Updating the Hornby Class 67 with a Zimo based sound system



The availability of a new Zimo based Class 67 Protodrive sound system from Paul Chetter is an opportunity to upgrade the Hornby model once again. This is also a chance to refine and simplify the somewhat "over the top" original interface circuit between the Hornby negative ground lighting boards and the decoder.


Hornby's 1/76 scale DB Schenker all-red Class 67 number 67018


Revised Circuit Proposal:

Hornby use their well known common negative approach to the lighting PCB assemblies. So I have to use a pair of inverting PNP transistors (BC559C or similar) to enable the decoder function outputs to control the positive feeds to the Hornby boards. A pair of diodes (1N4148 or similar) connected to FO3, are used to disable the rear lights when the locomotive is pulling a train. (When FO3 goes negative, the diodes starve the rear light LEDs of voltage). Cab LED and detonator flash LED are connected to the decoder via their series resistors in the normal way. The night time running light switching lines are paralleled and driven via FO1, with a 100k pull-up resistor to hold the Hornby MOSFETs on in daytime operations.  (If I had used this arrangement with the earlier LokSound V4, There would have been no need to add the extra FL4 function decoder!)


MX645 diagram showing the location of the connection pads needed for this project.


Additional wires soldered to the decoder.


Original DCC interface circuitry on main board replaced with the new decoder circuit. (Less detonator flash LED at this stage.)

It looks a bit of a mess!  Key question is:  Will it work?


Initial Mechanical and sound checks:

The loco chassis assembly was checked before fitting the upper body shell, to check basic operation and investigate the sound system.

First, to check communication and make sure we don't damage the speaker:

Before we fire it up......   master volume is: CV266  Default 65 is max ......reset to this to 25......  O.K.

Check basic motor control.....  Looking good! ........ Activate sound....  Excellent. All appears to be working OK...... except for occasional brown-outs. Track and wheels cleaned, much better, but I think some stay alive capacitance would be prudent, but where to put it?

Ah Ha! Room for 2000uF under the plastic cover over the number 1 end bogie (marked with a white number one in the photo above.) Unfortunately the corresponding number two end cover is a metal casting which protrudes too far down to leave room for capacitors behind the plastic side grills, fixed to it. 2000uF is not really big enough to make a huge impact, but it is certainly better than nothing!

Two 1000uF 25V capacitors connected in parallel to provide 2000uF of stay alive capability.


No more brown out incidents since fitting the caps!


Adjustments to movement related CVs:

Operation is smooth, without any obvious judder or hesitation effects, but starting seems a little abrupt in train pulling mode and extremely abrupt in light engine mode, so an increase in CV3 looks a good idea. slowing down seems rather abrupt in light engine mode (with brakes not really required.) So a slight increase in CV4 and more dramatic adjustment in the difference between train pulling and light engine momentum effects also seems desirable (via CV390).

High value CV3 & CV4 enables me to enter step speed changes when operating under PC control. This may not suit everyone!


CV Original value New value
2 1 3
3 40 100
4 90 90
5 1 1
6 1 1
9 0 0
57 140 140
390 30 150



Re-Programming the Decoder Function Mapping:


Sound Key assignments:

Key Initial assignment Updated assignment
F0 Lights BIS / Primer (on)  Primer alarm (off)
F1 Sound on/off Sound on/off
F2 Brake key Brake key
F3 Horns Two tone horn
F4 Horn Single toot horn
F5 Switch to Light engine Engine speed up
F6 Engine to idle Reserved
F7 Engine speed up Reserved
F8 Short Air Release Reserved
F9 Flange squeal Reserved
F10 Brake release Reduce momentum
F11 Brake squeal Flange squeal
F12 Spirax valves Spirax valves
F13 Horns Fan
F14 BIS / Primer (on)  Primer alarm (off) Buffer-up
F15 Fan Radio check
F16 Horns Door slam
F17 Guards whistle Guards whistle
F18 Door slam Triple Detonators
F19 Fade all sounds Brake release
F20 Triple Detonators Brake squeal
F21 Radio check lights control
F22-F26 Reserved lights control
F27 Volume Down Volume Down
F28 Volume Up Volume Up


Note: The data in the table below was established by reading the appropriate CVs from my decoder and maybe specific to that device. If viewers wish to remap a similar 67 project, then please read the necessary CVs in your own device, the values may well be different from mine!


Sound Changes Sound slot CV SS Value Volume CV Volume Value repeat code CV rc Value
BIS, Primer/ Primer alarm (ex F14) to F0 570 42 571 0 572 8
2 tone horn  to F3 519 25 520 0 521 0
Fan  (ex F15)   to F13 549 38 550 91 551 8
Flange Squeal  (ex F9)   to F11 543 20 544 0 545 72
Brake Release  (ex F10)   to F19 567 35 568 91 569 0
Brake Squeal  (ex F11)   to F20 673 37 674 91 675 0
Radio check  (ex F21) to F15 555 43 556 0 557 0
Door slam (ex F18) to F16 558 23 559 0 560 0
Triple detonators (ex F20) to F18 564 44 565 0 566 0
Buffer-up to F14 552 24 553 0 554 0
Remove sounds from F8 534 0 535 0 536 0
Remove sounds from F9 537 0 538 0 539 0
Remove sounds from F10 540 0 541 0 542 0
Remove sounds from F21 676 0 677 0 678 0
Remove sounds from F20 567 0 568 0 569 0


Sound Change enablers  CV  Value  CV  Value
Remove FO actions from F0 33 0 34 0
Remove sound fader from F19 313 0    

Some sound slot volume levels were adjusted to increase relative levels and the updated values are quoted above.

All working well !


Swiss mapping for lights control:

The Lighting Function Outputs will be controlled using ZIMO's "Swiss Mapping". The following table outlines the content of each group:


Group Description Fwd 1 Fwd 2 Rev 1 Rev 2
1 Day  light-engine running lights Headlight FO   Rear lights FO  
2 Night light-engine running lights Headlight FO FO1 Rear lights FO FO1
3 Enable hook end  running lights (if no train attached). FO3   FO3  
4 Cab light (driver end 2 only) FO4   FO4  
5 Detonator flash

(not yet fitted)


These will be assigned to keys 21-25


CV Values are tabulated below:


Description CV Value
Group 1      F-Key 430 21
M-Key 431 0
1st fwd FO 432 14
2nd fwd FO 433 0
1st Rev FO 434 15
2nd Rev FO 435 0
Group 2     F-Key 436 22
M-Key 437 0
1st fwd FO 438 14
2nd fwd FO 439 1
1st Rev FO 440 15
2nd Rev FO 441 1
Group 3      F-Key 442 23
M-Key 443 0
1st fwd FO 444 3
2nd fwd FO 445 0
1st Rev FO 446 3
2nd Rev FO 447 0
Group 4      F-Key 448 24
M-Key 449 0
1st fwd FO 450 4
2nd fwd FO 451 0
1st Rev FO 452 4
2nd Rev FO 453 0
Group 5      F-Key 454 25
M-Key 455 0
1st fwd FO 456 2
2nd fwd FO 457 0
1st Rev FO 458 0
2nd Rev FO 459 0

Groups 1-4 working well ! (Additional hardware needed for 5.)


Locomotive on the test track with the upper body shell now back in place.


Full Function Listing:


F0 BIS / Primer (on)  Primer alarm (off)
F1 Sound on/off
F2 Brake key
F3 Two tone horn
F4 Single toot horn
F5 Notch 1
F6 Notch 2
F7 Notch 3
F8 Notch 4
F9 Notch 5
F10 Reduce momentum
F11 Flange squeal
F12 Spirax valves
F13 Fan
F14 Buffer-up
F15 Radio check
F16 Door slam
F17 Guards whistle
F18 Triple Detonators
F19 Brake Release
F20 Brake Squeal
F21 Light engine day running lights
F22 Light engine night running lights
F23 Enable hook end lights (when not pulling a train)
F24 Cab light (driver cab at end 2 only)
F25 Future Detonator Flash (Needs new hardware)
F26 Future lighting reservation
F27 Volume Down
F28 Volume Up

I may reshape this to maximise commonality with the 66s later......but this will do fine for a bit of video work!



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