Peavey KB300, EQ mids are not working.

[SeBas]

Well the title gets straight to the point. So I have a Peavy Kb300 model KB-300-BW/ and I have no schematic and I have looked for the schematics and the one thats on this site is not the one. I will call Peavey tomorrow maybe they have it.
So I pull the amp out and proceed to clean the all jacks and the pots. This amp has never been maintained so it needed it, but surprisingly clean on the pcb and components.
After testing all the pots I discovered that the "mid" level which is the outer dial on these dual pots with concentric shafts. Channel 1 and channel 2 have no levels change when "mid" is turned, only a pop when the pot is set to 0 position.
Now that may be an indicator itself that it could be the pots themselves. Unfortunately I cant test the resistance of the pot without removing the board because the contacts are not visible, only the contacts for the center shaft.
Channel 3 works great and also lends to the idea that its bad pots on the other channels.
Does anyone know off hand if each channel has its own eq circuit or do they all share it. This would help determine if the pots may actually not be the problem. Thats it for now thats all I have. Any help is appreciated.

Also if anyone know where I can find these knobs please let me know,

and also where to find these pots as I have one that has a broken center shaft, and also why I need the knob
Thanks

[The Dude]

............Unfortunately I cant test the resistance of the pot without removing the board because the contacts are not visible............

See those 3 rivets on the backside of the pots in your pictue?

That said, it could just as easily be a solder issue, so it wouldn't hurt to remove the board and check anyway.

Edit: Each channel will have its own EQ circuit.

[Enzo]

Parts department at Peavey is the first place to look for parts. Unfortunately if the dual concentric pot is bad, those are long gone. I no longer have them either.

Peavey has all the schematics too. You have the 1989 version.

It isn't hard to pull the preamp board to check the pots, but even right as it sits, if you cant get at the pot itself, just go to the various components that connect to the pot, they are way more accessible.

On each channel, the dual pot and one single pot are your tone stack. Note they are 50k each, but the three are wired in parallel, so reading any one will result in a reading like 16k.

[SeBas]

On each channel, the dual pot and one single pot are your tone stack. Note they are 50k each, but the three are wired in parallel, so reading any one will result in a reading like 16k.

That would explain the 21 Kohm reading I got across the 2 outside posts. I wonder how they connected that in parallel, time to google parallel resistance theory so I can better understand it. Good idea with taking the reading off of the components that lead to the pots. two calls and 4 emails later and I'm still waiting on Peavey for the schematics. Don't know if I will be able to find the e.q. on the schematics will most likely need help with that.

[SeBas]

I got the schematics for the amp.KB300-1989

As I mentioned in my last post, i get 21Kohms on the outside post of the pot. If I measure between the center post and any other will the value change only if I turn the shaft for those posts or will the value change if I turn the other shaft in this concentric pot. ?

I now have to find out how this circuit works for the mids. Not sure still how a single pot and a dual stack pot in parallel make up a tone stack. As a matter a fact I don't know what a tone stack is.

[g1]

I got the schematics for the amp.KB300-1989

That is a private link that requires sign in which many of us do not have. Can you set it to 'public' or something?

[SeBas]

I had a feeling that might happen. Lets try again.

KB-300-BW 1989

I also updated the link in original post.

[SeBas]

That would explain the 21 Kohm reading I got across the 2 outside posts. I wonder how they connected that in parallel, time to google parallel resistance theory so I can better understand it. Good idea with taking the reading off of the components that lead to the pots. two calls and 4 emails later and I'm still waiting on Peavey for the schematics. Don't know if I will be able to find the e.q. on the schematics will most likely need help with that.

Like I said before I am not pro so maybe some of the stuff I do is pointless. 21k is from outside post to outside post (rivets on the post that I can see). I got the schematic and tried what enzo suggested to find the component tied directly to the pot, so in this case grounded my black DMM lead to the ground point on amp. I used the red lead of the DMM to the center point of the channel 2 pot that i could gain access to and engaged the center shaft from min to max and got 13k up to 31k. I then found the center point of for the mid (outside shaft) leads to what appears to be c125. Then moved the shaft from min - max and got 13k at min and as soon as I get past min the DMM gives me an Open Loop. I guess that pot is shot. I did the same to channel 1 pot and got the same readings for both center and outer shaft. I used the contact at c111. Open loop again. I tested channel 3 as I know it works and used contact at c136 and it worked fine, 13k -31k.

So if everybody here agrees its time to get some new pots, which are not made anymore...


Got a silly question, I thought r#'s were resisters and c#'s are caps but the c125, c111 and c136 appear to be resisters and the schematics says .0068. I put the DMM on it and got no readings. I found that odd and then randomly tested other resisters and got no reading. I did get a 1k reading on a174 or is it r174, its hard to tell on this drawing but I didn't understand why this was happening so I stopped and figured I ask you guys for you take.

Thanks

[The Dude]

Those are indeed caps. Just a different style. If the board designation is "C" it's a cap. You'll find those often in Peavey gear.

[Enzo]

The little ceramic caps look like resistors, yes, but I think in most cases, the caps are green and the resistors another color. In any case, whatever is there is not "wrong".

The 71190556 is the part number. Look for that in a search, someone may have some for sale.

It is a 50k pot, and the bad section is your mids? If you cannot find a new one, consider this fix: remove the pins from the bad section, you now have three holes. Get two 22k or two 27k resistors. Install one from each end hole to the center hole. The rsult will be electrically the same as a centered mid control.

[SeBas]

The 71190556 is the part number. Look for that in a search, someone may have some for sale.

Peavey explained that this pot is no longer available due to the fact the vendor went belly up. So now my radical approach comes into play. Could I not repair the pot. If the resistive material on the b side of the pot is gone or shorted then replace it. Where would I get the part you ask, well i might be able to pull it out of a single pot of the same amp. I've pull pots apart before and reassembled them, just not dual stack ones.

It is a 50k pot, and the bad section is your mids? If you cannot find a new one, consider this fix: remove the pins from the bad section, you now have three holes. Get two 22k or two 27k resistors. Install one from each end hole to the center hole. The result will be electrically the same as a centered mid control.

That makes sense. I was thinking of maybe adding a push button with two different resistances so I can have at least two settings for the mids.

Most likely I will just find a different double stack pot that fits and rig up the knobs because I have a broken shaft on one of the good dual pots.

THanks

[52 Bill]

Peavey explained that this pot is no longer available due to the fact the vendor went belly up.

Most likely I will just find a different double stack pot that fits and rig up the knobs because I have a broken shaft on one of the good dual pots.

I didn't know that CTS went out of business.

Try and find a dual concentric pot that will fit in the space that you have that will mount to the front panel. Then run jumper wires to the board to make the connections. Not very elegant, but it will work.

[Enzo]

Maybe they meant CTS went out of the dual pot of that sort business... PV would have to make an order for a few thousand of them to get any.