MK III mode switch

[gator7]

My back up MK III transmitter has a issue w/ a carrier that is not the spot switch stuck or not working. You can see and hear the spot come on when pushed. The carrier shows no RF output on the Meter. Its not the relay. It works fine.
But when I move the mode switch from am to side band. I loose the carrier. Also, the AM light will only light up when you key the mic. This makes me think I have a short in the mode switch?
I have removed all four wires from the spot switch. One at a time, and the carrier is still there.

Help!!

[gator7]

I guess no one has any ideas??

[BionicChicken]

Is it a carrier or just the meter needing to be zeroed? Does it interfere with stations you are listening to? What happens when you key the mike? One other thing...is the the transmitter meter or the receiver meter?

BC

[mark4]

Sounds like a funky mode switch.
Try cleaning it with a good contact cleaner that doesn't leave residue. That will just attract dust and cause other problems.
I have had great success using a knife edge exacto blade between the rivets on the mode switch. Saved countless switches that are still going strong years later. Catch it in time and you can get many years in allot of cases. But not all. These switches tend to break down from all the years dust and dirt. Invisible path ways start to develop between the rivets. Causing all kinds of problems. And when they totally break down poof! A dead short between the contacts and a burned mode switch! Not to mention other parts that it will take out meter-tanks and so on.

[gator7]

B C, It is a carrier that give about 20 db on the meter. When you push the spot, you can hear it, and see a small increase in the meter movement.

Mark 4, Thanks, I will try the exacto blade trick. I did clean the contacts. That didn't work.

Thanks to both of you.

Gator

[BionicChicken]

I have used (as a last ditch effort) muratic acid on the contacts. Dilute it down half and half (or more) with water and apply with a brush or spray bottle. I have used this on ceramic wafer switches with good results but above all keep off paint or plastic. Usually I use it when the switch is out of the equipment so there is no chance of messing up. It will take all tarnish off the copper contacts. Let it set 20 seconds and then rinse it off with contact cleaner, water, WD 40. You will need to lube it afterwards as it will remove all of it.
This is to be done at your own risk. I have done it many times and have not had a problem yet but the possibility is there.

BC

[gator7]

I am hopeful it will not require that extreme. But, I now know what to do if needed.


Thanks.

[2600]

Ahh, BC, the contacts on that switch are supposed to be SILVER plated. Removing the silver will require that you clean the switch about once a week. Exposed copper will tarnish and get noisy a LOT faster than silver does.

The most common issue is the brown bakelite wafer insulation breaking down so that separate circuits on the switch will "leak" current from one switch circuit to another. This is what causes the meter to peg when you select "Mod" on the meter switch Also will cause a half-Watt resistor connected to the 3-section filter cap to roast, every time you replace it.

I have tried grinding all the black carbon away from the bad spots on these, but my success rate was dismal. Even if it worked, it would be for a short time only. Once that brown plastic starts to go bad, there seems to be no stopping it.

73

[BionicChicken]

Ahhhh, 2600, the acid mix will not harm the silver coating with the exception of a slight discoloring. A diluted mixture is used sometimes to clean fingerstock on FM transmitters I am sure that you know. It's a shame that the EPA took away methylene cloride as a contact cleaner. It would remove most of the oxidation on these type switches.

It's been a while since you have been around here. It's good to see you come out of hiding and show yourself every now and then. I hope that things have been well with you.

BC