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View Full Version : Help! challenging tx troubleshoot for DIY'ers


jay flay tway
May 19, 2005, 01:41 AM
This may be a bit off topic but it seemed the best place to get an expert opinion.

Laser 4 40 mhz tx with hfs-04mg single3-conv. rx, 6 months old, never dropped.

It worked fine until...
I put the 600mah nicd batt pack from my optic 6 in the laser 4. (I use 2500mah batts in the optic now.)
I Hooked the Laser 4 up to the Optic 6 with the trainer cord. Worked one way(the way I'll never use it!) but not the other(the way I want to use it, with the optic as the master. I'll never put the cheap rx in a plane I don't want to crash!).

I Charged the laser 4 with the jack on the tx, using the wall chrgr. that came with the optic. The charger is very low current and designed for charging with the batt in the tx i think. Designed for that particular battery, too. NOthing got hot when charging, either.

NOw, the laser 4 DOES NOT work at all. It has power but it only makes the servos move JUST BARELY and both stick directions make the servos move the same way (the direction changes at half-travel too, weird).

Here's the weirder bit: When the laser 4 is the master and the optic is the slave(trainer), with the trainer switch held, the optic works fine (normal servo movement). I think this means that the "signal sending" components on the laser are fine, but something is wrong between the sticks and the sending parts.
With the trainer switch off, the laser 4 doesn't work (just as without the cable attached).

With the Optic as the master and the laser as the slave, the optic works(trainer switch off) and the laser doesn't(trainer switch held).

Very confusing, very weird, and frustrating.
It was bad enough that the LHS sold me a 40mhz tx with a single-conv. receiver, now it's useless for the last thing i thought i could use it for.

HOpe someone has some ideas about this. I'd appreciate this a lot.

Malc C
May 19, 2005, 03:54 AM
Not sure, but in some cased you have to remove the battery from one of the transmitters when you buddy them up as the one set will power the other through via the buddy lead. It may be that you have damaged the laser 4 by keeping the batteries in both sets ??

Mr.RC-CAM
May 19, 2005, 12:46 PM
I suspect there has been damage to the 5V supply that feeds the encoder. I believe that the encoder used in this Tx is a GMS87C1202 microcontroller (confirm the part number on it). A data sheet is here: http://systemic.hei.co.kr/sp/databook/GMS87C1202.pdf

Using the data sheet, identify the power pins (Vdd & Vss). You should have 5VDC across them. If it is radically different, then find the 5V Vreg and troubleshoot it.

EDIT: Update. I found the power supply / encoder schematic to the Laser 4FM. If it matches what you own then you should have no trouble completing the repair.

RC-CAM

Comatose
May 19, 2005, 09:55 PM
One thing I ran into on my laser4 is that the battery connector is VERY easy to damage by plugging and unplugging the battery. The PCB it goes into is single sided and I managed to make it delaminate while plugging the tx batttery in. Periodic powerdowns and general glitching until I fixed it. Just something to check.

jay flay tway
May 19, 2005, 11:11 PM
Wow, thanks a lot. I knew the DIYer's would have something useful to say about it. I'm about a 2.5 out of 10 for elec knowledge and troubleshooting. I'll take a look at the circuits inside.

I don't think it's the battery connector, although it sometimes doesn't connect when I put the batt in. It's either on or not and when lthe problems mentioned are happeningk, power is there and steady.

thanks again.

SlopeKing
May 20, 2005, 03:55 PM
I too have a laser 4, it functions completely fine, except for channel 2, if i push the stick one way (cant remember which) the servo moves about 1/3 travel, if i move it the other, it runs the SAME way 1/8th travel, any ideas, it would be nice to have my second radio back...

Mr.RC-CAM
May 20, 2005, 04:04 PM
I too have a laser 4, it functions completely fine, except for channel 2,Using a voltmeter, how many volts is present on the two outside terminals of the bad channel's stick pot? Should be about 5VDC.

If it is, then does the voltage on the middle pot terminal change linearly as the stick is moved end to end? I would expect that you would see under 1.5VDC at one extreme, over 3.5VDC at the other. Center stick should be about 2.5VDC or so.

If all is good, then you have a problem with the OPAMP on that channel. The schematic above will show you more.

RC-CAM

SlopeKing
May 20, 2005, 04:07 PM
The pot on the channel is fine, I tried it with the throttle stick, which does work... will have a look...

SlopeKing
May 20, 2005, 04:09 PM
Where do I find the opamp on the board?

Mr.RC-CAM
May 20, 2005, 04:12 PM
The pot on the channel is fine, I tried it with the throttle stick, which does work... will have a look...Use the voltmeter test. Do not rely on plugging the pot into another channel. The test is more than that.


Where do I find the opamp on the board?You'll have to ID it using the schematic's details. The LM324 is labeled as IC2.

SlopeKing
May 20, 2005, 04:25 PM
I get 4.83v on the outside terminals, and anywhere between 1.8 and 2.8v on opposite ends of the stick, change is pretty much linear... now the problem is diagnosing and, if needed replacing that opamp... which would just HAVE to be a quad wouldnt it, couldnt just be easy and be 4 small chips

Mr.RC-CAM
May 20, 2005, 05:18 PM
Before you swap out the OPAMP it would be good to check its output while you move the stick. You should see the voltage smoothly travel from ~0 to ~5 VDC, or nearly so. If the "bad" channel looks good there, then check the respective input pin on the microcontroller. If the signal looks good there as well then the microcontroller is confused.

You can use the other channels as a references for these additional tests.

RC-CAM

jay flay tway
May 22, 2005, 08:14 PM
Hi Slopeking. Just wondering if you ever either:
used a trainer cable with it?
or
charged it with the included wal-wart charger (if so, did you take the battery out?)

wondering which wrecked mine. It's too much of a coincidence otherwise. It stopped working right after i did those two things.

SlopeKing
May 22, 2005, 09:09 PM
Err... i frequently used a connection to the trainer port, for the FMS interface... mine didn't come with a charger (I opted to buy only the TX and RX).. but I bridged the diode and used my field charger... never had any problems... mine simply stopped working when I stopped using it... i'm now going to use one of the gimbals from it to control a camera in my RPV project (which requires the modification of my 9C)... so the repair project is cancelled