Old Peavey KB 300 Farts w/compressor indicator

[groovyfingers]

Title says it all, any ideas for tracking this down? Guess I'll need a schematic, call Peavey and give 'em my model and serial number? It probably won't be my problem, but has anyone chased a similar problem down where the compressor circuit engaged and caused break-up/distortion? What was it that you found and what fixed it? Thanks for your consideration.

[groovyfingers]

Found this but mine is not subtle at all.

http://music-electronics-forum.com/t5420/

Crackling break-up not subtle distortion creeping in. It used to work fine but has been this way for years now. Hope to get to the bottom of it.

[oc disorder]

Hi umm err Groovy Fingers...

The schematic for that one can be found here (no xlr) Categorized Schematics and Service Manuals for free download O-R

I presume that is the black faced one with the patch jack on ch 1 and dual concentric pots.
I found the knobs impossible to get now.
If I may make a sweeping generalization suggestion... change/replace the main filter caps if they haven't been replaced.
Its my hunch the comp lights up on the peaks um.. which happens to be when the amp is under the most strain.
I presume the dual throat piezo is ok too they can cause unusual noise Enzo suggested a while back buying a couple
of ksn Motorola ones or whoever makes piezo's now and raiding the drivers off the plastic horns to fit to the KB300
Of course they may be fine just thought I'd throw everything in...
Those old Peaveys they just keep going... and going ...and going... good luck !

[groovyfingers]

Thanks for the link!

You're right on the black faced one. Knobs all good, knock on wood.

Your generalization is well taken, I believe the caps are original. Have done the same with old Fender Princeton, Deluxe, and Bassman that I have. Good place to start.

The horn has been replaced with a smaller unit, could be a problem but seems to be the 15" speaker that is most offensive. Not a black widow, but has a good size magnet on it regardless. Was a good amp until this problem surfaced.

[Enzo]

Correlation is not causality, we need to find out if the amp is making a funny sound every time the compressor is triggered, or if every time it makes a funny sound it triggers the compressor. In other words, the two may be linked, but cause and effect we are not sure of.

Isolate the problem. Connect the amp chassis to a different speaker. That will tell us if the speaker is involved. For that matter, give it the Enzo whack. Ball up your fist and whack the top of the amp. Does that trigger any noises or other reaction? Any rection means a loose connection inside somewhere. Speaker vibration could be involved.

You should have the 260C power amp in it, used on any number of models.

First things first inside, check power supply. There will be +V and -V for the power amp, looks like 42v. Make sure both are about the same, relatively close to the 42, and free of ripple. There are two sets of 15v supplies. One pair is regulated and is sent to the preamp via that corner connector. The low voltage stuff on the power amp itself runs off +/-15v zener supplies. Make sure all four of them are up to voltage and free of ripple. Just takes a few seconds.

It may be a good idea to replace the filter caps, but I really recommend finding the problem first, before we start throwing parts at it. You won;t find 5000uf 55v can caps. They are long gone. Replace them with 4700uf 63v caps. I use snap-ins myself, but whatever you use, you will probably need to drill a new hole or two in the pc board. Watch where you do it.

To see if the compressor itself is involved, look on that board for a four pin Molex connector about a quarter of the way across the board. SHould be a couple wires to the front panel LED and a short wire connecting the remaining two pins together. Pull that plug. That disabled the compressor feedback. Still do it or no?

You can leave that connected, but pull the small IC on the power amp. There are only two, the small one and the larger TL074. The small one probably says 87478. Just remove it. Now the peak light should still work, but the IC gone means it cannot affect the signal. Does the LED still come on? Is there any audible event when it does? ANy symptom left? You could simply have a bad 87478.

But the amp could be crapping out and the compressor just follows along. Near where the speaker wires leave the power amp board, shoulod be an ugly brown coil of wire thingie. Wiggle it. Is it breaking loose at all? Look at its solder while you wiggle, see any motion of the leads?

You have a bunch of patch jacks, use them. Is there 15v on the preamp out or power amp in jacks? Shouldn;t be.

Send the preamp out signal to some other amp for a listen. Does the noise come out the preamp over there? Also, crank some good strong signal into the power amp in jack. Does that make it do it? We want to see if it is only power amp, only preamp, or both, doing this.


There, some starters.

[groovyfingers]

Amp was gifted to me and I was told the speaker was blown. To verify, I connected 15" speaker to another bass amp, and no issues. Then I connected the KB300 amp to a 4x12 cab and there was the noise. 15" speaker and horn totally bypassed. At this point I created this post.

So I tried the Enzo whack this morning. Had amp on for some time to warm everything up. No issues. Then dimed level knob for the three channels and the master. Still nothing when whacking the cab vigorously. Then turned the reverb up and noise, ugly noise.

[groovyfingers]

From the Peavey Forum: Enzo..."You have a Power Amp In jack. Plug your keyboard into that and play loud. Does it still crackle that way? (You are now bypassing the preamp, so none of the controls will do anything.)

And that jack can cause trouble, so plug a spare cord from preamp out jack to power amp in jack. Plug the keyboard into one of the front channel inputs. Does this stop the crackle?"

Just tried this and bypassing preamp doesn't affect noise, still there when compressor indicator lights. On the second suggestion, I plugged a short shielded ts cable between the preamp out to the power amp in on the back of the amp. Really compounds the noise/compression indicator light problem.

Very eager to try unplugging the 4 pin Molex connector to see if this stops the problem. Thanks Enzo!

[groovyfingers]

Ended up just pulling 87478 and problem gone. Put ic back in and problem back. Tried pulling 4 wire molex and problem gone as well. Think I'll just pull the ic and leave the molex connector plugged in. I'll see the led work, but not have any compression going on. I'll check tomorrow if Peavey has a replacement available, what the cost is, and see what I'm going to do. Thanks again, Enzo!

Oops! Just put back together, and played bass through amp with 87478 gone out of amp and molex plugged back in, and bass is distorted. It's not crackling, but it's not clean. So I guess my quick fix was anything but. Will check voltages and try the preamp suggestions to try to isolate the problem.

[groovyfingers]

Finally back to this amp. I had a local tech look at it twice and his equipment/diagnosis said it was working fine. But when I'd put it back together it was still distorting badly. So today did some of the checks recommended. Hit every jack with deoxit and worked with 1/4 inch patch cord to clean. Then used them to try to isolate problem. Power amp in is clean. Works well, used my KB100 pre-amp out to drive the KB300 power amp. Clean, no issues. Then used the KB300 pre-amp out to feed the KB100 power amp. Distortion and ugliness. Seems to indicate the KB300 power amp section is good and the pre-amp section is where my problems lie. I am trying to understand where I can make my voltage checks and how to do it. So I'm unable to report that information now.

[Enzo]

The amp has several input channels, do all of them act the same? If so that narrows it down to the "master" portion.

The preamp needs clean power, so are the +/-15v rails up to voltage and clean?

The amp is based on op amps. Even if the powr supplies are OK< check right at the power pins on each IC for proper supply reaching them. The go down the ICs and look for unwanted DC offset at all output pins. That is 1 and 7 on the 8-leg, and the corners on any 14 leg.

[groovyfingers]

Yes all inputs act the same. I'm trying to figure out where I can access test points for the 42 and 15v supplies. Thanks, good info as I try to come up with a plan.

Forgot to mention that I checked for unwanted voltage coming out of the power amp in and the pre-amp out. None detected so that's a plus.

[Enzo]

The 42v supplies run the power amp, and you told us that part works, so the 42v must be OK.

Look at your schematic, the 15v rails come onto the preamp board on that cable from the power amp in the corner. In fact the pins are labelled on the layout drawing that comes with the schematic. In any case, however it gets to the preamp board, it ultimately has to get to the power pins of the ICs, which I described above. if the op amps have +15 and -15 on their pins, then it must have gotten to the board OK.

Your whole master section is pretty much just U6.

[groovyfingers]

I thought the same thing concerning the 42v positive and negative, that if the power amp sounded good it ruled out a problem. Great! The Molex connector in the corner of the power amp board has -15.4v on the green wire and +15.6v on the red and it is carried over to the pre-amp board just fine. The U6's have -15.4v on pin 4 and +15.61 on pin 5 (I think, if 5 is directly across from pin 1) so power feed-wise, they seem problem free. I checked with my dvom, and switched from dc volts to ac volts to check for ripple. AC was 0.00v so just sitting there idling, no ripple. But maybe as in automotive I should check with the circuit loaded. Checking for unwanted dc offset at output pins 1 & 7 is a little over my head. I have a dvom and an automotive digital storage oscilloscope that I have not played with in this field. Might you elaborate, or is it a lost cause with my equipment and limited knowledge?

[Enzo]

With pin 1 upper left, they count down the left side and UP from the bottom on the right. Pi 8 is the upper right, across from pin 1. Pin 8 should have +15, pin 5 is lower right, and should not.

Pins 4 and 8 are the power pins of a dual op amp (8 legs). Pins 1 and 7 are the outputs of the two halves. 1 is upper left, and 7 is one down from upper right. Neither pin 1 nor pin 7 should have any DC on it. Oh a few millivolts won;t matter, just not several volts. Measure the voltage on those pins just as you did for the power pins.

Just for me, plug a cord into the preamp out jack. At the free end of the cord, measure for DC voltage between the tip and the sleeve of the plug. There should be zero DC volts. If you have 15v there, then one of your clamping diodes is shorted, CR9, CR10. Pin 7 could also be causing that. If we pull U6 from its socket and measure the voltage at pin 7 of the socket, that would tell us if any 15v is caused by the diodes or the IC.

Mind you I am not saying there WILL be any DC, but if there is, it points to the IC or a diode as the issue. The IC is suspect anyway.

[groovyfingers]

K will check DC voltages on output pins 1 and 7 tomorrow. So I have -15.4v on pin 4, and +15.61v on pin 8. I guessed poorly how they numbered the side opposite pin 1, shown on the schematic. I now know which pins are 1 and 7 and how/what to check, so tomorrow I will use my dvom and check for any trace of DC on them.

I put my dvom negative lead on the chassis, what I assumed would be ground, and plugged a patch cable into both the preamp out jack, and the power amp in. I checked the tip of the patch cable and had zero volts. I'll check tomorrow the way you described and see if I measure any voltage.