Peavey KB300 need schematic

[Luthin You]

Hi I need a schematic and PA layout for KB300 which is a 260C , I can only find a 300CH on here . It is all biased off and Compressor light on . It has been doing this intermittently but now it just off . Len

[Enzo]

Call customner service at Peavey, they will send you a copy.

But...

Just try a new TL074 in the long socket on the power amp. Note which way it goes in. The small IC on the power amp is the comptrsssor IC, try just removing it and see if the amp wakes up.

[Luthin You]

Call customner service at Peavey, they will send you a copy.

But...

Just try a new TL074 in the long socket on the power amp. Note which way it goes in. The small IC on the power amp is the comptrsssor IC, try just removing it and see if the amp wakes up.

Hi Enzo , I tried the TL074 , and have pulled the 4 pin compression plug . Its not easy for me to call pv as Im in Australia . Thanks for fast response !

[Enzo]

customerservice@peavey.com


So did pulling the small IC make the audio work again through the power amp?

[TomCarlos]

Hi Luthin...

I found a copy of a KB300 schematic. I hope this matches your amp. I believe this is a 1979 version (give or take).

Tom

[Luthin You]

Hi Luthin...

I found a copy of a KB300 schematic. I hope this matches your amp. I believe this is a 1979 version (give or take).

Tom

Yes thanks Tom , thats the one , PV sent a different one . Pulling the small IC doesnt change the problem , nor does the 4 pin plug . The problem is that the amp mutes and the comp. light comes on also . It may be a bad solder joint as it sometimes comes and goes when you push or pull the board to stress it . I have tried resoldering some joints on back but no change . The amp works perfectly when it is working , and mutes completely otherwise , which corresponds to a DC voltage of about 180mV on speaker .

[Enzo]

Peavey sent you the 1989 version drawings, you needed the 1983 set.

The last thing in the compression chain is the small IC, so if removing it does not restore the sound, then the compression circuit is not causing the issue.

So the other circuit is the power up mute or antithump. At the input to the power amp is JFET Q12. Is the gate voltage missing when the amp is muted? Or just remove the JFET and see if it wakes up the amp. Note ther is also Q13 a little higher up. Both those have their gates wired together, so a bad one can affect the other. They get their gate voltage from the Z circuit, which is a cap charged by the +16 and +15 supplies.

[Luthin You]

Thanks again Enzo , I have removed Q12 and 13 and 4 pin plug ,the muting problem persists , all I can notice is that pin 14 U1D rises to +5V during the muting , the inputs dont change ( pins 12 and 13 ) and its a new IC .

[Enzo]

If Q12 is removed and the little U2 is removed, then the amp is not muting, it simply has a power amp problem.

Those four little parts at the input OK?

Pull the cable from the preamp off, and touch the input pin on the Power AMp card with your finger. Hum? Clip some signal source to that pin, does the power amp amplify it?

The +/-15 for the preamp comes from a pair of regulators. The op amps in the power amp have their own zener derived +/-15v rails. Do the IC get good 15v rails right at the ICs?

[Luthin You]

Thanks Enzo , the problem is that the PA intermittently dies , the comp LED comes on but it seems to be in response to the fault . 15v are within 0.5v and as I said the small IC and Q12 and 13 dont effect the fault . the input to PA board is dead when the fault happens . Note that pin 14 U1D rises to +5v during the quiet period this takes a second or two . I replaced the cap in Z ct for fun. thanks again Len

[Enzo]

But we do know that the actual muting circuits are not involves, because they only have two tools to use, and you have disconnected them. At least I HOPE they are still disconnected. Q12 and U2.

You mentioned the 5v thing a couple times. I believe you that it happens, I suspect it is the symptom rather than the problem. My reaction is the input side is seeing an open. SO when it goes to this condition, quickly remove power and take resistance from pin 12 to ground. Do you get about 122k?

Since it comes and goes, turn it on, and when this happens, ball up your fist and whack the amp. Does it react? Fir that matter, when the amp IS working, whack the thing and see if you can trigger the problem. Any reaction to whacking tells us there is a loose connection somewhere. Tender fists? Use a rubber mallet.

And we have not asked that I recall: What happens at the output? Does any DC offset appear there? So far I don;t think we eliminated the high current circuits.

[Luthin You]

The problem IS very much effected by a whack or twisting the board and there is a rise to +180Mv on speaker . I suspect a bad joint but im not sure how capacitors react to a knock . It is hard to see where the +5 is coming from as there is no positive source around there (output U1D) . Do IC sockets ever go intermittent ? Now thinking about it , it is a bit strange that the comp light triggers , I only have a tiny signal so it doesnt bug me , whatever is causing the dropout is triggering the comp also.

[Enzo]

I suspect your op amp 5v is coming from an open. The thing gets to float to whatever it wants, in this case 5v. That is why one of my first tests would be those resistors to ground at the input. There is or at least should be both 15vs right into the IC, so if it is allowing 5 of the 15 to ooze out, there it is, It didn;lt have to come from outside

Do IC sockets fail? Of course. But not really very often. Usually pulling the IC out and pushing it back in cleans the contacts. I'd be more concerned with its solder. If the thing is sensitive to mechanical shock, then while it is running, grasp the TL074 and try twisting it a little. Trying to do this without flexing the rest of the board.

I have mentioned the input resistors, but there are also feedback resistors for the IC, R17,18. There are not a lot of direct connections to this IC, but always remember that solid state amps are one big loop. So for example, if the output is drifting, the op amp tries to correct it, and so it MIGHT be putting 5v out from that IC because that is what it takes to keep the speaker jack close to zero.

Have you chopsticked it yet? Use a wooden chopstick from a Chinese restaurant, or some other INSULATED thing to push gently on each and every part on that board, looking for one that reacts.

[Luthin You]

Yes I did the chopstick thing but could not close in on the area , I decided to re-solder the whole small signal side of the power amp and now it is fine . Thanks again.Len

[TomCarlos]

Good job Len!!