The rotary encoder in my Wavetek 75 arbitrary waveform generator is
physically broken, and it's impossible to hook anything up to the broken
legs to probe the specs.
I'm totally new to encoders and I'd appreciate if anybody can offer any
advice on finding a suitable replacement. I couldn't find the service
manual for Wavetek 75, so all the information I have is the encoder
itself:
http://www.stanford.edu/~wonghoi/DC/...coderFront.png
http://www.stanford.edu/~wonghoi/DC/...ncoderBack.png
http://www.stanford.edu/~wonghoi/DC/...EncoderCap.png
Hmmm.
Based on the photos, it appears to be a mechanical rotary encoder,
with a two-bit quadrature output and a common terminal. By my count
there are 13 or 14 contact fingers per output per quarter-revolution
of the control... this would allow for 50-some pulses per revolution
for each of the two channels, or around 200 counts per revolution if
you're doing the usual sort of quadrature decoding.
I suspect that a direct replacement may be tricky to find. In looking
through the DigiKey and Mouser catalogs, it appears to me that
mechanical-contact encoders with this large a number of counts per
revolution are quite unusual. Mechanical encoders these days seem to
max out at around 32 counts per revolution, and usually have
detents. I'd guess that you'll probably want a detent-less encoder,
with a counts-per-revolution value not too different from that of the
broken one (which may very well be a custom part). That's probably
going to mean switching to an optical encoder, one way or the other.
I can think of at least two ways to do it, depending on how the
circuitry in the Wavetek 75 works:
- If the existing encoder's common wire (the one going to that
sliding contact) was grounded, and if the two quadrature leads were
of the "external pull-up resistor to +5 volts, and the encoder
pulls the quadrature output down to ground", then dropping in an
optical encoder could be fairly easy. Rotary optoencoders
generally need a +5v feed for their LED (many have build-in current
limiting resistors for the LED) and have phototransistor
pull-downs... so, substituting one of these might be as simple as
wiring it in with the common/ground and quadrature-outputs as
before, and adding a single lead for +5 for the LED drive.
- If the existing encoder was wired up any differently, you'd need an
interface board (transistors, relays, or some such) to take the
open-collector output of a rotary optoencoder and drive the Wavetek
circuit as it wants to be driven. Probably not hard to build on a
bit of perfboard once you know what you need to do.
In either case, having a schematic for the Wavetek 75 is likely to be
essential. There's a Wavetek test equipment special-interest mailing
list at YahooGroups - somebody there might have a schematic or even
the whole service manual.
As to specific encoder lines: Grayhill 61K or 61R series, Clarostat
600 series, or one of the Bourns encoders (the Bourns 14mm optos
carries in the new Digi-Key catalog are under $20) might all be
candidates.
--
Dave Platt AE6EO
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