My legacy LA960 has all neccesary connections made but no voltage present @ either channel output

Hi, When an two channel (stereo) amplifier is connected in bridge mode the polarity of one channel is reversed, for this example say channel B. An identical (mono) signal is applied to both channel inputs.
The output is now connected between the two channel +ve outputs. As channel A's output goes +ve , channel B's output goes -ve, thus doubling the output voltage.
On this Behringer powered mixer, the bridge switch simply connects the power amp section inputs, the required way.
The bridged output is wired to the B output between pins 1+ and 2+ of the 4 pole Speakon connector. (Because usually pins 1+ and 1- are used, stereo operation is unaffected.)
So.... If you connect a speaker from the A output alone then I would expect it to work, BUT if there is already a speaker connected from the bridge output, then what happens will depend on the amplifier design.
At best the A channel will see too smaller impedance, and hopefully shutdown.
This is a Behringer, it has digital (class D) power amps, and it does Not claim any amplifier protection !!!
If the powered mixer works in normal stereo, then it is probably OK.
If it then does Not work in bridge mode it is likely to be the connector wiring.
All the best.

Gain access to the electrical connection with voltmeter set to 110 v ac check for voltage on both sides of switch. If voltage is present on outlet side follow output wire to start relay ck for power on out put side of relay. if power follow output wire to motor if voltage is present replace motor ! Be careful where you touch when probing contacts !

There could be other electronic components failure



• need to open again the unit

• get a multimeter and measure the DC voltages present on

right amplifier output stage.

• it should be no more than 0.7 V, if it s, remove those pair of power

transistors and test again for output voltages.


the negative probe

of the multimeter to the chassis of the unit.


• if the problem goes out ( more than 0.7V ),

assume one of the extracted pair has gone bad.


you can test it individually since it is already outside the circuit.




• Check for VAS circuits and Differential Input Stages


Let me know how you make out with the inspection.

It is a good chance you will find something.


hope this helps, thanks for using Fixya!

All you need to do is to get a multichannel TUNER which can be fitted to the present postion in the mother board. Make sure that you remove the present TUNER and ask for a HYPER BAND TUNER. The only issue here is that you will have only 29 positions to the memory and so that makes it important to only MANUALLY tune the required ones to the 29 positions.
The other solution is to add on a 200 channel expansion processor with tuner board to the existing motherboard. You will need to connect the voltages, the vertical and horizontal feed for the OSD- on screeen display and the IF, AFT outputs.
The last option is to get a set top box to expand the channels which comes with a remote as the output can be connected to the TV video/audio inputs.
Just contact any electronic store for the above mentioned units.
Thank you for using fixya.

I suspect one of the upfront IC amps has been damaged. This can happen if people power the mixer from a receptacle that is different from where other connected equipment is powered such as amp. For example, mixer is at one end of the hall and other inouts such as an output of a guitar amp is plugged in. If a ground fault or spike occurs, teh ground bounce can exceed the common mode voltage of the input amps.

In your case, the inpout amps are discrete transistor types.

A voltage surge into the inputs of your unit would LIKELY pop two bypass caps that are 330pf. For channel 2 these are C205 and C206. Check those for shorts.

VERIFY audio is available at the "send" (tip) of the channel insert connector... this verifies the preamp stage. You can PROBABLY plug a set of headphones in to check this... ONLY one side will have sound of course. Verify this is a good test by checking a working channel. The level will be quite low and we are looking for just some signal.

IF you do have signal on the channel insert send which is the "tip" of the channel insert with the trim turned up, let me know and I will check the schematic for the next thing to check farther down the line.

The cables from the amp to the speakers could be shorting out, the speaker(s) could be shorting or going bad. The impedance of the speakers could be to low for the amplifier. The amp could be overheating because of any of the above. Turn the power off and try disconnecting one speaker, make sure no bare wires are touching. Reapply the power and observe. If it still does it , reconnect the speaker and preform same test to the other channel. The previous test can determine the side that has the problem. If the problem is still present the speakers and cables need to be checked with an ohm meter. If the cables and speakers check out then the amp speaker outputs need to be checked to see if there is any DC voltage present. If DC voltage is present on the outputs then the amp output section needs to be repaired.

you have an unloaded channel,so the voltage is exceeding what the protect circuit is supposed to see.you have to connect an 8 ohm resistor across the output of the unused channel.
do not drive a signal into it or you will smoke the resistor.
a 10 watt will do ok with zero signal input

Unit is going into protection mode because the amplifier output stage is shorted. Measure output voltage of the speaker lines coming out of each output channel...front/center/rear...take measurement before the channel speaker line protection relay. If any voltage is present then that is the output channel that is at fault. Replacement parts are available to the consumer through tritronicsinc.com. If you have experience with a volt meter\checking transistors\soldering it is something you can do. If not it should be left to a service shop. Have a great day!!

barneyluc

If you've got a standard 4 speaker system (2 front, 2 rear), your best option is to connect both left speakers to the left channel amp output and both right speakers to the right channel amp output. The only thing you lose is the ability to fade the speakers front to back.

check all op amps for correct voltage swing +-should be equal,test output of op amps no voltage should be present, use chasis gnd as tester neg.check voltage regulation especially 12v or 15v +-.note some amps show unequal led indications when only 1 channel connected to a load.I will try to get some more info on this specific amp because i don't know this circuit well,keep in touch as you update though.