Behringer BXL 1800A

[Carlosraj]

Dear Friends,

An old schoolmate of mine brought this amp to me to help him fix it. I realize that the “Gain 2”, “ Shape “ and “Level” is not working, however the “Gain 1” is OK. The button between Gain 1 and Gain 2 is lit red and when pressed, it does not change. I guess it is stuck on Gain 1. I could not get the schematic for this amp. Could someone please advice, what could be the problem and where can get the schematic, Please help.

Thanks

Carlosraj

[Chuck H]

It seems that the switch simply isn't switching some stuff. It could be that the switch is lighting the led as an independent circuit but isn't performing it's intended signal circuit changes. This could be a switch failure, a transistor failure, an open power supply circuit, a broken board trace or pad, a bad remote switch (relay/LDR/transistor), etc. You only need to determine what's NOT happening at the switch itself to begin isolating the problem. The press switch is almost surely supposed to pass voltage to a remote switch circuit. Is it? If not then it's the push switch, broken pad, etc. Is there voltage entering the switch? If not then it could be the switching circuit power supply, etc. It doesn't seem like should be too hard to trouble shoot once you get your head around what's NOT happening and you need to poke around with a DMM to find that out.

[Jazz P Bass]

Behringer BX1200 Schematic

The closest schematic to the BXL1800 is the BX1200 (attached).

Have you tried the switching with the footswitch?
The Gain LED should only illuminate when Gain 2 is called for.

The LED's are controlled by a clock pulse on input pin 11 of IC 12.
When the input pin toggles between a high & a low signal, the output pins (9 & 8) should toggle the LED's in response.

Gain LED's.pdf

The actual switching of the signal itself , from Gain 1 to Gain 2, is controlled by the switches themselves, acting on IC 9.

Gain Switch.pdf

IC9 is a multiplexing IC.

CD 4053b __IC9.pdf

The very first thing I would suggest is to prove that the Gain switch works correctly.

[Carlosraj]

It seems that the switch simply isn't switching some stuff. It could be that the switch is lighting the led as an independent circuit but isn't performing it's intended signal circuit changes. This could be a switch failure, a transistor failure, an open power supply circuit, a broken board trace or pad, a bad remote switch (relay/LDR/transistor), etc. You only need to determine what's NOT happening at the switch itself to begin isolating the problem. The press switch is almost surely supposed to pass voltage to a remote switch circuit. Is it? If not then it's the push switch, broken pad, etc. Is there voltage entering the switch? If not then it could be the switching circuit power supply, etc. It doesn't seem like should be too hard to trouble shoot once you get your head around what's NOT happening and you need to poke around with a DMM to find that out.

Thanks Chuck

[Carlosraj]

Thanks Jazz, your info will be of tremendous help to me.
Thanks again

Carlosraj

[Carlosraj]

The closest schematic to the BXL1800 is the BX1200 (attached).

Have you tried the switching with the footswitch?
The Gain LED should only illuminate when Gain 2 is called for.

The LED's are controlled by a clock pulse on input pin 11 of IC 12.
When the input pin toggles between a high & a low signal, the output pins (9 & 8) should toggle the LED's in response.

[ATTACH]32346[/ATTACH]

The actual switching of the signal itself , from Gain 1 to Gain 2, is controlled by the switches themselves, acting on IC 9.

[ATTACH]32347[/ATTACH]

IC9 is a multiplexing IC.

[ATTACH]32345[/ATTACH]

The very first thing I would suggest is to prove that the Gain switch works correctly.

Dear Jazz,

I have not tried with a footswitch yet, will get one and try it out. There is one more problem I just realized. The ultrabass and it's switch is also not functioning. Just a guess, looking at the "Gain LED.pdf", I realized that both the gain switch and ultrabass switch (switch 2 and switch 4) are linked to IC 11 and IC 12. Could the problem be related to either of the ICs or the components associated with them? Need your advice.

Thanks

Carlos

[Jazz P Bass]

Easy enough to check the circuit.

IC11/ Pin 1 is the Gain input.
It should be High when the Gain switch is open & Low when the switch is closed.
If that is true, then look at IC11/ pin 2, output pin.
It should toggle Low/ High with respect to the inputs.
(IC11 is an inverter, so the output will be the opposite logic level of the input.)
http://www.diodes.com/datasheets/74HC14.pdf

The same holds true for the Ultra switch.
IC11 pin 3 & 4.

[Carlosraj]

Easy enough to check the circuit.

IC11/ Pin 1 is the Gain input.
It should be High when the Gain switch is open & Low when the switch is closed.
If that is true, then look at IC11/ pin 2, output pin.
It should toggle Low/ High with respect to the inputs.
(IC11 is an inverter, so the output will be the opposite logic level of the input.)
http://www.diodes.com/datasheets/74HC14.pdf

The same holds true for the Ultra switch.
IC11 pin 3 & 4.

Dear Jazz,

I tried with the footswitch but no difference. I have replaced IC12(74HC74), IC12(74HC14), IC6(CD4053) and IC5(CD4013). The rest are all op amps. Now the ultrabass and gain switches LEDs are constantly "ON". Pressing them doesn't change anything. While removing IC12(74HC74) pin 2 and pin 8 trace on the pcb came apart. I notice that this 2 pins are not connected to any other component on the pcb. Looks like they are unused. Pressing the gain switch or ultrabass switch doesn't show any changes on Pin 1, Pin 2, Pin3 and Pin 4 on IC 11. Any suggestions?

Thanks

Carlos

[Jazz P Bass]

Have you checked the switch itself?

I was rather hoping that you where going to troubleshoot the circuit, rather than shotgun all of the ic's.

If the footswitch or the front panel switches do not send an input to IC11 , pin 1 & 3, then you need to find out why.(the pins are held high & are pulled low by the Ft Sw or panel switch)

The FtSw jacks should both be at 5Vdc.
Check that.
If they are not, then you need to check the resistors & the supply (5Vdc) that hold those points high.(R127 & R128).

[Carlosraj]

Have you checked the switch itself?

I was rather hoping that you where going to troubleshoot the circuit, rather than shotgun all of the ic's.

If the footswitch or the front panel switches do not send an input to IC11 , pin 1 & 3, then you need to find out why.(the pins are held high & are pulled low by the Ft Sw or panel switch)

The FtSw jacks should both be at 5Vdc.
Check that.
If they are not, then you need to check the resistors & the supply (5Vdc) that hold those points high.(R127 & R128).

[/I]


I did remove the gain switch and checked that it does makes contact when pressed. I will check whether the 5Vdc is present at the switches. Just wondering, there is an optocoupler on this board. Do not know what is its function.

Thanks

Carlos

[Jazz P Bass]

The optocoupler (OP1) is used on the "Limit' circuit.

[Carlosraj]

The optocoupler (OP1) is used on the "Limit' circuit.

Dear Jazz,

There is 5V present at both the Gain and Ultra switch. On pressing the switch, the voltage goes to zero. When pressing the Gain switch, only pin 11 on IC12 goes from Hi to Lo but there is no change on all the other pins of IC12. On pressing the Ultra switch, pin 8 goes from Lo to Hi and pin 13 goes from Hi to Lo on IC12. There are no changes on any pin of IC 11 when either of the switches are pressed. I do not have a good knowledge in digital systems, only the basics.

Thanks

Carlos

[Jazz P Bass]

I think that you need to refocus.

Forget IC12.
IC 11 is the 'master'.

The logic is simple.
It is an 'inverting' circuit.
High in/ low out.

IC11 pin 1 & pin 3 are the inputs.
Pin 2 & pin 4 are the respective outputs.

If the Gain & Ultra switches have +5V on them & they are able to pull that +5V low, then you need to verify that that signal does indeed reach IC11.
It appears, from your reply, that the signal is Not reaching IC11 pin 1 (Gain)

[Carlosraj]

I think that you need to refocus.

Forget IC12.
IC 11 is the 'master'.

The logic is simple.
It is an 'inverting' circuit.
High in/ low out.

IC11 pin 1 & pin 3 are the inputs.
Pin 2 & pin 4 are the respective outputs.

If the Gain & Ultra switches have +5V on them & they are able to pull that +5V low, then you need to verify that that signal does indeed reach IC11.
It appears, from your reply, that the signal is Not reaching IC11 pin 1 (Gain)

Thanks Jazz, I will focus on that. I think its not only the Gain but the Ultra as well, because pressing either of the switches does not produce any changes on any pin of IC11. I will trace the circuit from the switches to IC11

Carlos

[Jazz P Bass]

IC12 is a flip flop circuit.

It will (when operating properly) turn on either the Gain or the Ulra circuits & LED's.

It can not do the 'flip flop' if it is not getting a signal from IC11 gain/ ultra outputs.