Surgery on AM radio to fix on-off switch

[dmjlambert]

OK here I go opening up my AM radio. This is the 67-72 style stock AM radio. My truck has a mangled dash that I'm going to repair and I bought this radio on eBay to fit in the new hole. I bench tested it and it will not turn on. So I removed the top and bottom plates and having a look see.

[dmjlambert]

The 12V+ comes in on terminal 1, then goes over to this thin rectangular device attached to the case on point 2. I think that is some sort of filter. It does not ground the wire, but it is close to ground. Perhaps some sort of spike protector. The other side of it is a blob of solder on the case outside under the label. From there it goes to the switch that is on the end of the 2 gang potentiometer, point 3. When the volume knob is turned up to click the radio on, I should get 12V on point 4, but I do not.

If I connect 12V+ directly to point 4, the radio tests OK, it just won't turn off without me disconnecting the battery.

There are 2 wires connected to point 4, to go to the radio light bulb which goes outside of the case and around back of the radio on the brown wire (5B) and to go to the choke (5A). Coming out of the choke the red wire 6 goes to one of the terminals on capacitor 7, visible from the top side.

So that rectangular plate, choke, and capacitor appears to be noise suppression and filtering at the input power supply.

It looks like I would have to unsolder about 5 or 6 connections on this gang potentiometer and switch before I could remove it to get to work on it. There may be some tabs I can bend to take it apart and examine the inside of the switch. Maybe I can remove 50 years of tarnish and varnish and make the contacts work.

[Dead Parrot]

There used to be stuff called "TV Tuner Cleaner" in a spray can. It was great for things like this as it was designed to clean off moving contacts. It usually wasn't a permanent fix but if a spray on a suspect part made things better, you at least knew where the problem likely was.

Not sure what a similar thing would be called today. "TV Tuner cleaner" pretty much died off with the old CRT TVs.

[Ziegelsteinfaust]

Wouldn't contact cleaner be close to the TV stuff?

[dmjlambert]

I will need to find a modern equivalent. For the on-off switch contacts WD-40 will probably be good enough or perhaps carburetor or brake cleaner. I need to see what it looks like when I get that pot assembly removed and can make a close examination of it. I saw on another post forum member truckin 79 recommends DeoxIT D5 and I remember seeing a mention from somebody about CRC QD Electronic Cleaner. I may get a can of something like that.

[RustyPile]

Originally Posted by dmjlambert

I will need to find a modern equivalent. For the on-off switch contacts WD-40 will probably be good enough or perhaps carburetor or brake cleaner. I need to see what it looks like when I get that pot assembly removed and can make a close examination of it. I saw on another post forum member truckin 79 recommends DeoxIT D5 and I remember seeing a mention from somebody about CRC QD Electronic Cleaner. I may get a can of something like that.

I hope I'm not too late to the party.. I'm an Amateur Radio Operator and I dabble quite a bit in electronics... DO NOT use brake cleaner or carb/choke cleaner.. That stuff can damage certain electronic parts... As you mentioned, DeoxIT D5 is ideal for cleaning switches, relays, connectors and contacts.. It's not necessary to remove or disassemble the switch.. Just spray through the little openings and cracks.. Work the switch back and forth while you spray.. Don't worry about using too much, the access will just "leak" out..

[dmjlambert]

No, not too late to the party. Thanks, I appreciate the info. I have not yet removed the pot assembly or picked up any DeoxIT D5 yet. I will get a round tuit. I don't see any cracks or little openings from where it is installed. I'll probably need to get it out of the radio and into my hands so I can examine it closely.

I noticed the mechanical push button tuning is sluggish and does not work well, although the tuning knob does seem to work. I do not see any conventient way to disassemble enough to get a good look at the push button inner workings. Do you think WD-40 is good to try for those mechanical parts of the tuner?

[RustyPile]

Originally Posted by dmjlambert

No, not too late to the party. Thanks, I appreciate the info. I have not yet removed the pot assembly or picked up any DeoxIT D5 yet. I will get a round tuit. I don't see any cracks or little openings from where it is installed. I'll probably need to get it out of the radio and into my hands so I can examine it closely.

I noticed the mechanical push button tuning is sluggish and does not work well, although the tuning knob does seem to work. I do not see any convenient way to disassemble enough to get a good look at the push button inner workings. Do you think WD-40 is good to try for those mechanical parts of the tuner?

Just looking at the closeup pic of the switch, there's plenty of openings for the DeoxIT to get in.. Place some paper towels on the table/bench and place the radio on them.. Have a spraying good time, the paper towels will soak up the access.. I don't like using WD-40 on electronics.. It's intended not so much for lubrication as an agent to absorb moisture, doesn't leave much residue behind.. You need to use something that will leave behind some "slickness".. DeoxIT should also work for that...

[dmjlambert]

Thanks. I've ordered some DeoxIT D5 and will resume the operation when it arrives.

[dmjlambert]

OK as it turns out that was not a very involved surgery. I left the pot/switch in the radio and just sprayed it. I connected a multimeter on ohms setting to the switch terminals and turned it on and off a couple times and it started working almost immediately. I sprayed the pushbutton controls and they started working also. Then I bench tested the radio and it's alive. Thanks for the input folks. Now I know this thing is worth trying to find a speaker and bracket.