Bachmann Ivatt Class 2MT

 

Another loco aimed mainly at our extended "Little Histon" layout, for an early 50s time line.

Bachmann photo of 46460

Locomotive Application:

Our village layout models Histon Station over a number of time periods. The Ivatt 2MT was used to haul passenger services in the 1950s, between Kettering and Cambridge several times a day, passing through Histon station, but only stopping upon the request of on-board passengers. The line passed over some very fragile bridges between Huntingdon and St Ives. (The remains of which can still be seen from the footpaths near Houghton Mill.) This placed severe limitations on the maximum weight of locomotive able to use the line. Hence the choice of the Class 2MT to haul the service.  Bachmann have just re-released their 2MT model in early crest livery, so it was a natural choice for our project.

 

Hatton's photo. (The model was pre-ordered from here.)

 

DC Running Test:

To make sure there are no inherent mechanical or electrical issues with the drive system, the loco was placed on a two yard test track and powered from my DC bench power supply. Movement was OK with quite reasonable slow running performance, which should only improve when powered instead from the back-emf motor control circuit in the DCC decoder. 

 

DCC Sound Fit:

The only available space for the decoder, speaker and stay-alive arrangements is in the tender. Although this is quite long, there is limited head height and after good results on the LMS bachmann Class 3F (which we use for Midland trains in the 1930s period on the layout) I've decided to take a similar approach here. A Zimo MX648 sound decoder will drive a "sugar cube" speaker within the tender body and an AVX 6800uF 15Volt super-cap with zener diode charging voltage protection will provide the stay alive storage medium. I'll source the decoder with a suitable sound project on board, plus the super-cap from Digitrains and I already have a Zimo sugar cube speaker in the spares box. 

The locomotive tender.

 

Tender Modifications:

Curiously, although an 8 pin DCC decoder connection is provided within the rear of the tender, there are no holes in the mouldings to allow sound to escape. So the first modification I have to make is to drill a matrix of holes between the wheels in the tender floor.

I've removed the ballast weight plates and the 8 pin socket, with its blanking connector, as the decoder will be hard wired in place. The sugar cube speaker has been glued to the coal hopper underside. There should be ample room left in the lower tender body to accommodate the MX648 and super-cap circuit. This will leave the coal hopper free to be filled with steel "liquid gravity" balls to provide an alternative ballast weight. After finding a couple of escaped balls magnetically attached to the cone of the LNER green B12 speaker, creating audio distortion, I'll be careful to more thoroughly glue ALL the balls in place this time!

As received

As received

 

Showing the speaker glued in position and the new hole matrix.

The decoder track and motor wires will be soldered direct to the small PCB fixed to the tender 4 way socket.

(In place of the 8 pin socket assembly wiring.)

 

 

Decoder in place (top left) and stay alive super-cap with protection circuit (top right).

 

 

Initial Testing:

The initial test was somewhat brief as the locomotive refused to move when the Lenz throttle was advanced. The sound system was working and the stay alive system kept the sound active long after the off switch had been thrown. As the loco behaved quite well under  DC control, I wondered if there might be an intermittent break in the motor leads or the loco-tender connector. So I removed the upper body from the loco chassis and made a direct connection to the motor.  Still no sign of life after an initial half turn or so. Checking in DC again by connecting my bench power supply direct to the motor, revealed smooth rotation, drawing only 60mA, so nothing wrong there!

A brief call to Digitrains solved the problem. It seems that MX648 decoders can't drive motors with a 10NF capacitor in parallel. Snipped the capacitor lead and magic! we have movement. 

Not quite as smooth as I'd hoped, but I've not tweaked CV9 or done any other "housekeeping" yet..... so early days.

 

 

Lighting Arrangements:

For expediency, there will be no lighting installed initially. Instead white disc(s) will be used for daytime headcode purposes. Firebox lights and headcode lights maybe fitted later when time and other commitments permit.

 

 

Decoder CV Adjustments:

The usual Zimo housekeeping tweaks (CV9=87) and CV267=60 for a 4 per driving wheel revolution chuff rate.

 

 

Adding the bagged items and re-testing:

Sound quality:  Good:  The sugar cube speakers work well when enclosed in a tender on sound projects!

DCC Motor control: Fine now (helped by a tweak on CV9 described above).

Checking on the return loop curve suggests that the longer draw bar (as shown) is probably the expedient choice for operations on the village layout although it doesn't help the appearance of the cab.

 

 

 

With thanks to David for further useful info:

Essential data for a future weathering project!

 

Supplier website links:
Hattons    The 2MT was sourced from Hattons
Digitrains    Source for Zimo stay alive super cap and Zimo DCC decoder

 

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