I am trying to use an XPS module (graupner IFS) to convert an old retro 2ch 27mhz transmitter. I managed to find a PPM signal inside the TX but it’s not being accepted as a valid PPM signal by the module. Can the XPS module be used on a 2-channel radio or am I on a dead track?

If it is possible I need to figure out what is wrong with the PPM signal (it is not particularly clean) and perhaps add some electronics to tweak it. I am stubborn to get this retro TX running again.

Attached a picture of the module for reference

Thanks!
Dave

The Graupner module is set for a specific signal (logic) level on the PPM input. If your 27MHz radio happens to use a very low current or really low voltage, the Graupner module will not work. Our Universal Module can be used with any PPM level.

Thank you. So if I can tweak the levels of the PPM signal there is a chance it could work? I mean the 2 channel only is not the problem?

The PPM signal that goes into the RF part of the electronics looks like this


Compared to a standard PPM signal the pulse width is shorter (about 0.2ms). Total frame length is 18 ms and the voltage level goes from about 4v to 9.8v (2.7v at the lowest). Now this is all analog components and when I connect the module the signal changes into this:


The amplitude drops and signal is between 2v to 4v. Not ideal.

These are PPM high signals. I believe Graupner uses PPM low (inverted) signals. On the RF section in the transmitter I managed to find an inverted PPM-ish signal that looks like this once the module is connected:



9.8V high and 0.2V low. I thought this would work but it did not.

Any thoughts? Is the problem voltage levels, frame rate, pulse width or "curvy" shape? I figure I can tweak the signal to some extent by replacing some components on the board but what should the signal be like to work? Or is using a pullup resistor on the module something that could help in accepting any of the above signals?

Regards
Dave

It could be current. The input to our RF module has to have sufficient drive current (the input is a transistor). The "low" value needs to be as close to 0v as possible. 0.2v is about the absolute maximum low pulse you can get away with.

you probably want to do it yourself, but Tony Stillman has converted a lot of these old metal box transmitters to XPS. The Krafts he's done for me have performed well. And old gold Futaba with the plug in module hasn't worked as well (rudder and elevator will only move from center to full deflection in one direction)

Tony’s conversions are pretty slick. I’ve never seen one like this with the module not hidden.
Mine, on the other hand, retains the old appearance minus the long antenna.

Old school r/c!!!!! (8 min 2 sec)

Tony’s website.
He’s a big advocate of the Nano.

https://radiosouthrc.com/