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Thanks to all for advice on the sine wave circuit. I have now located the PCB for the ancient (80's ?) Hornby Sound box. This is the one I pinched the ceramic caps from for the sine-wave circuit.
I think it could be fun to try and resurrect it...the board is edge marked "To Track" in one place suggesting there maybe some I/O voltage /current sensing or similar. Speaker is 8ohms and power 15v ac.
With DCC sound chips being so expensive, perhaps DC sound has something to offer at lower cost ? I've yet to find a schematic for the Hornby box, so I'll probably just try out a few audio-type circuit ideas and see what I can come up with.
Cheers for now,
Dave
That sounds like the R396 sound box.
The "Track" connection was to synch the chuff rate with the loco speed.
Somewhere around here I have a circuit of a similar DIY project (think it was only the chuff sound), I'll see if I can dig it up.
If you can supply good clear photo's of the top and bottom of the pcb, I might be able to reverse engineer it if you really want to get it going.
Thanks Terry, I'll re-fit the caps and try it out with a 15v ac supply from the controller. Yes, some sort of 'loco synching' seems most likely. Modern audio circuits seem to favour op amps with a feed-back loop rather than the NAND gate C-MOS chip fitted to the box.
Good opportunity for me to update my camera skills (?), but I'm not sure about getting them onto this forum page..!
Cheers, Dave
Dave,
If you can't get the photo's on to the forum, send me a message through the PM system and I will send you an email address you can send them directly to.
I did hunt up the circuit I mentioned and it is meant to be mounted in a piece of rolling stock and uses a mechanical system to sync the sound, so not applicable to your unit.
A bit of digging did come across a circuit that uses a cmos chip to make the sound and it was synched via the track voltage. I'll dig it up again and put it here - just need to remember where I saw it.
Found it, it's from the Babani book BP 384 Practical Electronic Model Railway Projects.
This is just the chuffer, the whistle is a seperate circuit.
Steam chuff 1.pngSteam chuff 2.png
Thanks Terry -those circuits look very interesting. All the caps are re-fitted to my unit except a de-coupler for the 15v ac rail (I'll probably use a 0.1 mf for that.)
I also found a demo /video of the Hornby box which shows it is pretty basic ! Also, I now have fly leads for track voltage, external speaker and the 15v ac supply, so should be able to start experimenting with 'add-ons etc' quite soon.
A quick update : I now have the Hornby box working with the exception of the loco-synched 'chuffing' sound.
Next job is to get the 'scope on it and trace some signal paths around the pcb.
Today's test showed that the motor pulses are definitely there on the 'scope. Current draw is between 50 and 200 ma dependent on controller setting. The Hornby controller puts out a fairly constant 18-20v. I'm told that it's a 'feedback type' controller and seems to vary track current to control motor speed. I wonder if this is a clue as, back in the 80's, basic trainset type controllers may have just been variable voltage devices. I also noticed the "volume" trimmer on the box is noisy and this varies with motor speed.