Blown x-tals?

[Ogier]

Hi all,

I was having a big issue with an iWaver 01 that I just bought.
I originally tested the car and it worked OK, only issue was a busted steering servo gear.

I’ve fixed the gear and tried to test it and nothing happens. I finally realised that when I tested it the first time, I used different crystals. It seems I’ve got a set of crystals that are somehow damaged.

I checked resistances on both the TX & RX crystals with my multi meter, but get nothing out of either of them. What type of a circuit/device is in these xtals?
Is there any way I can properly diagnose if it’s the RX or TX is broken?
At this stage, I don’t have access to another set on the same channel.

The electronics side of these things is keeping me very busy 😉

Thanks in advance for any help.
Graham.

[betty.k]

not sure how to diagnose a dud crystal but i know they are fragile items. dropping them on the ground can damage them :8ball:

[ph2t]

Unfortunately Graham, the crystals won’t reponsed to a continuity test as you’d like them two. They appear as an open circuit for DC current tests….

You’re best bet is to just get another pair.

In the world of RC, crystals are like Pet Hamsters…..don’t get too attached to them, lol…..

ph2t.

[Ogier]

Its weird. I did a bit more testing today, with someone else with the same frequencies. It turns out these crystals are OK, the same controller and crystals works on another car.
So now I guess I’ve got to go hunting through the components on the PCB to find out why it’s not working or only working sometimes. It actually worked for 1 second before stopping today, then again for 5 whole seconds a minute later. 🙁

Thanks for the tips on crystals! Graham.

[trash]

Crystals don’t blow. The best way I can describe them is they are a rock with a wire connected to each end. Actually, that’s exactly what they are.
If you were to pick up a rock off the side of the road and measure it with a multimeter, you could expect the same results.
Xtals are high impedence devices, so they are very difficult to damage.
It is possible to break them physically by hitting them with a hammer etc or dropping them, but even that’s hard to manage.

Xtals do actually require to be tuned. In most hobbiest cases, your transmitter/receiver is tuned wide enough to accept a range of frequencies at the expense of efficency and stability. Generally compare your new frequency to the old one.
If it is 27.095 and the new frequency is 27.145 then they should without any fuss. If however the new frequency is something like 27.850, then this might be a little too far off, but should still work in most cases.

If however you’re trying to use 29.550 or 36.300MHz these are too far off frequency or in a different band all together. On top of that, there are different xtal types and cuts. But most RC xtals are all cut to the same way.

There are some simple xtal testers that you can make that just have a simple LED to show they are working. If you have a CRO, you can see your transmitter’s carrier… you won’t see the actual sine waveform, but you will be able to see the carrier power. If you have a frequency counter or spectrum analyser, then you’re a god.

A simple xtal tester should cost about $10 or less to build.
A CRO (Cathode Ray Oscilloscope) can cost anywhere from $30 – $1000
A frequency counter $10-$500
A spectrum Analyser $300-$2000

A CRO you can buy from Jaycar or Dick Smith new, but second hand ones are easy to come by. I often see them sold for ~$50.

Frequency counters are also easy to come by. You can get cheap hobbiest ones that work up to 50MHz for less than $20 at flea markets, but if you have the cash, you can buy some really nice 3GHz units for about $100 like the star-tek or optronics of ebay.

Spectrum Analysers are usually expensive, a new one can cost about $30000, and most serious ham radio hobbiests will spend $3000. But there is a lot of older spec-an’s that do not cover over 1GHz and therefore are only of limited used. You can pick them up for as cheap as $300. Ebay is a good place to find them, but you have to consider shipping costs.
You can also make a spectrum analyser out of an old CB radio.
I’ll try and find the URL for the planes.

http://www.vkham.com classifieds is another good local place to look for gear.

[Ogier]

An interesting thing happened yesterday. I was describing this problem with the car to a guy at work, and I showed him that the car didn’t work with it’s own crystals (Channel 9). I then put other crystals in (Ch 2) and the car worked just fine. I put channel 9 back in and absolutely no reaction from the car, I pulled Ch 9 out and put them straight back in and the bloody thing worked!

I can only think that the connectors on the crystals was a little oxidised and the act of swapping them a few times has allowed a better connection.

So now I’ve got a spare car that’s up for sale or as a backup for parts if I need it (which I may just do considering the speed & damage I’m causing with my car).

Cheers + thanks for all the help so far! G.

[Author]

Posts