Peavey TNT 150 Hum Problem

[Billy Penn]

Hey all. I found a few threads regarding this issue but none helped. I have a Peavey TNT 150 with a hum problem. Its a low volume background hum. Ground switch does not help. I disconnected the 15 volt wire (with red, green, white & bare lead) from the amp board to the preamp board and the hum goes away. I do have the +15v & -15v rails from the amp board to the preamp board. No AV volts on the speaker terminals. Turn Post to zero and it still hums. No pots, EQ or chorus affect the hum at all. PCB mounts read zero ohms to ground so there is no oxidation or anything like that. Has anyone ever had this problem? Any help/suggestions/advice would be greatly appreciated.

[Jazz P Bass]

I would suggest that you go back & measure the + & - 15 volt rails for any Vac ripple.

Ideally there should be none.

You should also measure the main power rails the same way, although there will be a small amount of Vac ripple.

A large Vac reading is a good indication of bad filter caps.

[Billy Penn]

Can I measure Vac ripple with a DVM? No scope at this time.

I would suggest that you go back & measure the + $ - 15 volt rails for any Vac ripple.

Ideally there should be none.

You should also measure the main power rails the same way, although there will be a small amount of Vac ripple.

A large Vac reading is a good indication of bad filter caps.

[The Dude]

Can I measure Vac ripple with a DVM? No scope at this time.

Absolutely.

[Enzo]

Depends on your meter, try it and see. If your AC volts scale reads some small voltage, it works. If your meter on AC reads more than the DC voltage, your meter cannot be used for this. Usually it is inexpensive meters that can;t do it.

[Billy Penn]

Depends on your meter, try it and see. If your AC volts scale reads some small voltage, it works. If your meter on AC reads more than the DC voltage, your meter cannot be used for this. Usually it is inexpensive meters that can;t do it.

I have a Fluke 117 with an AC millivolt setting. Black lead to chassis ground red lead to + or - rail, correct? I don't have much solid state amp experience. More tube amp experience. I will try this tomorrow.

[Enzo]

Tube or solid state doesn;t matter, a power supply is a power supply, and ripple is ripple.

[Billy Penn]

Tube or solid state doesn;t matter, a power supply is a power supply, and ripple is ripple.

Ok, thank you. I will check it out tomorrow. Thank you very much for your reply. You've helped me in the past and do appreciate it!!

[Billy Penn]

I tested for ripple using my Fluke 117 set to AC volts and also tried millivolts and both read zero volts. Still have this low volume background hum. Checked the 15v and main power rails. Anything else I should look at? Maybe just change the main filter caps anyway?

[bkahuna]

Try tightening the screws attaching the preamp pcb to the chassis. Also, those attachment points are typically riveted to the chassis, and may become loose over time. I had one unit where I had to run a wire from the preamp ground to the chassis to keep it from humming because of loose rivets.

[Billy Penn]

Try tightening the screws attaching the preamp pcb to the chassis. Also, those attachment points are typically riveted to the chassis, and may become loose over time. I had one unit where I had to run a wire from the preamp ground to the chassis to keep it from humming because of loose rivets.

Ok, I will check this tomorrow. How loud was the hum that you were experiencing? This is a low volume background hum. I don't think it's normal.

[bkahuna]

It wasn't very loud. Just enough to be annoying.

[Billy Penn]

It wasn't very loud. Just enough to be annoying.

Also when I disconnect the wires that feed the 15v rails to the preamp board the hum goes away making me think that the issue is on the preamp board. Wouldn't that make sense?

[The Dude]

Also when I disconnect the wires that feed the 15v rails to the preamp board the hum goes away making me think that the issue is on the preamp board. Wouldn't that make sense?

Chicken or the egg? You are disconnecting power. Is it the power or the preamp? If you supply poorly filtered power to a circuit, it's likely to be noisy. If you supply clean power to an audio circuit that has problems it's likely to be noisy, nonworking, etc. It could be either and that's what you need to find out. Before you go to the trouble of pulling and replacing parts, a quick way to narrow it down would be to get yourself a couple of clip leads and clip in an electrolytic of appropriate value from your rails to ground. See if anything changes. If it does you'll know you have bad caps. If not, then you can move on to the preamp.

[Billy Penn]

Chicken or the egg? You are disconnecting power. Is it the power or the preamp? If you supply poorly filtered power to a circuit, it's likely to be noisy. If you supply clean power to an audio circuit that has problems it's likely to be noisy, nonworking, etc. It could be either and that's what you need to find out. Before you go to the trouble of pulling and replacing parts, a quick way to narrow it down would be to get yourself a couple of clip leads and clip in an electrolytic of appropriate value from your rails to ground. See if anything changes. If it does you'll know you have bad caps. If not, then you can move on to the preamp.

I will try this and check the pcb grounds. The pcb ground issue with these amps seems to be a common thread. The hum is low in volume but annoying like bkahuna said.