Old PCB design?

[Dave Plowman (News)]

I've been looking at a Smith's revcounter from the '80s which has an unusual (to me) PCB. The visible components are all standard - not surface mount - but there are resistors and small caps which appear to be part of the conductive layer. Not the usual surface mount stuff.

The actual board is some form of white plastic. There appear to be three layers on the surface. Coloured light green, which covers most of the board, a darker green which is the tracks and a deep green/black for the resistors, etc.

Anyone throw some light on the technology? I'll put a link to a pic if needed.

[Bob Minchin]

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

Bob

[Andrew Mawson]

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

[Andy Burns]

Thick film hybrid circuits?

[Dave Plowman (News)]

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.

[John Williamson]

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.

[The Natural Philosopher]

Thin film

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

[Dave Plowman (News)]

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.

[Dave Plowman (News)]

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.

[The Natural Philosopher]

they really only made one design!

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

[The Natural Philosopher]

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

we bought a few hundred...

[Dave Plowman (News)]

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.

[The Other Mike]

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

[NT]

FR3

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

NT

[Dave Plowman (News)]

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.