Very noisy KB100

This forum is for talking about Peavey keyboard amplifiers.
Post Reply
zuglufttier
New Member
Posts: 4
Joined: Mon Feb 17, 2020 3:18 am

Very noisy KB100

Post by zuglufttier » Mon Feb 17, 2020 6:22 am

Hi all!

I just got a KB100 (made in 1989 according to a time stamp on the amplifier board) in a pretty good shape - at first I thought dirty potentiometers would be all I have to fix... But once I cleaned the pots, I realized that they are not the only problem.

When I start the amp, it's emits a pretty strong (white?) noise and humming can be heard that gets louder once you turn up one of the three inputs. Apart from that, everything else seems to be working. The noise is on the headphone out as well as on the preamp out.

And another question: Is there a replacement for the volume pots on the three inputs? They are made by Alps and cannot be easily opened and cleaned.

User avatar
Enzo
Member
Posts: 9089
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: Very noisy KB100

Post by Enzo » Mon Feb 17, 2020 4:51 pm

They can be cleaned, you don't have to take them apart. They are not sealed. There is usually a gap where the legs come out of the body, there is usually a hole in the rear cover where it is pushed in to ast as a rotation stop. 99% of the time they clean up. Spray some DeOxit into each pot and turn them back and forth a few times.

If one channel is noisy it is likely a noisy op amp in that channel. If ALL channels are noisy, it it more likely an op amp after the channels merge.


The individual channel volumes are part # 71190522, which may be changed to 31190522. That is for the 1988 version.

The 1984 version uses 10k with pull switch. Does your have pull switches? Those are 71190414

zuglufttier
New Member
Posts: 4
Joined: Mon Feb 17, 2020 3:18 am

Re: Very noisy KB100

Post by zuglufttier » Tue Feb 18, 2020 2:47 am

Thanks a lot! I measured them yesterday and they do have an impedance of 1 megaohm, so they are the newer kind, I guess.
I poured some isopropyl alcohol into them and turned the knobs a few times, that did help a lot.

Now, onto that op amp: The noise is happening an all channels even with the volume turned down on all three channels. Can you give me any hints which parts exactly need to be replaced?

zuglufttier
New Member
Posts: 4
Joined: Mon Feb 17, 2020 3:18 am

Re: Very noisy KB100

Post by zuglufttier » Tue Feb 18, 2020 12:50 pm

And another question: Should the unit be quiet if I remove all op amps? Because there's no change at all if I do that.

User avatar
Enzo
Member
Posts: 9089
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: Very noisy KB100

Post by Enzo » Tue Feb 18, 2020 12:52 pm

No, you need to isolate the problem. We don't even know that it is a bad part, could be a dirty shunt contact in the patch jack.

zuglufttier
New Member
Posts: 4
Joined: Mon Feb 17, 2020 3:18 am

Re: Very noisy KB100

Post by zuglufttier » Fri Feb 21, 2020 11:11 am

After cleaning all the pots, I had a partial success ;) Noise and humming are staying at the same level even if the inputs are turned up fully.
But it is still easily audible if the amp is turned on and sometimes there's a plopping sound if the amp is turned off again.

Post Reply