Peavey KB100 Hum

[52 Bill]

Not sure what to test next.

Have you checked the input jacks? They are supposed to ground the input when there is no instrument plugged in. If the contact becomes dirty, the input will be open just like having a cord plugged in with no instrument.

[misterc57]

Have you checked the input jacks? They are supposed to ground the input when there is no instrument plugged in. If the contact becomes dirty, the input will be open just like having a cord plugged in with no instrument.

I did. Jack one did not have good contact. I adjusted it to make contact. Jack 2 and 3 were good.

I had a thought... perhaps I have DC ripple on the 15 volt rails. I assume that C60 and C62 are the filter caps for those rails? They are 25V caps. Those rails were running at just over 25V for who knows how long. Do those Zeners regulate to 15 volts back at those caps as well? I am going to check for AC on those rails next and check C60 and C62 for good contact on the board.

Thank you.

[g1]

Each input is consistent, no noise until the volume is around 5 (or around 3 with the volume knob in the pulled position).

The amp is very loud around 3 or 4 (volume knob pushed in) with no noise. Playing a keyboard, and it sounds great!

So, when you do have an instrument plugged in, like the keyboard, all is fine up to 4 on the volume, but at 5 there is way too much noise? (please ignore what happens with no input for now)

[misterc57]

Certain things have come to light.

Forget ripple; There is no measurable AC on my 15VDC rails.

I plugged in a guitar and there is no hum noise.

For some reason the noise only happens when my keyboard is plugged in. And a correction to my earlier notes; the noise begins past "2" (knob pushed in for no gain) on the volume knob.

Yet if I plug the same keyboard (and same cable) into my Peavey XRD 680S PA amp I do not hear any noise.

[g1]

Ground loop? Does the keyboard plug straight into 120VAC or does it run off a 'wall wart' type AC adapter?

[misterc57]

Keyboard plugs into the wall AC. I have created a 40 second video of this issue. Funny, the limited number of videos I have created all deal with noise

https://www.youtube.com/watch?v=yhyg7Cz-tQo

If I unplug from the KB100 and plug into the XRD680S there is no noise until you go over 8 on the volume where it is very faint (same noise) on the PA amp.

I do have another identical keyboard that I will dig out for a comparison.

Thank you

[Enzo]

Is your KB100 missing the third prong on its power plug?

[misterc57]

Is your KB100 missing the third prong on its power plug?

It is not missing, and I have continuity from the ground pin to the ground side of the 4 input jacks on the front panel.

I tried a direct box and that got rid of 80% of the noise, it also lowered the volume level of each input.

What is weird is that I have had at least five KB100 amps in the past that did not exhibit this issue, and they were used in the same location with the same keyboard.

Thank you

[g1]

Continuity may not be good enough if it's just a meter beep. Some meters will beep as high as 40 ohms. Check on resistance range. There may be several ohms or more and it may be a clue.
There are several places on the schematic shown as connected to earth/chassis ground, rather than just common ground. Those should be checked. Some of them are the rear panel jacks which probably rely on the star type washers for chassis ground connection. Check and clean them.
Also the main board probably has a ground buss that is connected to chassis by a trace that goes to chassis via a board mount screw. Check that the board mount screws are tight. And sometimes Peavey makes that board screw connection through a strap/nut that is riveted to chassis. Sometimes that rivet is not making a good enough connection to the chassis.

[misterc57]

Every ground connection to chassis was tightened.

I checked every earth ground in the pre-amp circuit, everyone was 2 ohms to the ground pin of the AC cord.

Every resister I came across was reading per the schematic except R47 (a 22 ohm resistor tied to U6 pin 3) read 7 ohms.

J4 and J5 (preamp in and out) tips have about 1 ohm resistance between each other and both are 10K to ground.

edit: I also tried my other identical keyboard and had the same results. Sounds like a ground loop.

[misterc57]

Hmmm on Hummm

So I have learned a lot of theory about ground loops in the last few days.

Looks like it is not un-common to have this issue between a keyboard and amp. On stage I always used a direct box to avoid it. However I have never experienced it at home before and I have had a lot of
KB amps pass through my hands. The KB series has always been my favorite keyboard amps.

There were some suggestions on how to handle ground loops;

1. Plug both the instrument and the amp into the same power strip. That did not work.
2. Use a 2 prong power cord adapter on either the keyboard or the amp. That did not work on either the keyboard or the amp.
3. Use a direct box. That works, but does not eliminate 100% of the noise, more like 80% in my opinion. Also the direct box cuts the volume level significantly. The direct box has a ground lift switch and
setting this switch in either position does not change the noise level, I found this curious. I confirmed that the switch works with my resistance meter reading ground continuity and had "OL" with the
switch set to the LIFT position and about 0 ohms in the GROUND position.

Some other observations all with the amp ON (volume set at 5) and an instrument cable connected from the keyboard to the amp;

With the amp ON, and the keyboard OFF, and keyboard plugged into the power strip; there is ground hum.
With the amp ON, and the keyboard ON; there is louder ground hum.
With the amp ON, and the keyboard unplugged from the power strip, there is no hum.

When plugging the keyboard directly into the Peavey XR680S PA amp there is no hum (however it is not nearly as loud as the KB100, it is about the same volume level as when I use the direct box into the KB100).
I wonder if the XR680S has circuitry to help eliminate the hum.

I plan to try a guitar amp next just to see if it picks up any hum.

Anyone have any other ideas?

Thanks again! MC

[52 Bill]

Does the amp have a polarity switch? Some Peaveys have a three way switch just for ac polarity.

[misterc57]

Does the amp have a polarity switch? Some Peaveys have a three way switch just for ac polarity.

It does. All 3 positions were tried with no difference.

Edit: You got me thinking. I am going to check that polarity switch and its surrounding circuit.

[misterc57]

Polarity Switch

All connections/paths around the polarity switch circuit seem good. Only thing I did not do is a cap check on the C64 and C65 caps. I will need to lift these out.

They are connected to what I think is called a solder cup. It looks like it is crimped and soldered. I have never serviced one of these. Do I just heat the outside of the cups and pull the cap pins out? Or do I have to work under the board?

What is a DVL cap?

Is there a way for me to test these caps without removing them? Perhaps monitor AC voltage frequencies in different switch positions? If so where would I setup my probes?

Thank you

[Jazz P Bass]

If the ground switch is a 3 position type, then placing the toggle in the middle position will disconnect the capacitor from the circuit.