How to adjust truss rod on fender duff mckagan bass

I suspect the H is really an N, which would place it.1990-91. As long as the adjustment screw for the truss rod is at the base of the neck. The plugged top end isn't a concern. Your best source of info is to contact Fender customer service directly. Here's the link-

Contact Fender Musical Instruments Corporation

Edgar,
How are you determining that the neck is not straight? If you are thinking the neck is not straight because the strings are at different heights along the neck, the problem may be neck tilt or saddle and/or bridge height. You can raise or lower the bridge system or string saddles to add or subtract string height along the neck. In addition, neck tilt (particularly easy to adjust on bolt-on necks) refers to the angle that the guitar neck exits the neck pocket on the guitar body. This also effects string height.
Conversly, if you see a distict bow or bend in the neck when you look down the top edge of the neck, the truss rod may indeed be involved. Generally, most players like a little bit of "relief" in the neck. That is to say, a little bit of a concave bow. I prefer a relatively straight neck with very little relief. Just preference.
However, you mentioned you already adjusted the truss rod. Make sure the truss rod is functional. If you loosen the bolt or hex on the truss rod and the concave bow on the neck gets more pronounced, it is likely that you truss rod is ok. If you make adjustments on the bolt and there is little or no change on the neck relief, it is possible that your truss rod is not functioning or is broken. It is very possible to break a truss rod by over tightening but it would take some real effort to do. Still, it is possible.
Check out those points and see if it helps.
Confirm truss rod is working.
Check bridge height and saddle height.
See if adjusting neck tilt is possible on your model guitar.
Good luck.

Any inbus the right size will do the job. Every respectable guitarshop should be able to tell you what that is for free. Don't even think of using anything even a little smaller: it will ruin the truss rod's head.

What you are describing is normally referred to as 'high action'. There are a couple of reasons this can occur (I'm assuming that the bass is not broken in any way). Either the neck is bowed forward too much, or the bridge saddles are set too high. If you have recently changed strings to a thicker gauge then the neck bow will need to be adjusted to compensate for the increased tension. If you are not experienced it can be risky attempting to adjust the truss rod by yourself so only proceed if you are happy with the risk!

On the headstock, just behind the nut is the truss rod cover. Remove the screws and cover. Underneath will be a nut, or bolt depending on the instrument. Allen key bolts are most common, but some use a regular nut. Make sure you have exactly the right size tool before you start! With the strings at pitch, make a 1/4 turn clockwise (tightening). Retune the strings and check the action. If it has moved but not enough then try another 1/4 turn. If it has not made any difference then you may need to seek the help of a pro, as the truss rod could be broken.

Again: don't try unless you can afford to risk breaking something! If in doubt take it to your local pro guitar shop and they should be able to help.

I own an Epiphone Casino and if I needed to adjust the truss rod I would take it to a luthier (stinged instrument repair person). There is a good write up on it here. http://www.fretnotguitarrepair.com/trussrods.htm

Hope this helps.

Bob

Try this site:http://www.violins.on.ca/bass_repair.html

Check for a problem with the neck or the truss rod. Look for anything loose or cracks and especially at the joint between the neck and the body.

Old or bad strings are the first thing to check of course. Use the right tension strings as they come in different weights. Use the recommended ones.

No the truss rod is usually in more than that. Take it to a reliable Musice Store and they shold be able to adjust it for you.

I can understand it a bit of a daunting task setting up these guitars (I have bought at least 5 like this) But yes it might be the truss rod ... Without looking at the guitar myself its hard to say ... If you tighten the truss rod remember how much you turned the allen wrench because if it doesnt work your going to need to reset it and start again ... Remember to turn it the right way! and if all else fails ... If your near me (Bournemouth, South of the UK) then you can always pop on over to mine and I can take a look at it.

Failing that just take it to your nearest guitar repair shop and a setup should cost you about £30 if your in the UK.