Help Needed to Find Where Crackling Distortion Sound is Coming From on 5 volt PS-2 Dryfus

Dave, you did all the right fixes.  Go for the cap and let us know how you made out.

Thanks Marty.

I have seen this once and still can not figure out where it is coming from on the board.  Hopefully the cap resolves this.   G

That cap has been a problem on a number of boards I've seen.  I've successfully replaced it on two boards and brought them back to life.  Every one I've seen that cap leaking or swelling has had the WINCAP brand cap on them, however not every WINCAP brand cap has exhibited any visible symptoms.

I have an in-circuit capacitor tester, and when I service a 5V board, I test that cap to see if it's value is significantly lower than the 330us rating.  If so, I replace it.

With all that being said, I have no idea if the cap is the root of your problem, only that it is certainly a problem if it's swelling or leaking.  Luckily, it's close enough to the board edge that you can reach in and replace it.

Update: I replaced the cap and the distortion is still there. Kind of like there is an electrical storm going on inside a component on the circuit board. The sound resembles the little AM radios, that we carried around as kids, when there was lightning striking in the distance on a hot summer day, or when the volume pot would start getting dirty and old, and if you rotated it, it would crackle. Could it possibly be something with the volume pot even without touching it?

Random pops is not typical of dirty pots but just rotate it a few times to rule it out. Moving the slider will remove some oxidation. If it does make a difference then hit it with some contact cleaner.

Pete

The speaker, the pot, and then the audio amp chip in that order would be my next suggestion.

Thanks. I'll give it a try.

Update 2:  Is it correct to assume the pot is only operational when running in conventional?  I adjusted it while the sound was playing in DCS mode and it had no effect on the static sound. I also unplugged it while in DCS mode and the sound remained the same. I then tried adjusting it in conventional and it does work to raise and lower the volume like it should, but the static stayed right along with it. Now my next step is to move on to the audio amp  chip like John has suggested. I'm not sure which chip that is. John, can you guide me to its location and is their a part number on it?  If I can access it I may be able to remove one from a bad board that still has a good chip and swap it out.

Can't miss it, it's a big sucker on one end next to the connectors, the TDA7056A

Thanks John.

Dave,

Is it correct to assume the pot is only operational when running in conventional?

Yes, that's correct>

Let me know if you need an amplifier chip, I have some of them.

Thanks for your reply Barry, and yes John, it would make the job much easier then trying to remove one from a used board. Please send me an email.

Dave, On the one I had the audio amp did not resolve either.  Some other component induces this distortion.  Go and try it, but may not resolve this.  G

Well, he'll have the audio amps in a couple of days.  It's somewhat of a shot in the dark, but it's certainly a possibility.  I'm not sure where I'd go next if that's not the issue.  I suspect maybe replace any electrolytic capacitors you can reach, they're always a good bet on stuff this old, they dry out.

Thanks G, Maybe I could try a test while I wait on the new amp coming from John. I want to try bypassing the tether and the tether plug board and hook the speaker directly to the PS-2 board. I'm wondering if the interference can be getting into the speaker wiring on it's way to the tender. I'm thinking this could be a possibility since the speaker wire is not shielded.

Anything is possible, but the speaker signal is very low impedance, and it would take a lot of energy to put really noticeable noise into that signal.  I'd suspect this is happening in lower signal amplitude circuitry.  If it's not the amp, my guess is it's before the amp.

Thanks John.  Question: When the sound volume is adjusted with the remote, the static volume will increase and decrease along with the engine sound and can be eliminated totally when volume is decreased to nothing. But when you turn off the engine sound, with the on / off engine sound button, only the engine sound shuts off and the static remains. Any thoughts on why this is?

Hmm...  Hard to say.  The amplifier module has a DC volume control, but I don't know if the same mechanism is used for command and conventional.  The chip is fairly simple in concept.  If the DC volume control is used for both command and conventional, then I'd suspect the amp is not really the issue.

Interesting John. I feel the same. I think GGG suspected this too.

Hard to know exactly where that might be, the circuit is undocumented, at least for us peons.

I had a problem like that with a steam loco that had the PS2 3V boards in the boiler. Ended up being solder/tin whiskers/ resin flux on the tender tether receptacle under the cab. Once I cleaned that up by running a hobby knife between the solder pads and cleaning with some contact cleaner all was fine. 

 https://en.wikipedia.org/wiki/Whisker_(metallurgy)

Have you checked to see if you've picked up a piece of metal in the speaker?  I had a few small pieces, makes a big difference.  See Eric's video https://youtu.be/JrwoqXIYNB4. 

Best of luck.

I was using a bench tester which eliminated all the other potential issues except the PS-2 board.  Changed all caps that could be reached, and audio amp.  Reloaded sound file.  Still had the static.  Could be corrupt memory chip, other components, etc...  Only seen one like this, and it still sits on the bench!  G

You may have a chance to see another one.

Casey Jones hit the nail on the head. Thank You Casey. Not only did the static go away, but the volume is much louder too. I would have never thought hardened flux could do that. All I did was take a small screw driver and scratch the varnish like flux off. GGG, take that engine down off the shelf and give it a try. Thank God for OGR and the wonderful people who love to help. I'm also very happy with 2 amps for the shelf, from John, I'm sure they'll get used.

I think George had his on his test fixture, so the only thing he was testing was the actual board set.  That's one of my first steps if I have an elusive issue, start testing individual parts.

One thing that might make sense is to use a solvent and scrub the board and see if that removes any contamination, can't hurt.