Behringer GM110 permanent low volume but surge in volume on switching off power

[DIYGuy]

My problem is that my Behringer GM110 amp is very soft in volume. I have tried all the things Enzo suggested in the old post ie check the jacks, use the fx loops etc. The problem seems to be with the power section - I have also used the preamp send to another amp and it works fine. I have basically checked all the caps (with an esr meter), the resistors and and even replaced the 4701 chip but made no difference. The only thing that I have not done is test (don't know how) the 074C smd op amp but plan to replace it anyway - could this be the cause?

When I switch off the power, there is also a momentary surge in volume. Any help would be most appreciated

[Enzo]

You reported the line out was diminished, and that comes off before the power amp. So I suspect your TL074 may indeed be the problem, particularly the section pins 10,9,8.

[DIYGuy]

DIyGuy

You reported the line out was diminished, and that comes off before the power amp. So I suspect your TL074 may indeed be the problem, particularly the section pins 10,9,8.

Is there some way to test that on circuit?

[Enzo]

Pin 8 is the output for that section. Is there unwanted DC offset there?

Considering they only cost a few cents and my labor sells for a dollar a minute, I'd replace it instead of spending half an hour trying to test it. But that is just me.

[DIYGuy]

Pin 8 is the output for that section. Is there unwanted DC offset there?

Considering they only cost a few cents and my labor sells for a dollar a minute, I'd replace it instead of spending half an hour trying to test it. But that is just me.

I'll check for DC and get back. Basically, I am just trying to learn about electronics as I go through fixing guitar amps hence my question about testing the chip. I have actually ordered the part a week earlier (waiting for it to arrive) and was going to replace it anyways and yes, timewise it is not worth the time to repair it but from my learning perspective it is worthwhile.

Once I get the part and replace it, I will let you know how I get on and in the meantime, I will test pin 8 for any DC. Do I expect to see any DC? And if so what does it mean?

[Enzo]

No, I expect NO DC offset. If it is there, we suspect the IC. Unless there is also some DC on the input pins.

[Mick Bailey]

Opamps fail in a number of ways but the usual ones are;

1. They become noisy and produce rustling, frying or crackling noises - sometimes intermittent. The trick is to eliminate the possibility of noise on either of the inputs. If the inputs are quiet and there's noise on the output, the opamp is on its way out.

2. They can short or present a low resistance to either or both supply rails. Usually the offending device will get warm or hot and can pull down the supply voltage. Other times it can be more difficult because it may not get hot enought to feel much different to the touch. Because all your opamps are likely to be strung out in parallel across the supplies it can be tricky to identify a problem.

3. Opamps amplify the difference between the inputs. In an audio circuit with no DC present on the inputs, there shouldn't be any DC on the output. As the opamp is imperfect by design, there may be a few mV but no more. Any significant DC means the opamp is most likely to be faulty, but there are a couple of things to watch. Many opamps will go into saturation if both inputs are shorted and the output can be driven close to the supply voltage, either + or -. Also, many opamps will latch up if the input is driven too close (or exceed) the supply voltage and give DC on the output.

If you're looking at pedals or other devices that use a single supply (say, 9v), one of the inputs will be usually biased at 1/2 the supply voltage (4.5v) and the output will be at that voltage too, with a blocking capacitor to prevent DC from affecting the next stage. This is to give a mid-point so that the signal can swing both positive and negative.

4. They can just go dead.

[km6xz]

If is sound normal when it raises in level when the power is turned off, I would suggest the opamp is fine. I would search for a muting circuit problem. I do not have the print for that model so do not even know if is has a mute circuit but it is being biased off for some reason and when supply voltage begins dropping signal returns to normal briefly. Unless you really are good at SMD chip desoldering and replacement it is too risky to willy-nilly replace parts just for the heck of it. You do not have to guess that a chip is bad, noticing input and output will be enough to tell whether it has a problem.
Do you have a scope? If so, disconnect the speaker and monitor the output using the scope with no load until you determine it is working properly. That is to protect the speaker and output transistors if a problem in the preamp is being probed and overloads the output amp/speaker. If you have a scope and can display the schematic for us, one or two quick measurement points can pinpoint the problem in the preamp.

[Enzo]

From the INSERT RETURN (FX return) the signal path goes through one op amp buffer stage and directly to the 4701 power amp IC. Ther is no discrete mute circuit. The LM4701 itself does have a mute pin though, pin 6. The circuit has a pullup (pulldown?) resistor to V-, R11 4k7. And also a cap up to V+, C2 47uf/50v. So when first powered, the empty cap holds pin 6 at relatively positive, and when it charges, R11 pulls it negative to operate. That is a turn0on thump mute. Could be involved.

However that mute only affects the PA IC itself, so if the line out is affected, I still like the TL074 stage. The line out is a balanced out driven by a dual op amp, the signal coming out of the same pin 8 the signal to the powr amp does on the TL074

http://www.ti.com/lit/ds/symlink/lm4701.pdf

I'd run a signal into the INSERT RETURN and see what it looks or sounds like at pin 8 of the 074, and at the input of the 4701, also pin 8.

[DIYGuy]

If is sound normal when it raises in level when the power is turned off, I would suggest the opamp is fine. I would search for a muting circuit problem. I do not have the print for that model so do not even know if is has a mute circuit but it is being biased off for some reason and when supply voltage begins dropping signal returns to normal briefly. Unless you really are good at SMD chip desoldering and replacement it is too risky to willy-nilly replace parts just for the heck of it. You do not have to guess that a chip is bad, noticing input and output will be enough to tell whether it has a problem.
Do you have a scope? If so, disconnect the speaker and monitor the output using the scope with no load until you determine it is working properly. That is to protect the speaker and output transistors if a problem in the preamp is being probed and overloads the output amp/speaker. If you have a scope and can display the schematic for us, one or two quick measurement points can pinpoint the problem in the preamp.

As a real beginner, I am only learning as I go. For a start, I have had no success in getting the schematic after trawling through the internet, which is what I normally do in the first place. I don't think it is easy or even possible to get schematics for Behringer amps. And when it comes to desoldering SMDs, it will be my first time....

Yes, I have recently purchased a scope with the intention of learning how to use it so again another first. Plan to trace the signals using a wave generator with an electrical engineer friend in the next few days based on what you and Enzo below have advised together with looking for the DC offset, if any. Will keep you all posted.

[km6xz]

The 4701 has a mute function but it is 80-120 db attenuation plus is too late in the signal path. A lot would be explained by seeing the signal at the TLO74 pins 1, 7 8 and 14 with a DC coupled scope. I have lots of Behringer schematics but not this one. If the signal is weak but undistorted there are fewer variables, and the IC is not likely the problem if it remains undistorted in the operating and in power supply sag when turning off. If there is DC offset, that clears as the supply drops, it would indicate the opamp is doing what the input of the chip is telling it to do.

Is it anything like a GMX110? The GMX110 I have info on but it may be a more complex amp since it uses a TDA2050 50 watt power IC and the input of the preamp has a muting circuit activated when the 1/4in phone plug is removed.

[Mick Bailey]

Sometimes troubleshooting an opamp can be as simple as touching a probe to either of the input pins then the output pin in turn. Not shorting them, you understand - just touch an ungrounded lead to the pin and hear if you get a buzz on the equipment output with the induced noise. The 'disturbance test'.

[Enzo]

Sorry, GMX is a lot different. I have been looking for an online place to link for this schematic. I can't post mine.

[DIYGuy]

Hi Enzo,

Tried to replace the chip but in the process damaged the contact point below pin 10 so there is nothing for pin 10 to solder onto. Can I use a wire to join that pin to the next component where it is meant to go to? If so, which one as I have no schematic to refer to. I would like to test continuity for each of the pins to the next component each pin connects to - can you help on that?

[Enzo]

Uh... I'll have to get out my file again.