, watch out for the small diode that clips into the heatsink, as the leads are prone to breaking off. These diodes are made of unobtainium and will be difficult to replace.
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| Re: K250-1 Popping [message #9950 is a reply to message #9916] |
Fri, 21 November 2008 13:39 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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ellum68:
I assume you mean the power amp board and not the pre-amp?
Any cap that measures a constant resistance value is bad and should be replaced.
C3 is actually used as a filter cap following the voltage dropping resistor R7-39K. If the cap is shorted, it may have caused R7 to overheat and go off value, so check it with your meter.
Check all of the other tantalum caps C5, C7, C8 as well, because they have been known to fail. The ceramic discs don't fail very often, so leave them for now.
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| Re: K250-1 Popping [message #9952 is a reply to message #9951] |
Fri, 21 November 2008 15:28 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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No, if you've inspected all of the normal things, I'd replace the bad cap and see if it clears up your problem.
I'd probably replace the matching C7-27uF/35v as well as long as I was in there. If one failed, the other is probably near death as well.
[Updated on: Fri, 21 November 2008 15:29] Report message to a moderator |
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| Re: K250-1 Popping [message #9953 is a reply to message #9952] |
Fri, 21 November 2008 15:41 |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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Yea, I was thinking of getting a pair as well. The closest I can find from Mouser in a radial style cap are 22uf or 33uf. I can get a 27uf@60v wet tantalum axial mount though. Which one should I go with?
I'm glad I've had this little adventure though. I'm now comfortable replacing componets on these PCBs. I'm thinking I'll replace the first few gain transistors in my B-4 to try to get rid of the ungodly hiss it has. Of course, I want the 250-1 to be done first.
[Updated on: Fri, 21 November 2008 16:18] Report message to a moderator |
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| Re: K250-1 Popping [message #10014 is a reply to message #9916] |
Fri, 28 November 2008 22:04 |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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Got it running tonight. I swapped out those two caps with new ones and the only distortion I get is natural break up at high volume. On a side note, I found some switches that work rather well in this thing. They're Mouser number UB216KKW015C-1JB-RO . They are square, push button, LED lighted switches rated 5amp. They take a lil' work to pull it off, but not bad. You may have to do some very mild filing of the switch opening for them to drop in and use a 2k resistor to knock down the voltage for the lights.
[Updated on: Sat, 29 November 2008 12:17] Report message to a moderator |
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| Re: K250-1 Popping [message #10018 is a reply to message #10017] |
Sat, 29 November 2008 12:54 |
Crazy Joe
Messages: 18
Registered: November 2008
Location: Enon, OH
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Junior Member |
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Oh and by the way, usually you can find shorted tantalums (or other caps, including electrolytics) without removing them. 90% of the time you can just put your meter across them in-circuit and tell right away, because bad ones will measure a couple of ohms or less, and there's almost never enough other circuitry in parallel with them to read a resistance that low (not to mention that most DVM's intentionally don't have enough voltage to turn on a p-n junction, unless you click on the setting with the little diode). Now if a cap has gone *open*, then you usually have to do some signal tracing. Techs, you agree with me?
Crazy Joe
The King of Nerd-A-Billy
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I started pulling caps on the preamp tonight. Capacitor "C3" that is rated 
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