Garage Door Remote Repair Advice  - Page 1
Forum section: electronics / beginners

author: michaelsalley
author_rank: Newbie
timestamp: March 19, 2015, 03:51:30 pm
content: I'm posting in "Newbie" instead of "Repair" because it has to be one or the other, and I'm a newbie to thisaspectof solving this sort of problem even though I do have a little bit of prior knowledge. Normally I know enough to find bad capacitors, replace them with new ones, and give up if that wasn't the issue. I own a multimeter (Extech 430) but no oscilloscope at this time.I'm trying to repair a Sears Craftsman garage door opener from a few decades ago. It does not use rolling codes or any newer securities such as those; it has a simple 9-way DIP block which provides for a finite number of variations on the signal. (I set my switches to a different configuration for these pictures so as not to give away my code, but I've already checked the obvious; it's the same code that works correctly on the other remote.)I do own an identical twin to this unit, which does work correctly, so I do at least have that for a reference point. I can do comparisons between them as needed.How I think it should work... if I'm wrong here then perhaps my tests were invalidIf I understand this thing, when either switch (S1a or S1b) is closed, the block of 9 DIP switches (they have 3 positions each) provides input to the 125C58 chip, which uses that input to create a specific variant of the output waveform which is then transmitted by the thing that looks like a wingnut with an allen bolt through the middle of it at the opposite end of the board from the battery input. (Is that wingnut thing an "oscillator"... a "transmitter"...?)The rest of the components on the board are just for voltage/current management and the LED. Fundamentally, the transmitter has to transmit what the chip tells it to transmit, and the chip makes that decision by the positioning of the nine switches.Am I right, generally? Have I missed something fundamental?The overall symptom is that when I push the button, the red LED lights but the door does not respond.What I've done so far1. I haven't desoldered every component to test them and get actual readings yet (my soldering is marginal and I don't want to cook components just messing around with them), but I have the same output voltage (about 6.6VDC) when reading from battery negative to either of the two largest contacts on what I think is the transmitter. The other remote control (identical to this one, except it works correctly) shows within a tiny margin of the same voltage when reading across the same points. I'm taking that to mean that there isn't a serious problem with any of the capacitors, resistors, transistors, or diodes.2. I've checked S1a and S1b switches with the ohm meter - they both open when released and close when depressed as expected.3. I also checked the resistance of all 9 switches, in all 3 positions, on both boards. Every single one performs consistently with its neighbors and also with the unit that works properly, so I am taking that to mean that the 9 switches are giving thecorrect inputto the TI chip. One of the three positions gives very low resistance, another gives nearly infinite resistance, and the other gives a finite and consistent resistance which is much higher than the low, also consistent one. No appreciable difference between either of my two boards in this test.4. On the assumption that the Allen screw inside the plastic "transmitter" might be some sort of trim or drift adjustment, I took it out to the garage and tried several different positions on that adjustment and never did get the door to respond. I tried to put it back close to where it was, but I may have lost the original setting there while experimenting. Hopefully it's something I can get back.5. I did see on the back of the board that it looked like some previous repair effort had been made; several solder joints look rather messy and have clearly been worked on in the past (not by me). The worst looking joint of all is the one that looks like it's been patched over at the base of the transmitter, but I used the meter and it's continuous to the pins below it. As cracked as it looks visually, it appears to be making solid contact everywhere it should be. That doesn't mean I haven't missed a physical solder joint problem but I *think* everything is fine in that regard.~~~At this point, I only have two theories. Either (A), the 125C58 chip is not producing the correct output given the correct inputs from the DIP switches, or (B) the transmitter either isn't receiving that waveform or can't transmit it correctly. I don't know how to test either of those things which is where I need help.I've gotten myself very confused trying to research where to go from here. At first I thought I needed an oscilloscope, but then I got to thinking that this thing transmits at 390 mhz (if my research has been correct) whereas most of the affordable used scopes on eBay are in the 10-100mhz range. I don't know enough about this to know whether that means I can't use that tool on this project or not.I've read several threads on other sites about using audio input on a nonvaluable PC and a scope program, or Audacity sound recording software, to mock up a "scope" enough for this type of project, and I'm willing to try, but thereagain I am low on knowledge as to how to do it safely. I understand the principle and I have the "mechanical" ability to rig it up - but I have conflicting information on what voltage is too much for such a contraption to safely measure, and I have conflicting information on what size of resistor to use in between the test lead and the device to reduce it below that threshold.I looked at devices such as the Bus Pirate as cheap ways to slightly more safely rig up a similar test environment, but then it seems to max out at 5V and again, this thing is using 9V input and outputting > 6V.I can follow instructions but I can't seem to think my way out of this hole at the moment.I know I need to read what this chip is outputting to the transmitter (somehow) and I know that it would be nice if I could read what the transmitter is actually broadcasting (harder) but I am not sure what I can do, on a budget, to figure those things out.I did look for a datasheet on the chip and it is not available. Looks like some kind of custom project for Sears, probably. Here is the thread I started on the TI forum about that, for what it is worth:http://e2e.ti.com/support/logic/f/151/t/407522Yes, I know the whole thing is easily replaceable. It's more about learning to repair this (and feeding an interest in the general field) than about getting the opener working ASAP. I can use the other remote indefinitely while I work on this project.Any pointers on what I need to test next, and what the bare minimum equipment I can do it with might be? If I need to rig up a sound card scope then I am going to need some very specific help on what components to use to reduce the voltage and in what order to connect them.

author: eejake52
author_rank: Contributor
timestamp: March 19, 2015, 04:56:20 pm
content: Very nice writeup - you obviously put some time into it.Here is a quick summary of the other parts.L1 (shaped like a wing-nut) is a variable inductor (i.e. a coil with a movable slug); this and a capacitor form the 315 MHz oscillatorQ1 is a plain transistor which turns the oscillator on or offYou are mostly correct about the operation of the circuit, but here is some further explanation. The whole circuit is powered on by S1a or b. When power is applied, the TI chip reads the 9 inputs, then encodes each bit as either long&short, or short&long, then all those long/short patterns are used to turn on/off a 38 kHz square wave which is then fed to the 315 MHz oscillator. Then the whole process repeats for as long as the switch is pushed.I think it will be tough to diagnose without some further test gear. So, if you had a scope, you would look at the base of the transistor and see if there is anything resembling 38kHz, in an on/off pattern that repeats. But you don't have a scope, so your options are limited. Here's what I suggest:a) double check all those solder joints, and clean excess solder between joints; especially around L1b) find someone who is a ham radio operator, and who has an RF frequency counter, and ask him/her to verify that the unit is putting out RF, and at about 315 MHz. If the frequency is close, but off, adjust L1 until it is 315. [NB: Canadian remotes from that era use 433 MHz; so if you're a canuck - it might be 433]. If you can't find a ham operator, some radio repair shops will help - if you can get past the front desk and into the back where the techs are.c) buy a 'learning' remote at HomeDepot (or perhaps use the one built into your vehicle). If it can learn from the known-good unit, but not from the known-bad unit, then you most likely have a problem with the oscillator.

author: Fank1
author_rank: Regular Contributor
timestamp: March 19, 2015, 09:49:24 pm
content: Assuming that it did work at one time ?The first thing I would check is the switch.A simple test would be to "carefully" short out the switch and see if it works.

author: amyk
author_rank: Super Contributor
timestamp: March 19, 2015, 10:48:12 pm
content: Based on the dust pattern on the switch sliders, I'm going to take a guess that your code was 0+++0+0--.Of course it is of no use as I have no idea where you live, so don't worry.The IC appears to be a remarked SN77702 (datasheethereand attached); at least the pinout matches, and it was made by TI. The 390k/100p combination on pin 14 is also unique.

author: rastro
author_rank: Frequent Contributor
timestamp: March 20, 2015, 08:41:20 pm
content: You could try an inexpensive ~$20.00 USB DVB-T as a rudimentary spectrum analyzer. Compare the good unit RF signal to the failed unit. You should be able to verify if there is any RF output relative to the good remote.start here:http://sdr.osmocom.org/trac/wiki/rtl-sdr

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2015, 08:57:58 pm
content: It's likely any scope probe held near will give some indication that it is transmitting OR not.

author: rastro
author_rank: Frequent Contributor
timestamp: March 20, 2015, 09:08:23 pm
content: Quote from: tautech on March 20, 2015, 08:57:58 pmAgreed - but OP says he doesn't have an o-scope.

author: tautech
author_rank: Super Contributor
timestamp: March 20, 2015, 09:15:34 pm
content: Quote from: rastro on March 20, 2015, 09:08:23 pmShit forgot that, been a day or so since I saw this thread.Everybody needs a scope.

author: kingofkya
author_rank: Regular Contributor
timestamp: March 20, 2015, 09:20:14 pm
content: Maybe a fequancy counter might give you an idea if its transmitting or not? Some multimeters have em. But as others have said check that switch its one of the easest things to check.

author: rmel
author_rank: Regular Contributor
timestamp: March 21, 2015, 04:23:31 am
content: Check if your garage door opener has an LED that lights up when it detects a compatible signal. When I was adding a new remote, I noticed that the LED would light up when I pressed the remote buttons, even before the remote was "paired" with my opener. This would at least tell you your remote is transmitting.

author: SeanB
author_rank: Super Contributor
timestamp: March 21, 2015, 05:45:57 am
content: If it ever got damp I would suspect a through hole via of being open circuit. I have one at work with at least 2 vias open circuit, making the RF section inoperative. They supply the 12V supply from the A23 coin cell stack to it permanently, so the damp caused the via to corrode internally and the track as well to corrode under the solder mask. It was easier to simply use another remote and delete this one from the systems instead, as I have numbered them and marked the number inside the housing as well. The cost of the more complex receiver ( able to do logging if I buy the expensive RS422 cable and the software which is nice but not exactly cost effective) and the ability to have 1000 individual remotes ( I have 40, just more than the standard one can handle) is a plus.

author: michaelsalley
author_rank: Newbie
timestamp: March 24, 2015, 07:02:34 pm
content: Thanks to all of you (I won't do an itemized list) for the suggestions, the datasheet, and the assistance understanding what all the things on the board are and what they do.A few folks suggested checking the switch - it does work properly. In addition to checking the switches out with an ohmmeter, I also have experience to suggest they work: when I press the switches, the red LED lights up on the unit. It's just the frequency/transmission which isn't happening or isn't correct -- the switches are making contact and the LED is illuminating so the circuit is at least trying to activate.I've got a scope on the way from eBay (20mhz, analog, dual trace... philosophically, would rather start with the "basics" and then work up)... assuming it arrives in one piece and is what it is supposed to be, I'll post back after I have some results from that (or after I get stuck trying to use it.) Until then, everything but rechecking the joints (which I'll do the next chance I get) is probably speculative at best.Has anyone ever tried one of the cheap eBay frequency counters? I know full well that they're probably not well built, not very accurate, and not pro-grade equipment... but this is to diagnose occasional problems, not to start a career. I'm talking about devices likeeBay auction: #400569532720oreBay auction: #281423258318.Are they even worth the fifteen bucks for issues like this? I'm assuming they might be if a certain amount of luck is involved.

author: rastro
author_rank: Frequent Contributor
timestamp: March 24, 2015, 07:17:31 pm
content: Quote from: SeanB on March 21, 2015, 05:45:57 amSean,This is a good observation. I've seen 2 different RF outdoor temperature go bad after a couple of years - even though the main indoor unit Clock/temperature still operates. I'm beginning to suspect this might have done in those units as well. I think I still have one. I'll have to take a look at it.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 24, 2015, 08:24:07 pm
content: Quote from: rastro on March 24, 2015, 07:17:31 pmExcept for the fact that this is a single side pcb...

author: kolbep
author_rank: Frequent Contributor
timestamp: March 24, 2015, 08:52:11 pm
content: Wash the board, then rinse with alcohol or distilled water, and let it dry...I had a situation once where a small tin whisker, or maybe some sea spray, was touching 2 pins together. That caused it to, when you push button 1, it transmits the codes for a button 1 and 5 together (a shift type of function). A good wash and scrub with a toothbrush sorted it out