Comments on: Calling all electronics gurus - Any ideas why this thing isn't working?

By: Violet W

Violet W — Tue, 02 Mar 2010 19:38:27 +0000

You could check the POT with an ohmmeter (or DMM). With the battery disconnected, measure across the white wire and green wire where they connect on the circuit board, and move the pot. See if the reading changes from 0K to 10K or thereabouts, or in reverse. Then check similarly across the white wire and the blue wire, move the pot, and confirm that the readings change in reverse (decrease when the other reading increased, etc.)

If you deformed any of the coils, or changed adjustments on the coils, then the transmitter might have drifted off of frequency or its signal is being damped.

This is where it would good to find an interested person with an oscilloscope.

The bottom third of the circuit appears to apply an adjustable voltage to the middle third. The middle third contains a low-frequency multi-vibrator oscillator. The adjustable voltage fed from the lower third probably changes the duty cycle of the oscillation. That signal is fed into the upper third of the circuit, to the 27 MHz RF oscillator on the left (crystal, transistor, L1, caps). The signal probably gates the oscillator on and off. The two transistors on the upper right comprise an RF power amplifier. L2 (and cap) is an adjustable filter that kills upper harmonics. If it is detuned, it may reject the 27Mhz signal. With an oscilloscope, it would be pretty easy to following the signal around through the circuit and see where the signal was lost. My advice would be to find an interested person with an oscilloscope.

I hope you are successful in returning to flight.

By: Eric

Eric — Tue, 02 Mar 2010 19:38:27 +0000

Did you check for continuity between the leads of the pot?

You need to see if the pot is working — its a variable resistor, use a multimeter to read the resistance as you move the stick back and forth.

It’s also possible that adding the new stick to the controller overloaded a transistor in the circuit if the resistances were out of range (although this would probably have tripped teh fuse — did you check the fuse???)

I may be able to continue recommending things to try…feel free to message me

By: Roger

Roger — Tue, 02 Mar 2010 19:38:27 +0000

Since you have the schematic all you need to do is find someone knowledgeable about electronics in your neighborhood. It should be repairable. I would guess when you replaced the switch you got the wrong type. Your switch is probably 3 ganged SPST switches that all switch on together. The switch in the schematic has two ganged switches that switch on while the third one looks like it is supposed to switch off when the others switch on. That’s a unique switch I hope you saved it. I would replace the new switch with the original that might fix the remote.

By: lee26loo

lee26loo — Tue, 02 Mar 2010 19:38:27 +0000

This was a simple transmitter about 1970 or older product. To trouble shoot more easily,you may use a scope to touch the antenna to see it produces 27.145 mc carrier and modulated with your voice as you speak. And measure its RF power output ( about 5 watts at 50 ohms load). Perhaps the 10k old potential meter ( you call it stick) is defective (no contact) because this is the only thing you did not replace. Check its resistance and at the same move the potential meter to see it has variable resistance at both ends between the center pole. Re-check every thing that you touched. Since you move the antenna to a new location,it requires to re-tune the coil L2 to do a matching (de-tune antenna shall kill all the RF output power feeds to antenna). You need a RF watt meter to do this fine adjustment.