Old PCB design?

Sounds a bit like thick film hybrid technology. Resistors were printed with resistive ink and then if needed laser trimmed.

Bob

There were 'printed resistors' which were laid down with a silk screen, so I suppose there could have been printed capacitors, but I would imagine the values would be small.

AWEM

Thick film hybrid circuits?

Right - I've heard of that. Is it still used? And wonder why they chose that - there is any amount of space inside the instrument, so it could easily have been a conventional PCB.

Light green is the protective varnish, looking darker where the copper and carbon show through.

More reliable in high vibration applications, due to the lack of soldered joints, and cheaper to make, if you're making thousands of identical units.

Thin film

I might just know where a few brand new mechanisms of those exist if it is needful.

There are actually more soldered joints on this PCB than integrated components. Vibration not really an issue.

Right. But all the other electronics in the car use 'conventional' PCBs - including the Smiths OBC, which is a far more complicated device. But I suppose the PCB in question could be used for all the various designs they made.

Can easily get a brand new one at low cost - it's on the SD1. But the one I've stripped was for a 6 cylinder so no use for mine anyway - which still works fine.

It's just one of these projects I so like. The original runs from the coil negative. I'm looking at making new electronics so it works from much lower level pulses. Like the tach output from an ECU driving COP etc.

they really only made one design!

IIRC it was a 10ma 270 degree meter..with a crappy pulse counter on it.

yeah. Mine became the 'analogue dial' in one of the firsts ever digitally tuned radios..

we bought a few hundred...

Not so - the first electronic Smiths tach (RV1? used two germanium transistors. Mine has one IC. Which I suspect is just a 555 in drag.

That was going to be another question - how to measure what the movement is.

More than that, they also made the RVC which iirc was a single transistor design.

FR3

Printed resistors are simple enough. Sounds like yours has gone one step further with thick film circuitry to give embedded caps too.

NT

I'm not certain they are caps as I can't get any reading on my DVM. Whereas I can on the resistors. There also appears to be some form of transformer which feeds in the ignition pulses.