As very much a new boy to the world of DMU preservation I feel a bit nervous when I start waxing technical, but I wanted to say something about the tacho circuits and the related Start Isolation Relay, as well as a few spin-off subjects. I offer these ramblings because, having had some problems starting my 117 set, I was unable to find out very much in the TRA literature, and felt that a void needed to be filled. Having said that, I've little doubt that if I get it wrong, all those who know the answers (but failed to tell us - or at least, me), will be goaded into telling me how wrong I am, so that one way or another the facts will emerge. I stress that what I am describing relates only to my own unit, and may differ from other units - even other 117s.

Outside of each power car, on one side, below the solebar, are a pair of small equipment boxes, long in the vertical, carrying the legend BUT and below that 'Smiths'. I had been told that these had something mysterious to do with the tacho. Opening one up reveals, at the bottom, a series of terminals, above that a transformer, and above that, some more terminals, and above that some electronics mounted on a printed circuit board (PCB). I soon realised that whilst the terminals receive signals from the engine tachogenerator (mounted on the engine), these signals are passed, both to the desk tachometers and also provide inputs to the transformer. However the output from the transformer which drives the PCB and the PCB itself do absolutely nothing for the tacho system. The output from the PCB simply drives the operating coil of the Start Isolation Relay (SIR).

And what does the SIR do? As designed, it simply prevents the starter from being operated. All that gubbins just to prevent operators doing what they shouldn't do in any case, that is, trying to start the engine when it is already running, However, incorrect operation of this equipment can prevent attempts to start an engine by disconnecting the starter motor

Looking at what appear to be later designs, the SIR relay has an additional function, and will shut a running engine down. Where is the SIR relay? Not in the tacho box but in a larger box, again, under the solebar containing, not only the SIR relay, but the the train stop relay and local start relay. The SIR relay differs from the others in that (certainly in my unit) it is a more modern open frame relay, as opposed to the archaic Stone's 'Tonum' relays, and carries two pairs of changeover contacts.

Before we start, (and at risk of teaching grandmother), I'd like to explain briefly what a relay is. At it's most minimum it's a device with four terminals. Two of these terminals ALWAYS go to the operating coil. When current is passed through the coil (by applying a voltage ACROSS the coil) a magnetic field is set up and an 'armature' moves in the field. This armature can move a a pair of contacts, in like manner to a simple switch. At it's simplest the contacts can be made to CLOSE when the coil is 'energised' (they are said to be 'normally open' or n/o). The simple alternative is to arrange the contacts to OPEN when the coil is energised (they are said to be 'normally closed' or n/c). A combination of the two with three terminals allows a common contact to close with on one side whilst opening the other. This is called a changeover (c/o) contact. The Tonum relays can cause confusion since one side of the contact is often the frame. In the case of the SIR relay (in a form made famous by Thorn Industries, and thus often referred to conversationally as a Thorn relay irrespective of the manufacturer). This more modern relay has allowed two changeover contacts to be incorporated. To return to the normally closed relay again, it's worth noting that a certain amount of mental agility is needed to keep track of what is going on, since, generally the relay is energised in order to STOP something happening.

Anyway, let's look at normal operation of the equipment. Let's assume that everything has been properly prepared and we are ready to press the local start button, outside a motor car. At this stage the voltage at the output of the tacho box (on the terminal marked ES on wire 68 or 69) is at 24volts (24v) relative to the local negative. If you are using a multimeter, the black probe should normally go to the local negative, in work of this type. Wire 68 (or 69 for No2 engine) goes to one side of the SIR's operating coil. The other side of the coil goes via wire 51 to the local positive (also 24v). There is thus 24v either side of the coil, that is zero volts ACROSS the coil, and the coil is thus DE-energised. Now read this paragraph again ...it's tricky!

Now the SIR uses one side of one of its changeover contacts wired to act as a NORMALLY CLOSED contact. This contact is wired in a serial chain with contacts from other relays, in order to feed the starter solenoid (which directly actuates the starter motor), so, providing the SIR contacts are closed, the starter will crank and, all being well the engine will fire and run

Within a fraction of a second of the engine firing and starting to run the output from the tacho box will fall to close to 0v relative to the local negative. There is thus almost 24v ACROSS the SIR coil, so it will energise. It's normally closed contacts will open, and any attempt to operate the starter will fail.

So long as the engine runs the relay operating coil will be energised (the coil may well be warm to touch after a while). If, for whatever reason, the engine stops, then there is a lag of about five seconds (generated by the tacho box) before its output returns to +24v, the SIRs coil de-energises, the contacts relax to their closed position and the engine can be re-cranked. Sometimes it is possible to hear the relay de-energising if you listen carefully.

Assuming everything else in OK (Batteries OK, BIS ON, Fuse 7 OK, Train Stop Relay de-energised, FG1 desk key ON, Engine Isolation Switch ON (square box outside operated by carriage key), , Water Level Relay energised, Fire Alarm Switch OK), then wait a few minutes and feel SIRs operating coil - is it warm? Yes? Then suspect the tacho box.

It all gets a bit technical now, as we have to get to grips with the PCB. Fig1 shows the PCB as it is seen looking directly into the tacho box. If the PCB is released by undoing the nuts on the four mounting pillars, then, by turning the PCB round we see the components shown in stylised fashion in Fig.2. There are not that many different types of components mounted on the PCB, so it's wotrhwhile trying to describe them. Those rectangular ones marked R1, R2 etc. are resistors. R1, the big green resistor looks as if it has been installed to limit current and prevent serious damage in case of failure further into the circuit. For ease of identification the local positive conductor is shown in red (also that apart upstream of R1 is also shown in red, since, in electronic terms, it will also, normally, be close to +24v. Conversely the local negative line is shown in blue. All other conductors are shown in grey. Also shown as rectangles are diodes, and a special form of diode referred to as a Zener diode, the bigger 'Tophat' Zeners are shown as barred arrows. Unlike resistors diodes need to go in the right way round. The sausage shaped components are capacitors. Resistors, diodes and capacitors are all pretty reliable in circuits such as our tacho circuit. The large circular device towards the top marked IC1 is a type of integrated circuit referred to as an operational amplifer or, simply, op-amp. It is of an extremely widespread type referred to, loosely, as a 741. They come in umpteen shapes and sizes - but unfortunately the round eight-leg shape used here is obsolescent and direct replacements may be very tricky to find. Note that of its eight legs, only numbers 2,3,4,6,and 7 are connected. It is, of course, important that the right legs go in the right holes! This is the clever device in the circuit - and probably the least reliable. However, fear not. There are two other devices shown as circular in the diagram, onr e is a variable resistor, about which nothing further will be said. The other is a transistor, of variety NPN and type BFY51, and again, very common. This final component is in rather an exposed position electronically, and although well protected by Zeners (which is mainly what Zeners are for), it can still be damaged, and, on balance, is probably the device in the circuit most liable to fail. It has three legs called base, collector and emitter and so marked on the diagram.

It's actually very difficult to work out what a circuit does by simply looking at it, and even more difficult to propose failure mechanisms and consequent repairs, there is however a way. The way is to unscramble it and redraw it as a so-called circuit diagram. A few re-works and the form starts to emerge, and this is seen in Fig3. Without going into details, the transformer lower down in the tacho box feeds an alternating signal (that is, AC) which increases in both frequency and amplitude with the engine RPM, to the input components of the PCB. The input components are arranged so that the DC voltage on pin 2 of the op-amp attempts to fall as speed increases. The op-amp is configured as a so-called Schmitt trigger, so that with a falling voltage at pin 2 the ouput voltage at pin 6 snaps from close to 0v to approaching 24v. This turns transistor TR1 on, which conducts between collector and emitter, so that the voltage at the output terminal ES falls to close to 0v. If the engine stops, the voltage at pin 2 remains low until the charge on capacitor C1 leaks away (about five seconds). The voltage at pin2 increases, the output at pin 6 snaps low, transistor TR1 turns off. Its collector voltage increase to +24v and the SIR de-energises

So there it is. I hope this ramble has not served to simply confuse further, if you have specific problems I will try and help further on receipt of an e-mail. Having said that, I have no wish to set myself up as some sort of expert of the subject - I haven't yet had time to fix my own tacho box, and I've got a lot more to discover. But how fascinating it all is!