There is Indeed an DCC Installation Gotcha Always Lying in Wait!

[tehachapifan]

Been doing my own N scale DCC and sound installs for years now and had pretty much figured I'd learned the common ways to let the smoke out along the way. More recently I had been adding power keepers to my custom N scale SW1500 fleet and had already done 3 with zero issues. Today I decided to tackle the 4th and everything went along as normal, checking for common gotchas along the way.

Before I buttoned everything back up, I placed it on the track for a quick test and noticed the cab beacon came on steady (no blinking) and with just track power. Quickly pulled the loco off, figuring a wire to the beacon is contacting the frame or something, but couldn't find a short. Went for another test to see if the other lights worked and the did. Wait! Whoa, the decoder is heating up! pulled the loco again and then stared at it, scratching my head for several minutes trying to come up with what the problem is.

Started doing an investigation looking for what had the be a short somewhere and just couldn't find anything. Pondered and pondered more, then figured I'd look at where I attached a wire to the decoder. Looks OK. Look even closer (this is all with an Opti-visor). Still don't see anything but I keep looking as there is a row of pads in the area that are EXTREMELY close together here. Then I finally spot wat has to be the tiniest (almost microscopic) finger of solder that was bridging to the adjacent pad after I attached the new wire. This was quite surprising, as this is one of my typical gotcha checks, but I initially missed it. I removed it and went for another test. Beacon and all lights working normally. Awesome! Throttle up to test motor and no movement. Hmm. Try reverse, works fine. Try forward again, zero movement.

Thought maybe I was going to get a decoder to survive after releasing some of its smoke (well, there wasn't any actual smoke, just heat), but the fact remains that decoder death is instantaneous with an installation error! There are no warnings and no second chances!

[Jim Starbuck]

Decoder installation can be a humbling experience but very gratifying when everything works as planned.
I was told once there are two kinds of installers…those that have blown up a decoder and those that are going to.

[tehachapifan]

Ha! Ha! Truer words have never been spoken!

[peteski]

I think everybody who does what you do Ross has a similar experience.  Hint:  maybe use more flux.  It prevents those thin whiskers from forming because it causes solder to bead.  Based on my own experience.

You did get lucky!

[tehachapifan]

I think everybody who does what you do Ross has a similar experience.  Hint:  maybe use more flux.  It prevents those thin whiskers from forming because it causes solder to bead.  Based on my own experience.

You did get lucky!

Not lucky enough, unfortunately. Still gotta pull and replace the decoder.

I think my iron tip needs replacing too.

[peteski]

Not lucky enough, unfortunately. Still gotta pull and replace the decoder.

I think my iron tip needs replacing too.

Somehow I must have glossed over the sad part of your post.  Sorry to hear about the bum decoder.

[tehachapifan]

Somehow I must have glossed over the sad part of your post.  Sorry to hear about the bum decoder.

Well, after one last hard look at the decoder before I pulled it, I found another TINY bridge I created across an ultra-tiny (1-2mm?) component (resistor?) that was butted up right next to the solder pad. This was a rectangular component with soldered ends that I bridged. I was able to flick it away and all is well again!

While I still believe I need a new soldering tip, it had been a while since I did and precise soldering like this and I noticed I was not as surgeon-like as I had been in the past.

I do have to reprogram all my CV settings, as I tried a hard reset before I found the bridges, but at least I don't have to replace the decoder now!

This is a Zimo MX621 decoder, BTW. Great little decoders!

[peteski]

That's a great news Russ, and yes, ZIMO and ESU are excellent decoders.  Several years ago they were pricier than domestic ones like Digitrax, but nowadays their price has come down to be within few dollars of the domestic brands.

Oh, and as you noticed, we aren't getting any younger. I have to wear the Optivisor for all my decoder installs, and even then, I sometimes also use a loupe for close inspection. Then for really small precision work I have my stereo microscope. Of course good lighting of the work area is also vital.

[MK]

This stuff happens from the factory too.  Once I had a Digitrax decoder that had a solder bridge across two SMDs.  Turn on track power and off it went at full speed regardless of throttle position.

I found the bridge by examining the board with a 10x loupe.  It was tiny!  The other decoder, bought at the same time, was fine.

Hard to imagine a wave soldering machine creating a solder whisker bridge.

[peteski]

Hard to imagine a wave soldering machine creating a solder whisker bridge.

Wave soldering went away pretty much with through-hole components.    SMDs are mainly soldered using hot air or just radiated heat.  First, solder paste (mix of microscopic solder balls mixed with liquid flux) get first stenciled onto the bare PC board onto the solder pads, then SMDs are placed on the board by a robot, then the board is passed through an oven which melts the solder paste.  If you look under magnification, you can often see tiny stray solder "balls" leftover around the components.  Due to that soldering technique whisker-type solder bridges are very rare. But I guess they can happen too. Usually created when hand soldering wired, or other components.

[MK]

As always, thanks for the education Peteski! 

[Dwight in Toronto]

I just had one of those ‘pad-spanning solder whisker’ incidents a few days ago. 

I found the short by testing continuity with my multimeter.  But honestly, even with the highest optivisor magnification, I could not see the solder - it was thinner than a hair.  A few careful passes with the tip of an Xacto blade cleared the path.

[peteski]

I just had one of those ‘pad-spanning solder whisker’ incidents a few days ago. 

I found the short by testing continuity with my multimeter.  But honestly, even with the highest optivisor magnification, I could not see the solder - it was thinner than a hair.  A few careful passes with the tip of an Xacto blade cleared the path.

If this was between adjacent pads you soldered yourself, more flux would have likely prevented it.  Flux can always be washed off after soldering.

[milw156]

Tehachapifan, I just had that happen with a Lokpilot to which I had soldered no wires to. Guess Im gonna be testing before installation from now on,