Atlas SD60 & SDN144A1 - Stay Alive

[Jamesn320]

Hi All,

I've got two N Scale Atlas SD60, both fitted with Digitrax SDN144A1 decoders.  And I'm having trouble with them!  The issue is they appear to have bad pickup so they move a few mm and the power cuts off then comes back right away but the loco then goes through its start up sequence, then starts moving and cuts off again.  Very frustrating.

Ive tried:

Cleaning wheels,
Cleaning track,
Removing wheels and cleaning axle points.
Removing wheel contacts and cleaning.
Cleaning pick up strips which sit on wheel contact strips.
Cutting back and soldering a wire between the wheel contacts and pick up strips.
Changing the supplied capacitor for a 330uF tantalum.
Turning off the back EMF by setting CV57 to 0

Any other ideas?

With the sound off it does run a bit better and on DC (that is with a DC circuit board installed) there is no issue.

What I want to try and do is use 3 x 330uF tantalums (plus resistor and diode) as a stay alive however I know the connections where the supplied capacitor connects are just for the sound side of the decoder.  Am I right in thinking this?  A true stay alive needs to connect to the main DC + & - . I know where the +common (Blue wire) connection is from the Digitrax manual for this decoder (See attached image) but can anybody point me in the direction of the -DC point?  The bridge rectifier is a bit hard to spot on this one.



Cheers,

James

[nscaler711]

Burn it and go buy a Loksound decoder. 
I'm being serious here... They are phenomenal decoders, and the sound is a 100x better...

Sorry I can't actually help though...

[jdcolombo]

Try putting a coat of solder on the pads that fit under the frame "fingers" for electrical pickup.

Many of Digitrax's "drop in" decoders are just a tad thin and don't make good contact with the frame pickup "fingers."  Use a thin coat of solder on the pads to make sure that the decoder is very tight under those fingers.

Honestly, though, the advice from the previous poster is correct - next time, buy ESU LokSound.  There's really nothing to recommend about Digitrax's sound decoders.

John C.

[Jamesn320]

Hi John,

Should have said, I have done that as well.  No issue with the connection from the chassis to the board.

Mainly looking for the -DC point. 

I know ESU are better, I work with them all the time and are my preferred decoder but for reasons I wont get into here on this project this is what I have to work with. 

Cheers,

James

[peteski]

The stay-alive caps you often see people install (and post the info here) are less that 1000uF and they don't have enough energy to power the motor. They just have enough energy to keep the decoder electronics powered during very short power drops while the locos inertial still keeps it coasting through the dirty track.
If that is all you want then just replace the 100uF cap with a larger one.

We recently discussed this in https://www.therailwire.net/forum/index.php?topic=43939.0

If you want to install a keep-alive SuperCap circuit which has enough energy to  keep the entire model powered up for a longer time then you would need to install it right at the output of the bridge rectifier.

[tehachapifan]

Any chance the area of track you're testing these on is remote from any track feeders? I still have a hunch that sound dropouts can also be caused by too low of track power, which I believe may be caused if there are not enough track feeders and/or they are some distance away. The fact that you said it performs a little better with the sound off makes me think this could potentially be on the list of possibilities. I have areas on my layout that are pretty (actually, really) remote from track feeders and I will get some sound dropouts if I try to run a 3 sound unit lashup through these areas. Wheras, a 2 sound unit lashup will perform much better through these areas.

[AKNscale]

I used to have these in two of my SD50s. They were terrible runners like you’re describing, and I’m pretty sure it’s that the caps aren’t good enough combined with the Digitrax stall. I took these out of my 50s, but I just changed the caps in a friend’s 70MACs(Digitrax sound decoders) with the same issue and it has made them much better runners. I used two 16V 220uf tantalum caps per loco. That was the discussion peteski put on here.

[Jamesn320]

Hi Guys,

I did change the standard cap with 990uf and it had a little effect but not much.

Tehachapifan, every rail section on my layout is fed but thanks for the input its always worth checking.  This loco does this one a yard of programming track attached to my Sprog!

Anyway there is now an ESU Micro V4.0 in there and without any capacitors it runs flawlessly so its simply the decoder!  Shame, but I would still like to know where the negative side of the bridge is if anybody knows?

Cheers,

James

[peteski]

Hi Guys,

I did change the standard cap with 990uf and it had a little effect but not much.

990uF?  That is a really odd value.  Where did you find that cap or how did you assemble your bank of caps? What cap values did you use?

If you really added a 990uF (micro-Farads) cap, and that has not improved the the decoder operating during short power dropouts, then I think that you have other issues.

[Jamesn320]

330uf Tantalum x3 with diode and resistor.  I've done that a lot and it works well.  And yes I agree, the chip has issues.  But is the original cap connections the true + & - DC points from the rectifier?

[peteski]

330uf Tantalum x3 with diode and resistor.  I've done that a lot and it works well.  And yes I agree, the chip has issues.  But is the original cap connections the true + & - DC points from the rectifier?

I don't have that decoder so I cannot answer that question.  It seems probable though.  The other possibility (unlikely) is that it is a coupling capacitor between the output of the audio amplifier and the speaker.
330uF tantalum caps?  What is their voltage rating?  16V? Must have been pretty pricey.

[Jamesn320]

I must admit I haven’t checked this batch to see if they really are 330.

https://www.ebay.co.uk/itm/20pcs-Tantalum-Capacitors-337C-16V-330uF-Type-D-SMD-7343-10-Surface-Mount/322335558548?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

[peteski]

I must admit I haven’t checked this batch to see if they really are 330.

https://www.ebay.co.uk/itm/20pcs-Tantalum-Capacitors-337C-16V-330uF-Type-D-SMD-7343-10-Surface-Mount/322335558548?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Well, those are 330uF 16V.  Personally, with DCC, I wouldn't use tantalum caps rated for less than 20V, but I'm extra cautious. 

[reinhardtjh]

They are from the same Ebay seller as the "Cheap Chinese Caps" in this thread https://www.therailwire.net/forum/index.php?topic=43648.0  Hopefully they don't blow up as easily.