Deep Fried LM393 - Keithley 2110 - Page 1
Forum section: electronics / repair

author: beenai2018
author_rank: Contributor
timestamp: October 15, 2024, 05:33:52 pm
content: Twas supposed to be a simple day. Check and Calibrate a BK 1602 Valve power supply (with the keithley 2110 ) to sell.. Got the schematics and the procedure (just twiddling some pots) .I got a whiff off something heating up in the power supply when setting a meter cal ,and I thought "Hey ,The 2110 has a temperature second display while measuring volts.. So lets turn that on ,and touch this transistor ... " and Zap!!!?? The temp input has opto isolation and should be isolated from the Main measurement side ..Now, The keithley 2110 is going nuts on the display with no input.. and lets look at the BK 1602 .. won't reach full 400V without the current limit tripping ..(with no load!!) .. Kill two units with one thermocouple..Im wary of ground short circuits via probes after I watched Dave's "Dont blow the ass out of your scope ...video" way back , and In this case my beliefs were that the thermo inputs were isolated ..The aftermath .. I didn't realize this at first but there was probably a mini fire in the scope when it was letting its magic smoke out .... and the smell .. OMG , phenol ,phenols all the way ..And then I opened the unit (smell to infinity ) ... and prominently left the space.. Two days of airing out the room and running a mini ozoniser later .. lolSo , it looks like an LM393 is visibly dead and maybe others bits around it . Looking at forums it looks like another member (with a 2700) had one go out as well .QuestionsThe pads have delaminated and I have probed the circuit for connections and Annotated them.I have been slowly scraping away the blackened section ,but it looks like I may have to clear out the traces and leave the area unpopulated.I cant cut a hole out in the pcb because there are traces on the bottom . There may be hidden traces as well .Since the pcb is charred and shows a 6 M resistance on the fiberglass is it worth reusing the area for new parts ? ,or do a riser / daughterboard away from the burn spot ?If I can relocate the circuit ,should I dremel out as much as possible to prevent leakages etc?The top phenolic layer is gone and Im seeing fiberglass mat .Im thinking this is the start of problems and more will reveal themselves (whack a mole ,lol)

author: Kleinstein
author_rank: Super Contributor
timestamp: October 15, 2024, 06:04:57 pm
content: The circuit directly at the LM393 is not that sensitive to leakage. These comparators are usually used to control JFET switches, by pulling the gate low to switch them off. The input signals are logic signals and thus not that sensitive.Chances are that the 2 JFETs controlled from the chip can also be damaged. This are likely some mmbf4392/3.The signals below the chip look like the supply and digital signals towards the LM393 - so non critical signals when it comes to leakage.The aera around Q208 may be the most critical part with leakage in the charred area. This part may want a good clean and maybe cuts under the transistor for extra isolation or maybe more this part to a small riser.It looks like the thermocouple part does not have that much protection and is using the same ground / supply. It would be nice to have that part fully isolated, but is obviously is not. There could be an issue in the TC part of the circuit too.

author: beenai2018
author_rank: Contributor
timestamp: October 15, 2024, 07:17:41 pm
content: Quote from: Kleinstein on October 15, 2024, 06:04:57 pmThanks !! Gives me a bit more confidence to tackle this and trust its future measurements.Q208 was replaced previously as causing an offset on all readings with a 2N7000 .I will look at further moving it awayThe 2 JFET are "M6G" - 4393 . I have discretes to bodge in if needed .Regarding the thermocouple input .. Whilst the meter allows thermocouple as a second display to voltage ... the manual warns of dire consequences .. or maybe I'm misreading it.

author: Harry_22
author_rank: Frequent Contributor
timestamp: October 15, 2024, 08:15:06 pm
content: Hi!There are no hidden traces visible. Almost the entire topology of this place is made through vias.You probably need to partially remove/bend the top traces. Remove the coal almost to the bottom layer. Restore thickness with epoxy. Drill vias. And carefully restore the top layer with reference to vias.Good luck!PSPlease see in attachment animated Gif.https://drive.google.com/file/d/1BoWv6qVmAhurE9w8fAfhqcgL8M3zqWl9/view?usp=sharing