RCS Guidance System Not Working - Help Required Please

Button pushers
Stew
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Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

RCS Guidance System Not Working - Help Required Please

Post by Stew »

Hi all.

Does anyone know why a 27mhz RCS transmitter will not actuate my escapement off the tone button, but when I work the on/off switch it does! Any clues?
If it helps I'll post a pic of the circuit diagram.

Stew.
Last edited by Stew on 11 Feb 2022, 11:09, edited 2 times in total.
Stew
Posts: 495
Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

Re: RCS Guidance System Question

Post by Stew »

I'm thinking ACY22 transistor off the tone button bust?
(Diagram courtesy of NORCIM website)
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Pchristy
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Location: South Devon, UK

Re: RCS Guidance System Not Working - Help Required Please

Post by Pchristy »

Really need a bit more information! It sounds as if the likely candidate is the push button being short circuited.

Tune a VHF radio to around 108 MHz (4x27!) and switch the RCS on. You may need to fiddle with the tuning a bit, but with luck you should hear the "tone". This should only be heard when the button is pressed. If it is on all the time, the button is almost certainly short circuit. Check with a multimeter.

--
Pete
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Mike_K
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Re: RCS Guidance System Not Working - Help Required Please

Post by Mike_K »

Hi Pete

Out of interest, I wonder if this old trick still works with modern domestic radio sets that can be tuned to the 108MHz "FM band"? I just tried with a domestic Bose FM radio trying it with a MacGregor S/C set that I know works and I couldn't get a peep out of the FM radio, but the Minimac Rx relay was operating every time the transmitter was keyed.

Mike
Stew
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Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

Re: RCS Guidance System Not Working - Help Required Please

Post by Stew »

Hi chaps.

I don't have a vhf radio, but I tried the push button switch with a multimeter and it works fine. I tried my other RCS Tx board and same reading from the switch on that one when pressed, so the push button switch seems good.
Stew
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Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

Re: RCS Guidance System Not Working - Help Required Please

Post by Stew »

Here's the little fellas side by side for testing.
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Stew
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Re: RCS Guidance System Not Working - Help Required Please

Post by Stew »

Am I right in thinking that the escapement clicks when the Tx is switched on because of the carrier, and then pressing the button should produce a tone and properly actuate the escapement, but isn't?
It's odd that it suddenly ceased to work though.

Luckily it's brother does work, so at least I can compare readings from the two.

It may be obvious I know little about such things, but I am making an effort to learn, it's a hobby after all! Perhaps it's ceasing to work is the radio gods making me figure it out!
Stew
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Joined: 02 Mar 2018, 10:21
Location: Staindrop, Darlington.

Re: RCS Guidance System Not Working - Help Required Please

Post by Stew »

Push button removed and replaced with another push button. Still no joy.
I'm still thinking the transistor near the switch on the schematic but I'm not 100% certain of its job.
Pchristy
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Location: South Devon, UK

Re: RCS Guidance System Not Working - Help Required Please

Post by Pchristy »

The problem is that without proper test gear - even the most basic - you are working in the dark!

The two transistors on the left of the circuit diagram are a multivibrator that generates a tone when the button is pressed. The one immediately to the right of them is the crystal oscillator that generates a low level RF signal (27 MHz). The one on the extreme right is the power amplifier that boosts the signal up to the required level for transmission. The transistor at the very bottom modulates the output by switching the power amplifier on and off and is driven by the tone generator.

The problem is that without even basic test gear (VHF radio?), you can't tell if its actually radiating any RF signal. That could be down to the oscillator (dead transistor or duff crystal), the power amp (dead transistor) or the modulator (ditto). If it IS producing RF, but still not working, it will probably be the multivibrator.

If the relay clicks momentarily when the Tx is switched on, then it is probably producing RF, which leads back to the multivibrator or modulator. Again, without an oscilloscope, it could be quite tricky to determine which it is.

If you can find an old fashioned crystal earpiece (as used for crystal sets, beloved of children of yesteryear), you could try connecting it between the red dot on the diagram and ground. You should hear a tone when the button is pressed. It needs to be a crystal (high impedance) earpiece, not a modern (low impedance) earphone. The latter will load the circuit too much and stop it working anyway!

If you have an analogue voltmeter, you could try connecting it between the red dot and ground. With the button not pressed, it should read a high voltage. When the button is pressed, it should drop to about half that voltage. That would indicate that the multivibrator is doing its thing, but it might not be the right frequency, mark/space ratio, or a multitude of other things, but at least it would indicate that it is working. It may not be working properly though. To test the rest, you need at least some kind of radio capable of tuning to 27 MHz - or one of its harmonics - or a field strength meter.

Without any of this, you are searching for a black cat in an unlit cellar! :?

--
Pete
Stew
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Re: RCS Guidance System Not Working - Help Required Please

Post by Stew »

Thanks Peter, I really appreciate that, it will give me something to go on. I am actually enjoying the process as I'm learning as I go, albeit on a very basic level...

Stew.
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