Treble knob has no effect (active) - Problem with pot or printed board ?

PaulPosition

PaulPosition

Dec 26, 2007
6
1
4,551
Everwinter Montréal
#1
[Disclaimer: I know (almost) nothing about electronics. If I say stupid thing is because I am stupid]

Okay, so I've got this cheap-y Yamaha TRBX304 bass, which is active, but the treble knob has no effect at all on the sound that's output. Physically the knob is alright, hasn't been ripped off or turned further than advisable, so I suspect the problem is more with the board.

How can I try to troubleshoot the problem? Can't afford to get it repaired right now but just knowing what's up would be great. Would allow me to plan a solution, at least. -- Alternatively, any "safe" way to make sure that if the whole treble cut/boost circuit is borked, it doesn't affect sound (ie isn't locked at full cut or full boost)? Like, deactivate it... (?)
 
#2
Okay,

So I went in and checked for loose wires (none), shorts (none that I could see), pulled and reset all connections to the circuit board, looked for obvious failed capacitor or burn mark, and checked with a multimeter that the pot itself was operating correctly (unplugged, with supplied voltage from mm's battery, it gives different values center-to-clockwise than center-to-counterclockwise, which change when I turn the knob)...

So I'm thinking it might really well be the board itself. But then again, using the 5-way switch "Performance EQ" (readymade tones, basically) it DOES seem to make the sound brighter. Would there be TWO independant EQs on the board??

Any good troubleshooting tip? Or explanation...?

Thanks guys...
 
#3
Like many people here I am unfamiliar with this bass and with Yamaha in general. From the product description I would guess that the treble knob only works in the center or "flat" position of the five way switch but I am far from certain of that. It could be that what you describe is normal for most settings of that switch, there may only be one position where the treble control works. Does the bass control work?
 
  • Like
Reactions: PaulPosition
#4
Thanks,

Yeah, not the most well known basses, obviously. Here's a lousy picture of what the wiring looks like, in case it looks like some other better known system...
DSC_3123.jpg

Anyway, the bass boost/cut works wonder. From lock to lock it almost kill the signal or boosts it to high heavens. And it DOES work whether the 5-way switch is set to flat or to any of the preset EQs.

I've played some more with my setup, though, and I'm starting to think I may be crazy, deaf, or both.. Or I was completely misunderstanding how sound works, which is a possibility. -- Anyway, I find that playing with the treble ON MY AMP (a cheap bass cube from Rolly') doesn't change the sound much either. As if there wasn't much in that band that could be amplified to begin with (?!?!?) Does that even make sense? Are low and high bands split and then mixed back together, but I'm somehow loosing the highs somewhere along the way? It's not like high notes are rendered inaudible, they play well though they may lack some shine... dunno.

Damn I hate the guy who stole my El Capitan and condemned me to that Yamaha... :(
 
#5
Oh, hey, Yamaha Canada was nice enough to send me the Service Manual for the guitar. I'll be studying it with attention (and confusion, unfortunately), but in case anyone wants to have a look, lend a hand...
 

Attachments

  • TRBX304_305_504_505_C.pdf
    2.8 MB · Views: 867
#7
PaulPosition said:
Oh, hey, Yamaha Canada was nice enough to send me the Service Manual for the guitar. I'll be studying it with attention (and confusion, unfortunately), but in case anyone wants to have a look, lend a hand...
Click to expand...
Its been a while since I picked up a Yamaha, but as I recall the EQ controls only worked when the 5 way was in the center position. The other 4 positions, 1,2, 4 and 5 bypassed the manual EQ controls in lieu of the presets. I could be wrong, but it sounds like its working as it should.
 
  • Like
Reactions: PaulPosition
#8
Okay, well I fell like bashing myself silly over the head with a bass (the fish, that is)... (see below)

Meghans Dad said:
Its been a while since I picked up a Yamaha, but as I recall the EQ controls only worked when the 5 way was in the center position. The other 4 positions, 1,2, 4 and 5 bypassed the manual EQ controls in lieu of the presets. I could be wrong, but it sounds like its working as it should.
Click to expand...
Thanks but, well, as I posted:
PaulPosition said:
Anyway, the bass boost/cut works wonder. From lock to lock it almost kill the signal or boosts it to high heavens. And it DOES work whether the 5-way switch is set to flat or to any of the preset EQs.
Click to expand...


RobbieK said:
try gently tapping the top of the pickup with something steel, like a small screwdriver. have your amp fairly low for this. even with a fairly crappy amp/speaker you should hear if your treble control is working.
Click to expand...
Well, what would you know, it seems that it *DOES* indeed work, just in very subtle amount. Ticking at the pickup and looking at a VU meter, I'd get difference of about ~30 decibels playing with the bass boost/cut and only maybe about 10-15db from the treble knob. Still it works, my ears were bad - and my old dirty strings probably didn't help.

Thanks a lot for helping clearing that out.
 
  • Like
Reactions: Meghans Dad
#9
it *DOES* indeed work, just in very subtle amount.
Click to expand...

It's amazing how often an active treble works much higher than the resonance of the pickups. The response of pickup coils drops very dramatically just higher than the resonant peak. It's not uncommon to have a treble control that that peaks an octave or more above the pickup resonance. There's not a lot to actually boost. (Or cut for that matter.) Especially with old strings and an amp with no good treble response.

I've only glanced at that circuit, and without a bit of work, simulating etc, it's hard to recommend mods off hand, but if you want a brighter tone at the input of the preamp (to give your treble control a little more to work with), then the first thing I'd do is to snip the earth wire from the blend pot. However if the blend pot is a/c, this may effect the blend sweep of the pot. Try playing while touching and removing that blend pot earth wire. You might be surprised at how much this can open the tone.

You could also try increasing the input impedance of the pre. It has a fet front end, so this is simple. You can do this in two ways. Either increase both R101 and R102 to say, 2.2 meg. Or remove R101 altogether, and run R202 to the 4.5v rail (pin 3 of the opamp) instead of earth.
 
  • Like
Reactions: jmwreck and PaulPosition
You must log in or register to reply here.
Top Bottom
Back