This will be the 3rd Marantz time I've had this problem on personal units. If you have something similar happening when playing your unit and it is driving you nuts you are not alone. It seems like a simple issue at first glance. You're thinking it stems from a dirty switch, push button or slide type, maybe single ended (RCA) jack or speaker cable connection. Check all these things, get the can of Deoxit out, clean all switches rotating, lever, slide, or push button types. Next the volume, balance and tone sliders. On the 1200, the 10X gain switch gets a shot of Deoxit as well as the preamp to amp jumpers and sockets. When doing all this, the issue is still there on the left channel, it 's time to go to the next level. Remove the retainer and unplug each pc card. They will disconnect from the motherboard. Do then one at a time. Clean the male end on the cards, and female connectors, then reinsert when done.Next, it's time to clean all the single ended connectors on the rear panel including , speaker outputs, I found broken grounds where the metal outside single ended jack breaks then loose continuity, when moving of rotating whenever the cable is moved or twisted. Intermittent loss of signal is still present. Time to pull the schematics out for the unit. I have them printed and scaled up from smaller 1 or 2 page fold out, to 24" by 36" single sheet, printed off a PDF file by the local Office Depot.Keep them stored in a poster tube. Now you can see everything really well. I easily followed the signal paths from the inputs forward. To my surprise and embarrassment, I discovered where the signal path travels thru the Dubbing Jacks on these units DOH! This applies to all models that have Dubbing TRS 1/4" in and output jacks. The jack terminals break contact whenever a jack is inserted. The dubbing signal both input and output feed the preamp. They do not the mix dubbing signals in and out with rear in/out jacks simultaneously as I used to think. These connectors are important because when you normally use the unit, no jacks plugged in to dubbing . The signal passes across small delicate spring loaded tin tab connectors for the left/ right channels and ground, both input and output. These connectors are not shielded or sealed when nothing is inserted into the jack, thus they are totally exposed to the environment in the unit, in your house, cooking grease, pet dander, humidity, pollution, pollen, dust dirt etc. when windows are open,...you getting the idea?. It doesn't take much buildup and tarnish to break continuity of these little buggers. After 40-50 years, more than enough tarnish or buildu[ is there, and it's even worse on units with unused jacks which still corrode naturally even when unused. In my situation, the audio level loses gain on one or both channels, the issue coming and going intermittently. You may not notice it, simply turn the volume up more, or adjust the balance. That sort of works until a channel stops working altogether, or drop outs intermittently happen all the time as mine did. After checking everything else, the schematics took me straight to the Dubbing jacks. You hardcore Marantz guys knew were this was leading from the first few sentences. Clean Clean Clean those dubbing jacks and connectors. It is easier to bench the unit and pop the top instead of spraying deox blindly into the holes then inserting the Christmas tree scrubbie stalk. I did mine benched from inside plus the 1/4" Headphone and Mic jacks. Doing it on the bench )or dining table kitchen counter if you don't have one) keeps the inside of the unit cleaner. Put a paper towel or rags all inside to stop excess from pooling on the the bottom or overspraying, getting crap on the other pc cards, wiring case, etc. Results don't lie. The unit sounds better with connections clean and tight wherever they connect or make contact. For those who never use dubbing jacks, you could install a jumper for each contact,eliminating a possible failure point as well as eliminating the need to deep clean the connector contacts every few years. Speaking of contacts and points of failure due to buildup on contacts; any of these units with the speaker protect relay board, the relay points tarnish and burn over the years. The failure type is the same, only this time it's affecting the power side of the audio signal not the preamp side. These relays are hard to get hold of today NOS prices are high WHEN you do find one. It's best to remove the board and desolder the relay. Doing it otherwise you may damage other components close by. Using a small relay burnishing tool, clean the points and contacts. Next, use a piece of cardstock the thickness of a business card, dip one end into 99% denatured or isopropyl alcohol. Carefully insert between the points and fixed relay contacts, applying very light finger pressure the contact arm in order to squeeze the card between the movable points and fixed contacts. Straight up and down, or side to side motion is fine. Continue using additional pieces dipped in alcohol, until the paper stays clean. If you are not familiar with working inside these units or working with high voltages and capacitances containing LETHAL VOLTAGES that can harm or kill you, it is recommended you leave the job to a pro. Hopefully this will save someone some time chasing intermittent loss of audio signals or reduced levels. If nothing else, you can clean the Dubbin jack terminals if not done before. ZEven if there is nothing wrong today, you will be eliminating a failure tomorrow. If your model Marantz is more that 30 years old and has these TRS Dubbing in/out, Mic, or HeadPhone jacks, clean them along with the single ended jacks on the rear. Consider this preventative maintenance. I would recommend cleaning all connection once a year, if possible. Poor connections or bad interconnect, even speaker cables are one of the highest failure points for any audio gear.