I have a Peavy Special 130 guitar amp that hums loudly with volume set to zero and nothing connected. I ran a test to disconect the guitar preamp and the noise went away. The unit has two zeners that are supposed to control two different 15VDC circuits for the preamp. Voltages are very low in the preamp circuit. Measuring across the zener reveals one at roughly 2 volts and the other at 7 volts. AC is roughly 2 voltsto lower. Could bad zeners cause this issue? Or should I be looking for something else lugging down the voltage. No fuses have blown. The amp section has roughly the required voltage near 42 so do not think the problem is there also based upon the disconnect test.
It could be the diodes, but whatever is the cause it's before the pre-amp section. Generally a loud hum such as that with no effect by volume control, is mostly caused by the filter capacitor in the power supply. And that hum is the 50HZ mains hum getting into your system!
SOURCE: A peavey special 130 guitar amp with excessive hum
I used to have one of these years ago!
What do you mean when you say 'switch to the off position'? Do you mean it hums when switched off?
Because the sound does not fully follow volume, it is likely a bad connection or power filter capacitor is not working.
Either way, it means a trip to a repair shop. There is nothing in there you can fix unless you are a repairman. Your local music shop should be able to coordinate a repair.
If this amplifier is like the 1983 version I had, its built like a tank and the fix should be easy.
Hope this helps!
Testimonial: "Yeah, it was a filter cap ( electrolytic )that was at fault. I replaced it my self, and now the amp is working fine.I suspected a bad cap, thanks! "
SOURCE: crate gtx 65 guitar amp mains hum
I had the same problem on my VTX65. I noticed it would oscillate with different effects so I removed the effects module. Problem solved. Now it's dead quiet. I use a Digitech RP500 so removing the effects module was a non issue.
430 views
Usually answered in minutes!
Thanks. It appears that the circuit had minimal AC in the 15 VDC power supply that powers the pre amp. What bothers me is that would the low voltage cause the hum or just make the preamp inoperative. The amp section with preamp disconnected is stone quitet (no hum). Now could defective filter caps lower the voltage to make it appear that the zeners were bad and not regulating voltage. Before I started to completely disassemble and start removing parts I wanted to be sure there was good likelihood that the parts were the issue. From old radio repair experience, filtter caps can always cause hum. But can they cause a low voltage in the preamp power supply?
Yes the capacitors can cause power loss, when they dry up. I even checked it up in an old book on Radio Repair just to confirm it!!
Thanks. I will pull the capacitors for the 15V circuit first to check them for leakage and only then go after the zeners.
×