Soundlab G742BA2 Powered Mixer

[GRAHAM99]

I have acquired one of these but do not have any speakers for it.
Can I test the mixer without having speakers connected?
There are out puts for main mix but I assume this also sends power to the speaker sockets.
Do the speaker sockets need a load?

[Drew Stephenson]

Pop your headphones in and you'll be able to test pretty much everything except the final power-amp stage.

[Hugh Robjohns]

Can I test the mixer without having speakers connected?

Yes.

There are out puts for main mix but I assume this also sends power to the speaker sockets.

The power amp inputs come directly from those mix outputs, so yes, the loudspeaker outputs would be carrying a beefed-up version of the same signal.

Do the speaker sockets need a load?

No. Solid-state amplifiers like the ones in this powered mixer don't mind having no load connected. You'll do no harm by not connecting speakers... other than waste some power from the mains supply as heat.

What can cause harm is connecting speakers with too small a load, or even shorting the speaker terminals together.

Most valve amps will get upset and even go 'phut' if you run them without a speaker load connected, but that's not the case here.

[GRAHAM99]

Thanks for this I am much more comfortable now!

[GRAHAM99]

I have got the mixer section working by replacing some capacitors.
I need to test the speaker outputs but I only have one speaker cab.
Would it be ok to test each side separately, switching off before changing the jack plug from one output to another?

[Hugh Robjohns]

Yes.

[GRAHAM99]

Great!!

[GRAHAM99]

No signal at all from either channel!!
More investigations needed.

[Rene Asologuitar]

No signal at all from either channel!!
More investigations needed.

****************
Very interesting.
Pls post your findings.
Rene

[ef37a]

I would like to qualify the advice that solid state power amps do not need a load because of a couple of, I admit probably rare, experiences.

If you drive the almighty living **** out of the amp in SOME cases the resistor in the almost universally fitted Zobel network across the speaker output will cook. Poor design yes but 'it'appen. The problem was further compounded because the capacitor in the network was tight against said resistor and its casing melted then it shorted. Result, catastrophic transistor failure.

Such very large signals can result from acoustic feedback or an electric guitar feeding back or an input jack cable where the plug tip touches some random piece of metal.

This situation can easily be avoided with the OP's mixer by keeping the master faders that feed the power amps shut down.

The 'no load' issue with valves is I think well known but even here it is signal level that is the destroyer. Most well designed valve amps are quite happy without a load UNLESS you put a signal through them. In other words, provided you shut down the input, hot swapping a speaker jack will do no harm but of course, best not to tempt fate!

Dave.

[GRAHAM99]

I have got the mixer part to work by replacing a lot of the capacitors on the power supply board.There is still a slight hum at higher volumes though maybe more capacitors need replacing.I tested it via headphones and mixer outputs.
When I switch on the unit, the fan on the power amp board fires up very briefly and then shuts down.No output from left or fight side.There are two fuses on this board that are then blown.
A couple of capacitors look a bit dodgy so I will replace them and try again.
It is costing me a fortune in fuses though!

[ef37a]

I have got the mixer part to work by replacing a lot of the capacitors on the power supply board.There is still a slight hum at higher volumes though maybe more capacitors need replacing.I tested it via headphones and mixer outputs.
When I switch on the unit, the fan on the power amp board fires up very briefly and then shuts down.No output from left or fight side.There are two fuses on this board that are then blown.
A couple of capacitors look a bit dodgy so I will replace them and try again.
It is costing me a fortune in fuses though!

I suspect Graham that you are on a hiding to nothing there. Assuming the PAs are built using discrete transistors the most likely cause of fuse blowing is shorted power transistors but they will have taken out much of the rest of the devices in the circuit. Rebuilding complex DC coupled power amps can be soul destroying. Hours spent sourcing the right devices and soldering only to have the whole thing fail at switch on!

If you don't need the power sections I would simple remove the power from the amps. If you do want the PAs, strip out the old stuff and fit a ready made PA module.

Dave.

[GRAHAM99]

I got it for £20 including a flight case and I am using it to improve my knowledge of electronics!
I would like to get it all working again but I know what you mean about spending ages on it.
Would it be easy to source and replace the power amp modules?
I think they are 200w each.

[ef37a]

I got it for £20 including a flight case and I am using it to improve my knowledge of electronics!
I would like to get it all working again but I know what you mean about spending ages on it.
Would it be easy to source and replace the power amp modules?
I think they are 200w each.

A laudable aim G but do you really need 200W internal amplifiers? Do you have speakers that can handle that? You could get a pair of kits or a stereo amp kit from Vellemann or similar that could give you 15 or 20W per channel which would be easily enough to tell what is going on with the mixer. 15-20W per ch' into very modest hi fi speakers will knock spots off an FSTV sound system and drown a computer 'media' speaker system.
First thing is to isolate the faulty PAs from the supply rails them measure those. From that you can look for suitable amp modules although I suspect if the mixer delivered a GENUINE continuous 200W sine into even 4 Ohms, those rails will be too high for most lower power modules...Still! You want to learn!

I take it you have a half decent digital multimeter?

Dave.

[James Perrett]

The most important thing to check when thinking about replacing power amps is the power supply. Does it have +/- and 0V rails or does it just have + and 0V rails? What voltage is it giving out and do you know how much current it can supply (look at the fuse ratings for a very approximate guide)?

[GRAHAM99]

I replaced some capacitors on the power board and it now cuts out after about 20 seconds.
One of the capacitors was in a poor state.
Onwards….

[GRAHAM99]

I do have a digital multimeter!