Medical Lift, Repairing the PCB, cap issues. - Page 1
Forum section: electronics / beginners

author: Chris_top_he_r
author_rank: Newbie
timestamp: March 16, 2021, 04:50:48 pm
content: Hello EEV.I've been trying to complete a pro bono repair of a medical lift for a client. This means trying to dust off decades old electrical knowledge, if any was ever there. The project has been posted at my URL of wigglemylegs.com where you scroll down to "Medical Lift Aid" project. At the bottom of the thread there is a section called "Documentation" which has a PDF schematic that I hope I did correctly, and an Excel spreadsheet BOM with links to datasheets. The schematic layout is in a similar fashion as the physical board so that I could find things while hooking up the traces. Sorry. Now that it's done I see I should arrange it so that it would be easier to read electrically. The battery connections are in the lower left quadrant of the schematic. The extreme lower left is the ground tab for the negative post, and a little bit northeast of that is the thick green line labelled "red wire" which goes to a fuse and the positive post. Individual photos of each PCB component are at the bottom of Documentation area. Same photos also embedded in the BOM spreadsheet.The URL says the fix was to replace a blown out C5. Please note C2,C3,C4,C5, & C6 are all spec'd the same at 0.1 uF. The originals are rated at 63V and available new replacements are rated at 50V. I hope that 50V spec is alright as it allows me to get the radial package for this PCB. I think I have to go SMD to get 63V today. This fix for C5 lasted a couple weeks and the client called a few days ago as it quit again. I now have the PCB back here again to examine. The first repair I could not get the C5 to test bad out of circuit with 9V battery. When testing in-circuit on the first repair, all caps tested with behavior of a good cap according to analog meter volt/ohmmeter showing charge and decay. As mentioned there, I replaced C5 anyway due to physical appearance and the Medical Lift was repaired. The perplexing thing for me is trying to prove these caps good or bad with a 9 volt battery and my new DMM. When testing in-circuit with new DMM, the caps C4. the newly replaced C5, & C6 caps test as OL on Farads making me think they are now blown, and two of them, C2 and C3, test at 0.8 uF rather than their spec of 0.1 uF. Maybe due to age from 1996, or in-circuit issues. Don't really want to get an ESR meter or a scope. Plan to replace all components I can get my hands on, and have client replace 1996 AC adapter battery charger at client's site. Normally the unit runs off a car battery that travels across room in the same housing with the PCB and the motor, and then to charge, the user plugs in the AC adapter into this PCB at the J6 barrel connector in the upper right vertical edge of schematic. C4, C5, & C6 are located sort of together in the upper middle area of the schematic.Seeing these three caps looking dubious, I now suspect the old adapter charger has aged into an out of spec problem. Probably should be replaced. I think my new DMM has a way to use Min/Max to monitor the adapter charger for a while but I have to learn how to use it. I think I should tell client to get a UPS to condition the power from Edison at the wall outlet and maybe also find a way to prevent surges coming out of the charger and running into the PCB. Also thinking of possibly adding a switch to the housing to allow switching battery out of circuit, then plugging in the new charger until charged, and then switching the battery back into the circuit when charging is complete and its ready for use.Can anyone tell me how my caps keep testing good when out of circuit? I figure it is because my test voltages are not high enough. I guess before posting this I should have done more testing. While waiting for reply, I'll take out another cap that currently says OL such as C4 or C6. I will also re-install the same replaced C5 and see if it tests OL in-circuit again. As I mentioned, all caps tested in-circuit as functioning with analog meter during first repair. The newly replaced C5 from the first repair is testing on spec at 0.1 uF out of circuit.Any advice on testing caps and on clean power charging?I also wonder if this device has a motor capacitor somewhere or other failing circuitry. For weeks I couldn't get a response from the motor company Von Weisse after email and attempted phone calls. I finally clicked the "News" tab at their URL today and it says they sold and moved manufacturing to Mexico in 2018 and this morning I left new voice mail and email to a new contact and he already responded with a motor drawing that calls out the connectors, and I then thanked him and also requested the pin outs. I suppose I could go to client and remove the entire assembly from the cross beam and bring the battery and motor and all back here if I must. The sticker on the motor assembly has the following info.Von WeisseCL&E Motors - Linear ActuatorsModel: V06965AB62;12 VOLT DC;1/6 HP;19 AMP;SERIAL J205;10 OUTPUT RPM;REVERSE INTERCHANGE LEADS 133:1 RATIO;Thanks for listening.Chris