fitting A1/A3 sound decoder

[DaveU]

I have fitted the above decoder and speaker to a flying scotsman changed id etc but no sound functions appear on the railmaster buttons. So I tried installing it as the FS tts model, the buttons were then labelled but no sounds. The train moves but no sound. Any ideas?

 

[WilliamDavid]

If you have it set up using the Flying Scotsman TTS setting, you should indeed get all the sounds.

Assuming the speaker is attached, it could be a faulty decoder or a faulty speaker.

I have had a couple where the sound failed after originally working. I suspect duff decoders.

I suggest you contact Hornby.

[DaveU]

Sorted today, deleted the loco, reinstalled and reset the chip to factory default - reloaded as FS tts and hey presto - it worked perfectly! However, 1 slight problem has developed - on 2 of my points it now cuts out, does anyone know if I can fit a 'stay alive' to the new Hornby sound ship to overcome this. (It seems the new chip has made the loco more sensitive to the short break in power on these 2 old points - previous chip was a hattons special)

[Howbi]

@DaveU..........It is possible to connect a Stay-alive to a TTS decoder but it is a very delicate task to solder two wires to the correct components on the surface of the decoder........there are 4 bridge rectifiers surface mounted and it is critical to identify the exact connections to solder to........this blow-up photo shows the components but I am not qualified to advise which bits to solder to.........HB

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[Howbi]

@DaveU..........The size of capacitor you use is important both in dimension and value.........these are 20mm x 10mm rated @ 2200uF which is close to the minimum value to have any meaningfull effect.......HB

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[Chrissaf]

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Try the solder pads indicated above. Check with a multi-meter set to DC Volts and you should get a reading about 14 volts DC across these two points. Note: following the track connected to the red circle ends up on the Blue +ve return wire solder pad, thus that is definitely the +ve side of the bridge rectifier.

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Reposted as image upload working this morning.

[DaveU]

So, if I pluck up the courage - can I just connect the circuit to the blue +ve wireon the harness and the negative to the solder pad on the bridge rectifier? ( shame there is no wire on the harness to minimise risk to the chip!

Thanks

 

[Chrissaf]

Yes.

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But do check the voltage with a digital multi-meter first to confirm the connection points. i.e the about 14 volts value quoted.

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Basically, DCC decoders that are 'Stay Alive' ready have the additional wire on the negative of the bridge pre-provided.

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PS - You need a capacitor with a minimum 16 volt rating, but 25 volts would be preferred if physical space permits. The higher voltage caps are bigger.

[DaveU]

Looking at one of your posts last year you talked about the diode/resistor circuit providing protection for the bridge rectifier and you suggested lower value capacitors might not need this (ie not super capacitors) - did I read that right? I'm going to have a go on a cheap decoder, firstly if I can, with just a capacitor 4700uf 16v but if essential will add diode/resistor circuit. You mention making it switchable, can that be on either +ve or -ve or does it need to be double pole? Does the length of connecting wire matter? As may need to put the circuit away from the decoder due to space considerations.

Thanks

[Chrissaf]

.....you suggested lower value capacitors might not need this (ie not super capacitors) - did I read that right?

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Yes. Super Capacitors have extremely low internal impedance and due to their high Farad value (relative to Electroytics) take longer to absorb a charge. Thus the high inrush current can last significantly longer and need the diode protection. A 4,700uF Electrolytic capacitor will probably be OK without the additional diode protection.

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You mention making it switchable, can that be on either +ve or -ve or does it need to be double pole?

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You only need to break the capacitor circuit path. Either capacitor leg will do. Neither does it need to be a physical switch. It could be done as simply as an inline micro miniature connector that you pull apart prior to programming (body off CV configuration would probably then be needed**). Test the decoders ability to accept CV programming with the 'Stay Alive' in circuit first. Only consider fitting a 'break circuit' capability if you experience programming issues. Although the capacitor is on the DC side of the decoder rectifier bridge, it needs to be charged up before the full supply voltage is across the decoder IC where the CV configurations take place. The power supply current provided by the programming output of Hornby DCC controllers is very low indeed. It is possible that the 'Stay Alive' capacitor may take too long to charge up and CV programming will fail due to DCC configuration packets being lost whilst the capacitor is still charging. This is just a theory, AFAIK decoders that are made commercially fitted with 'stay alive' don't need any disconnection options.

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Does the length of connecting wire matter?

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Not really, the capacitor is operating as a DC device and is not being used in the DCC signal path.

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Note** having the body off to perform CV programming (to allow capacitor disconnection), shouldn't be a major issue. I would expect CVs would only be configured once during set up, after which the configuration is stable and wouldn't need changing (in an ideal world).

[michael_winstanley]

I've been wondering whether to go DCC... after reading these posts I think I'll stick to analogue! Thanks everyone you've saved me a lot of money.

[Fishmanoz]

May I suggest that, if you are making your decision on just this topic, it would not be a “sound” basis on which to make such a decision.  Have you done a forum search and looked at any of the many threads which discuss the merits of DC v DCC?