Channel A and B left side output has no output and nothing but a buzz on the earphones for one channel - assume left channel Meter for left side shows no level. Checked the 2 side by side fuses and both are testing Good. Right side works good.
Inspection shows no burned parts. Circuit board looks good. Rotary switches Look good. Volume controls measure same Resistance
Two variable resistors on the board measure about the same Resistance between terminals.
Do you have schematic or suggestions on what to test next?
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Re: Optronica amp one channel out.
Disconnect any input cable and test-----------if you still have a problem connect your right speakers cables(which supposed to be good)instead of left speakers cable and test------ if the problem solved you will have a problem in left side(cables or speaker)-------if not solved you will have a problem in your amp it self. and needs hardware repair.
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As the channels are identical you can use the good channel has a guide to the bad one. First thing is to look around see if anything does not look the same as the right channel.
If nothing obvious stands out, with the power on, using an insulated screwdriver touch the two channels one at a time. Keep away from the power supply and anything that is common to both channels. The channel will buzz once touched. Depending which points you touch them. The right channel will tell you if it should buzz at that point. Work backwards from the speaker outputs if you can. If you hear a buzz on the left channel then it is working at that point, if it produces the same sound on the right.
Using this method you should track down the part that has gone.
As stereo amps are two mono ones put together you can use the good channel as a guide to tell you what the bad channel is not doing. You just need an insulated tool. With that and the amp on touch the left and right channels with the tool. Keep away from the power supply parts as it's nothing to do with them. You will hear a buzz if the channel works. Remember if it buzzers in the good channel it should buzz in the bad one in the same spot. You should soon find the faulty stage and part.
Did you try to switch the loudspeakers? Before changing anything always lover the volume and switch off the amplifier. Assuming you tested already different inputs, you can be sure no cable from signal source is out or defect. (or did you nit check that first?). When the amp is switched off, disconnect the speakers and switch left to right and right to the left output.. If the other speaker is quiet now, you are sure it is in the amp. Still his can be anything. You can check the connections between the pre and main amplifier on the back side. When you have an extra set of interlink cables, perhaps you can cross the in and output there too. Take care every time you change something, lower the voltage and switch off the amplifier. Sudden clicks when switching on or changing anything can damage the amp and or the speakers.
Perhaps when you checked this you can locate where the problem is? Speaker, speaker cable, power amp, pre amp, (only a part of the pre amp.
Yes amps buzz when something gets near them - say a pen or probe and if you know what it is safe to touch a finger! Fortunatly for you the left channel is the same as the right. So if you believe it's the pre-amp work backwards from the where the power amp connects to the pre-amp, going through the pre-amp stage by stage - prodding the parts. Remember a buzz in the right a buzz in the left! No buzz in the left but one in the right fault found! Ignore the power supply it's nothing to do with that as it's common to both channels.
If you are saying that it only works on say the left channel in stereo mode, but both channels come on in mono, then there's a fault in the amplifier's pre-amp. These days they are an IC, it can be traced by following the input wires to the point on the PC Board and then looking around for an IC with all inputs going in. If you are lucky there might be two! One for each channel. Replace the one on the dead channel. You can test it, either the single IC or both, touching it's pins with an insulated tool while the amp is on. It will buzz on the channel. Stereo amps have two sets of identical parts, except when an IC is involved. But even then that will have two halves (inside it). You just need to touch the pins of the IC, with the tool, but don't short the pins together! You should hear at least 4 buzzers. One at the right input, one at the left input, one each at the left and right output. If you only get the left an right output buzz, plus the good channel input buzz, you know it's the IC that's gone.
The good thing about stereo amps are they are actually two mono amps joined together. So when there's a fault in one channel you have the other channel to compare it too. For example the voltages should be the same. You can also touch the working parts with an insulated tool. When you do it will buzz in the channels in places. So if it buzzers in the good channel in a certain place then it should in the bad in the same place. You can search backwoods or forwards to determine where the signal is lost in the bad channel. It should take you to the culprit part.
Don't bother with the power supply parts as they are common to both channels. Just buzz test the bad channel and compare with the good.
You have a blown channel. If you are experienced with electronics repair, use the right channel to assist in diagnosing the left. Both channels should be efectively identical. In most cases, the defective parts are the output transistors. You should be able to A/B the output amp sections.
Feed the amp with the left channel preamp to the right channel input on amp and check output.
If you get output from amp then pre amp nedds to be checked out if still no output then amp is suspect.