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T-40 Troubles

GreggBummer

GreggBummer

Jan 16, 2008
1,790
624
Southern New Jersey
#1
I have a 1980s model Peavey T40 with the blade style pickups. Recently, I noticed that when the pickups are in single coil mode (tone knobs above 7) they have become very noisy. I am think it might be single coil hum, but, when the pups are set at the same tone and volume levels they still sound noisy. When the bass is in humbucker mode (tone knobs at or below 6) the noise disappears. The noise decreases if I roll off one tone knob only. Sometimes, if I move the bass to a different position, the noise is exacerbated. I have plugged the bass into various amps and used several different cables.

On a recent trip to the guitar shop I had the pots cleaned. I asked the tech what he thought and he seemed to think it was normal. I am not sure about that, I think this noise is a recent development.

Suggestions? Thoughts?

I love this bass too much to it sit idle.
 
#5
I had a very similar issue - turned out to be our practice space, where there was a ceiling fan light dimmer on a remote. If the lights were at any setting other than 0 or 100%, I'd get a nasty hum in SC mode. I used the T-40 in there a lot, so I initially thought it was the bass.

If this is happening at multiple locations, you can probably rule it out.
Best of luck - I agree that the T-40 is a fantastic bass.
 
#6
Is it both pickups? or is it just one? My neck pickup had a similar situation, it was the lead going into the pickup at a sharp angle. One of the wires was broken there.
 
#8
ack said:
I had a very similar issue - turned out to be our practice space, where there was a ceiling fan light dimmer on a remote. If the lights were at any setting other than 0 or 100%, I'd get a nasty hum in SC mode. I used the T-40 in there a lot, so I initially thought it was the bass.

If this is happening at multiple locations, you can probably rule it out.
Best of luck - I agree that the T-40 is a fantastic bass.
Click to expand...

I had the same thing happen at our old space - the motor on the 'fridge went bad and caused all kinds of buzzing issues with the PA, bass & guitar amps. Took us a while to figure it out...
 
#9
The hum is in both pickups. Again, only in single coil mode.

Also, the hum seems to be in multiple locations in my home and also in the bands practice space. Same volume and buzz.

I will pull the pick guard (...all those screws :crying:) and check the wires to include the ground.

Since posting, I just noticed that when I touch the pup selector switch the noise changes but doesn't go away. So the ground may be the issue.

I work tonight and tomorrow all day (I am an EMT/Firefighter in my professional life) so I won't be able to pull the PG until Wednesday at the earliest. I'll update then.

Thank you all for your advice.
 
#10
Here's the schematic and it shows all the grounds.
mech
2_zps1027b5b2.jpg
 
#16
The problem still exists. When the bass is in "single coil mode" (tone knobs above 7) then there is a hum, even when the knobs are in the same place- like all the way up to ten. When the bass is in humbucker mode (tone pots below 6) the noise disappears.

After parking the T40 in a case for several months, I took the bass to two separate techs, both of whom well trained and fantastic guys I've used for years. They both confirmed that it is "single coil" hum. No ground issues, all joints and connections checked. The bass is well shielded. I have tested the bass in multiple locations, with multiple amplifers, with several cables.

So, when the tone knobs are both above 7, there is a remarkable hum. I directly compared it to my jazz bass. When the volume pots are not in phase, it hums just like the T40. The T40 is somewhat louder.

So, my follow up questions

1) If the tone knobs are both up FULL to 10, then both neck and bridge pickups should be in phase and should hum cancel. Correct? They obviously aren't.

2) Could one of the pickups be installed "backwards"? Would this cause a constant single coil hum?

3) Could on the tone pots not go high enough? Meaning that if the bridge pot is at 100% the other pot is inefficient and working at 5% less, thereby keeping the whole system out of phase.

4) Am I making a mountain out of a mole hill? Should I just suck up the single coil hum and use a noise reducer, like the ZNR available on my Zoom MS-60B?

I am NOT very good with electronics. I will be honest, I know enough to be a danger to myself.


Thank you in advance.
 
#18
You said "recently ... they have become very noisy" implying that you didn't have this issue before.
If that is the case, then something must have changed (certainly not improperly installed pups).
Possibly a bad pot, since you tested all the wiring.

It seems more likely a ground wire has snapped or corroded and no longer makes proper contact, possibly the all important bridge ground wire.
 
#19
kcole4001 said:
You said "recently ... they have become very noisy" implying that you didn't have this issue before.
If that is the case, then something must have changed (certainly not improperly installed pups).
Possibly a bad pot, since you tested all the wiring.

It seems more likely a ground wire has snapped or corroded and no longer makes proper contact, possibly the all important bridge ground wire.
Click to expand...

to add to this, do you have a resistance meter? that way you can test the ground wires from end to end. Also look at those four caps. Make sure they're not corroded where the leads enter into the ceramic.
 
#20
GreggBummer said:
1) If the tone knobs are both up FULL to 10, then both neck and bridge pickups should be in phase and should hum cancel. Correct? They obviously aren't.

2) Could one of the pickups be installed "backwards"? Would this cause a constant single coil hum?

3) Could on the tone pots not go high enough? Meaning that if the bridge pot is at 100% the other pot is inefficient and working at 5% less, thereby keeping the whole system out of phase.
Click to expand...

1. Even with everything totally balanced the two T-40 pickups will not form a perfect humbucking pair when both are in single coil mode and full on due to the distance between the pickups. Whatever hum field being picked up must be picked up equally by each coil to be canceled. The distance between the pickups, as on a Jazz, precludes them from being as good as a self contained humbucker.

2. Since the pickups are self contained humbuckers it makes no difference which way the pickups are installed.

3. There is a possibility one of the tone or volume pots has developed a problem at the end of rotation that is keeping the system from being balanced with all controls set to 10. This normally shows up as some static or the sound dropping out when the pot is rotated.

Even though the solder joints have been inspected I would reheat all of them. A high resistance solder joint would do the same thing as a pot that was not all the way on 10...raise the resistance in the circuit and unbalance the system. The solder joints are about 30 years old and may not have been totally perfect when they left the factory.

None of the capacitors will cause the hum problem you are experiencing.

mech
 

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