Ashbourne Junction Tonefan

I have a Dapol Class 22 which I have just chipped with a 21 pin Zen Black decoder. Whilst I can get the directional lights and headcode lights working fine, and the loco runs perfectly, I cannot for the life of me get the cab lights to operate. Does anyone know if I’m missing something or if there might be a compliance issue with the lighting on this loco. The loco is brand new today. I have tried to set the CV settings in accordance to the instructions below, but as yet no joy. I’d read somewhere that other Dapol locos have unusual lighting settings trying to be all things to all men. I also found this in another forum regarding cv8 and wondered if the zen black 21pin share similar characteristics?

‘I succumbed to temptation and bought D6327 in blue with small yellow panels from Hatton's. I fitted a 6-function Zimo MX638D decoder after testing on DC. So far so good. She ran well with the Zimo decoder.

I like the way Dapol have arranged the lights: F0 works the headcode plus marker or tail light directionally at the A end, F1 works the headcode and marker light at the B end, and F3 works the tail light at that end. However, the cab lights refused to work at all. Then I realised the MX638D uses two logic outputs for the 5th and 6th functions. However, a useful trick with the MX634 and MX638 families is that the user can change the logic outputs to full powered outputs with a simple CV change, changing the decoders to MX634C or MX638C. The trick is to program CV8 to 3 for powered outputs (as the 'C' variant), or CV8  = 4 for logic outputs (as the 'D' variants).

Anyway, the cab lights at each end now work on functions 4 and 5, respectively.’ 

Thanks for any help offered.
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Ashbourne Junction Tonefan
I have swapped the decoder (though I absolutely completely love the black zen and have it in most other locos) for a Zimo 638D for a test and then found the same problem. So I reset CV8 to 3 and the cab lights worked immediately. The only explanation I can find is this is in this forum post: 

The 21 pin socket is a bit of a mess standards wise - there are two versions of the standard (oh joy!). 

The first four outputs are fine (front light, rear light, output 1, output 2), those are all normal full-power outputs.  The problem is the outputs 3 and 4, which can be either full power or logic level. 

But the documentation implies a CV change could fix things:



First question to Dapol:  which version of 21pin, "logic level" or "full power" ?  I can't find an online instruction sheet. Dapol's instructions for the Imperium decoder says "Aux 3,4,5 & 6 are Logic level outputs".   As that's the decoder Dapol would recommend, it implies that the 121 is expecting "logic level" for output 3. 


Zimo cover both versions of the 21-pin standard.  They use the designation "C" for logic level and "D" for full power on the 21-pin decoders.

The MX631D is "wrong decoder", that has full power on outputs 3 and 4, you need the MX631C which has logic level on outputs 3 and 4. 

The MX634D and MX638D decoders can be changed between the two different 21pin standards by a CV change.   CV8=3 gives logic level on outputs 3 and 4 (what appears to be required for the Dapol 121) and CV8=4 gives full power outputs (as shipped condition for MX634D or MX638D). 


So, a documentation hunt says a CV write of CV8=3 will change the Zimo MX638D to provide logic level output

So problem solved, but strange stuff with this loco indeed. Dapol’s answer was ask your decoder manufacturer. I have yet to try changing the zen blacks CV8 to 3, but if it works I will swap back as I am a DCC Concepts loyal user. Hope this helps anybody in a similar predicament. I will send this info to Dapol as their answer to me was completely useless and yet they are the guys who choose to use this logic! Still the loco is great nonetheless. 

further: the Zen Black manual states CV 8 can only be configured to 36 for a factory reset. It says range is n/a. Having tried to set the Black to CV8: 3 as an experiment, I found no success in the same way I did with the zimo. It has not damaged the Zen Black and I can still use it in other locos, but if cab lights are needed in the Dapol 22, the two seem incompatible. So in summary, check outputs 3 and 4 are logic level on your decoder if you want to use this loco on DCC and have FULL functionality. I still think overall Zen Black is a much superior decoder in all other respects. I subsequently have learned about these powered outputs and differing standards, but I still feel this wasn’t made clear in the class 22 documentation or is communicated clearly for modellers without some searching.


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Well done for getting to the bottom of it and many thanks for taking the time to share it here for others to benefit from.  I'm just glad it wasn't me as I would have never got that far!!
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