Why Are Most CB Radios Built Upside Down? - Page 1
Forum section: electronics / rf-microwave

author: German_EE
author_rank: Super Contributor
timestamp: March 23, 2017, 06:37:05 pm
content: I watch a wide variety of technical video channels including Buddy on 'The Radio Shop' and 'Mike's Radio Repair'. Both of them, especially Mike, show repairs on CB radios and something has started to bug me. Take a CB radio apart and the PCB will be at the top with all the components pointing downwards, they all seem to do it, why?It puts more stress on the solder joints, everytime you go over a bump all those big electrolytics and transformers will give their joints a little pull.It's awkward to service as the front panel is upside down as you work your way around the circuitOverheating components will cook the board as the heat rises and has nowhere to go.So CB radio experts, an explanation would be appreciated. Why do CB radios use 'Australian' construction?

author: G0HZU
author_rank: Super Contributor
timestamp: March 23, 2017, 07:01:13 pm
content: Because a typical owner will want to put it into a vehicle either on the bracket provided or sometimes flush underneath a shelf.The CB radio has an internal speaker and the best place for it is facing down towards the floor of the vehicle. So all of this means that most CBs have an upside down PCB at the top. The speaker fits to the lower case of the CB and the conical shape of the speaker with its coil will be arranged to poke up into an unused part of the main PCB. On some CBs they still have to put a (30mm dia?) hole in the main PCB to allow the speaker coil/magnet some extra room.

author: djacobow
author_rank: Super Contributor
timestamp: March 23, 2017, 08:50:38 pm
content: Designed in Australia, maybe?<ducks>

author: darrellg
author_rank: Unknown
timestamp: March 24, 2017, 05:32:14 pm
content: Quote from: German_EE on March 23, 2017, 06:37:05 pmI'm not sure that I agree with this premise. Most CB radios are built on single sided boards. The most common problem with single sided boards is the copper separating from the substrate (especially during rework). When the unit is upside-down, the components are pulling the copper closer to the substrate. If the components were on top, unless they are perfectly flush with the board, gravity would be trying to separate the copper from the substrate, causing cracks around the annular rings.

author: W9GFO
author_rank: Regular Contributor
timestamp: March 24, 2017, 05:42:17 pm
content: Quote from: German_EE on March 23, 2017, 06:37:05 pmI sincerely doubt that gravity has a significant effect on the components. In vehicles, especially commercial trucks, it will be the vibrations that are the dominant forces acting on the little hardware bits in the radio. Those electrolytics and transformers are wiggling back and forth every moment that the engine is running.

author: KE5FX
author_rank: Super Contributor
timestamp: March 24, 2017, 05:50:56 pm
content: Any debris that might be shaken loose during vibration is best kept away from the board. Could be an especially important point if there are any ventilation openings. Vehicles are messy environments...

author: james_s
author_rank: Super Contributor
timestamp: March 24, 2017, 06:12:03 pm
content: I'm pretty sure G0HZU hit the nail on the head. It's so the speaker can point downward, nothing more. I don't think gravity is really an issue, these things seem to be about as reliable as anything else.

author: Nusa
author_rank: Super Contributor
timestamp: March 24, 2017, 06:37:13 pm
content: It's not true of the desktop versions. They were generally built right-side up.On mobiles, speakers tended to be on the bottom. I agree with that.Board mountable switches/lights/displays tend to be on the top, above the more bulky mic jack and selector switches/pots. That means the board must be high too.It's more likely to survive the environment if there are no openings on the top of the case. CB's were big before cars came with proper cupholders. Sometimes the CB WAS the cupholder/sandwich shelf.

author: rsjsouza
author_rank: Super Contributor
timestamp: March 24, 2017, 08:10:23 pm
content: Quote from: W9GFO on March 24, 2017, 05:42:17 pmThat reminded me of a friend that worked for the military division of Philco radio. He told me how they used single-sided boards but with metallized holes going all the way through the other side to guarantee extra solder robustness against vibration.

author: G0HZU
author_rank: Super Contributor
timestamp: March 25, 2017, 02:03:41 am
content: Quote from: james_s on March 24, 2017, 06:12:03 pmYes, I think it's because of the speaker. Having the speaker on the bottom suits a typical mobile CB installation because it will fit tightly under a shelf and you can also fit/use a slide mount if the speaker is on the underside.When it comes to vibration etc, a typical FM CB radio will use melted wax around the main VCO components in the PLL to prevent microphony as the vehicle is driven. Also, CB radios have always had fairly decent (and complicated) noise blanker and noise limiter circuits to prevent interference from the poorly suppressed ignition system of 60s, 70s and 80s cars and trucks. They were generally quite rugged. However, if you were to remove the PCB from the rigid chassis of a typical 1980s CB radio you would find that the PCB material was quite fragile when exposed like this. It was very easy to flex and break the PCB apart into pieces as the circuit boards were usually made of a paper based material.

author: G0HZU
author_rank: Super Contributor
timestamp: March 25, 2017, 02:07:38 am
content: QuoteThat takes me back...google forSommerkamp coffewarmerThis was one of the more desirable export CBs that had a high power amplifier/heatsink at the rear. Ideal for keeping your coffee warm...

author: voltz
author_rank: Frequent Contributor
timestamp: March 27, 2017, 10:00:33 am
content: just a +1. Speaker placement pointing down hence PCB at the top. Simple as that really.

author: CJay
author_rank: Super Contributor
timestamp: March 27, 2017, 10:30:22 am
content: To be fair, a lot of CB radios were built on exactly the same PCB material as a lot of Ham radio and even PMR radios of the time, the difference was generally in the quality of the design and components, modern CB stuff is SMD for the most part and is on far better quality fibreglass using, mostly, much better quality components.I think the main reason for their being built 'upside down' was everything to do with the speaker, if it were for mechanical reasons then car radios would have been built that way round too and my memory tells me there was a mix of both even from the same manufacturer so I'd suspect the orientation of the board in car audio is chosen purely for mechanical/manufacturing convenience.And that Sommerkamp, yes, most desirable but what a pig to work on, nightmare delicate connections to the LED frequency counter display as I recall but a lovely PLL design. The Belcom LS102 was similar.