Re: DRO

clarkf@... wrote:

> From: clarkf@...
>
> Hi all I am new to this list, My name is Frank from Melbourne, Australia.
>
> I have acquired a Teledyne Gurley Pathfinder DRO with one of the readout bars faulty.
> Comparison with the good bar does not help as the circuit and optics are different.
>
> Have contacted T Gurley with no reply.
>
> Any one have any details which might help - the problem is in the bar as I can interchange the connections and the control box works OK The glass scale is Ok as I can swap the entire head and get it to work.

Well, you've narrowed it down pretty far. There isn't much in the read head. I had some decrepit
Gurley scales a while ago, and tinkered with them some. Basically, there is a light source in one
side of the read head, and 2 photocells in the other side. The photocells have 'analyzer' gratings
in front of them, which have the grating mask facing the grating that is part of the spar. there are
some teflon or similar plastic sliders that keep the gratings from actually rubbing together. occasionally,
a piece of swarf gets into the spar and knocks one of the sliders off. then, the analyzer grating gets
worn off against the main scale. Check yours to see if this has happened. If so, then the grating
has to be replaced. it is generally a piece identical to the main scale, so you might be able to cut
a piece of the end of the scale off and install it there. Then, you have to esablish the quadrature
phase relationship.

But, hopefully, that is not the problem. First, check the light source. if it is a light bulb, make sure
it glows. In general, they run the lamps very low, in the yellow range, so they last a long time.
The sensors are sensitive to red to infrared light, so that is fine. If this is an LED head, then you
probably won't be able to see the light, because it is almost certainly IR. You can check that
it is drawing current by looking for a voltage drop on the series resistor, which is almost always
right nex to the LED.

If all this looks OK, then the problem is with the sensors. generally, these are photodiodes,
with one end grounded and the other end biased to +5 Volts with a resistor around 10 K Ohms.
With the head connected to something that allows very small movements, use an oscilloscope
(preferred) or a voltmeter and check the voltage on the photodiode lead that is biased to the
+ voltage. As you move the scale in very tiny increments, the voltage there should go up and
down, as it responds to the varying light falling on the diode. I suspect you might find that one
of the photodiodes is bad, or possibly the circuit that converts the varying voltage to a digital
level isn't working. Anyway, you have a working scale to compare signals to, which is a great
help.

If you can make some of these measurements, but don't know where to go next, let me know,
and I'll try to advise further.

Jon