| Buzzy output from k200? [message #9373] |
Mon, 31 March 2008 10:29  |
NathanielR
Messages: 4
Registered: March 2008
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Junior Member |
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Hi everyone! I'm in the process of bringing a k200 back to life. It appears to be a bass model, since it has no effects. Right now, the normal channel works, and the bright channel has some barely inaudible output with the volume and treble cranked. Also, when playing there is faint distortion underneath the clean notes at any volume, as well as a fair amount of hiss. A couple of things:
1)I replaced all the se4002 transistors with 2N3904 on the preamp boards, which restored output to the normal channel. The bright channel seems to need more work, but that isn't my biggest concern right now.
2) The buzzy distortion must originate in the power amp section, as the RCA line out is completely clean. Very little hiss, as well.
3) I haven't replaced the filter caps. The red dots on the top haven't bulged, so I assumed they were fine? However, before draining the caps I measure their voltage, and they never seem to retain anything above 1v. Is this possible, or am I metering incorrectly?
4)Haven't replaced the 2-prong cord or bridge rectifier
I found a post describing this problem as "crossover distortion" from mismatched output transistors. Given the scenario I've described, would you agree that is the problem? Would I simply need to put it matched nte181 in the output section?
Also, I've found several posts on replacing the bridge rectifier with fast recovery diodes. Any opinions/instructions on that as well? Thanks for your time! The board has been a huge help in getting me this close!
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| Re: Buzzy output from k200? [message #9374 is a reply to message #9373] |
Mon, 31 March 2008 12:42  |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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Welcome to the board!
"It appears to be a bass model, since it has no effects."
Yes, some people use these for bass, but just like the Fender Bassman, they're great for guitar as well.
"..bright channel has some barely inaudible output with the volume and treble cranked."
The bright switch on this channel is actually like a bypass switch on an FX pedal, if it becomes oxidized it can cause the problem that you describe. Spray a little contact cleaner in the back, and try switching it back and forth a few times to see if this helps. This could also be caused by other things, but check the switch first.
"...when playing there is faint distortion underneath the clean notes at any volume, as well as a fair amount of hiss."
As you noted, this is probably a power amp problem. I'd first check for any really off value components, bad solder joints and loose connections, before replacing output transistors.
"...the filter caps...never seem to retain anything above 1v."
When you turn off the amp, the circuitry still draws power and will bleed off the voltage. Try the same test without the speaker load connected, you should see a much slower drain on the filter caps. The relief valves are only one way to judge a cap, they can be dried up and off value without ever showing any physical signs.
"Haven't replaced the 2-prong cord or bridge rectifier"
Changing the power cord wouldn't be a bad thing, but would entirely be up to you. If the bridge rectifier is working, why replace it?
Bill
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| Re: Buzzy output from k200? [message #9380 is a reply to message #9378] |
Tue, 01 April 2008 16:24 |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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"I am still suspicious of the filter caps. I tried leaving it on for several minutes with no speaker load connected, and metering the remaining voltage immediately after turning it off. One cap has consistently shown no voltage, the second no more than 2 volts."
That is faster than I would think, but I haven't measured that in a while. I'll do a little checking tonight.
"Could I meter this while it's turned on to confirm that it's operating correctly?"
If you do, you should find about + and - 45 volts. Be careful when working on any amp while it's plugged into a live outlet.
What transistor was the lead broken off of? I don't know of any easy way for you to break one off, I have seen them rust off though. Is it one of the original parts with the attached heatsink or have they already been replaced?
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| Re: Buzzy output from k200? [message #9382 is a reply to message #9380] |
Wed, 02 April 2008 17:13 |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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Here's an update.
Last night, I powered up the my K200B-1 and checked the power supply voltages. My amp has the original Mallory 4500mF 50v caps installed. I got + and - 39.5 volts.
Reading the positive supply, when the amp was turned off it took approximately 30 sec for the voltage level to drop to 1 volt (no speaker load), (3 sec with speaker load). The negative supply drops almost immediately due to the pilot lamp being wired to the negative supply.
I can't say that this has anything to do with your amp, as there could be something drawing the voltage down faster than normal on yours, but here is what I measured on mine.
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| Re: Buzzy output from k200? [message #9384 is a reply to message #9382] |
Sat, 05 April 2008 03:23 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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First off I agree with Bill about the voltages. If I read your first post correctly, you said that there is ample sound out of the Normal channel but little or no sound out of the Bright channel. If that is the case, then the power amp is not the culprit. That's not to say you shouldn't change the filter caps. Also, like Bill said, the voltage decay time is a function of the capacity and the age of the caps. I sometimes replace mine with caps that are up to 4 times the original value. That allows for more headroom before power supply compression and distortion. I don't want to start an argument with Steve, but I can't see how changing the bridge to fast recovery diodes can affect the overall sound. I don't mean to berate you but just make sure your meter is set for DC. If you have it set for AC and reading voltage at the caps. you MIGHT read up to 2 or more volts AC of ripple. Keep on plugging away. Speaking of plugging away, the 2 wire to 3 wire cord change will do nothing to help the sound of the amp. It will just keep your lips from getting fried on your microphone.  The underlying distortion you hear is definitely crossover distortion. That means that when your signal to your speaker changes from positive to negative, it doesn't do it smoothly. It jumps in a large voltage change from positive to negative and back again. The output bias of that amp needs to be checked. Maybe that broken transistor is the culprit after all. If the bias is wrong, the amp's output will not be 0 volts DC at no signal input. This would also affect the discharge time of the caps too. If the amp does not "thump" when turned on, I would suspect the caps.
Conrad
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| Re: Buzzy output from k200? [message #9397 is a reply to message #9384] |
Tue, 08 April 2008 09:47 |
NathanielR
Messages: 4
Registered: March 2008
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Junior Member |
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I was out of town for the weekend, so I haven't had a chance to work on it. Thanks for all the help, it's been a very learning experience along the way! After further inspection/cleaning of the board, it looks like some SE4002 transistors were replaced along the way, and I've replaced them all with 2N3904's. I found one trace on the power amp board to be nearly black, perhaps from a component that burned out? That section of the board seemed to have replaced components, just by the differences in soldering. If my thorough cleaning/re-soldering didn't clear it up, I will move on to replacing suspect components.
re: filter caps
I did have my DMM on DC, but the readings seem in line with the ones bill got. My suspicions were pretty non-technical: I'm just used to hearing how those caps can hold their charge for weeks and still zap ya! Larger caps might ultimately be desirable, since I'll be using this for bass and headroom might be an issue.
re: output bias
I will take a reading at the amps input to confirm if it's not 0 volts DC.
re: 3 prong cord
My Princeton Reverb put me through enough lip-frying to realize this "upgrade" is not optional!
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| Re: Buzzy output from k200? [message #9399 is a reply to message #9397] |
Tue, 08 April 2008 11:58 |
chicagobill
Messages: 2005
Registered: April 2003
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Senior Member |
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The output transistor bias is basically set by the string of diodes and resistors located in the driver stage. This includes the one diode that is thermally mounted on the output transistor heat sink.
If there is a problem with any of these parts, the output transistors can either draw too much or too little current at idle. If one of the output transistors turns on too soon or too late it can sometimes cause odd distortions.
If the power amp has been worked on before, I'd first double check all of the previous repairs. There may not be any issues with the work done earlier, but I always check to see what was done before, so that I can rule it out as the source of the current problems.
If the driver or output transistors have been changed, make sure that they are of the correct type and ratings. Kustom power amp designs are fairly immune to transistor mis-matches, but I've seen a few cases that required replacement with higher gain transistors, in order to correct output signal problems.
Keep us informed on your progress.
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