T-40 Electronics Problem

[Bobster]

Hi,

I'm getting an 80' T-40 that has an electronics issue.

When the neck pickup is in humbucker mode (7 or below on the tone knob) the output is very low. The rear pickup doesn't have the issue.

I don't have easy access to this bass yet, but I'm wondering if the experts can point me in the right direction for troubleshooting when I am around it?

I know the first step is to verify both of the pickup's coils are actually working. Then, I'll look for shorts.

After that, I'm a little lost and it's been hard to search for this specific issue, although I have seen a few posts in different places with this issue on either the neck or bridge pickup.

Bob

[VincentSalizeri]

Don't know too much about T40 electronics, but do you know if the pickup is in series or parallel? Parallel usually has a lower output than series. Shouldn't be tough to check for.

[mech]

Although it would be a very rare thing to happen, it sounds like the second coil that is brought in to make the PU humbucking is open. Another possibility is that the red wire is shorted to the silver wire in the cable from the PU. This happens if heat is applied for too long a period of time while soldering and the silver wire melts through the insulation of one of the other wires. Strip off about 1" of the outer insulation on the cable where the wires exit to check.

The red wire is the center tap between the coils and is grounded by the tone pot to make the PU single coil.

mech

[Bobster]

Hi Vince,

Both the neck and bridge pickup are the same, but the behavior is different for the two. The neck is much quieter than the bridge with the tone down.

Mec,

Thanks. Since there is a little sound when tone is at 0, hopefully it's not the pickup itself. The only other incident I found like this was the result of the phase switch grounding out on the shield.

I'm sure a look inside will make this a bit simpler.

Bob

[mech]

The phase switch only affects the bridge PU. For humbucker mode the coils are in series.

mech

edit: There's no simple way to look inside a T-40.

[Bobster]

Mech,

Looking at Chip Todd's 11/2010 wiring diagram, it looks like I should see the neck pickup's red wire going to the tone pot (right leg), the white going to the volume pot (center leg), and the black and bare wires going to ground.

Does that match your experience?

Also, any idea of the impact of losing the caps (failure) on the neck vol and tone controls?

Bob

Thanks

[mech]

That wiring is what I've seen.

The most common failure of a small value cap is to fail open. It's very rare for one to short.

The cap on the Vol pot is for high end bypass, which gives a little more presence when the volume is turned down. If that cap fails "open" you probably couldn't hear the difference. If it fails "shorted", that Vol control would act like a master Vol for both PUs when both are selected.

If the cap for the Tone control fails "open" the high end roll-off doesn't work. If it fails "shorted" the Tone control will act as a volume.

Re-reading your 1st post, there may be a short between where the Neck tone cap attaches or the cap may be bad.

mech

[Bobster]

Thanks again, Mech.

I'll do a tap test on the neck pickup coils and then give that neck tone cap a good hard look.

I'll let you know how it goes.

Bob

[Bobster]

Well, it turned out the bridge pickup pot was slightly loose and was shorting out one of the neck pickup wires.

Straightened that out and it's sounding great, very aggressive and a huge range.

Thanks!