(Topic ID: 130497)

TerryB's Guide to Logic Probes


By terryb

4 years ago



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    #1 4 years ago

    Logic Probe Guide

    One of the simplest and cheapest tools you can include in your test equipment arsenal is a logic probe. Although a lot of people seem overwhelmed by logic probes, they are actually very easy to use. In regards to their purpose, consider a logic probe as a bridge between a meter and a scope.

    While a meter is great for reading constant voltages (see first image below) they fall short when a signal is pulsed (see second image below, which is a 12 volt pulsed signal from the switch matrix). What the meter will try to do with this signal is average it and give you a single voltage reading, which is not very helpful. Of course a scope works great on pulsed signals, but is much more expensive, and complicated.

    Specific to pinball, both the lamp and switch matrices are pulsed, plus circuits on the cpu, display and sound boards. While you can infer readings from the switch matrix, for example, using a meter it is much easier to just use a logic probe.

    12-volt-signal.gif

    Switch-Matrix-Pulse.gif

    #2 4 years ago

    Buying a Logic Probe

    In the image below you can see my recommended logic probe, the Elenco LP-560, available at Amazon for $17. You can spend more, but this is really all you need. In addition to all of the standard features (which we'll discuss more in a little bit) it also provides an audible tone in addition to the led's. While this will not benefit you much initially, as you become more proficient there are times when the audible tone provides a better indication of a pulsed circuit than the leds.

    The only function that it does not have is a pulser, which allows you to apply a signal to a circuit. This is a fairly advanced technique and most hobbyists will never have need for it.

    Elenco-LP560.jpg

    #3 4 years ago

    Logic Families

    Note: The information in this section has been simplified in order to align with the goal of a beginner level guide. For example, both CMOS and TTL gates have different input and output logic levels, although we will consider them as being the same for our purposes. While not necessary, if you want to fully understand the differences between TTL and CMOS logic levels see the following article at All About Circuits.

    http://www.allaboutcircuits.com/textbook/digital/chpt-3/logic-signal-voltage-levels/

    There are different logic families, or generations, of integrated circuits. Each logic family has different behavior and within each logic family there can be subsets with different characteristics. The only two we need to be concerned with in regards to our discussion are TTL and CMOS.

    TTL chips use a nominal Vcc (Vcc is the fancy term for the supply voltage) of 5 volts and the inputs and outputs are always binary (low, high or pulsed). TTL chips typically, but not always, use a standard naming convention of 54XX or 74XX.

    On the other hand, CMOS chips can use a Vcc ranging from 3 - 15 volts and depending on the chip can have either binary (low, high or pulsed) or analog inputs and outputs. CMOS chips typically, but not always, use a numbering convention of 40XX or 45XX.

    One example of CMOS in a pinball machine is the LM339 voltage comparator used in Williams/Bally switch matrix circuits. We'll discuss this in more detail as we get into the switch matrix examples, but for now the important part is to be able to recognize whether an IC is TTL or CMOS. If in doubt, you can always check the datasheet for any given IC.

    #4 4 years ago

    Logic Families cont.

    Based on the logic family of the chip there are different voltage ranges that are considered to be low or high in a digital circuit. In the case of TTL the low range is 0 - .8 volts and the high range is 2 - 5 volts. So any reading between 0 and .8 volts is considered a logic 0 and any reading between 2 and 5 volts is considered a logic 1.

    The specification for CMOS circuitry in a 5 volt circuit is a low range of 0 - 1.5 and a high range of 3.5 - 5. For a 10 volt Vcc the low range would be 0 - 3 volts and the high range 7 - 10 volts. The high and low voltage ranges scale linearly across the possible supply voltages of 3 -15 volts.

    Thankfully you don't need to remember all that though, since there is a TTL/CMOS switch on the Elenco (and in fact all logic probes except for those that are auto-sensing). Put the switch in the correct position (based on the previous information about CMOS and TTL) and it will correctly read low and high signals for that logic family.

    #5 4 years ago

    Logic Probe Features

    The first thing you will notice is that the logic probe has two wires (red and black) with alligator clips at the end. This is where the probe gets it's power and they must be connected to ground and supply voltage. If you're testing a 5 volt circuit, the red lead goes to 5 volts and the black lead to ground. If you're testing a 12 volt circuit (parts of the switch matrix, for example) the red lead goes on 12 volts and the black lead on ground.

    The pointy thing at the other end from the two wires is the probe. Unlike a meter this single probe is all you need to take your readings.

    There are two switches, TTL/CMOS and MEM/PULSE, that will need to be set properly. If you're analyzing a TTL chip, put the TTL/CMOS switch in TTL and when checking a CMOS chip, put the switch in CMOS. The MEM position on the MEM/PULSE switch will capture a pulse and retain the reading, which is advantageous in some rare situations, but for our purposes here you want it set to PULSE.

    The last, and most important part, of the logic probe are the HI/LO and PULSE led's. The red (HI), green (LO) and yellow (PULSE) led's are used to indicate the state of the measurement point. Note: Some logic probes use different combinations of lights to indicate the status, so just a reminder, I'm specifically talking about the Elenco logic probe here.

    In the first image below you can see the various signals that can be indicated by the led's. In most cases you can narrow these down to three issues: is the line high, is the line low or is the line pulsed. The next image provides another representation, comparing the led's to what you would see on an oscilloscope.

    Elenco-Indicators.gif

    tools-logic-probe-hi-lo.png

    #7 4 years ago
    Quoted from flipnout1:

    Very nice Terry!

    I figured it was about time you learned how to use a logic probe. I'm sure they didn't give you fancy pieces of equipment like this in the marines.

    #8 4 years ago

    recently purchased this probe with the hopes of figuring out how to use it. thanks for doing this

    #9 4 years ago

    What's a good probe to get with a pulser? I could've used that feature a few times and it's about time for me to upgrade.

    #10 4 years ago
    Quoted from fattdirk:

    What's a good probe to get with a pulser? I could've used that feature a few times and it's about time for me to upgrade.

    I'm not aware of any probe/pulser combos that have an audio beeper. Other than that, the Elenco LP-900 should get the job done. It is also 50MHz instead of 20 if that matters to you. I should also add the LP-900 does not have a ttl/cmos setting. As far as I can tell it uses a fixed logic threshold for both. This generally should not be an issue, but it's worth mentioning.

    The other choices would be Tenma or B&K Precision.

    #11 4 years ago

    Switch Matrix Example

    Now let's look at a real world example (Williams WPC in this case, but the theory is the same on other games) to see how the logic probe works when testing the switch matrix. Note: It is beyond the scope of this article to cover how the switch matrix works. See the following link for more information on the switch matrix.

    http://pinballrehab.com/1-articles/solid-state-repair/repair-guides/146-switch-matrix-theory-and-troubleshooting

    The first image below provides a generic WPC switch matrix circuit and we'll walk through what each test point should look like, starting with the column, or send, signals.

    The ULN2803 is a TTL chip that uses 5 volt logic on the input (point B) and controls a 12 volt signal on the output (point A). So the logic probe should be set to TTL and the red lead connected to 5 volts when testing inputs and 12 volts when testing outputs.

    Tip: If you look at the second image below you will see three red circles with pull-up resistors and a supply voltage within them. If the pull-up resistor is connected to a 5 volt supply you know you are working on a 5 volt circuit and if it's connected to a 12 volt source you know you're working on a 12 volt circuit.

    With our logic probe connected to 5 volts and the probe on point B we will get a green light and the yellow light will be pulsing. This indicates a low signal with high pulses. This signal is a constant timing pulse and will not change based on the status of the switch.

    The circle shown at point A tells us that the output signal from the ULNL2803 is inverted. So a high input provides a low output, and a low input provides a high output. Therefore, with our logic probe connected to 12 volts and the probe on point A we will get a red light and the yellow light will be pulsing. This indicates a high signal with low pulses.

    Note: Thanks to zaza for the corrected switch matrix diagrams (these are incorrect in the WPC manual and show an additional connection on the 10K resistor that does not exist).

    Switch-Matrix-Circuit.png

    Pull-Up-Resistors.png

    #12 4 years ago

    Switch Matrix Example cont.

    The row side gets slightly more complex and the readings will change based on the status of the switch. The first part of the circuit we are concerned with is the LM339. It is a CMOS chip that takes a 12 volt signal on the + input (point C) and provides 5 volt logic on the output (point D). So the logic probe should be set to CMOS and the red lead connected to 12 volts when testing inputs and 5 volts when testing outputs.

    With our logic probe connected to 12 volts and the probe on point C we will get a red light with the switch open, which indicates a high reading. With the switch closed we will get a red light and the yellow light will be pulsing. This indicates a high signal with low pulses.

    With our logic probe connected to 5 volts and the probe on point D we will get a red light with the switch open, which indicates a high reading. With the switch closed we will get a red light and the yellow light will be pulsing. This indicates a high signal with low pulses.

    The 74LS240 is a TTL chip and since there is a circle on the output we know that the signal is inverted. So with our logic probe set to TTL, connected to 5 volts and the probe on test point E we will get a green light with the switch open and a green light with the yellow light pulsing with the switch closed. The former indicates a low reading and the latter a low reading with high pulses.

    The image below provides a graphical representation of the logic probe led status for each test point.

    I've really tried hard to keep this at a beginner level while still providing the necessary information and would appreciate any feedback on stuff that needs clarification or is confusing.

    Switch-Matrix-Readings.png

    #13 4 years ago

    I've really struggled to explain logic probe troubleshooting to people and this is an excellent reference for it.

    This is a great contribution to the community. Thank you Terry!

    -Jay

    #15 4 years ago

    Thanks for doing this. Added to favorites. This is a helpful practical discussion on the use of logic probe for pinball.

    #16 4 years ago

    This will be sent to many people in the future. Thanks for taking the time to do this.

    #17 4 years ago

    Great post - thanks!!! How does the rate of the blink on the Pulse LED relate to the actual frequency. In the one diagram it says "the rate of which indicates the frequency of pulses". It couldn't be a one for one relationship or you wouldn't see it blinking (in the case of the switch matrix for example)?

    #18 4 years ago
    Quoted from not4tilts:

    How does the rate of the blink on the Pulse LED relate to the actual frequency. In the one diagram it says "the rate of which indicates the frequency of pulses". It couldn't be a one for one relationship or you wouldn't see it blinking (in the case of the switch matrix for example)?

    The signal is manipulated before it goes to the led, for example there is a pulse-stretching circuit so fast pulses can be seen. This is why the audio can often provide more information once you get a good ear for it. At a certain point though, it will just look like the light is on solid. I should probably clarify my comment on the drawing since it is a little misleading.

    #19 4 years ago

    Pro Tip

    I often hear people comment they are concerned about sticking a probe or lead into the backbox while the game is on. Here's a couple of products that most professional technicians carry that will help make your life easier, and safer. In both cases you can setup all of your test points with the game off, and then turn it on to take your readings.

    I should clarify by safer that I meant for the game, as in not shorting a couple of pins/components together. As long as you keep your fingers away from AC and the DMD high voltage section you've got nothing to worry about.

    The first is a minigrabber test clip, which has a small J shaped, spring loaded clip for attaching onto resistor, capacitor and transistor leads (see image below--the bottom test clips are the minigrabber type). Due to their design they have little chance of shorting to another component once installed. Buy ones that are long enough you can place the other end on the playfield glass.

    The next item is a DIP clip for testing IC's (see image below). The clip is placed over the IC and provides extended test points where you can use a minigrabber or your test probe to take readings. They come in a variety of sizes, but if you don't want to buy a full selection, you can use a 14 pin test clip which will fit on IC's with more pins, although not all pins will be available for testing at one time.

    In some cases you can also use a larger DIP clip on a smaller IC if there is no physical obstruction on the board (since the clip will extend beyond the IC).

    Ed at Great Plains Electronics sells the 3M product at a great price ($18 for a 14 pin).

    https://www.greatplainselectronics.com/products.asp?cat=6

    Test-Clips.jpg

    DIP-Clip-and-J-Clips.jpg

    #20 4 years ago

    DIP Clips make it very easy to test DIP style IC's so I started selling them awhile back. The ones I have are the 3M clips. Nice but a bit higher priced than the blue Pamona's:
    https://www.greatplainselectronics.com/products.asp?cat=6

    Ed

    #21 4 years ago

    TerryB, If you want I can make this little mistake (pics post #11 and #12) disappear in this 99% perfect guide

    edit: done

    #22 4 years ago
    Quoted from zaza:

    I can edit and upload the 3 pictures in this post and remove them here afterwards when you have put them in place. Idea ?

    That would be great zaza. Thanks for catching that.

    #23 4 years ago
    Quoted from G-P-E:

    DIP Clips make it very easy to test DIP style IC's so I started selling them awhile back. The ones I have are the 3M clips. Nice but a bit higher priced than the blue Pamona's:

    I would suggest people buy from Ed and support our community. The 3M is a better product than the Pomona so you're getting your money's worth, plus that is a great price that Ed is offering.

    I added the Great Plains Electronics link to my post above (sorry Ed I didn't realize you carried them).

    #24 4 years ago

    Thanks zaza, I updated my posts with the new images.

    3 weeks later
    #25 4 years ago

    I'm still seeing a lot of questions on logic probes, so a little bump is in order. This is really a must have tool if you're going to do board repair.

    #26 4 years ago

    I believe the best setup is a separate pulser from the logic probe. You can inject a pulse(s) up-circuit and measure with the logic probe for the expected logic result down circuit.

    #27 4 years ago

    Great thread Terry.

    How about another "example". Suppose you had a Space Shuttle that was having an issue with the lamp column IJ7-9. The transistors and resistors tested good in the circuit so you wanted to test the PIA U14.

    What reading on the the logic probe should we expect at pin 17? How do we know what pin is the input for pin 17 and what reading would we expect there?

    Should be a great learning experience for all, or at least me

    image.jpg

    #28 4 years ago

    Thanks for the DIP clip info. I didn't know about these!!

    #29 4 years ago
    Quoted from terryb:

    ....
    With our logic probe connected to 5 volts and the probe on point B we will get a green light and the yellow light will be pulsing. This indicates a low signal with high pulses. This signal is a constant timing pulse and will not change based on the status of the switch.
    Switch-Matrix-Circuit.png
    Pull-Up-Resistors.png

    I'm a little confused on how you are supposed to know what the probe should be showing, and how to figure that out. I know you said, "In the case of TTL the low range is 0 - .8 volts and the high range is 2 - 5 volts." but I cannot relate how that coincides with the schematic.

    #30 4 years ago
    Quoted from smailskid:

    What reading on the the logic probe should we expect at pin 17? How do we know what pin is the input for pin 17 and what reading would we expect there?

    In regards to pin 17 you want the game to be in a situation where that lamp should be going on and off (attract mode is typically a good choice). Or you can go into lamp test and turn the lamp on and off while monitoring pin 17.

    Over time you'll learn what state a line should be in when it is enabled or disabled, but when you're starting just look for the state to change when you change the input.

    There is not a one-to-one relationship between pins on a PIA. It taks in 8 bit data and converts that to individual outputs. If you were missing a data/address line you would see multiple problems.

    #31 4 years ago
    Quoted from laanguiano:

    I'm a little confused on how you are supposed to know what the probe should be showing, and how to figure that out. I know you said, "In the case of TTL the low range is 0 - .8 volts and the high range is 2 - 5 volts." but I cannot relate how that coincides with the schematic.

    I think you're asking how do you know if the test point should be low or high?

    If you don't have extensive electronics experience (to the point where you can just read a circuit and figure out what is going on) there are a couple of choices. Probably the easiest is to compare the circuit in question to a known working circuit. The next is as I described above, is to change the input and see if the output changes.

    For example if you are working on a solenoid problem. Put your logic probe on the test point and then activate the solenoid. If the reading was initially low it should go high and if the reading was initially high it should go low.

    If you want to get a little deeper into it then the following series of articles should help with a basic understanding of electronics.

    http://pinballrehab.com/1-articles/solid-state-repair/tutorials/147-solid-state-pinball-tutorials

    Also remember that IC datasheets are your friend. The datasheet for any IC will typically provide a truth table that will tell you what the output should be based on the possible inputs.

    If I missed the point of your question let me know.

    #32 4 years ago
    Quoted from smailskid:

    Great thread Terry.
    How about another "example". Suppose you had a Space Shuttle that was having an issue with the lamp column IJ7-9. The transistors and resistors tested good in the circuit so you wanted to test the PIA U14.
    What reading on the the logic probe should we expect at pin 17? How do we know what pin is the input for pin 17 and what reading would we expect there?
    Should be a great learning experience for all, or at least me
    image.jpg

    Logic Probe U53 P4. That should be pulsing always when the lamps are strobing (use attract mode or lamp test mode). If you have pulses there. The PIA output is good.

    Next logic probe U53 P6. You should have pulses there identical to what you saw above on u53 p4. If no pulses the blanking/buffer 7408 IC at U53 is bad

    Then you can logic probe q22 for pulses. only thing left after that is the main strobe transistor q30.

    If you are unsure, you have sever other identical column circuits to compare to.

    2 weeks later
    #33 4 years ago

    Sometimes, when I poke at a chip with my probe, a pin might be neither high nor low. It's just... dead. I can kind of understand how an input pin might read dead (based on the state of the circuit leading to that pin) but why would an output pin on something like a 7408 or 7402 read anything but low at the very least?

    #34 4 years ago
    Quoted from UvulaBob:

    Sometimes, when I poke at a chip with my probe, a pin might be neither high nor low. It's just... dead. I can kind of understand how an input pin might read dead (based on the state of the circuit leading to that pin) but why would an output pin on something like a 7408 or 7402 read anything but low at the very least?

    Can you provide a specific example? Also keep in mind that sometimes not every gate on an IC is used.

    #35 4 years ago

    I'm working on a Firepower right now, and IC7 (which is a 7408 quad AND gate) has:

    No signal at all on pin 4 (Input A)
    A long-low-short-high pulse on Pin 5 (Input B)
    No signal on pin 6 (The Output Pin)

    Pin 4 is connected to Pin 19 of PIA 4 (the Solenoid PIA). Why would that line not give a reading? And why would the output pin also be dead? Should that at least be low, since one of its two input pins is dead?

    I guess what I'm wondering is this. Of all the states a logic probe is presented as possibly being in, "no lights or sound at all" isn't one of them. Under what circumstances is would a logic probe legitimately show no activity whatsoever?

    #36 4 years ago

    I'm no system 6 expert, but from what I see in the 6802 datasheet I would expect to see a valid logic level on pin 4 (CB2) and thereby pin 6. Do any of the switches work?

    Quoted from UvulaBob:

    I guess what I'm wondering is this. Of all the states a logic probe is presented as possibly being in, "no lights or sound at all" isn't one of them. Under what circumstances is would a logic probe legitimately show no activity whatsoever?

    No lights and no sound would indicate a bad logic level or no input (see the first image under logic probe features). There are some rare instances where you will see an input or output that was left floating (by design) and you could get wonky readings. You seldom see this though since it is not generally recommended to design a circuit this way.

    Maybe someone with more system 6 experience can jump in here and be more specific in regards to your situation.

    2 months later
    #37 4 years ago

    Terry asked me to share this story. I picked up a cherry looking Whirlwind a month ago, and had yet to play a game. The right sling shot locked on at startup, and once I solved that, the REAL problems surfaced.

    I have literally spent the past 4 weeks troubleshooting two seemingly unrelated issues with the MPU, because a.) I want to learn board repair b.) I got a good deal and didn't want to spend $3-400 on board repair or taking the easy way out with a Rottendog. It turns out that had I sent my MPU in for repair, it would have been sent back to me.

    During this exhaustive and frustrating process, spending seemingly hours in the garage, I became an expert at schematics, logic chips, circuitry, flow, and could probably fix any System 11B board haha.

    Here were the two issues with the game.
    1.) Sound Overlay Solenoids (3 lightning flash lamps, topper blower and playfield spinner discs) all locked on at startup or never worked at all. Which of these occurred was seemingly random.
    2.) The music/speech didn't work, but the second test for the sounds all worked.

    The System 11B MPU board had prior battery corrosion, but it was neutralized (and I reneutralized it). The traces looked OK, and all pinned out OK with continuity. With my initially limited understanding of schematics (never had to deal with much beyond blown TIP102/TIP36C), I posted a tech thread here and got little response, so I took my college physics knowledge and went to work.

    I created a neat molex connector that went from an arcade switching power supply to 1J17 so I could work on the bench.
    I replaced and socketed U42 (unnecessarily) -- although was on the right track. It was near the corrision area and goes to the sound solenoid overlay board.
    I replaced SRC 1 and 3 (possibly unnecessary, but in the corroded area so this was easy and worth it).
    I took all the boards except the MPU up to the arcade. Everything swapped into another Whirlwind and worked perfectly, meaning both the solenoid group lock on and sound problems HAD to be with the MPU.

    OK, time to open the Elenco logic probe which I've had in the drawer for two years and never needed. This thing is stupid easy to use (and never even saw this thread before). Read the back of the package and find the ground and +5 test points to clamp onto. Now's where this thread gets interesting.

    I replaced the blanking circuit at U43 and Q50 -- thinking the behavior was some bizarre timing issue. The blanking circuit would begin HI, instead of LO then HI after the CPU booted. This was a good fix, but unrelated.
    I purchased Leon's TEST ROM and everything checks out perfectly at the PIAs out to the pins. WTF. Now I was getting pissed. How was this possible?

    Last night, I was using the sound solenoid overlay board to try knock out the bizzarre solenoid issue. Again, they either didn't work at all, or locked on when I turned on the machine. I knew the problem was with the MPU, but I figured I'd work backwards. The Whirlwind manual provides a nice flow chart on how it the solenoids are powered. All the transistors were good, taking me back to the 74LS374 flip-flop logic chip, U1, immediately downstream of the input ribbon cable. I pinned out U1 on the sound solenoid overlay with the logic probe with the solenoids in the non-working state and wrote them down. I did the same with the solenoids locked on (pulling the 50V connector to avoid blowing a fuse). I then did the same with the ribbon cable input at J2. I couldn't figure out how the U1 74LS374 chip was ending up in a "locked" state with the clock signal HI. The outputs on U1 for the solenoids were either all HI (off) or all LO (on) and wouldn't change. Even more bizzarre, NOTHING changed on the input pins from J2 in either the locked-on or working case. So I googled 74LS374 and pulled up the data sheet to see what this chip is supposed to do. Stumped, I went to bed.

    I re-read it again this morning, and it kept talking about the clock (pin 11) going low to high in transition to latch the input and output to LO or HI. This got me thinking this morning that maybe the clock pin was supposed to be pulsing and not HI.

    Looking at the schematics from the MPU to the Sound Solenoid Overlay board... MD0 thru 4 coming off pins 3-7 on 1J21 on the MPU are the solenoids, while MD5,6 and 7 (pins 8,9,10 on 1J21) all connecting via ribbon cable to 5J2 on the sound solenoid overlay board. According to the schematic, MD5/6/7 (whatever they are) all control both the clock signal on U1 and are also somehow used at U20 to work with the sound board. AHA!!!

    Oddly enough, when I probed all the pins on ribbon cable input J2 last night, I made an annotation next to pin10 (MD7) because the probe read HI, but it was "fuzzy/scratchy" sounding as if there was a subtle pulse in there. The "low" LED kept glowing very faintly every second or so. AHA!!! I checked it again just now and sure enough, same thing.

    OK, back to the MPU. 1J21 pin 10 for MD7 comes from U42, pin 17, which is PULSING CLEANLY!! I then check SRC pack 1, pin 9 next in line (which I had replaced last week). CLEAN. I check 1J21 pin 10. CLEAN. I check the other end of the ribbon cable at pin 10. HI and scratchy. ARE YOU F--KING KIDDING ME?!!

    The problem: A bad ribbon cable. I jumped 1J21 pin 10 to J2 pin 10 and now all the music speech works perfectly and the solenoids behave perfectly!!

    Lesson Learned: Logic probe signals should be clean - LO, HI, PULSE. Visibly and audibly. I'm not sure how I could have saved any time here. None of my other games used a 20-pin ribbon cable to borrow from. In this case the test ROM passed, no errors with the game ROM, which despite my thinking the issue was at the PIA , pointed to something else. One would have easily suspected the corroded area, and despite confirmations of a strong continuity signal across the MPU traces, I replaced parts that looked suspect (probably good to get rid of the corrosion anyways). After that I used the game manual to understand the flow to the solenoids, and noticed that 74LS374 appeared "locked" in the turn on state, which is not how a flip-flop chip should behave. I noticed that the MD7 signal seemed off, and the key to all of this was the logic probe. After noticing the LS374 clock and the music were joined at that MD7 signal, it was then finding out how I was going from strong pulse to "scratchy hi".

    #38 4 years ago

    Bookmarked this

    #39 4 years ago

    Awesome! I still need one

    #40 4 years ago

    Thanks for sharing that wxforecaster. While the theory shit is great a real-life example is worth a 1,000 words. I mentioned this earlier, but this is why learning how to "read" the audio will sometimes give you more information than the led's.

    Good troubleshooting, btw.

    Don't have any soldering stories do you???

    #41 4 years ago

    Excellent writeup of how to diagnose your problem. Thanks for taking the time to do so.

    #42 4 years ago
    Quoted from terryb:

    Thanks for sharing that wxforecaster. While the theory shit is great a real-life example is worth a 1,000 words. I mentioned this earlier, but this is why learning how to "read" the audio will sometimes give you more information than the led's.
    Good troubleshooting, btw.
    Don't have any soldering stories do you???

    Elenco LP-560 logic probe. Bought it on Amazon for like $30 on sale a few holidays ago. I clearly saved like $200-$300 in fees sending a board out for repair (who knows what it would have been returned with -- correct answer is no change), and I learned a ton about LS chips, transistors and schematic reading w/o paying for professional courses.

    As for soldering. I'm a self taught "expert". Practice, practice, practice. Seriously, I am fixing my kids stuff weekly. You have no idea how badly the Chinese solder garbage thin-strand 22 gauge wire and hot glue gun it together. Sooooo many throw-away products can be fixed in the time it takes to heat up the soldering iron. Guitar Hero drums, outside solar LED lights, daughters LED changing night light pet, etc.. My findings are pretty simple -- soldering is all about temperature and using 60/40 .031" solder. I have a temperature controlled Tenma that I bought via referral from rec.games.pinball like 15 years ago. It still works perfectly and I'm still on the original tip!! I found that 650 degrees is the perfect operating temperature for pinball boards. I see soooo many destroyed through-hole plates with those "on/off" XX WATT radio shack soldering irons. They're either too cold or too hot. I don't even use a desoldering iron to remove stuff (although I plan on getting one soon) -- the Tenma and my $3 solder sucker works great.

    3 weeks later
    #43 3 years ago

    What a fantastic guide! Thank you TerryB!

    1 year later
    #44 2 years ago

    Terry, three years ago someone asked about which probe to get & you recommended an Elenco LP900. He asked if it was audible & you said that it's not necessary. Is that still true? If it's better to get one with audio, can you recommend a cheap one?

    Quoted from terryb:

    It is not audible. I've never considered that an important issue and most of the companies that made audible logic probes in the past have discontinued them. I'm sure you could look around and find one, but there's none I'm familiar with.

    #45 2 years ago
    Quoted from Topher5000:

    He asked if it was audible & you said that it's not necessary.

    It is a nice feature if you work with a logic probe a lot and get a good ear for the tones. In some circuits you can hear problems that you might not catch using the led's. In my case though I usually just move on to an oscilloscope on those types of circuits. So bottom line it can be nice if you use a logic probe a lot and have a good ear, but is not really necessary for most people.

    The Elenco 560 I recommended at the beginning of the article has audible tones.

    1 month later
    #46 2 years ago

    Good night, I did not understand very well the part of knowing if the integrated is cmos or ttl.

    How do you know if the integrated is cmos or the integrated is ttl?

    For example: to test the array of switches to see if the signal arrives at the pin connector how do you do it please?

    #47 2 years ago

    If you're checking the row or column at the connector that goes to the playfield that is CMOS. So connect the power to 12 volts and ground and set the switch to CMOS.

    The first clue is the signal is 12 volts at that point and TTL chips are limited to an output of 5V. If you want to confirm just google the datasheet for the associated IC. If you're working on a WPC game that would be a ULN2803 for the send and LM339 for the receive. Both are CMOS.

    #48 2 years ago

    Sorry, I was to see the datasheet of the integrated ones and still did not understand very well.
    The plug j205, j207, j209 are tested with the logic probe in ttl ?

    See attached image

    Capturar (resized).JPG

    #49 2 years ago
    Quoted from terryb:

    In the case of TTL the low range is 0 - .8 volts and the high range is 2 - 5 volts.

    So yes, you can derive the answer from those numbers. The high range is a minimum of 2 volts and the low range is a maximum of .8 volts so that is a TTL chip. The chip inputs/outputs would be tested with the logic probe set to TTL and power from 5 volts (the chips Vcc). Normally the datasheet will state whether a chip is TTL or CMOS, but that one does not appear to have that information.

    Quoted from pedroborges:

    The plug j205, j207, j209 are tested with the logic probe in ttl ?

    At J206, 207, 208 and 209 you would set the logic probe on CMOS and get power from 12 volts.

    Have a look at the image in post 12. The 74240 chip is TTL with a Vcc of 5 volts, but the chip that reads the signal at the connector is an LM339 which is CMOS and the Vcc is 12 volts. Different chips on the send circuitry, but the same issue applies. It is not uncommon for TTL and CMOS chips to be intermixed in the same circuit.

    #50 2 years ago

    thanks
    But in j206 one should not choose TTL since the uln2803 connects in plug j206 ?

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