Crane Scale repair issues - Page 1
Forum section: electronics / repair

author: jerryk
author_rank: Regular Contributor
timestamp: March 28, 2017, 11:16:05 pm
content: I'm in the process of repairing an MSI 4300 2k crane scale. It had a significant alkaline battery spill that caused damage to three IC's a some other components that as a result were shorted out. All of the batter spill issues are resolved and new components have been installed and lucky me, it almost works with a couple of exceptions so far. One is that three of the segments in the LCD display are dead or malfunctioning. I have cleaned the circuit board pads with alcohol and made sure the locking tabs are secure and even when I attached the display. There are two rows of 67 pads and two controllers running the display. All pads are spotless and the rubber segment strips on the display both look clean without deformity. I'm new at this hobby and am not sure my terms are correct but is there any way to test this display to see if the error is in the display or in the circuit board IC'c?Thanks - Jerry

author: Armadillo
author_rank: Super Contributor
timestamp: March 29, 2017, 12:05:08 am
content: Did you install the old strips the wrong way around. Try flipping the strip 180 degrees around [those old indents don't like new sitting]. Also those holding screws should not be too loose. Don't use brute force though.

author: jerryk
author_rank: Regular Contributor
timestamp: March 29, 2017, 01:04:13 am
content: To make sure I understand. Can I pull the conductive elastomer strips off the lcd and flip them left to right, or turn them over where the part facing up is now facing down toward the lcd? The strips are in their original position as far as I can tell and have not shifted. They seem stuck to the lcd so I did not want to chance their removal since new parts are not available. I have blown the strips with compressed air but was wondering if a q-tip with alcohol is a safe way to clean the strips. I assume the other floating foam strips are to hold the EL lamp against the lcd and off the two controllers.Thanks - Jerry

author: Armadillo
author_rank: Super Contributor
timestamp: March 29, 2017, 04:43:17 am
content: Since the strips are in the original position, that's good and OK.Put the slanting foam back in straight alignment with the strip. Guide the LCD back into the guiding slots if there is any.Then just focus on any obstruction that will forbid the strip to evenly pressed onto the PCB pads, for example half of the strip will pass through the slot and half is somewhat restricted to come out of the slot resulting uneven pressing of the strip onto the pcb pad. You will need investigate this area. The mounting screws need to be evenly tightened to achieve even pressing also. Unless oily hand touch the strip but it's still safe to use IPA with q-tip to clean the resistive part of the strips anyway.

author: jerryk
author_rank: Regular Contributor
timestamp: March 29, 2017, 04:28:36 pm
content: I'm feeling like I'm on thin ice with these connectors. I have moved the display from side to side and had a little change in the display but the same three segments seem to always show fault. I have read some recommendations to remove the strips from the glass and clean that side with IPA, others say you can sand the nubs down, and even a recommendation to remove the strips and boil them. If the display fails my repair is sunk so I'm proceeding with caution. Is there a way to test each segment by applying say 3v dc to along the strip to light up the segments one at a time? I'm sure if the display gets it's ground from the surrounding frame or what voltage parameters are used to power each segment.Jerry

author: CatalinaWOW
author_rank: Super Contributor
timestamp: March 29, 2017, 04:48:33 pm
content: Some background information to help you think about the problem.http://www.fujipoly.com/usa/products/zebra-elastomeric-connectors/http://www.fujipoly.com/usa/design-guidelines/89.htmland the other big source of this type connector. (AFAIK Fujipoly originated this technology).http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=3-1773443-0&DocType=DS&DocLang=&s_cid=1046You can also buy some random replacement connectors by searching "elastomeric connnector" on Ebay. Pot luck on whether it will fit your application.

author: jerryk
author_rank: Regular Contributor
timestamp: May 03, 2017, 08:37:22 pm
content: Another issue with my repair of this crane scale recently surfaced. There is a slow discharge of the batteries over time. I assume that this means there is a short somewhere in the circuit. The scale works and gives accurate weights. I have never been able to solve the display issue stated above but it would be nice to be able to leave the two battery packs installed without them draining over time.The circuit board power comes from two separate battery packs of 4 D-cell per pack. They connect separately at U1 and the traces from U1 go in different directions and power different components of the circuit. As an example the negative side of one of the battery packs has continuity with the large electrolytic caps minus lead but the other pack does not. Yet this scale will run with just one (regardless of which one) battery pack installed. My assumption is the power to this board combines dual and single rail supplies to the various parts of the board and the weight sensor. Both battery packs get drained down evenly over time with the power off and the system will not work below 5.5v.As a possible culprit is another suspected issue I raised in this thread(https://www.eevblog.com/forum/beginners/how-to-determine-if-an-smd-board-has-shorted-component/msg1156767/#msg1156767) about a couple of smd caps that showed continuity across the terminals. I dismissed this as a possibility because of the last post in the thread and that scale works other that the display issue posted above.I do not have a schematic but here is a picture showing the power terminals at U1. Any likely places that this short may hide?