CW-80 and MTH GP-9 bug?

Check the back of your MTH owners manual.

 The Lionel CW-80 transformer is NOT on MTH's list of compatible/ recommended transformers.

Additionally, the resistive load thing is a CW- 80 design. A load is required just to get a correct voltage reading.

Its strange to me while Lionels most common entry level transformer was engineered with these oddities.

Heres something I posted to my own thread on the electrical forum while trying to descern these CW-80 quirks myself while using one to power accessories.

From Dale Manquens "Trainfacts "website:

"Each of the Triacs is bypassed by a resistor and capacitor in series to reduce transients across the Triacs. NOTE: These rather large 1 uF capacitors will cause misleading output voltage readings on a high-impedance digital voltmeter if there is little or no resistance connected from A or B to U at the output. The impedance of the capacitor, even with the Triac shut off, will be much lower than the input impedance of the digital voltmeter, causing virtually the entire available 18 VAC to appear across the meter. When testing this transformer, add a light bulb across the output being tested. The bulb will not only offer a low resistance shunt to help the meter read correctly, but it will also give a visual indication of what is happening with the output voltage as the lever is changed or the Accessory voltage is programmed."

Well, there's your reason WHY... it's a switching power supply, powered by a traditional iron core transformer.

It’s not a switching supply in the sense that the term applies to. It’s a triac controlled power supply from a robust transformer. The reason it needs a resistor is that the controller doesn’t go all the way down to zero volts and the engine can’t respond and change directions or go to neutral. It’s a well known possible issue with triac controllers. The old track lockons with the power is on lamp would be ideal for this transformer.

As cjack says this is a known issue.  If you open an MTH Z-controller which is a similar design as the CW-80 you might recognize that big white component

As to why the engine itself does not represent a big enough load to "tame" the controller output, most MTH engines have on-board power (battery, super-capacitor).  This on-board backup power takes over supplying power to the engine's electronics when you push the DIRECTION button or zero the throttle (i.e., turn off track power) on the controller.  Engines without this on-board backup power would provide a suitable load to tame the controller output because the track voltage might be directly wired to, say, a headlight bulb presenting a suitable load.