Trying to fix remote for Traktor RC Dickie Fendt Vario Favorit 926

[htm]

TLDR; Hi, i'm trying to find out what's wrong with this old remote. Below what i tested so far. i hope someone can point me to what to test next

The remote for this RC tractor stopped working


Before, when i pressed forward, it took some time for the receiver to pick it up.
(The remote doesn't do proportional control. it's either left or right, and center, forward1, forward2 and backward)
i thought it were bad contacts, so i cleaned the contacts, and cleaned the pcb a bit with alcohol.
After putting the remote back together, the tractor didn't respond anymore at all.

Could it be that components (like the elco) have degraded and make the signal distorted, or was it my cleaning?

The chip says "TX68 8421501 840". Couldn't find a datasheet for it.
The chip pins do register the stick movements, and the digital signal is generated as i do see a signal on the scope.(the place where i measured was at the black arrow in the "traces_w_components" image.
I'm not sure what voltage levels i should be seeing?
(i have an iffy scope, but the scale was at 0.5V/cm and the signal was half a cell high. i could recognize all controll states in the signal.
i only have a 10 Mhz scope, the remote is 27 Mhz, so i'm not sure i see the actual data or a samples version)

Does someone have advice what to test next?

[Swake]

Attached a datasheet: TX6/RX6 of Realtek Semiconductor who is one of the leaders of remote controller technology design of toy product.
It might not be 100% your chip, but probably close.

Page 11 has an application circuit. Seems to be very similar to your PCB, not same but many things are.

Found something on ebay too:
https://www.ebay.com/itm/TX6B-DIP16-IC-/323731601337

Start with the easy things: check power supply (9V battery?), check shorts or open connections, bad solder joints, antenna wire.

[htm]

Thanks

i checked the signal coming from the chip, and i can see the encoded keypresses in a bitpattern, so all that part seems ok i guess.
but i don't see/notice something like an oscillation pattern by the crystal. tomorrow i'll post a photo of the oscilloscope.

[htm]

Hi all. in hopes of finding the fault in this transmitter, i took some pictures of the signal going to the coil.

- a picture when not using the sticks "no signal", showing the 9 volt baseline
- a picture when of pushing the 'forward' stick, showing the 9 volt baseline
- a picture zoomed in when pushing the 'forward' stick. (i can actually see bits changing when moving the sticks)
- a picture shoing the measuring point.

Is this how a radio signal for a 27Mhz transmitter should look like? What else should i check?

[MathWizard]

All my old remote control toys got thrown out long before I knew any electronics, I wish I had them now. I should buy some old toy at a garage sale.

Looking at a datasheet for like what Swake mentioned, they say much lower frequencies, like fosc=128kHz.

https://elektrotanya.com/files/forum/2015/10/TX6-RX6%20vez%C3%A9rl%C3%A9s.pdf

[htm]

Ah, The chip doing lower frequencies explains why my oscilloscope shows something understandable i guess

The crystal label says "DY 27.125 Mhz". My oscilloscope is a 10Mhz one :/
I attached a picture of the crystal. it also shows the transistor which seems burned.

Any idea what i should/could test?

[htm]

To me, it looks like the signal on the scope is only showing the digital signal from the IC.

i've seen this video  about remaking a remote control:


That makes me think the signal coming from the crystal is missing. So i guess the transistor closest to the crystal is broken.

i guess i'd better draw the thing in an editor, so that i can better understand the components use, in order to determine a replacement

[fzabkar]

If you shake the remote next to your ear, can you hear the crystal rattling? Sometimes the crystal shatters when the remote is subjected to physical abuse.

[fmashockie]

I have attached an application circuit for the TX-6B (its not TX-68).  You can see from the circuit it incorporates the ~27MHz crystal.  You could test the crystal out of circuit with an oscilloscope and wave function generator. 

You've clearly thoroughly checked the transmitter.  But have you checked the receiver circuit?  When troubleshooting I always check the easiest things first.  Crystal oscillators are pretty durable and last decades.  So unless it has taken on some significant mechanical stress, there's probably nothing wrong with it. 

Here's a link to the TX6B datasheet as well. https://datasheetspdf.com/pdf-file/1077448/SilanSemiconductors/TX-6B/1

[htm]

Thanks i'll try to check it.

1)There's no rattling.
2)(actually, since i'm an electro noob, i made pictures and tried to measure things, but i didn't really know what to specifically look for.)
i'm not sure how i could test the crystal by using a signalgenerator (i'll google the details)
I have an old 10Mhz scope and recently got me this device Joyit JDS2915(signalgenerator and frequency measurement device). Will that be sufficient?

[fmashockie]

I think  you need to take a step back.  Forget testing the crystal at this point.  But unfortunately, just like your scope, you might not have enough bandwidth to test this 27MHz crystal with that signal generator since its only up to 15MHz.

I think you should take a look at the receiver circuit on the tractor.  Start with the easy stuff first.  Do a visual inspection.  Check to make sure the motor is working.  Feel free to share pics. 

Since you said you have little experience with electronics, I can tell you 90% of the time problems like these are usually not nearly as complex as you think (a cold solder joint, a loose connection, low battery, etc.).  So start with those easy things to check first before digging in with an osilloscope. 

[htm]

Hi. Thanks for your input.

i realized we had one toy with a 27Mhz remote. So i checked that one on my 10Mhz scope. on the antenna i do see the image resembling the previous youtube video when reading from the antenna. Even though the transmission circuit may be different, i do see a 27Mhz wave(images attached)

But on my defective remote, i don't read anything on the antenna.
On pins of both transistors, i do see the pulse-signal(this picture of before) from the chip encoding the joystick positions.
So it seems the defective remote is only having the pulse train, but there is no crystal signal to be mixed in.
That would explain the signal not passing the high-pass filter(capacitors) towards the antenna i guess.

As you proposed, i think i go the route of checking the crystal out of circuit with an oscilloscope and wave function generator.

[fmashockie]

Nice work! I'm not sure how much luck you'll have performing that test on the crystal without a scope and function generator since the frequency you wish to measure is not within the bandwidth for both of your test devices, but good luck!

[htm]

#update
i found another old 27Mhz RC transmitter with a crystal in it. (Turns out the newer 27Mhz cheap ones don't have a crystal).
its crystal says 27Mhz. The original crystal in the remote has 27.125 written on it.

After reading a bit about oscillators, and checking the scope differences between the good and bad remote, it seemed the crystal was dead.

After replacing the defective crystal, even though it isn't an exact match, the remote can control the RC Dickie tractor again!
i guess i'm lucky that the old receiver isn't that selective

If you shake the remote next to your ear, can you hear the crystal rattling? Sometimes the crystal shatters when the remote is subjected to physical abuse.

Thanks for the suggestion. i shook it and didn't hear anything. i took the crystal appart just now, and it shows one of the pins came loose from the strip on the crystal.

[fmashockie]

Fantastic! Glad you got it working.  And thank you for updating everyone on the outcome!