Calling all electronics gurus - Any ideas why this thing isn't working?
The thing in question is an antique Ace R/C Pulse Commander radio. It is a single stick transmitter on 27.145 MHz. It was functioning just fine a month ago. Dad and I opened it up to replace the stick assembly since the original is in poor mechanical shape due to the age of the plastic frame.
To do this required:
1) Unsoldering the wire leads between the PC board and the potentiometer (for movement of the stick) mounted on the stick assembly. The locations of the wires were marked on the pot before removal.
2) Removal of the antenna mount from the stick assembly to mount it on the new assembly. The antenna is bolted to the plastic stick assembly with a wire lead between it and the pc board.
We also replaced the switch with a different one of the same type...sliding switch with 6 posts on the back.
Replacing the stick did not work (pot on the replacement assembly was too different) so we re-installed the original one and resoldered all the wires back to their original locations. Powered up the transmitter and its matching radio system and got no response. The radio is fine because another transmitter on a different 27 MHz frequency operated the radio.
We checked for continuity and/or power on the leads between:
1) Battery and the switch
2) Switch and the PC board
3) Antenna and the PC board
4) Pot and the PC board
Everything checked out perfectly.
You can view the schematic in the pdf file at this link:
http://www.e-radiocontrol.com.ar/downloads/vintage/rxtxpulso/27%20mhx%20Pulse%20Commander%20tx.PDF
My dad and I really want to get it fixed because it is sooo much fun to fly but since everything checks out and should be working we're not sure what's wrong. Do any of you have any idea what the problem is? Any help would be greatly appreciated.
Replaced all batteries today with fresh Duracells. The others I was using were new in package Rayovacs. Anyway, after powering up the system the batteries in the transmitter (6 AA batts) started heating up VERY fast, talking temps hot enough to cause a very mild burn in a little under 3 seconds. Dad says its a dead short. We have no oscilliscope and don't know anyone who does. Can't take the system to a shop for testing because it will cost more than the purchase price of a modern R/C system which we have several of. We do have other antique transmitters which work perfectly unfortunately they are not on the same frequency as the system we are trying to fix. Dad and I have decided to replace the crystal in the receiver so it will function with the other antique transmitters we have.
Replaced all batteries today with fresh Duracells. The others I was using were new in package Rayovacs. Anyway, after powering up the system the batteries in the transmitter (6 AA batts) started heating up VERY fast, talking temps hot enough to cause a very mild burn in a little under 3 seconds. Dad says its a dead short. We have no oscilliscope and don't know anyone who does. Can't take the system to a shop for testing because it will cost more than the purchase price of a modern R/C system which we have several of. We do have other antique transmitters which work perfectly unfortunately they are not on the same frequency as the system we are trying to fix. Dad and I have decided to replace the crystal in the receiver so it will function with the other antique transmitters we have.
This system is exactly like the system we were working on:
http://cgi.ebay.com/1976-ACE-1-STICK-R-C-RADIO-CONTROL-TRANSMITTER-RECEIVER_W0QQitemZ280410690255QQcmdZViewItemQQptZRadio_Control_Parts_Accessories?hash=item4149c792cf
These are like the other transmitters we have: http://www.rchalloffame.org/Exhibits/Exhibit26/index.html
...and dad has one of these actuators and I flew it once, VERY different. Designed for 3 channels using the silver transmitters in the 2nd link. http://www.youtube.com/watch?v=xBSRmlIGSWQ
Anyway, dad and I try to keep these old systems funcitioning because they are fun to fly.
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.
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
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.
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.