| Hum, hum, hum [message #9731] |
Sun, 05 October 2008 12:55  |
basshead
Messages: 5
Registered: October 2008
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Junior Member |
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Hello everyone. I'm new here, found the site a few days ago and I've been having a good read through the archives. What a great resource! These amps are so cool, I'm glad people are working to share the knowledge and keep them alive...
I've just bought a K200 with problems - a previous owner had unsucessfully attempted some repairs and damaged the output transistors. I've replace these today and redone some dry joints. I powered the amp up and I get a hum which is unaffected by the volume controls or anything running through the input. Both channels of the amp work though - running a guitar through them everything does as expected, and there's no obvious distortion. There is no voltage present on the speaker output, and when I turn the amp off the hum goes straight away rather than dying away.
The hum is 200hz, don't have anything to test the waveform (I'm in Europe so mains is 50hz here): is it significant that this is a multiple of mains frequency?
If I've interpreted what I've read in the archives correctly, I think I should be replacing the filter caps now. Is this correct, or am I likely to be looking at a bigger job? I'm reasonably competent with a soldering iron but don't know much about amps. If anyone has some advice for me, I'd be really grateful...
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| Re: Hum, hum, hum [message #9733 is a reply to message #9732] |
Mon, 06 October 2008 14:21  |
basshead
Messages: 5
Registered: October 2008
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Junior Member |
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Hey, thanks for the quick reply.
The sound of an instrument through the amnp is not distorted. I'd describe the permanent hum as a buzz rather than a hum, probably more of a saw than a sine wave.
The caps look pretty old, they look the same age as the rest of the components but the dots are intact.
Measuring between the red and green wires of both caps the reading is 87V DC! Measuring from eiter colour to ground is 43.5V.
AC voltage across the speaker output is settling at around 100mV.
And there's no reverb tray or anything to confuse the issue!
thanks so much for the help so far...
[Updated on: Mon, 06 October 2008 14:23] Report message to a moderator |
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| Re: Hum, hum, hum [message #9736 is a reply to message #9735] |
Tue, 07 October 2008 09:20 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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That model is a 2x15 CTS cabinet with a K200B1 head. That head has no effects or what would be classified as a bass head. There is no difference between the bass or guitar versions. The only difference is the speakers. I think the answer is in one of your earlier posts. If the sound goes away as soon as you turn off the power then it is not input induced. The filter caps are, more than likely, bad. After sitting for a long number of years and just age in general, the caps dry out and stop working. At least to thier nominal capacity. The fact that you have 43+ volts on each side is a good sign that there are no blown transistors in the output stage. The sound you are hearing is not a pure sine wave because the caps are working somewhat and altering the sinewave to more of a sawtooth. I would change out the filter caps. I believe those are 4500uF caps but I would replace them with 10,000uF caps minimum. The larger caps will give you more headroom before distortion. If the hissing sound drops out immediately after turning off the mains, then the caps are definately bad. But you probably can't hear the hissing, the definitive Kustom trademark.
Conrad
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| Re: Hum, hum, hum [message #9738 is a reply to message #9737] |
Tue, 07 October 2008 14:57 |
basshead
Messages: 5
Registered: October 2008
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Junior Member |
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Hey gents, thanks for the tips. The hissing does, indeed, stop the moment I turn the power off. There is no fade out whatsoever.
So, replacement caps have been ordered (I've even been smart enought to remember to get new brackets, too).
It's a credit to the webboard contributors that, with the mountains of information here, I was able to get close to solving the problem myself. And it's brilliant to have such knowledgeable chaps here to reassure you that you're doing the right thing!
Caps are due to arrive tomorrow. Will report back tomorrow evening. Fingers crossed this will sort it out!
best wishes
Dan
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| Re: Hum, hum, hum [message #9741 is a reply to message #9740] |
Wed, 08 October 2008 19:05 |
basshead
Messages: 5
Registered: October 2008
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Junior Member |
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Hey Steve...
Thanks for the tip. The caps arrived today butI went for a few drinks after work, so am leaving it 'til the morning. Already have my diagram drawn up in readiness!
best wishes
Dan
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