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Problem with RC Car Receiver

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DamoRC

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The range on my RC car has dropped from around 250 feet to less than 20 feet and I cannot figure out why. It's taken a bit of a bashing lately which is probably the cause. To troubleshoot, I have pulled the onboard electronics package including the receiver (Rx) and set them up with just the steering servo connected. I set the steering trim on the transmitter (Tx) to full lock on one side and powered up the Tx and the Rx. While the Tx is within a foot or two, the servo steers as it should. I then walk the Tx away from the Rx until the servo returns to center (i.e. the Rx no longer picks up the Tx signal). I put the Tx down and return to the bench to fiddle with the Rx to see if I can see something obviously wrong. I have replaced the antenna with no sucess and there are no obvious loose connections or components hanging off. So I haven't found anything obvious. The odd thing is sometimes, if I pick up the antenna, or hold the Rx board in a certain way or sometimes just cup my hands around the Rx, it will start to respond to the Tx properly. Initially I thought I was grounding the board somehow which was helping the Rx. But now I think that my touching the board or somehow getting between the Rx and Tx is modifying the signal so that the Rx is better able to pick it up. The Tx/Rx are AM operating at 49 MHz.

Thoughts? Comments?

p.s. - I have replaced all the batteries with fresh one (Rx and Tx) to no avail and I am pretty sure that this is not a Tx problem.

Thanks in advance.
 
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Have you tried another crystal? What do you mean you replaced the antenna, did you solder a new wire in place of the old one?
 
It doesn't have a crystal (it's not really a hobby grade car). It has a small coil connected to the antenna via some caps.

Yes - I soldered on a new wire and also trimmed it to the same length as the original - no joy so I put the old antenna back on.
 
It could be one of many things.
1. The first thing you do is get another transmitter and ascertain if the Tx or Tx is at fault.
The test-set has to be from the same batch.

2. Remove the receiver antenna and determine the range.
This will tell you if the antenna cap is open.

Go to my website and see "27MHz transmitters" for some ccts.

<mod edit: self promotion deleted. If you have content to share, please post directly to ETO. Do not link to your own personal website>
 
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Thanks Colin. I can't get my hands on a test Tx / Rx (at least not at this time) but I will look out for one.

With the antenna removed, the range is approximately Zero feet.

Will check out your link later. How does one check for an open cap?

I was hoping that the phenomenon of the Rx working when I hold or cover it with my hand would point to a specific type of issue that is well known (except by me of course).

Thanks again.
 
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If touching the receiver makes it work better, try closing in on the section that improves in perofrmance.
I suspect it is the actual receiver section of the board (the 49MHz) section.
"Home in" by holding a metal screwdiver near the parts and see if this "conducts the signal." Put a finger near the parts. Try to "home in" on the actual area that improves.

Move or twist the board slightly. Tap the board and the parts. Have you touched (screwed) the slug in the coil?
This is the only way to go about it, unless you have one of the Field Strength Meters as described in the 27MHz article, (link above).
It is very difficult to test for an open cap as they are only a few puff. The fact that the receiver degrades when the antenna is removed, indicates the cap connected to the antenna is ok. But you can connect the antenna directly to the coil to further check this.
 
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You could simply have a broken wire in the antenna. Is the antenna on the car a floppy piece of wire? It's easy for those to break internally, solder on a new antenna to eliminate that as a problem.
 
Cheap RC cars use a very simple Super-regen single transistor radio.
It is originally set to the hairy edge of oscillation (for sensitivity) and the slightest physical change can dramatically reduce the gain.
 
Colin - thanks - will spend some more time "probing"

Sceadwian - I have tried a new antenna and no joy.

Audioguru - thanks - I will see if I can learn more about this type of receiver. What is the purpose of the ferrite rod in the center of the coil - if this changes position will it affect the Rx significantly?
 
Does the ferrite rod look like a screw?

It adjusts the frequency to suit the transmitter (or receiver). That's how you "align" the two units to get the "sensitivity" (range).
 
Yep - it has a slot for a small screw driver. I have adjusted it in a controlled fashion (1/4 turn - check range - repeat) but it doesn't help much. Insetad, as I adjust away from it's original position the sensitivity gets worse.
 
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