Post Number: 140
|Posted on Monday, October 31, 2005 - 11:10 am: |
I own a 77 series bass.
I am getting a distorted signal the worst being on the neck pickup. I don't have a power supply. but the batteries are fresh. The distortion is worse with Q's in the 0db mode (all the way up) and also with the filter cranked. I have tried numerous cords and even tried headphones in hopes of the distorion being linked to my connection. I have played with the trim pots to lower the signal but to no success. Is this fixable by having "the upgrade" done or is this something totally differnt? I need help, my favorite bass needs to be recordable! Thanks in advance.
Post Number: 546
|Posted on Monday, October 31, 2005 - 3:06 pm: |
Check for problems with the jack, jack... It's an old enough instrument to have grown some funk in there...
Post Number: 2719
|Posted on Monday, October 31, 2005 - 3:55 pm: |
If the bass was stored for a long time or the controls were rarely moved, you might have a pot that needs excercising. Start with the filter pot for the neck pickup and rotate back and forth 50-100 times through the full rotation of the pot. It's very likely this will resolve the distortion you are hearing.
Post Number: 142
|Posted on Monday, October 31, 2005 - 8:16 pm: |
Thanks for the reply.
Though as of now the problem is still present. Could the pickup being microphonic be a contributing factor? What is the best way to test for a p/u being microphonic. Could this high frequency distorion be linked to RF noise? The humcanceller pots don't effect this problem one bit. The best way to describe it is to liken it to static. When the Q is on 0db and the filter cranked the sound is close to that of an old school fuzz box(kind of cool) but not COOL all of the time. Sigh..
Post Number: 2493
|Posted on Wednesday, November 02, 2005 - 10:40 am: |
My guess is that it is not RF noise; but I believe it to be the case that RF noise is directional, so you can test this by turning in different directions to see if the noise changes.
By the way, is the noise still there when the pickup selector switch is set only on the bridge pickup and only on the neck pickup?
- Exercise all the pots
- Exercise the jack
- Exercise the pickup selector switch
- Try another set of fresh batteries
- In the control cavity unplug one of the pickups and see if the noise is still there; then try unplugging the other. (Also note that there have been one or two club members who found that one of their pickup leads wasn't plugged in completely.)
- While you're in the control cavity, check that nothing is loose, and that a circuit board hasn't moved and is touching the shielding.
Post Number: 105
|Posted on Wednesday, November 02, 2005 - 2:08 pm: |
I'll throw in a thought here as well...
The sort of high frequency "hash" that you're getting is often the result of a feedback or gain problem that's outside the audible sound spectrum. You can't hear a 30KHz tone, but the opamp in your EQ circuit faithfully tries to reproduce it and may be driven into overdrive by something you can't hear.
When this happens, it's often because something is out of spec in the circuit and is causing feedback at some superhigh frequency.
One cause of this is faulty high-resistance connections. The previous advice hits a lot of the points to try here - turn the pots, exercise the jack and pickup switch. Over the years, a film can cover the contacts which will cause it to act like a resistor instead of a wire, and that resistance in the circuit can start the out of band feedback problems.
It is also a good idea to disconnect, clean, and reconnect the push-on molex connectors that connect the pickups and jack to the EQ boards. It will probably be sufficient to pull the connector and see if the pin that it's stuck on is shiny. If not, then use some contact cleaner or a polishing cloth to clean the pin surface. You might try something like Cain DeOxIt which is a red colored contact cleaner for this purpose. I don't know if I believe that DeOxIt is as magic as the manufacturer would have you believe, but you can buy it in a little pen with a fiber tip which actually does an excellent job cleaning small areas like this.
The other source of this sort of problem isn't as easy to deal with, as it may be a failure of a component on the EQ board. It's unlikely that a resistor or cap has gone bad in here (they are the usual suspects in high-power applications like an amp, but unlikely in a low power situation like this). One of the opamps may have failed through which would probably require a trip back to the factory to diagnose and repair.
You can think of the electronics in the bass as a chain of blocks, first the pickups and humcanceller, then the filters (which is also the first amplifcation stage), and finally a mixer to add the two signals together. Since the humcanceller trims aren't having any effect, then the pickups and humcanceller circuit are probably OK, and since the noise is coming in before the filters, then it's probably the early gain stage that has the problem.
Some stuff to try, anyway. Good luck
Post Number: 14
|Posted on Thursday, November 03, 2005 - 7:52 am: |
It might be just a case of a bad connection somewhere. On my essence I had a similar problem which I solved by spraying contact spray in every connector and then reattaching them. It seems to have something to do (with me at least) with a bad contact.
Post Number: 143
|Posted on Friday, November 04, 2005 - 7:26 pm: |
The only problem is on a bass this old the connections are not modular but put on with soder. So I am trying evevthing except disconnecting the wires.
Thanks any other advice is welcome. I will keep you all posted.
BTW: The think a seperate subdivison for trouble shooting is a great idea.
Post Number: 2750
|Posted on Friday, November 04, 2005 - 9:54 pm: |
The pickups are modular connectors, even if they don't look like it (as long as it's original). There's also a connector for the 14-pin ribbon cable between the harness and the preamp card, so do check those out.
Make sure you really rotate the controls a good 50 or even 100 times for a thorough cleaning.
You can test for a microphonic pickup by tapping the pickup with something non-ferrous while muting the strings. Make sure you've got at least the test pickup selected. If you hear a thumping sound, then the pickup is microphonic (just make sure your strings are really muted).
If all else fails, you can send in just your preamp card. For $75 we'll either fix yours or send you a rebuilt one. At least it will eliminate the ICs and FETs as the cause.
If after that you still have the distortion, we'll have to start looking at replacing the pots. I'm going to sound like a broken record here, but dad will rotate them before replacing, so make sure you really do that first - it often works.
Glad you like the new area, I'm hoping it will help us get needed advice out there more quickly.
Post Number: 2521
|Posted on Saturday, November 05, 2005 - 6:06 am: |
As Mica alluded to, the old modular connectors don't look like the newer ones; the older ones are thinner.
Post Number: 2770
|Posted on Thursday, November 10, 2005 - 6:31 pm: |
Are your cords stereo to mono? If so, are the signals resistively mixed like this:
Or is your 1/4" jack been modified to mono (a 1977 bass would have come with a stereo 1/4" output)? In that case, are there mixing resitors on the output jack?