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Hum, hum, hum [message #9731]

Sun, 05 October 2008 12:55

basshead
Messages: 5
Registered: October 2008

Junior Member

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 #9732 is a reply to message #9731]

Mon, 06 October 2008 07:08

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Is the sound that you are getting out of the amp distorted? Those two large filter caps if original will have small round dots indented in their tops, if these dots are popping up at all than the filter is dryed up and bad.
If you measure across the red and green wire of both caps you should have (well your on 50HZ) about 38 to 40 volts DC?
Check this and give a report back, with a meter set to low a/v volts (under 100)what are you reading across the speaker out put?
Also is this a K200 model with reverb?

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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

Junior Member

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]

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Re: Hum, hum, hum [message #9734 is a reply to message #9731]

Tue, 07 October 2008 07:41

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

Does the tag on the rear panel say k200B?
Does the amp have a grounded a/c cord/ 3 prong, if not does flipping the on/off/on switch to full left or full right change the buzz?

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Re: Hum, hum, hum [message #9735 is a reply to message #9734]

Tue, 07 October 2008 07:54

basshead
Messages: 5
Registered: October 2008

Junior Member

Hi Steve. Good morning!

The person I bought it off told me it was a K200. Tag on the back says 2-15C-1. However, looking at the pictures on here it seems to match this:

http://www.vintagekustom.com/images/newlit/kolorkatalog//03.jpg

Three prong grounded mains cord fitted...

Thanks for keepingup with me on this.

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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...

Senior Member

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 #9737 is a reply to message #9731]

Tue, 07 October 2008 12:24

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

If you turn brite channels brite switch on with the channel volume up to 1/2 or 3/4 you should have no problem hearing a dull hiss even thru those CTS bass drivers.

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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

Junior Member

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 #9740 is a reply to message #9731]

Wed, 08 October 2008 06:43

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

When you change over the caps be sure to label each wire and draw out a picture of how they where hooked up as reversing the positve and negitve power rails will in the least take out half of the output and driver transistors in the head.

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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

Junior Member

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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