Need a little help on some boards - Page 1
Forum section: electronics / repair

author: RyanG
author_rank: Regular Contributor
timestamp: December 01, 2018, 06:59:27 am
content: I had a guy drop some boards off to me. I work on bowling stuff and these are more of the same, but I have never worked on these before. I think they accept keyboard input to send to the main processor box for a scoring system.The board takes 24v input. It uses an LM2575HVT to step that down to 5v.The guy brought me 3 of them. One of them is known good while the other two are bad. The first thing I did was check the electrolytics, which were all good. Then I checked the resistance on the 5v section of each board. That is where I found the main difference. The good board had a resistance of around 1k while the other two were at around 67 ohms and 141 ohms.The main processor on the board is an MC143150B1FU1. The SERVICE pin on the processor is constantly high on the two bad boards, which is indicated by a red LED. The good board lights the same red LED very briefly when it is first powered up.I'm assuming it is just a short somewhere on the 5v, but what would be the best way to go about figuring out where that is on this board? I tried leaving it on for a while and nothing got really hot. The only thing that I noticed that got warmer on one of the bad boards were the LS273s near the center of the board. The two bad boards do draw more current. One draws only about 20ma more and the other is a bit worse with around 100ma more. That is with me bypassing the LM2575 and putting 5v straight into the board.

author: jfphp
author_rank: Regular Contributor
timestamp: December 01, 2018, 10:50:02 am
content: There are several techniques to find an abnormal current demand but the current here is not very high.1. Silly but long and not easy : thermometer. Probe on every component and comparison with the good board2. Infrared : you have to get a thermal camera (Fluke for instance), shut the light and carefully take views of the board,(comparison with a good one is better). You can find the hotspot where the current is abnormaly high (difference in color).3. Old HP technique : inject a pulse through a logic pulser 546A into the 5V and follow the pluses on the board with a 547A current tracer. You will find the problem, easier of course through a comparison with a good board.4. Specialized instruments to mesure mOhm resistances, inject pulses... Huntron, Bicotest... Not cheap and tricky to use.

author: Samogon
author_rank: Frequent Contributor
timestamp: December 02, 2018, 01:26:19 am
content: Polar Toneohm, can find lowest resistance with inductive probe.Now they too expensive for single project.

author: Shock
author_rank: Super Contributor
timestamp: December 02, 2018, 02:03:38 am
content: ESR meter that measures in the milliohms to check the resistance between rails at different devices or using a milli or microvolt meter and checking the voltage drop along the supply rail can also work.There is also hot and cold tests, if you are very systematic you may see the current draw change on the faulty component.You can also lift the supply pin to various components and remove and test components out of circuit and double check the supply resistance between them. Making a rough schematic can help with this.Don't forget to remove the regulator as a suspect.

author: RyanG
author_rank: Regular Contributor
timestamp: December 02, 2018, 07:50:55 am
content: I tried lifting the supply on one of the LS273s that had a bump in the center of it and the resistance on the 5v rail went up to about 660 ohms instead of the 67 ohms that I was getting. That is still lower than the 1kohm I get on the working one, but at least I found something. How would the ESR meter work since I am getting 67ohms across the rail normally? Wouldn't it just always read the ESR of the cap on the supply rail?

author: vk6zgo
author_rank: Super Contributor
timestamp: December 02, 2018, 09:16:50 am
content: Quote from: RyanG on December 02, 2018, 07:50:55 amJust chase it with a plain old ohmmeter.Have a look at whatever else is on the same supply rail as the LS 273, & disconnect things one at a time tillyou get the same resistance as the good one.If you get really frustrated, cut the track & see if whatever is loading it down disappears.I don't like doing that, unless I have to, because you then need to do a track repair.

author: Shock
author_rank: Super Contributor
timestamp: December 02, 2018, 10:37:48 am
content: Quote from: RyanG on December 02, 2018, 07:50:55 amThe ESR and volt drop methods are just other ways to find low ohm shorts but you also need enough resolution, which is the catch for ESR. Sorry if I didn't explain but certain ESR meters can be used similar to circuit tracers due to their test frequency and voltage (won't turn on junctions). You can also use ohms/diode tests in both directions to get a feel for components behavior.From what you are telling me the likely fault is the VR but being taken out by another component. Which was probably too much current either a slow overheating or fast death. In case you overlooked these steps look for dry/bad joints and cracked/damaged or discolored components under magnification on both sides. Normally if the board has many hours on it or several years old I'll check electrolytic caps as well (ESR and capacitance).Set up the good board like the bad board (VR removed) work out its idle current consumption and then current limit the bad board like it. Does anything get hot? Turn up the current limit a little, does current consumption keep increasing? Does hitting a component with hot or cold change the current consumption?You can use the voltage drop method in comparison to the good board to see if something is sucking current. Unless it had volatile config I would remove anything in socket as a way of narrowing down the troubleshooting.If in doubt measure it out. Start making notes U1=, U2=, U3= etc and compare the good and bad board. I'd avoid doing two faulty boards at once. Then if you are still clueless move onto component by component.

author: RyanG
author_rank: Regular Contributor
timestamp: December 02, 2018, 07:03:39 pm
content: I am working on only 1 of the bad boards at a time since the fault might be the same on both. I actually pulled both of the ICs in sockets with no change. With the VR bypassed 1 board pulls over 100ma more than the good one. That is the one with the bulged LS273. I took that IC out of circuit but the red indicator LED still lit. I looked at the datasheet for the processor. The LED is connected to the service pin which is high during initialization of the processor. I thought maybe the processor was continuously resetting but the reset pin has constant 5v. There isn't much heat generated by anything even if left on for a while.

author: Shock
author_rank: Super Contributor
timestamp: December 02, 2018, 08:35:56 pm
content: Yeah just work on one till you get stuck at least.Read this post about using the voltage drop method.https://electronics.stackexchange.com/questions/16398/find-short-location-on-pcbEven though it's not a very low resistance short same thing applies.Its about 0.5W and possibly about ~50 ohms in parallel with the working circuit. It "seems" present in the circuit powered or not. You can identify the difference in the PCBs by resistance so use that as a quick check with each change.Copy down those rotary switch settings on the faulty board and when there is no power manipulate them gently back and forth a few times back to their original setting.See if C20 and C21 are a part of the circuit and have voltage across them if they are pull them with C8 and measure for ESR/Capacitance as well as if the rail resistance suddenly goes back to 1K. Remove C34 and check the same. Check if that big cap (input side of VR) has voltage when running from your bench supply. You want to be sure it's not affecting the circuit your testing.Perhaps the Echelon FTT-10A Twisted Pair Transceiver has 50 ohm termination always active on it?Check if J6 (or any others J1-J5?!?) is open on both boards. Check the bodge resistor is the same value on both boards.Take a photo of the underside if it has anything on it.

author: neil
author_rank: Contributor
timestamp: December 03, 2018, 02:58:00 am
content: If you turn the photo upside down, the eprom seems to have the word "Bad" written on it. Maybe test current drain versus good board with this chip pulled on each? Or swap the eproms?Or perhaps "Bad" was just a general comment referring to the board as a whole.

author: RyanG
author_rank: Regular Contributor
timestamp: December 03, 2018, 03:01:20 am
content: I'm going to try the voltage drop method on some stuff soon, but I can answer some of the other things now.I have already spun the rotary switches around a few times without any changes.C20 and C21 are in series with the input connector pins, so they are not on the 5v rail. I pulled C8 and checked its ESR(I checked every cap), which was good.C34 has 3.1v on it on all 3 boards and the resistance measured across it reads OL on the meter.J6 is not shorted, but if I short it when power is connected the red LED gets brighter.There is nothing on the bottom of the board.The bad marking on the EPROM was by the guy that gave this to me to mark the entire board as bad.

author: RyanG
author_rank: Regular Contributor
timestamp: December 03, 2018, 04:32:16 am
content: I left the board on for over an hour. Both LEDs went out but the current was still being drawn by the board. The 4 ICs in a line and the GAL in the socket were all warm while everything else was still room temp.

author: SMdude
author_rank: Frequent Contributor
timestamp: December 03, 2018, 11:24:28 am
content: Is the oscillator working?

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 03, 2018, 03:44:38 pm
content: A thermal camera and oscilloscope with a "component tester" would be my tools of choice for this kind of work.Unpowered comparison of the LS273 outputs V/I curves WRT to GND or/and Vcc usually reveals where something has gone wrong quite fast. (Especially with a good board at hand!)Something going wrong in this case is likely to have happened on the IDC connectors i.e: Swapping connectors, shorting, ESD... Destroying one or more of the LS273 outputs.The quick solution would be replacing all the LS273.The GAL being a fast device, probably runs warm in normal conditions.

author: RyanG
author_rank: Regular Contributor
timestamp: December 03, 2018, 07:56:05 pm
content: I lifted the VCC pins of both of the LS273s. The resistance across the 5v rail only went up to 600 ohms, which is the same as when I only lifted the one on the bulged one.

author: shakalnokturn
author_rank: Super Contributor
timestamp: December 04, 2018, 12:19:49 pm
content: Depending on how it is wired and how well it has been destroyed internally you could have an input tied to Vcc giving you the unexpected result.Try also lifting ground pin, or as you will have to replace at least one anyway, remove it completely.