TBP-1 preamp: plug in foot switch causes amp to be mute

[LeeLuu]

Hi Forum. This Fender bass preamp comes with a 4 switch foot switch. As soon as I plug in a cable in to the foot switch socket the amp goes immediately mute and the indicator lamp is flashing at the front panel. As it would when pressing the mute button on the front for muting. Other then that the amp functions perfectly except if I plug in the foot switch cable. I did release (off position) all knobs at the front panel as it is mentioned in the manual for proper functionality of the foot switch.
By checking the control lamps at the front panel I can see that the first 3 foot switch knobs do work except the mute switch is not triggering. The problem I thing is on the amp itself since it already mutes by plunging in the jack at the back (with ore without the foot switch attached). What could be the cause?

By looking in the electrics I can see a connection which I'm not sure it should bee like this. Maybe a professional electrical person could tell my if this is suppose to be like this.
Thanks a lot!

[nevetslab]

Hi Forum. This Fender bass preamp comes with a 4 switch foot switch. As soon as I plug in a cable in to the foot switch socket the amp goes immediately mute and the indicator lamp is flashing at the front panel. As it would when pressing the mute button on the front for muting. Other then that the amp functions perfectly except if I plug in the foot switch cable. I did release (off position) all knobs at the front panel as it is mentioned in the manual for proper functionality of the foot switch.
By checking the control lamps at the front panel I can see that the first 3 foot switch knobs do work except the mute switch is not triggering. The problem I thing is on the amp itself since it already mutes by plunging in the jack at the back (with ore without the foot switch attached). What could be the cause?

By looking in the electrics I can see a connection which I'm not sure it should bee like this. Maybe a professional electrical person could tell my if this is suppose to be like this.
Thanks a lot!

Welcome to the forum!

That resistor lead of the 15k resistor connected to the inverting input of the 4560 dual op amp, pin 2 is a proper trace, and not a solder short.

You didn't mention the model of this Fender Preamp. Their current trend in some amps is to use an inexpensive non-shielded 2-conductor molded phone plug cable, though older designs used a 3-conductor type. The 2-conductor type makes use of an AC voltage source, and with the switching both in the pedal and in the amp, the potentials are separated by the functions to yield the four switch modes. You may have an issue with the switching circuits....a common fault in the Fender Hot Rod series amps that use this method.

Does the front panel mute switch work? (obviously the pedal wouldn't be connected in this case).

[LeeLuu]

Hi and thanks, good to hear that tis is not a short. Yes the front panel mute switch works perfect. Original I got the amp with a non-shielded 2-conductor molded phone plug cable (tried different ones as well, same result). The wired thing is that as soon as I plug in it triggers the mute / no sound. So what would i do to check the switching circuits? This common fault, do they show the same issue: foot switch cable, plugin triggers the amp to be mute?

[nevetslab]

Hi and thanks, good to hear that tis is not a short. Yes the front panel mute switch works perfect. Original I got the amp with a non-shielded 2-conductor molded phone plug cable (tried different ones as well, same result). The wired thing is that as soon as I plug in it triggers the mute / no sound. So what would i do to check the switching circuits? This common fault, do they show the same issue: foot switch cable, plugin triggers the amp to be mute?

To go further, we need the Fender Model #, as well as the Pedal model # (might not have one), then need to come up with the schematics to figure it out. Right now, we're all shooting in the dark. It could be a problem with the pedal, as well as a problem with the amps' foot pedal circuitry. You've already identified the panel switching works, and 3 of 4 pedal circuits work, so it could just be a pedal issue. Open it up carefully and have a look, post photos.

Is this a new preamp, or something you bought used on-line or elsewhere?

Are you skilled in Electronics, properly equipped with the basic tools & meter(s), or just starting out, hoping to be lucky to figure it out, or one of us do so and back-seat drive you carefully (which often is the case)?

[nosaj]

To go further, we need the Fender Model #, as well as the Pedal model # (might not have one), then need to come up with the schematics to figure it out. Right now, we're all shooting in the dark. It could be a problem with the pedal, as well as a problem with the amps' foot pedal circuitry. You've already identified the panel switching works, and 3 of 4 pedal circuits work, so it could just be a pedal issue. Open it up carefully and have a look, post photos.

Is this a new preamp, or something you bought used on-line or elsewhere?

Are you skilled in Electronics, properly equipped with the basic tools & meter(s), or just starting out, hoping to be lucky to figure it out, or one of us do so and back-seat drive you carefully (which often is the case)?

Just for grins what happens if you only plug the cable partially in?

nosaj

[g1]

I think when you plug into the footswitch jack with an open cable (no footswitch) it may be normal for it to mute.
The problem may still be a faulty footswitch.
The easiest thing right now for you would be to try your footswitch with another TBP-1, or try your preamp with another footswitch of the proper type.

[LeeLuu]

Thanks so fare, all of you. I'll post pictures later. I'm equipped with the basic tools & meter(s) and do understand a little electrics. Just a quick thought, since 3 are working what do you think if I just remove the mute switch from the foot switch - would that disturb the switching circuits?

[nevetslab]

TBP-1 Bass Preamp Owner's Manual attached

Thanks so fare, all of you. I'll post pictures later. I'm equipped with the basic tools & meter(s) and do understand a little electrics. Just a quick thought, since 3 are working what do you think if I just remove the mute switch from the foot switch - would that disturb the switching circuits?

My apologies.......you have the Fender Model # right in the Subject line. I just downloaded it, read thru and noticed the comment concerning the MUTE function. Manual says on all of the foot pedal functions, for the pedal to work, the Overdrive Enable, VariQ Enable and Mute switches all have to be disengaged (button out) on the front panel to work. I assume the Mute button IS out.

TBP1_Preamp_BassAmp_ OM.pdf

Have you opened up the foot switch? While the OM doesn't comment on the actual pedal unit, there's a picture of it behind the preamp on the front page of the manual. Looks like the aluminum extrusion style pedal used in the Hot Rod Series. I'm sure the switches and the assembly are the same type, no doubt with 2-color LED's, so there would be the usual diode network on that board. We've had these switches fail, as well as a common ailment is solder joint fractures on the pedal PCB, so I'd suggest looking into verifying the Mute Switch works electrically before just removing it. I don't think removing it from circuit would cause the rest to misbehave, but........I haven't seen the schematic on the unit. Probably very similar in switching logic to that of the Hot Rod series.

[LeeLuu]

I still have the original manual and did read the section about the position of the switches - thank you. I learned that it is mostly the footswitch not the amp so I'll check on the footswitch first, you are right on that. I found somme electrical pictures about the TBP-1.

http://pick.elbow.be/TBP-1/schalter.jpg

http://pick.elbow.be/TBP-1/electric.jpg

[nevetslab]

I still have the original manual and did read the section about the position of the switches - thank you. I learned that it is mostly the footswitch not the amp so I'll check on the footswitch first, you are right on that. I found somme electrical pictures about the TBP-1.

http://pick.elbow.be/TBP-1/schalter.jpg

http://pick.elbow.be/TBP-1/electric.jpg

Both useful documents, thanks for posting those. From the Foot Switch jack of the preamp, there will be an AC voltage, sourced by some resistor value. AS each string of switches on your foot pedal have steering diodes (one diode passing positive-going voltage, the other passing negative-going voltage, the combination of the switches will yield a unique voltage on this jack for the internal switching logic. I've attached the service manual for a Hot Rod DeVille for reference, which also contains the schematic for the two-button foot switch. There's a good description on the schematic for all of the switch combinations to help troubleshoot the switching circuit. Your preamp will be doing something similar, though more complex as there are two more switches to contend with. I've never seen the schematic for your preamp, but it too would no doubt have the rest of that detail.

Hot Rod DeVille Svc Manual.pdf

[dmeek]

Here's the complete service manual with a circuit description

https://music-electronics-forum.com/...1&d=1561470138

[nevetslab]

I still have the original manual and did read the section about the position of the switches - thank you. I learned that it is mostly the footswitch not the amp so I'll check on the footswitch first, you are right on that. I found somme electrical pictures about the TBP-1.

http://pick.elbow.be/TBP-1/schalter.jpg

http://pick.elbow.be/TBP-1/electric.jpg

With the complete service manual that dmeek just provided this morning, and a bit of patience, you should be able to navigate thru the test points and see what your unit is NOT doing with regards to the voltages provided at the test points for the Mute. I've never worked on this unit before, but have on countless Hot Rod series amps with channel switching issues, all in similar circuits, and Fender has provided very useful voltage readings per switch position. Happy hunting!

[LeeLuu]

A professional did check the foot switch now and found out that everything in the electronic is ok but the inscription of the outer body case is not matching the one written on the circuit board (picture)!
"Gain, FX Loop, Vari-Q and Mute" would be correct for the TBP-1 but the board inside indicates
"Channel Select, Drive/More Drive, Effects, Reverb". So my conclusion: they messed up at the production but the foot switch should actually work on a different amp (but which one?).
So my question is now: Any idea where I can get a right circuit board ore is there a simple fix/batch I can do on this board it works with the TBP-1?
To find an buy only the correct footswitch is like winning a jackpot.... :-/
Thanks for the extended manual!

[LeeLuu]

If there is anybody out there selling a TBP-1 foot switch - let me know.

[g1]

edit: nevermind
the board in that footswitch does not match what is shown on the schematic.

You could probably modify that one to work. It would involve changing some of the diodes and LED's, and probably cutting some traces and running jumper wires.