Lima cd converted Cl 31. Do I adjust cv’s?

[George-351466]

Converting a Lima Class 31 to HM7000 is delightfully straight forward: bags of space for a big speaker and power bank. I had previously fitted a cd motor (with voltage droppers) when running on an analogue layout and it worked very well.

In converting it to HM7000 mode it still runs well and the sound is great (Hornby’s Class 31 profile). However, take off and deceleration are a little odd. On starting the loco revs up nicely, but then leaps forward like a sprinter, whilst deceleration (ie slowing down) is is very casual: unless I kill the power half way round the layout, the loco just drifts on as if the brakes are failing.

It’s not a disaster, as running, pulling, basic speed setting and sound are all great, but the start off and deceleration are just irritating. Obviously the Hornby profiles and cv settings are not designed for a Lima cd converted motor. I have gingerly ventured into the cv profiles and fiddled with both the accelerating and deceleration figures (decreasing them), but to little noticeable effect.

Is this an unavoidable consequence from adapting old technology, or do I need to get more adventurous with the cv’s - a world in which I’m still rather out of my depth.

Thanks, as always, for any helpful suggestions.

As a post script, it’s just about my first anniversary of taking the HM7000 plunge and I’d just like to add my congrats to Hornby’s development team for bringing it to the market. The Bluetooth control app is brilliant and has transformed my operating enjoyment of the hobby.

[P-Henny]

I suggest reading pages 132 & 133 of the HM7000 Decoder Reference Manual regarding running the 'Auto Calbration' utility built into the decoder. This utility runs the loco on a test length of track, it measures the motor characteristics and sets appropriate CVs to suit.

If you didn't know such documentation existed, its all on the link below:

https://uk.hornby.com/hm7000/hm-dcc-guides

[96RAF]

If you have had to fit voltage droppers (presumed to be diodes) to the CD motor then it is likely a 6-volt motor which doesn't leave much for the decoder to play with.

I suggest if auto-calibrate doesn't work out that you look at fiddling with the complex speed curve after enabling it in CV29.

You need to change the lower end values to give you a wider spread across speed steps, i.e. make the 0-6 volts range cover about 3/4 of the speed curve then the rest can go linear up to about 9 volts as the motors will take some over-volting depending upon make of course.

Or go linear from 0-6 volts across the full range. For guidance the curve is in speed steps not volts so 6-volts will be around 128 speed steps, then straight line the value from there to 1 speed step. You could even stick with the simple speed curve and simply write value 128 to CV5 and value 64 to CV6.

[DJB61]

Came back to the hobby just over 2 years ago and I've upgraded all my 90's Lima locos with CD motors and DCC. Used the motor kits from Strathpeffer Junction which are mainly 6v so they recommended the fitment of diodes to drop the voltage albeit only slightly. Also installed 8pin sockets as I don't want direct wiring. They all have a piece of stripboard bolted in with the socket and diodes. Adjusted Cv5 and Cv6 otherwise they go too fast and you risk applying too much voltage to the motor. Set those Cv's to 135/45, or 150/80, so you need to test on your layout. 

They all have apart from one, DCC Concepts Zen decoders with SA's however one has a TTS. The latter doesn't have Cv5 or Cv6 so it's at top speed just over speed step 35. Not upgrading any more of these to sound as I've got newer locos with sound or have fitted either TXS or Zimo decoders including a Plux22. Also made a bigger slot for the front bogie wire (if fitted) as I found they could break off after a lot of running around curves. Picture is of a Class 37 bought in 2023 which was new but old stock. This has also been fitted with front lights not shown.

[George-351466]

Many thanks for helpful suggestions. After a good deal of delving into the detail of cv’s and advanced settings, as per ideas given,  I was still not getting satisfactory results, so I went back to read the detail of the supplier’s (Strathpefferjunction) notes and concluded that it might be the voltage droppers (diodes) which were interfering with smooth operation. On analogue the loco had worked brilliantly, but dcc operation can allow for limits on power etc via basic cv settings. Anyway, I deleted the diodes and, hey presto, perfect, smooth take off, slowing and speed control. Conclusion: if converting with an 6v cd motor on an dcc layout, don’t use reduction diodes, but limit load on the motor by cv settings, weight and speed of train, plus times of continuously running - and all should be fine. Now, suitably enthused, I’m going to attempt a Hornby tender drive conversion 🤞