Kraft Series 71 3 channel radio

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Canardlyfly
Posts: 2
Joined: 25 Jun 2018, 17:10

Kraft Series 71 3 channel radio

Post by Canardlyfly »

Hi I recently won the Series 71 3 channel set on Ebay. A complete set TX, RX RX Nicad 3 servos charger switch harness and the instruction manual. The batteries were in the set. So I need to find a 110v supply. Wrong, read the instructions which explain how to change the charger from 110v to 220v. very simple un-solder one wire and solder it to another terminal and therefore connecting the positive wire to the 220v winding in the transformer! Can't imagine that instruction appearing in the manual these days. So out of curiosity I put the set on charge. And guess what it charged and whats more actually worked on 72.96Mhz! So question 1 Other than 'mobile' Ofcoms listing for 72Mhz does anyone know what/who uses 72 Mhz in the UK? However the batteries were clearly U/S because the servos chattered and the range was about one foot. I stripped the TX down to remove the nicads, slightly leaky but no damage but I suspect some black wire corrosion is lurking. It became clear during this process that the set had seen little or no use. Removing the stick assembly revealed no grime around the edge of the stick bezel or grime in the serrations on the on/off switch. Converting to 2.4 Ghz should be easy. I've just bought a Lemon module nice and small, probably not enough space to fit Frsky module. A convenient jumper link will allow simple disconnection/ re-connection of the logic to the 2.4 Ghz module. I shall use the original encoder because everything is connected to the PCB. The stick assembly the throttle lever, battery box etc. So question 2 other than electrolytic capacitors do any other capacitors deteriorate with age?
The set came with 3 servos KPS 8 or 9 probably. About the same width and height of modern standard servos but significantly longer. Now the clever bit, two servos were series 71 and have 4 wire amps the third a series 72 is a three wire servo. Now i can obviously swap the servo lead on the series 72 servo for a JR servo lead, but the series 71 servos will have motors more suited to 2.4 volts and would I guess be quite upset if I changed the amps to a three wire amp. and 4.8v to the motor. Unless someone knows better. All in all the set has obviously been stored carefully so deterioration of the plastics shouldn't be a problem. The set came from Mesa CA, not a million miles from where it was made.

Finally question 3 will I need to just remove the crystal to disable the RF or should i also break the power feed to the RF section as well?
Pchristy
Posts: 413
Joined: 16 Feb 2018, 13:57
Location: South Devon, UK

Re: Kraft Series 71 3 channel radio

Post by Pchristy »

72MHz is primarily used for mobile/base communications in the UK. This would include taxis, fire engines, ambulances etc. Emergency services (and Ofcom!) would likely take a dim view of anyone interfering with their communications, even if it was extremely localised!

Removing the crystal will certainly kill the unwanted 72 MHz output. However, there is a remote chance that it could still oscillate at some random frequency - unlikely, but possible. I would therefore recommend removing the power to the RF section by some means. This will also reduce the current consumption fractionally, by disabling any bias circuitry.

You could still use the 4-wire servos if you could supply them with a centre-tapped supply. However, bearing in mind how cheap servos are these days, and the age of 4-wire servos, I wouldn't recommend using them in anger! OK for a bench demo unit, but not for flying!

It is always worth replacing old electrolytic capacitors. The other types should be OK, but it is probably worth measuring the output pulses once you have it up and running, just to be sure.

Best of luck!

--
Pete
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