HELP! How to solder keep-alive to LokSound 73199 Select Direct Micro

[cberrya6e]

Hello!  I am hoping someone can give me some pointers as to how to improve my results adding a keep-alive to a LokSound Select Direct Micro 73199.  Thanks to the unbelievable work of Mr Peteski, I know where to place the leads.  My issue is trying to get a quick, solid solder bead on the outboard sides of the capacitors on the left side as depicted in his illustrations (Purple positive, Green ground).  It is near impossible to get a good solder bead as if it is coated with something.  I have used alcohol and a swipe of a file to get to bare metal, but this has not proven effective.  I am terrified of any prolonged application of the iron given that these decoders are made of glass.  Does anybody have any tips or process they follow to effectively attach keep-alive leads to these locations?  Thanks!

Regards,
Chuck

[peteski]

Chuck, glass "melting" temperature is the least of your worries - it is much higher than temperature used for soldering.  The boards are made from a glass/epoxy composite, and if drastically overheated, the epoxy is the first to go. That includes the epoxy adhesive that keeps the copper traces attached to the board. Then there is also possibility of thermal shock occurring when one side of the cap is suddenly heated. In practice however, those problems rarely occur.  Yes, these decoders are delicate micro-electronic circuits, but if you are careful and have the right soldering equipment, they are quite robust.

I have not seen any coating applied to those decoders.  However, the type of lead-free solder which is used nowadays on electronic circuit boards has a higher melting temperature than the old "electronic type" 63/37 Tin/Lead solder, so the soldering iron has to be adjusted to a higher temperature.  If yours is adjustable, I would set if for around 730 deg. F. The other piece of the puzzle is maximizing heat transfer.  Coating the cap's terminal  to be soldered with a bit of electronic paste flux (to aid in heat transfer) and making sure that the soldering iron's tip is clean and has a slight buildup of fresh solder is also important. 

I usually do the process above using the that 63/37 solder I mentioned, without actually soldering a wire, but just to build up some extra solder on the cap's terminal.  Then I apply more paste flux to the cap's terminal, make sure that the wire I'm soldering is well tinned (again with 63/37 solder). That allows me to quickly solder the wire to the cap's "primed" terminal.

Following those recommendation, you should have no problem soldering wires to the cap's terminals. Just do it quickly because if you dwell on it to long, the solder on the cap's other terminal will soften and the cap might start moving on the board, or even come off.

[Steveruger45]

Yes. Everything Pete said.  I’ve soldered on these boards loads of times with only one issue by shorting out the H bridge when trying to use the left and right motor tabs on top of the board.  My hand slipped.
 I use the smallest chisel tip in my Weller soldering station.
I tape the board to a block of wood to stop it moving around.   (I didn’t use the tape The board to block technique in my one failure to date, BTW).
I also use leaded solder 37/63 (Kessler 0.015” and I have used 0.020” too ok) and a very little non acid flux paste.  750 F iron temp, Tin the tip, tin the board connection point, tin the wire, apply a little bit of flux past again on the wire, bring the wire to the connection point and hit it quick with iron.
Clean after with an IPA dampened q-tip.   
Works for me.

[jdcolombo]

While I've never soldered to a 73199 board, I have done so on any number of other boards from a variety of manufacturers, including a Soundtraxx Tsunami and Paragon 2, which also use the "lead-less" solder.  Same advice as Pete and Steve:

1.  Set soldering temp at 750 degrees.
2.  Tiny bit of flux on the place you want solder to go.  Key here is "tiny bit." 
3.  Make sure your tip is clean, then coat it with solder.
4.  IMMEDIATELY after step 3, touch iron to wire/pad/cap end.  Don't let the fresh coat of solder you put on the tip linger there for more than a couple of seconds.  Prep and be swift.

Never had a problem getting solder to flow onto a pad/cap end in this way.  I use a 1/64" conical tip for this kind of work, but a very small chisel tip probably would be better for maximum heat transfer.  With my conical tip, I don't use just the point; instead I tend to try to use more of the cone, which maximizes heat transfer.

John C.

[cberrya6e]

Thank you all for the advice!  Will take it and run.

[GM50 4164]

There is a lot of room under the board to solder the negative to. Find a area that would be clear of the frame and scrape a small area of the green coating away to reveal the brass plate. I do this instead of soldering to the built on caps.

[Steveruger45]

There is a lot of room under the board to solder the negative to. Find a area that would be clear of the frame and scrape a small area of the green coating away to reveal the brass plate. I do this instead of soldering to the built on caps.

Yep, I’ve done that too on the 73100 decoder (never did it on the 73199)but I found you need a little bigger chisel tip to get a quick heat transfer going relatively quickly for a good solder joint than I would normally use on a OEM solder pad.

[GM50 4164]

Yep, I’ve done that too on the 73100 decoder (never did it on the 73199)but I found you need a little bigger chisel tip to get a quick heat transfer going relatively quickly for a good solder joint than I would normally use on a OEM solder pad.

I have some rosin flux from RS that I apply a little bit to the spot and it seems to work very quickly. I use a fine clean tip and I don't have to keep heat on it but for a second.