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Old Dec 11, 2014, 06:26 AM
Joined Dec 2014
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Fixing Modelcraft 4-CH TX RX 40.675Mhz Problem and Modification....

I brought Modelcraft 60.475 Mhz Transmitter receiver set t this during my stay in Germany for 42" Glow plug trainer , From Conrad... probably in 2009-2010. I never used...

Due to Recent need I had tried it, it has problem....with 3rd channel and 4 channel, ch-1 and ch-2 works fine.... here is video link
Modelcraft 4CH 40.675 MHZ RC TRANSMITTER PROBLEM (1 min 4 sec)



Observations please see in video (ref link above)
  • Neither servo nor ESC work at channel 3
  • Using throttle channel trimmer on top position( 4th on tx, Refer to attached manual page) make 3 servos(ch-1,ch-2 and ch-4(Throttle) works perfectly....
  • moving channel 3 will move all servos or at least two depending on channel trimmer positions....
  • Esc does not work at ch-4(which is throttle)... but servo works on ch-4 while trimmer is on top position....
  • Esc works at channel 1 and Channel 2 ( Manual says it should work with electric model.... too...)

Strange!!!
Rx has crystal of 40.220 and Tx 20.3375, I think both Rx and Tx crystal should have same frequency right ? and Frequency mentioned on set 40.675 Mhz

Already Tried[LIST][*]Potentiometers seems ok and checked variable resistance shows correct on multimeter[*]interchanged servos....and used different set of servos...[*]Low battries cause problems with servos.... I tried with new and full batteries....but no success /LIST]

If you have any idea how to fix this issue please advice. how to identify the ppm and bind wire going to tx.... there are four wires as shown in attached image Tx_module_marking_VDD_VCC_Fr.jpg .... i guess red(marked ad VDD on pcb) and black(marked with negative sign) for supply other twos, yellow marked as(VCC) and white marked as (FR), so which one is ppm and bind ?

Alternate Solution
  • Use only Tx and Rx module and create, Arduino pro mini based circuit or Arduino nano based circuit described here and connect ppm and bind to tx module...
  • Interchange Potentimeters of CH-4 and Ch-2 (so throttle works fine) and then use as 3-ch trans receiver....
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Last edited by svalambhia; Dec 11, 2014 at 06:39 AM. Reason: Images were not attached.....
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Old Yesterday, 03:08 AM
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Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Quote:
Originally Posted by svalambhia View Post
Strange!!!
Rx has crystal of 40.220 and Tx 20.3375, I think both Rx and Tx crystal should have same frequency right ? and Frequency mentioned on set 40.675 Mhz
FM Tx and Rx crystals are usually marked with the same frequency, but in reality they are different (and neither one is the channel frequency!). On 40MHz the Tx crystal typically operates at half the channel frequency. The Rx crystal is above or below the channel frequency, by an offset equal to the IF frequency (455kHz in a single conversion receiver).

Quote:
Already Tried
Potentiometers seems ok and checked variable resistance shows correct on multimeter
Have you tried measuring the voltage on each pot wiper while the Tx is turned on? This will tell you whether the problem is in the pot or somewhere else in the circuit.

To prove that the entire encoder is working properly you could view the ppm signal with an oscilloscope. If you don't have an oscilloscope then feed the ppm signal into a PC sound card and use a program such as winscope.

Another possibility is that the Tx and Rx are not on exactly the same frequency (modulator out of tune, bad crystal?). When FM TX and Rx frequencies are not aligned the signal gets distorted and you may see interaction between channels and/or some channels not working properly.

Quote:
how to identify the ppm and bind wire going to tx.... there are four wires as shown in attached image Tx_module_marking_VDD_VCC_Fr.jpg .... i guess red(marked ad VDD on pcb) and black(marked with negative sign) for supply other twos, yellow marked as(VCC) and white marked as (FR), so which one is ppm and bind ?
FM radios have no need for a bind function.

Vdd (Drain supply Voltage) is a regulated voltage (+5V?) which powers the logic chips on the encoder board, as well as the FM modulator on the RF board. This is used because the modulator needs a stable voltage to minimize frequency drift.

Vcc (Collector supply Voltage) is a higher voltage (probably direct from the battery) which supplies the RF power amplifier.

FR is the only wire that doesn't supply power, so it must be the PPM signal!
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