GK Backline 600 channel problem

[catstrat]

Hi everyone. I have a GK backline 600 on my bench and it is switching between normal and overdrive channels on it's own. I checked for cracked solder joints and changed out 2 j113 jfets and one tl 072 plus d4. Cleaned the footswitch jack, removed the pushbutton switch and tested the amp without it and you could see the distortion channel led coming on at will. I just tried to call GK and was met with a pre recorded message asking me to leave my mailbox number.WTH. I sure could use a schematic on the preamp section of this amp. I know some schematics are posted already but they don't seem to match what i have. Thanks.

[Jazz P Bass]

Are the specific channel LEDs changing when the problem occurs.
If not, I would suspect U5, analog switch.
By removing the channel pushbutton switch, that should have effectively killed any switching commands.

[MarkusBass]

Before changing anything, I would check the voltage that switches the analog switch U5. Maybe C21 gets shorted monentarily? Or R47 is not contacting?

Mark

[catstrat]

Hi. U6 is a 0612aa chip. I haven't checked with all data sheet yet but isn't that an analog switch? I changed c21 and corrected cracked solder joints at 0612 chip and resoldered r47 and no change. Are we sure the posted schematic is the right one? Let me check a few things and i'll get back to you. Thanks

[catstrat]

The Go612 chip is not listed as an analog switch with AllDataSheet. I really need some help now. Thanks

[Jazz P Bass]

All right .
Settle down.
There are two preamps on the schematic.
BL 250 & BL 150.
The BL 250 should match the BL600.
The schematic nomenclature does not match the board layout.
Sw3 is the channel switch.
U2 (layout) is the analog switch (marked U5 on the schematic)
It is directly across from Sw3 (towards the rear of the board.
Note:
Be mighty careful when working on this board.
The person who laid it out was insane!
The through hole pads are too small.
The connection pads are too small.

[catstrat]

I realized after the fact there are 2 schematics. The schematic shows a DG419DJ analog switch in the U6 position. I actually have a O612AA chip in that position so i'm guessing either that's an in-house number or a substitution. I got the pin out for the DG419 from Alldatasheet and did some voltage checks.Pin 4 is positive 14 volts. Pin 7 is negative 14 volts. Pin 3 is ground. Pins 1,2 6 and 8 seem to be signal carriers. Pin 1 is the common. Which pin activates the switch? Looking at the schematic i've checked everything in the positive voltage path and everything seems fine. There wasn't any dc voltage leaking between the pins that i could see but that doesn't mean the switch is good. Before i got the schematics i was shooting in the dark and i replaced U1 TL 072 and also diode D4.I touched up some of the solder joints which are in the switching circuit. D4 was a 1n4148 i replaced it with a 1n914. I haven't checked yet but isn't the !N 4148 a zener? Anyway, the led is still on constantly and could that LED be leakyThanks.

[Jazz P Bass]

I am confused about the IC's.
Which schematic shows the analog switch as U6?
The one I have shows that as a TL072 for the bass control.
Have you looked at the board layout that I posted?
Anyway, forget the numbers.
Find the Ch B LED. It gets it's power from the same point as the analog IC. (shown as U2)
Then the LED goes through the 22K resistor to U2 pin 6.
That is the input pin to the analog IC.
A low level input will turn Sw 1 on & Sw 2 off.
A high level Sw1 is Off & Sw 2 is On.
Thses are the signal routing switches.
Ch A signal (clean) or Ch B signal (distorted).
They have nothing to do with the LEDs being on or off.
The original problem that was stated was that the signal where switching by themselves.
I had requested whether or not the LEDs where switching or just the signal.
I have not seen an answer so I have no idea just what you are trying to fix.

[catstrat]

Thanks. Sorry. To clarify, the initial complaint were the channels were switching on their own. During test the distortion channel LED was flickering on and off and the channels were switching by themselves in unison with the LED.. The pre amp board in this amp has the analog switch Physically in position marked U6. I believe the 2nd and 3rd schematic that's posted show the same thing, if i'm reading it right. The LED should have nothing to do with the actual switching, i would think. I'm just grabbing for straws. I'm treating the analog switch as a relay and it takes two signal sources and routes them to the same point in the circuit depending on what position the analog ic is in. Only one signal gets through at a time. Am i right about this? It's either normally open or normally closed. This is kind of new to me. I'll spend a little more time with the schematic and try again tomorrow. Thanks for all your help.

[Jazz P Bass]

I just had one of these beasts that was doing the same thing.
Ch B LED & signal came on by themselves.
Odd was in Ch A mode the LED was just slightly on.
In my case something severe was plugged into the FtSw jack & blew traces off the board.
A new Sw3 & replacing the burnt busses (with wires) solved the problems.
Now, the circuit.
+15 Vdc through a 2.2K to the push sw & the jack switch, then down through the 221K to pin 6 of the analog switch.
With me so far?
At this point Ch A is On (because analog IC has a high on pin 6)& the LED is off.(it has no ground reference)
Grounding the push sw or the footswitch turns on the LED & sends pin 6 low.
The DG 419 has two switches.
Sw 1 & Sw 2.
Only one (in a good IC) is on at any one time.
As I said, a low on pin 6 will turn Sw 1 on & Sw 2 off.
A high level Sw1 is Off & Sw 2 is On.
You can prove the analog switch by lifting the 221K resistor from the LED to pin 6.
It should change from Sw 2 (clean) to Sw 1(distortion)

[Enzo]

When you get two symtpoms that occur together, like channel switchcing on its own AND LEDs blinking at the same time, don;t IGNORE one of them. You need to troubleshoot the problem. If the DG chip is what switches signal paths, then what CONTROLS that IC? And what CONTROLS the LEDs?

I am looking at the schematic, and I see U6 as the switching element, and I see the LED. The voltage to the control pin of the gate IC comes THROUGH the LED to get there. The footswitch or panel switch grounds the bottom end of the LED. That then turns on the LED, and also grounds off the control pin of the IC. SP that leaves us with possible switch and/or jack somehow intermittantly grounding that point ORRR when it is supposed to be grounding it, it sometimes loses continuity to ground. Hell, C45 could be trying to short, we could remove it and see if it makesd a difference.

But since the LED is a series element when the IC is activated, what if the LED itself is the problem? It could be intermittent, or its solder cracked. Set it so the LED is ON, then wiggle the LED. Does it flicker when wiggled?

I have attached the schematic I use. the BL 600 is one of the models using it.

[Jazz P Bass]

Rereading all of this I noticed that you had removed the pushbutton switch SW3.
As I stated the copper traced & pads are a tad thin on this board.
You just may have abad connection, torn pad, at the PB switch.
Specifically on the top layer.
The schematic that I posted (while being confusing) has the board layout.
You can see the upper (red) & lower (blue) traces.
I would prove out with a meter that the traced & connections are indeed intact.
The top layer traces go all the way over to C21 & R47 before going on to the FtSw jack (J5).
There is a through hole over at J5 that sends the upper trace down to the lower trace.
Make sure that is connecting.

[catstrat]

Thanks Jazz p. You mentioned something that i totally forgot and this board does have tracings on both sides. Very small tracings at that! Sometimes in an attempt to fix one problem you can inadvertantly cause other problems. At least that's happened to me before and it may have just happened again. For me what makes amps like this partcularly tricky is the board looks like it was soldered on a Wave soldering machine because you have the solder on both sides of the board and they are very sensitive to breaking a pad or a tracing just trying remove a componant. When i worked for Compaq computer, they would stuff the boards by machine and line the boards up on "set of rails" sort of, and turn on the machine and the boards would pass through molten solder that would solder in the parts. I think this would be easier to order a new board, if that's possible. These amps don't cost that much in the first place. I'll see what i can turn up and thanks for all the help.

[catstrat]

GK backline switching issue

Hello. I had to put the GK to the side so i could take care of other amps. Now i'm back on it. I did more test on it and i didn't find any cracked solder joints or damaged tracings, torn solder pads. I powered it up this morning and the thing i didn't mention before was the Distortion LED doesn't start coming on until the amp has been on for 9 minutes or so. It starts with a very dim,flickering glow and the longer the amp is on the brighter it gets until it is full bright after about 20 minutes. If i understand this right, U6 needs a hi signal to pin 6 to keep the amp in the clean channel. When the signal at pin 6 drops to certain low level the distortion channel is activated. I may be barking up the wrong tree and the problem may be something breaking down in the power supply to the switching circuit, like maybe a zener used as a regulator, and not the switching circuit at all. I'll test voltage at r15 over a period of 15 minutes and see what happens. I think i've done this but don't remember. Boys and girls, this is why we should write down our troubleshooting records when it starts getting involved.LOL. Could someone post the power supply schematic so i can have a look. Thanks for everyone's help. All of you are life savers.

[Jazz P Bass]

The amp that I had in for repair had very similar issues with the Ch B LED illuminating.
It was a bad switch.
The Ch B part was never fully open from ground. There was always some resistance to ground, which got worse as the amp heated up.
The switch I got direct from GK.