Help needed to fix missile launcher

[The Engineer]

Hello everyone, my first post here. I'm trying to repair a missile launcher which I bought second hand for my grandson but it's driving me nuts and I need some help from a technical expert!

 

 

The transmitter is a simple 27MHz circuit with a single transistor crystal oscillator feeding a single transistor modulator, driven by a PT8A9701 encoder.

 

The receiver is a single transistor feeding into a PT8A973 decoder.

 

The first problem was lack of range. The telescopic aerial was broken so I replaced it, together with the transistors in the transmitter and receiver. I adjusted the receiver tuning coil and it now has a reasonable range and responds to the buttons that play sound effects and launch missiles. However it only responds intermittently to commands that control the motors. Changing the encoder and decoder ICs didn't help.

 

If I monitor the output of the receiver that feeds into the decoder I get a continuous stream of PWM data, but the corresponding decoder output that feeds the motors either remains low or only goes high occasionally. The motor circuits are fine - connecting 3V to the base of the driver transistors has them spinning round nicely.

 

The transmitter and receiver circuits are as per this data sheet:

 

https://www.diodes.com/assets/Datasheets/PT8A9701973973L.pdf

 

If anyone has any ideas on where I can look for the fault I'd be very grateful as I am completely baffled.

 

Mike

 

Edited March 17 by The Engineer
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[The Engineer]

OK it looks as though I was in the wrong forest barking up the wrong tree 😒.

 

The brushed motors are clapped out.

 

Can someone advise me on a replacement for these please? The body measures 30mm long x 24mm diameter and the shaft is 2mm diameter. The motors run off a 9.6V NiCd battery pack.

 

What voltage motor should I buy and any idea what speed it should be for a tracked vehicle?

 

 

Mike

[Nitroholic]

Those look similar to the motors on Heng Long tanks.

 

I got some in the garage somewhere I can measure up if that helps

 

[The Engineer]

If you don't mind that would be very helpful, thank you.

 

I found these online which have all the correct dimensions but they are advertised for boats and the motor speed is 8224rpm at 12V. Not sure if that's what I need for this vehicle.

 

https://www.switchelectronics.co.uk/products/mfa-719re-280-1-12-24v-3-pole-dc-motor-8224rpm

 

Mike

 

Update: I calculated the gearbox reduction ratio as 20: 1 which gives a track drive wheel speed of 7 revs per second. Is that reasonable?

Edited March 30 by The Engineer
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