gk backline 600 not working right after replacing jacks

[electic]

I have a backline 600 with corroded jacks. It was working fine before I decided to do what I thought to be a simple task, replace all the input jacks in the preamp section. I can plug into the RETURN and get good sound and volume, bypassing the preamp. When I plug in the INPUT, its faint, and none of the tone knobs work in either channel. I've also tried plugging the bass in the input, and connecting the SEND to an external amp. Loud buzzing comes from the backline when I do this and a the bass comes through the other external amp, but with the same symptoms, no response from the tone knobs.

I used the desoldering copper to remove all five jacks, replaced them with new ones I order from GK parts online store. My first try was a real failure due to crappy soldering. I went back, and tested each jack, and think I've resolved my issues, but not quite sure. My question is, where can I start, from a diagnosing perspective?

[TomCarlos]

Have you tried to put a patch cable from the SEND to the RETURN and test the amp using the Input?

[g1]

Is this double-sided circuit board?

[electic]

Have you tried to put a patch cable from the SEND to the RETURN and test the amp using the Input?

I just tried that, with all volumes turned down to 0, i can hear the guitar pretty loud, and as i turn up the preamp channel b volume, it gets louder. still no response from the tone knobs

[electic]

Yes, it is. In my tracing/ testing, I tried to trace the leads on both sides of the board. I was able to successfully trace both tip and ring of each jack with a cable plugged in their individual components to each bus

[nosaj]

Yes, it is. In my tracing/ testing, I tried to trace the leads on both sides of the board. I was able to successfully trace both tip and ring of each jack with a cable plugged in their individual components to each bus

I suggest using an audio probe to trace where the signal gets bad. Could still be at the jacks, Pcb could've sustain'ed some damage due to "crappy Soldering"?
http://diy-fever.com/misc/audio-probe/
nosaj

[TomCarlos]

I should have asked earlier, and just as a double check, you used the correct replacement jacks for the Send and Return? Some jacks have a switch on them, so I want to rule out having used the wrong type of jack.

[Jazz P Bass]

If the tone controls no longer function I would suspect that you lost a ground trace that they rely on.

That is an old tube amp, ala Fender, 'boost' trick.
Disconnect the ground connection to the tone stack.

[J M Fahey]

GK typical tone controls are cut-boost active feedback type, not passive like F/M/V types, so the ground open trick does not work here.

Thgere might be an open track though, just not necessarily "ground" .

Also if I remember right tone controls use 3 Op Amps: 1 for Bass/treble Baxandall type controls and 1 each for Lo Mid and BHi Mid active.
IF the latter have a frequency varying knob, then they use 2 Op Amps each.

My point being that since itīs a complex active tone control, rather than a "simple" cut track we might have some problem affecting the whole tone control block.

Some amps offer a "tone bypass" option, Eden or SWR comes to mind, maybe modern Backline series incorporates some of that; should check user manual.

[g1]

Yes, it is. In my tracing/ testing, I tried to trace the leads on both sides of the board. I was able to successfully trace both tip and ring of each jack with a cable plugged in their individual components to each bus

I hope you mean you traced to each bus on both sides of the circuit board.

Can anyone post the schematic/service manual?

[The Dude]

Backline 600.pdf

[electic]

[ATTACH=CONFIG]54121[/ATTACH]

Thanks for the schematic, this was really helpful. All of the jacks are correct part number. I've learned that I botched 4 of the five jack soldering jobs, and damaged 3 pcb contacts, specifically related to grounding on the send/return and input side. I've repaired the pcb contacts, traced and corrected the solders of the input, send, and tuner jacks. I had to remove the RETURN jack, the schematic was a little bit different than the PCB, but I figured it out. I resoldered, more carefully this time, and the thing works again. I really appreciate everyone's help with this.

[DrGonz78]

Those boards are very difficult to work on and are just a real pita.