AF Model 8B transformer problem

Does the green light stays on with the train hooked up? Are you measuring current from the track  or transformer posts?

Gunny

Yes, the green light stays on at all times. I'm measuring voltage at the transformer posts.

The 8B is a simple transformer internally. From the description it sounds like there is a very high resistance connection inside the transformer, possibly the wiper arm. The meter draws a very small amount of current so it sees the winding voltage even with the high resistance. The train is trying to draw about 2A so all the voltage is dropped across the internal resistance. Try using a piece of wire to short across the variable and base posts. If the internal breaker does not trip then it confirms the problem.

Thanks for the suggestion. Internal breaker did not trip when shorted across either base and variable or base and accessory posts. Tried cleaning the top of the windings and the wiper contact with alcohol: no effect. Tried running a train using the 15v accessory post and base post: didn't move. Tried measuring the resistance across pairs of posts, but my auto-ranging meter didn't cooperate.

What next?

The 8B has about an 7.5:1 turns ratio to provide 15V from the old 110V house power. I do not have an 8B handy but I do have a 4B which has a similar 100W core. The primary measured 10 ohms DC. The secondary measured 1/2ohm DC. Any of the three output binding posts should be close to infinite resistance to a primary winding end. If the resistance measurements are not plus or minus 20% from those values there is something wrong with either the connections or within the windings.

So I started measuring resistance along the path from the base post following the wiring, and I strongly suspect the problem is the direction switch. I tried cleaning its contacts and succeeded in bending them so they didn't connect. Tried bending them back, reassembled things, and the transformer briefly worked. Your suggestion to look for a large (think infinite) resistance definitely led to this approach.

BTW, to overcome my meter's unwillingness to measure low ohms, I first measured a 350 ohm resistor, and then made all other measurements with that resistor in series. That's a keeper idea for the future.

Not home yet, but thank you so much. Your help has moved me forward tremendously.

Great, I hope you can get it working. The 8B is a good transformer.

I have an 8B and was having problems with intermittent operation.  Spraying contact cleaner on the direction switch corrected the problem.  I don’t know how that would apply to your current situation.

John

Well, too late for me since I used a Q-tip with alcohol and apparently bent the contact. But I will bend some more tomorrow and see what happens. I feel as if I am zeroing in.

It works! It was indeed the direction switch. Neither alcohol nor contact cleaner worked. I disassembled the switch/circuit breaker so I could see the contacts. They were black. I used a needle file on them until shiny and flat. It was tricky getting the thing back together. But it works.

FYI the green light is wired directly to the secondary windings and does not go through the direction switch or connect to the base post. The 15V post is likewise directly connected to one end of the secondary winding. The base post is connected to the direction switch directly.

For future reference, I measured the primary winding (across the wall plug) at 6.6 ohms, and the secondary at 5.1 ohms, but these could easily be plus or minus as the readings bounced around somewhat.

Thanks again for the help and support.

Thanks for posting the solution. That adds to the collective knowledge base for all of us.

@Ken Wing posted:

It works! It was indeed the direction switch. Neither alcohol nor contact cleaner worked. I disassembled the switch/circuit breaker so I could see the contacts. They were black. I used a needle file on them until shiny and flat. It was tricky getting the thing back together. But it works.

FYI the green light is wired directly to the secondary windings and does not go through the direction switch or connect to the base post. The 15V post is likewise directly connected to one end of the secondary winding. The base post is connected to the direction switch directly.

For future reference, I measured the primary winding (across the wall plug) at 6.6 ohms, and the secondary at 5.1 ohms, but these could easily be plus or minus as the readings bounced around somewhat.

Thanks again for the help and support.

Since the green light is always connected to the secondary, you’d have to disconnect it to get an accurate reading of resistance.

Yes, I thought of that after I posted (and reassembled).