Lionel 8142-T35 sound board(SOS) repair

I accidentally found the root of the A.C. buzz problem. It was a bad capacitor, but not the ones of which I was suspicious. There are also a couple of electrolytic capacitors on the board. The 47 MFD one had a bad lead. I accidentally moved it, which restored contact, and the buzz went away temporarily.
So it was off to Radio Shack for a replacement, which is now installed.

But now the volume seems low.......

I am still hoping that someone with an electronics background will chime in.
As I have already posted, my 8142-T35 sound board is now working, but at a low volume.
I got out another tender with the same board for comparison, and there is quite a difference in volume.
It turns out that the capacitor that I installed to replace the defective one is too large. The original was  47 Microfarad, 50 volt. The one I installed is a 470 Microfarad, 35 volt. Could this account for the reduced volume?
If so, is a 47 Microfarad, 35 volt capacitor likely to do the job? The voltage is still well over track level voltage. (That is what's available at Radio Shack)

I tried increasing the capacitance of the 47 MFD cap on the loud board by adding a 470 mfd capacitor in parallel. It did not reduce the volume, so I guess that's not the problem.

Still hoping for some thoughts / advice.

C.W. I wish I could help.  I had this same problem with the loco we run around our Nativity display and posted about it here on the Forum.  Someone put me in touch with a "guru" who repairs these boards.  This is what I wrote:

Thanks!

I guess I will try to figure out which cap is the speaker coupling capacitor and change that.

Success!
In the hope of making this information available to others, I changed the title of the thread in an attempt to make it more searchable.

Thanks to Ted, and whomever originally provided the information he quoted.

And thanks to those who participated in the older thread linked below.

I was able to figure out which capacitor was the output coupling capacitor (22 MFD electrolytic on my board) , and replaced it, using a larger one (100 MFD). The sound is much louder now. (Please let me know if using this size will cause a problem down the road.) I guess I should check the value of the cap on good board I have.

In summary (for this particular bad board):

AC buzz instead of steam sound - replaced 47 MFD electrolytic capacitor

Using a larger value seems to be OK.

Low volume - replaced 22 MFD electrolytic capacitor

Using a larger value makes the volume louder (Is this OK?)

General - resolder bad joints, and replace frayed wires.

Here is another thread from 2012 with some good information.

A picture of my board, with the replaced capacitors labeled is below.

Cap A - fixed the AC buzz - 47 MFD

Cap B - fixed the volume - original 22 MFD, replacement I used - 100 MFD

IMO, the 100uf cap probably won't be a big deal, I don't think that will be an issue.  FWIW, the original 22uf probably was on it's last legs and a new 22uf would have probably also brought the volume back.

Whenever you are working on electronics that is 30-40 years old, the first thing to suspect is electrolytic capacitors.  I've fixed a lot of antique equipment by simply going through and replacing the electrolytics.

Thanks John!

One reason I used the 100 MFD electrolytic capacitor was that I had it on hand. I don't have much in the way of capacitors. 

As you can imagine, I have what is most likely a better assortment.

Here is an important update:
After running the tender with the repaired sound of steam board for a while, the volume went again.
I opened it up, and while I was poking around I discovered that two of the transistors were getting hot. I am not certain whether I wrecked them. I guess I will try putting the proper sized 22 mfd capacitor in.

Probably a good first step.  There could be some other issue, though that board is pretty simple, not too much to go wrong...

Well, I got the board working again. Unfortunately, I did not go with my original plan to just replace the 100 MFD capacitor with one of the proper size.

Instead, I decided to change the hot transistors first.

I changed the transistors one by one, and briefly tested the board after each one.

So, I changed out all four of the 3-wire transistors - no change.

I installed a new 47 MFD capacitor (correct value) to replace the oversized 470 MFD capacitor that I had installed - no change.

I replaced the 470 MFD capacitor - no change.

I replaced the oversized 100 MFD capacitor that I had installed with the correct sized 22 MFD capacitor. - the board now works.

Interesting, I believe the 22uf cap is the DC isolation for the speaker, apparently the impedance of the 100uf one was sufficiently lower to overload the drivers.

Well, live and learn.
For me, it was an enjoyable effort.

I now have a list of components for those sound of steam boards. I am going to add them to the list of items to get from Digikey the next time I place an order.

Thanks again for the assistance!

I'm glad it finally all worked out.  As you say, it was an education, something I'm getting right now doing an upgrade on a tinplate set.  

Glad you got it working again.  Now I know what I have to change on mine.

One more question... Were you running the loco on a "chopped wave" power supply (like an MTH Z-1000 or Lionel Powermaster?)  We run our Christmas loop with an old K-Line transformer which puts out a very aggressive waveform.  Would the much higher (+/- 35V peak-to-peak) voltage be responsible for overheating the transistors and causing the board to fail?  -Ted

I have a relatively small layout, powered by a Postwar 275 watt ZW.

The next time I have a bad board, I am going to start by checking the diodes, then replacing the three electrolytic capacitors with new ones with the correct values.
Last step will probably be replacing the four 3-wire transistors. (if changing the capacitors doesn't fix the problem)

Currently, Radio Shack has the correct capacitors available individually. They do not have all of the transistors available individually, but by purchasing two different inexpensive assortments, I was able to get them all with some spares.

As I posted above, I do plan to buy a few of each component the next time I place an order with Digikey. I think I have also identified the correct spade connectors (male and female) on the Digikey site, and will order some of them too.

After the second failure, (with the wrong capacitors installed), the two transistors closest to the speaker wires were getting quite hot. Now they get mildly warm after leaving power continuously on for several minutes.
Since I changed the four 3-wire transistors and all three electrolytic capacitors, I don't really know what caused or solved the problem with certainty. I do know that the board wasn't all better until I replaced in oversized 100 MFD cap with the correct 22 MFD cap.

I just was looking up part numbers on the Digikey site.
Is it safe to assume that aluminum caps are ok (as opposed to tantalum)?

There are an awful lot of brands from which to choose.
Kernet, Panasonic Electronic Components, Nicholson, Rubykon, United Chemi-con, and more.

Any particular brands to buy or stay away from?

Virtually all the caps you run across in model train stuff will be aluminum.  Tantalum are for situations where you need higher capacitance in smaller spaces and wider operating temperature ranges.  They will be considerably more expensive than aluminum for the same ratings.  I used a lot of these in aerospace applications, space was at a premium in panel mounted instruments.

Thanks!

How about brands?

They are all about the same price.

Are any of them USA?

The brands at Digikey will all pretty much be mainstream stuff.  I don't know of many that are made in the US, virtually all of those are made in the Far East.  I use a lot of the Panasonic brand, but I've used most of the other brands at times.  I plug in the specifications I need, then look at what's available.  Typically, I just pick the cheapest one in the quantity I need.  For some of the surface mount stuff I've been using lately, there may only be one or two that are an exact fit.  For the thru-hole stuff, there is normally a ton of choices.

CW, What part numbers did you have for the transistors?  Some of these boards don't have legible writing on the transistors.  G

quote:

CW, What part numbers did you have for the transistors?  Some of these boards don't have legible writing on the transistors.  G

LOL, the transistors I purchased were the ones that you (GGG) identified in 2012 in this thread. (THANKS!)

I could not see the markings on the original transistors while they were on the board. As I removed them, I could see they were there, and matched your 2012 descriptions. The board I asked about in 2012 did not have marked transistors.

The attached document contains what I have so far:

It's been my experience that with many of these older boards that almost any generic transistor with similar ratings and of the correct type will usually function.  Frequently, I'll start with the 2N2222 for NPN and the 2N2907 for PNP replacements.  Much of the time, they do the trick.

OOPS!,  a lot of my early work was hand written on paper in a file,  been so long I don't know where I have those notes.  Glad it worked for you.   G