Circuit design error

[Pufango]

Hello lads. I’m having problems getting this circuit to work. Have built it three times,checked the components over and over again.Replacing every part two and three times! Still can’t get it to work. Have built several other similar designs without problems. Must say it’s driving me mad. Anyone have any ideas ? The actual design is a simple 27mhz single channel transmitter.The tone generator works. But that’s it Many thanks Tony Wright.

 

[Nigel Goodwin]

Where did you find the antique coils specified in the article?. Likewise, have you got the exact correct crystal?

However, it doesn't look like something you should be building anyway, as it's a horrible old circuit and will play havoc with any more modern 27MHz remote control models.

 

[Pufango]

Hello it’s not the coils ,I make them myself .Have never had problems with coils in any of my builds .Dont think there will be any problems with other 27mhz users ! I don’t use these vintage radio control systems for my models I use my 2.4 ghz computer systems .I just like to scratch build old electronics.

 

[Ramussons]

What is your 27 MHz receiver?
Disconnect the VT4 / VT5 astable.
Check whether your VT1 is oscillating.

 

[Pufango]

Hello have tried different receivers ,home made and commercial. No joy there. Allso no signs of oscillating.Must say it’s a total mystery ! Could there be an error in the coil specifications ? Think I will go back to building steam engines there a lot easer to diagnose problems

 

[JimB]

Hello, some nice construction you have there, both electronic and mechanical.

As to why your latest transmitter does not work, two things come to mind.

The coil L1 and capacitor C2, they should be resonant at around 27MHz.
Are you sure, do you have any electronic test equipment to test that they are resonant at 27MHz?

The crystal. This is specified as a third overtone type.
A third overtone crystal is one which mechanically resonates at three times its fundamental frequency.
If you put a fundamental mode crystal into that circuit it will oscillate at 9MHz, if you are lucky.

JimB

 

[Diver300]

You could use a 27 MHz fundamental crystal. They will oscillate more easily at 27 MHz than a 27 MHz 3rd Overtone crystal. 27 MHz fundamental mode crystals are more readily available than they were when the circuit was designed.

As JimB said, the LC circuit is tuned to 27 Mz, which should stop a 27 MHz, 3rd overtone running at 9 MHz. There is no such problem with a 27 MHz fundamental crystal, but the LC circuit will do no harm.

 

[Pufango]

Thanks lads .Think what I might do is to use an oscillating circuit from a design that i know is working and graft it on to this one that doesn’t work! To see if that does the job. Re Crystals I’m using standard model rc types,so I don’t think it’s the problem ? My test equipment is very basic just the usual multi meter and component testers. Have a kit for an osciliscope ,do you think it’s worth the bother to make to help diagnose the problem? Hope you don’t mind me asking these very basic questions ?