solved: using IPA on a high-impedance path, it may not evaporate that fast

[Enginerding]

edited:  this was solved, stupid error.  See my next post for explanation.  And thanks to those that got me thinking in the right direction. 

I’m trying to fix channel 1 on a LeCroy LC584 front-end. 

I’m getting a strange capacitive charging effect on the 1 meg input path, but can’t find the source and stuck as to what to do next.

Between Pin 6 and Pin 5 on RL1002 I measure 5.5 meg and slowly trickling up (at about 8.7 meg after 30 minutes). On channel 4 (good), it measures 175 meg (desoldered a leg on R1016). 

Also note, this is the schematic for the LC574, so some of the values are different, but the layout is the same.

Attachments:

1. Schematic, zoomed in on 1 meg relays (Everything circled is stuff I’ve desoldered while troubleshooting.)
2. Functional Block Diagram for front-end.
3. PDF of Front-End Schematic.

Full Service Manual on ko4bb: http://ftb.ko4bb.com/getsimple/index.php?id=download&file=LeCroy/LeCroy_LC574A_Service_Manual.pdf

[DaJMasta]

Does "capacitive effect" mean an AC-coupling like effect on the channel?

If that's the case, I would check trace continuity and solder joint quality for everything in the used signal path (if the 50 ohm side works fine, you don't need to check it or the connector itself).  High pass response on what's supposed to be a DC line, especially if it's centered around higher frequencies (10s to 100s of MHz) is often just a cold solder joint or broken trace/leg - very common with broken connectors as well, but the 50 Ohm input works it's probably not that.


If it's not that, measuring in circuit is probably not going to your analysis any favors.  I'd either remove parts or isolate the connections you're measuring by removing passives or whatnot, but trying to inject a signal and scope various points in the path can be useful too.

[edpalmer42]

If you haven't done so already, give the board a *really* good cleaning with 99% isopropyl alcohol.  Use a toothbrush or similar to scrub it and clean away any flux residue or other contaminants - particularly near the inputs.  It doesn't take much to create a multi-megaohm leakage path.  Then put it somewhere warm to dry.  An oven set at 100F would do the job.

If that doesn't help, you may have to consider removing RL1000 to clean underneath it.  Keep that as a last resort.

[Enginerding]

Well don't I feel stupid!  Thank you guys, you got me on target.  It was a lower-impedance path caused by isopropyl accumulating under the relays, allowed current to flow and took the appearance of a charging capacitor.  I'm going to leave this thread up as a monument to my stupidity, but may start a new thread to detail the rest of the front-end repair.

A few hours later I put the DMM back on and measured 13meg and trickling up.  Hmm it was stuck at 8meg before.  So I decided to shoot some IPA on it and it instantly fell to 0.5meg! There's a lot of nooks and crannies on this board, variable capacitors, and the relays on the underside, which IPA got under.  My oven only goes down to 180F so I used a blow-dryer on low and now it's >145meg.  I also think my 99% IPA has absorbed some water from the atmosphere.

Sweet!  One (self-created) prob solved.

[Yansi]

Shouldn't IPA be non-conductive liquid after all?

[aqibi2000]

What percentage was the IPA?

[Enginerding]

Amazon Basics "Solimo" brand 99%.

I'm not a chemist, but from what I understand the moment you crack open an anhydrous IPA bottle, it starts to soak water from the air until settling at around 93%.  I live in a humid place, the RH is around 70% always, so maybe the IPA becomes even more hydrous?

I don't usually work on boards with so many nooks and crannies, so it def threw me for a loop.  Where the hell did this low impedance path come from??  Trap for young players. 

[Electro Detective]

I learned my lesson the hard way as OP did 

and also found some IPA leaves a white residue as well, 
and some IPAs take forever to dry and go away   

If in doubt I will nuke areas that need to be clean and conduct free with a 'no residue' electronic contact spray that's 'safe on plastics'

I will test the product first to ensure it does what it says, and then stock a few cans of the same batch 

If I need to go beyond that, say under components, I will carefully shoot filtered (no oil or moisture) compressed air into the works, to ensure no undried drama

if that still doesn't do it, the hair dryer gets the gig.. 

 

[Enginerding]

what's your water/oil filter setup for your air compressor?

I've been thinking of trying an air compressor for a while; seems like a waste to use throwaway cans.  Whenever I use my air compressor, too much rusty water comes out for electronics.  Might have to use an aluminum compressor if I do it.