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I have an MTH 20-2040 Southern Pacific GP-9 engine, pictured below. I operate the engine conventionally on an isolated loop of tubular track, using a Lionel prewar type V 150 watt transformer. Except for the initial reset state, the bell rings constantly when the engine is under power. No instructions came with the engine, and I am not very familiar with MTH products. Is there a way to turn the bell off? Thanks.


Original Post

You have an "offset" going onto the track. The whistle and bell are normally controlled by a rectifier block that uses a series of diodes to offset the peaks and valleys of the sine wave by about 1.5 volts. A positive offset blows the horn/whistle; a negative offset rings the bell. I've seen this with some earlier MTH locomotives on the club layout. A pre-war transformer shouldn't present a problem as those were variable AC, so there is likely some DC leaking into the track from somewhere. I don't recall if the V-150 has a whistle controller, but if it does, you may have a leaking whistle rectifier.


Do you have any accessories tied into the track? Is the transformer shared by anything else like a TIU, or common-grounded to track powered by another transformer? An electronic transformer like a CW80 generates a chopped sine wave and that noise could definitely leak into the system. You may be able to clean that up with a 100uF 35V non-polarized electrolytic capacitor across the A-U terminals. I've attached a Proto-1 reset chart and the QS2Plus manual for how to program the resets. The easiest way is to find a friend with an MTH Z4000 which can enter the codes directly.




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  • Proto-1_Reset_Chart
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Thank you all for the replies. Here are the accessories that are connected to this track:


Lionel # 12782 Operating Lift Bridge;

Lionel # 24111 Operating Swing Bridge;

Lionel # 14167 Operating Lift Bridge;

Lionel # 153 IR Controller for a postwar banjo signal.

There are also two prewar 072 switches for a passing siding.


I presume from the comments to this thread that one or more of the above accessories could be outputting a DC signal to the track that would cause the bell to ring constantly?

The failed diode that I mentioned was one of the 4 located on the opposite end of the lower board from the capacitor and fortunately was visibly failed. I had previously tried all the normal diagnostic and reset procedures and the engine would run and respond normally except for that persistent bell. In this case, replacing the diode cured the problem.


Eric Hofberg


Last edited by chug

I did some testing on an isolated Fastrack oval with no accessories attached. Power was provided from the D-U posts (non-whistle control) of a Lionel postwar SW 130 watt transformer.


At first, the engine behaved the same way with the bell constantly ringing. After activating the reverse unit in the engine by moving the transformer handle to off five or six times, the bell stopped ringing and the engine worked normally.


After activating the engine's reverse unit by moving the transformer handle to off five or six more times, the bell started the constant ringing again. It continued ringing until I activated the reverse unit five or six more times, then went silent again. At this point, with a silent bell, I stopped testing.


It would appear from the testing that activating the engine's reverse unit by cutting off the power five or six times causes the bell to operate or not operate. Is there a better way to activate/deactivate the bell? Thanks.

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