No output from one ch on old JVC VN-300 audio amp in stereo mode, power stage seems ok as both ch work ok in mono mode.circuit diagram, anyone?

Stereo amplifies are just two mono ones joined together. So if you have a problem with the left you can always look at the right side if that is working good. As all the parts are the same and so are any voltage readings taken. A visual inspection is the first option, if something looks strange in the left channel compared to the right, then replace it with the same part you can see in the right. The only problem is when the amp uses one power amp for both channels. But in that case replace the thing.
Apart from that individual parts, such as the power supply, you can leave alone, as they would affect both channels.

is there Sound stage output is stereo if stereo then one side is missing or broken link from ic to speaker, If mono output then use with one side 2 speaker parallel.

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.

Generally mono outputs on this kind of mixer are usually balanced. This means all three poles of the connector are connected. The hot or plus signal (tip) will go to the left of your phones, the cold or minus (ring) will go to the right. This will be ok from a sound point of view But it may not have enough drive to make the phones loud enough.
If you only hear a signal in the left then the output is unbalanced.
A simple fix is a cheap headphone amp (less than 20 pounds) and a simple jack to two phono lead (wired as a split). There is a ton of old audio equipment about for very little money including some very small mixers, that have headphone outputs, that is what I have done a lot in the past.

very good ! there is nothig wrong with your system, buy a stereo to mono RCA jack (Rs10/-) and connect the subwoofer (if single coiled) L+ & R-- of your existing amplifier. NB: Please go through the manual of amplifier
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You, sir, are a rare adherent to stereo-only electronics. Are you replacing it for cause (showing its age)? The PM7003 is only 2 x 70 watts like the JVC but it also has separable preamp/amps so you COULD upgrade or augment the power output with external amps and still control it all locally.

Of course, any current AVR's do that too and bring digital sources and multichannel audio to the mix for as little as the PM7703 may cost.

All you should need is a "mono to stereo phono cable". The mono jack goes into your amp and the two stereo jacks into the sub.

Make sure the unit is on. Press the FM Mode /Mute -Demo button (right next to the power) press it untill you get to either mono or auto (-for stereo). That will take the unit out of the demo mode. next turn the music jog dial (located just above the preset buttons) untill you get to which ever setting you choose. Hope that helps. ( I can't find the manual on line either- not even from JVC! )

yes, it is a bridgable amplifier, if that is what you are talking about. Look at the speaker output terminal and it should have markings on it for stereo output and it will have a line between the 2 terminals to use for bridging it. When you bridge an amplifier it is mono. There may also be a switch on the amp for stereo or bridged. Put that switch in the bridged mode. Connect one speaker to the speaker output and you have your bridged mono amplifier. If you plan on using 2 woofers, it is better and safer to use it in stereo mode with one speaker on each channel.

Let me know if you need any more help,

Dave

use channel 1 and 2 as normal, use the positive from channel 3 and the negative for channel 4 as mono 1ch, your bridged output on the mono chan should be around 200watts at 4ohms.