Peavey Combo 300 problems

[Axtman]

My Peavey Combo 300 has some problems listed below:

1. It distorts easily when I turn up the volume. It sounds like a coil rub on the speaker but the speaker is fine.

2. It has a "rushing" sound when I turn up the volume with nothing connected.

3. It sometimes cuts in and out when playing. The sound is normal then gets low then normal again.

4. (This is a strange one.) It makes my volume knob on my passive P-bass have a "scratchy" sound when I turn it.

Any advice would be helpful. Thanks.

[Enzo]

3. Make sure the connector on the speaker wires has not spread the female molex pin. Make sure the large-ish coil on the power amp board - it is right next to wher the speaker wire plugs on - has not broken free at one end. Take a spare cord and plug from preamp out to power amp in. ANy help? Do the same thing at the patch jacks - send to return. ANy help?

4. Usually that means a little DC voltage is on the input jack. PLug a cord into the input jack. Turn the volume down on the amp so there is no noise. Now with a volt meter, measure for DC voltage between the tip and sleeve of the free end of that cord. SHould be none. Not sure how it would get there unless C1 was shorted or removed.

2. Turn the controls up and down. ANy controls that have any effect on the sound - loudness of it, tone, whatever - are after the source of the sound. ANy that do not affect it are before its source. That will help harrow it down. Most likely one of your op amp ICs is noisy.

1. Speaker is fine? You connected a different speaker to the amp and it does the same thing? And you connected this speaker to another amp and it is OK? DOn;t assume the speaker is OK just because you can;t feel it rubbing by pushing on the cone.

As we did before, plug a cord into every jack on it, one by one, and measure for DC voltage at each one. SHOuld not be any.

If the output is distorted, isolate the problem. PLug your guitar into the power amp in jack. Is the soumd still distorted? That would mean the power amp was at fault. If not, go to the patch return jack. PLug in there. OK?

Likewise, plug a cord into the patch send jack, and feed that signal to some other amplifier for a listed. Is that signal distorted from the second amp? How about the preamp out signal?

[Axtman]

Thanks Enzo!

My replies to your replies. (Please let me know if there is a better way to do this.)

3. I will open up the amp again and check the molex plugs/pins. I will also check the coil (choke?). I plugged a guitar cord from the preamp out and power amp in and there was not difference.

4. I checked ALL the input and output plugs for DC voltage. No DC voltage present. I also checked for AC voltage and got none....except when I "dime" all the controls with no input I got up to 31 volts AC and a very high pitched squeal. The "Presence" control affected the volume of the squeal.

2. The lower tone controls seem to affect the distortion more than the higher tone controls.

1. I will remove the speaker and check it in another (good) amp.

[Axtman]

I removed the speaker and plugged it into another amp. Sure enough there is some distortion at low notes. I removed the magnet off the frame and found some burned spots on the voice coil. Next step is to order a new Peavey Black Widow basket.

The amp does have a high end squeal when the amp is cranked.