What Electrical Diagnostic Tools Do You Use? / Suspicious Bell-Whistle Controller

I'd start by simply swapping the diode strings between the two positions and see if it moves with the diodes.  Could be as simple as a bad diode.

Last night I swapped the hot wires coming from the A & D posts of the ZW to the diode strings and the problem reversed! Now the train is creeping along with the handle on the right and fine with the handle on the left.

Now what?? If the diode string with the 6 pairs is causing the problem, how do I determine it it's one bad diode or do I simply remove the pair at the far right and see if that resolves the problem in that both diode strings will be in balance?

Just short out pairs of diodes, one pair at a time, when you get to the pair with the bad diode, it should work normally.

I'd probably just use my meter on the diode setting to test the diodes, that's another method.

Well, the mystery deepens. I'm not sure what you meant about shorting out each diode. Did you mean run a small piece of wire across the two leads of each one individually?

I did do a continuity test of each diode and they're all good! I tested both directions also, making sure that there was no current flow - to + for each one individually as well as flow + to -.

I'm at a loss other than to eliminate the sixth pair of diodes so that both sides of the transformer match and see if that accomplishes anything. The only other thing I can think of is maybe there is a problem with the left side of the ZW. I could tap the "A" post of the KW and swap it with the D post of the ZW.

Any other suggestions would be appreciated.

I was actually talking about shorting out a pair, one for each direction.  That way, there are still the same number of diodes in each orientation in the string.

From your description, I thought the problem followed the diodes.  If so, why would you suspect the transformer?

I'm just grasping at straws. The ZW is 60 years old plus or minus 5 years, and has never had any service on it. (Hasn't needed it, actually). Thinking about it, I did meter the A/U & D/U posts recently and they're both putting out 18 - 20 volts.

I'm am suspicious of the diode string with the six pairs. Can you briefly describe which of the two leads of each diode pair do I connect together? Also, wouldn't the continuity test I performed on each individual diode reveal which one was bad?

I don't know what test you actually performed, or exactly how it was performed, so I can't say if the test was valid.

My original suggestion was to simply move the diode strings between the two locations.  If you did that, you eliminated the transformer and the tracks, you stated that the issue followed the diodes.

How about a picture of exactly what you have for the diode strings that seem to be the problem?

When I get home from work tonight I’ll take a picture and post it. And yes I did swap the hot wire from each transformer post (A & D) and the problem moved to the opposite side.

In performing the continuity check, I touched the leads from the meter (set to Ω) to both leads of each of the individual diodes.  In one direction I would see current then when I swapped the leads from the meter there would be zero current. I did this on all 12 diodes.

There is usually a special "diode" setting for the meter which will properly measure diodes.

Sounds like you have eliminated the transformer, did you also eliminate the loop of track as the issue in the same test?  If so, you're down to something with the diodes.

Does the whistle/bell work as expected on the 6 diode ladder?  Does the train speed up or slow down when it is activitated? 

GRJ - I believe that both your's and Dale's should slow down. Is that right?

The diode strings slow things down all the time, when you hit the horn/bell, it shouldn't further slow it down as the voltage drop is always there.

Here is my Whistle/Bell controller. The white wire is coming in from the D post of the transformer, the black wire goes out to an old fashioned cab control matrix of 8 barrier strips & toggle switches, that flip control of a particular block from the left handle to the right handle. The smaller wires go to the terminals of a DPDT toggle to activate either the whistle or the bell.

Oh, and I should mention that yes, the horn or whistle & bell work fine.

What I'm saying is illustrated in the following picture.  Just work your way down the diode strings in the same manner, shorting one diode pair from the string for each test.  See if any single shorted diode pair fixes it.

My guess is maybe one of the diodes is not handling the current, even though it might look OK with a meter test.

OK - I'll give it a try. But just to be clear, when you say "short" the diodes, you mean to place a conductor between the two terminals of each diode, to work in pairs going down the string, running the train each time to see if that fixes the problem?

Correct, just jumper out one diode of each string and then try running.  Keep moving the jumper down one diode at a time and test.  If it's a bad diode, at some point the problem should go away.

I just wanted to finally report back on this. It's been a few weeks but my wife and I decided we couldn’t take the winter anymore (even though it was Spring) and flew to Turks & Caicos for some warmth.

Anyway, upon returning, I tested each pair of diodes as discussed previously and could detect absolutely no difference in train operation regardless of which pair I shorted out. As a last resort I completely removed one pair of diodes from the six-pair group that is connected to the “D” handle of the ZW, turning the six-pair controller into a five-pair (same as the “A” handle)and voila’, problem solved.

It’s hard to understand how one additional pair of diodes could make that much of a difference in the amount of voltage going to the track. I did meter it and it looked like no more than a 2 volt difference but regardless, it was just enough to stall a 2-motor post-war engine w/4 passenger cars (GG-1 Congressional Set). Strangely, with a more modern engine, a WBB U33C (so I could test the bell & horn) in place of the GG-1, the problem while still evident, was not as severe.