123 and tachometer

[christopher storey]

This is a great solution if you have a voltage sensing tacho but for those Series 1 cars with an electronic tacho and all Series 2, they have a current sensing tacho so this will not help or work.

I think there is may be a slightly misleading statement in your post. AFAIK, no Series 1 cars ever had an electronic ( i.e ignition driven ) tachometer. All S1 cars had the tachometers driven from the AC generator on the inlet camshaft . I suppose it is possible that some very late S1.5 cars changed from this, but I have not seen one

[Lost Horizon]

Just out of interest, does anyone have a circuit diagram for a Series 2 Tach?

[cactusman]

Everything you ever wanted to know about inductively triggered Smith's tachometer

https://accutach-public.sharepoint.com/ ... background

Not jaguar specific but they were all internally more or less the same...see if yours has one or two transistors...they will be little metal cans or glass painted black with three legs... then you can use the appropriate diagram referenced in the article...

And if your tachometer works but is wildly inaccurate the chances are the various resistors and capacitors have drifted with age. You may be able to adjust using the variable resistances or it may be wise to replace all the capacitors and resistors with modern components and then recalibrate.

If your tacho is dead the likelihood is than one or other transistor has failed. They are all long since out of production. EBay does provide a possible source but virtually any small signal germanium transistor will work. OC71 or OC81 types are still about and would probably perform just fine. Modern silicon types may not without significant alterations to the resistor values.

One other thing to note....the wires from the secondary side of the current transformer to the circuit are very thin and easily broken. Bugger up this part and you are in trouble

[Lost Horizon]

Everything you ever wanted to know about inductively triggered Smith's tachometer

https://accutach-public.sharepoint.com/ ... background

Not jaguar specific but they were all internally more or less the same..............

thx Julian. I have many years of old new stock, and multiple cross reference books to get from there to here for equivalent circuit substitutions.

Mine is a one transistor, and looks like it wants a 1A square wave input based on this info. This means the 555 circuit would work well with the simple addition of a suitable mosfet or whatever on the output stage to the tach if there are erratic noise triggering issues with the electronic ign and the coil couple on the primary side... The fettling scope will tell the tale.

An amp here, an amp there, and pretty soon it adds up to real power draw. We've come a long way since then, haven't we?

[cactusman]

Our sheds must be similar ...I too have draws full of bits...and a wardrobe full of lovely valves but that is another story! Things have come a long way although not necessarily for the better as most things today are sealed for life and not fixable. I could wax lyrical about old tek scopes and how fabulously they were built...

[Lost Horizon]

Our sheds must be similar ...I too have draws full of bits...and a wardrobe full of lovely valves but that is another story! Things have come a long way although not necessarily for the better as most things today are sealed for life and not fixable. I could wax lyrical about old tek scopes and how fabulously they were built...

I'm quite sure they are. I learned on 'valves' also. Graduated in 1969, specialized in Systems and controls when those marvellous things called FET's first came into being. Gave up my Curta for an HP RPN calculator with those 50 step magnetic programming strips. Still have the Curta tho. Must be the same reason why I'm compelled to bucket list a 1970 E-Jag.

I still get the odd 'old boy' call to do some custom NEMA interface for some old autopilot for some grand blue water cruiser who used a Micrologic Loran or some such and now has a modern Garmin that has no idea how to talk to it.

Take care.

[johnetype]

My tacho is a single transistor unit, described as "Gen 2" in the link below, so that's three cars the same. From that I think it's reasonable to assume that the Smiths tacho used in Series 2 cars is "Gen 2".

The only component in the tacho likely to age is the capacitor. I replaced it with a 2.2uf tantalum capacitor and my tacho works fine.

Everything you ever wanted to know about inductively triggered Smith's tachometer

https://accutach-public.sharepoint.com/ ... background

[cactusman]

Capacitors are the most likely to age but from years of experience mending vintage electronic equipment old carbon composition resistors do drift with age. In most circumstances a 10% change makes no odds....but it does sometimes...

[christopher storey]

Also, assuming that these tachos have thermistors ( some do and some don't ) these tend to age , with the result that variations in temperature produce variations in rpm readings ! I agree with Julian that it is best to replace all the components that one can

[johnetype]

Obviously it's a judgement call as to how far you go, everything ages to some extent but the resistors were only +/- 10% originally so their value isn't critical and I suspect any replacement thermistor will be NOS so just as likely to have aged. These tachos aren't super accurate, you just want them to work reliably and indicate within I'd suggest 10%.

The more you go in and replace, the more you risk damaging the circuit board or the current transformer and if you do that you have real problems as said earlier. The capacitor is the obvious item to replace as it's likely to have aged at least to the point of not working well or failed and it's off to one side on the pcb and easy to get at so you don't have to disassemble further than taking the movement out of the case.

[Lost Horizon]

..the joys of working on 50 year old stuff.. sometimes the integrated drift of all components works in your favor. Sometimes.

I always go the digitally programmed equivalent for fixes where you can with something like a parallax stamp or propeller Microcontroller. It's comforting to know that for any I/O it will be precise for life, and if the outcome isn't what you want, just reprogram it, add a couple of branching conditions, and you have your version 2.0 without touching a soldering iron.. But I digress, as any old guy typically does.

[cactusman]

Just be wary with the iron. Germanium transistors are very intolerant of over heating their 'legs' or reverse polarity. A puff of freezer spray is handy. As suggested...the thing is never going to be spot on. I refurbished my mgb one year's back...just replaced the electrolytic capacitors. Then 're calibrated initially using a pulse generator driving a power fet to switch a brake light bulb with the tachometer primary in circuit and then checked on the car using an electronic tachometer setting on a car multimeter. Well within 10% to 5000 rpm...good enough for me

[juicerider]

Hi
Yeah I’m in oz too and I didnt want to send my tacho away. I fitted the 123 system to my S2 works great. With the tacho i got this converter from eBay. This covers it from RVI to RVC . Just wire it up to the gauge on the original tacho and it bypasses the old original circuit. You will also need to run another wire from the coil to the box. It sits behind the dash taped to my tacho. Works very well, no problems.
https://www.ebay.com/itm/Smiths-RVI-to- ... SwnHZYSobE