It might be dirty contacts. Give it a shot of canned air before you do anything else.
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07-30-15: PtssNo, not from the front. Contact cleaner won't be effective unless it reaches the contacts.
Also, if the housing of the switch is completely sealed, with no slots or other small openings, spraying it from inside the preamp won't accomplish anything either. In that situation probably all you can do is to toggle the switch up and down many times, and see if that helps. If it doesn't, the switch would probably have to be replaced.
BTW, if and when you spray contact cleaner into whatever small openings may be present on the housing of the switch, there may be a temptation to look closely at what you are doing. Keep in mind that some of the spray will splash back from any surfaces it comes in contact with, and into your face and your eyes if they are too close. It may also emerge in a strong stream from any openings that may be present on the switch in addition to the one you are spraying into.
Thanks Al. This switch is on my treasured Spectral DMC20 preamp I've had since new. I do use the switch regularly as I'm sensitive to having correct phase in my Spectral/MIT system. I've called Spectral but I think they're on holiday as I reach an after hours message although I'm calling during business hours. Would it be suitable/effective to use just a drop of the Caig ProGold Gp5 that I've used for years for contact cleaning generally?
I suppose it would be reasonable to give that a try, Pete, assuming as I said that there is a way to get it inside the housing of the switch. I'm not completely certain, though, as Caig's D5 is more specifically intended to dissolve corrosion, while the G series is intended mainly as a contact enhancer/conditioner/protectant for contact surfaces that are already mostly corrosion-free.
The Toggle Switches on the front panel of your DMC20 are totally sealed, no amount of whatever liquid you spray on them will ever get onto the contact surfaces. Toggle Switch as an example, there are literally 1000's of these in many different configurations.
The only thing you can do is to replace the switch, which appears to be somewhat easy if your tech savvy, if not take it to someone whom is. The switches appear to be mounted on a PCB rear-mounted to the faceplate using the switches as standoffs / mounting hardware.
Spectral DMC20 Internal picture
Best of luck
In studying the picture of the DMC20, I don't think the toggle switch has anything to do with your channel dropout issue. The switch on the front merely control relays that do the actual phase inversion of the output, so its my somewhat qualified guess that one of the relays, orange cubes located towards the rear of the PCB to the right behind the output terminals, or the two located in the center just behind these are at fault.
Best of Luck
Kind thanks to all; particularly to Peter of Pbnaudio for the photos and explanations. I do remember now that the relays for phase switching were a noteworthy feature in the literature for the DMC20. I have zero tech savvy and a high respect for the design and build of my Spectral. I would prefer not to incur the risk of shipping to Spectral but have no idea of how to find someone in Vancouver,BC or perhaps as far away as Seattle, who could replace the relay 'if' I can obtain a part from Spectral.
I was very lucky (thanks to kind advice from Michael Percy, the well known and highly respected parts supplier) to obtain my Spectral DMC120 and DMA180 and early MIT i/c's and speaker cables as a result of an insurance settlement from a break in. Otherwise the cost was prohibitive. They have provided "exquisite" enjoyment for about 20 years now and power conditioning improvements from Equitech,MIT and Sound Application have yielded such dramatic refinements I remain satisfied with these treasures and wish to keep them impeccable. Aha! I'm here convincing myself I'd best return it to the good folks at Spectral! Yes; thanks to your inputs I believe I have come to the best decision. Cheers to our great group here at Audiogon!
Relays are readily available from for an example Mouser, Here is a suggestion, but need a little more information to find the correct type, which can be had from many different manufacturer of relays, which is a commodity. Although someone would like to think that they need to be infused with Audiophile Pixie Dust to sound "just right" :-)
Any reputable repair tech can procure a relay that would work, from suppliers like Mouser, Newark, Digikey etc. That where folks like Spectral and all the rest of us get them from.
Best of Luck