Wavetek 75 (or 75A or 23) Rotary Encoder

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:

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Thanks in advance!

Cheers, Hoi

Reply to
Hoi Wong
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Here's the cap for the rotary encoder, if that's useful at all:

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Reply to
Hoi Wong

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.
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Reply to
Dave Platt

Thanks a lot. I'll give it a shot.

Cheers, Hoi

Reply to
Hoi Wong

I've been loosely following this thread, trying to remember where I had seen a software program that would produce the code wheel for rotary encoders. I finally found it; it's called CodeWheel. The author has a web page at

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which pretty much describes the codewheel, design, theory, etc. It even describes how to build one using the innards from an optical mouse. Download the software from a link toward the bottom of the page. It's free.

From your photo, I don't see any score marks from the wipers on the wheel, so I'm assuming that you have an optical encoder. The CodeWheel program might be a great solution to your problem. If you still have the crumbled pieces of the wheel, you should be able to piece it back together well enough to determine the appropriate design of the wheel. If the codewheel segments are conductive (a mechanical encoder), then you might get enough conductivity from a laser printer. Alternatively, you might try your luck at etching some very thin PCB material.

I tried to locate a manual for the model 75, but no such luck.

Good luck on the project. I would love to hear the results of your efforts. Cheers!!!!

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Dave M
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DaveM

On Fri, 8 Aug 2008 17:52:49 -0700, snipped-for-privacy@radagast.org (Dave Platt) put finger to keyboard and composed:

The cap is stamped with a "50". Could that be the number of pulses? BTW, that's a Matsushita logo. I can't guess at what "6L" might mean, though. Could it be a part number series?

FWIW, there are some Wavetek manuals here: ftp://bama.sbc.edu/downloads/wavetek/

Would this wavegen have a similar encoder? ftp://bama.sbc.edu/downloads/wavetek/275/

- Franc Zabkar

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Reply to
Franc Zabkar

On Mon, 11 Aug 2008 06:52:50 +1000, Franc Zabkar put finger to keyboard and composed:

Or this one (Model 23)?

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Other Wavetek stuff:

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- Franc Zabkar

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Reply to
Franc Zabkar

I misinterpreted the image of the broken rotary encoder that the OP posted. I saw the broken part as the codewheel, not the wiper part with the contacts attached. Apologies.

I forgot that the subject line mentioned the model 23. Here is a link to the manual for the model 23:

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It gives you a pretty good description of the encoder operation, and a picture of the code wheel. Not much else about the encoder itself, though. The description of the encoder in the model 23 manual doesn't seem to fit the images of the model 75 encoder.

The only info given about it in the parts list is the Wavetek part number, which is 5109-00-0026. Wavetek is listed as the manufacturer.

The schematic shows two contacts in the encoder. That would seem to agree with the OP's thinking that it is a 2-bit encoder. The model 23 manual indicates that the encoder outputs 50 pulses per revolution.

Since there doesn't seem to be an indexing contact on the wheel, I think you could use an encoder with a lower number of steps per rev without any problem (electrically). It might, however, make the adjustment less sensitive due to the reduced resolution of the encoder. You'd have to make more turns of the knob with the lower resolution encoder.

Find one at Mouser or Digikey with the appropriate physical characteristics and highest resolution and give it a try. Nothing to lose at this point.

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DaveM

Hi Dave,

The part# on the service manual is no where to be found, neither does it contain much information:

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I just probed the connections to the Wavetek 75 board, and it seems like one wire is grounded while two are hooked up to +5V (My mechanical encoder was too broken to tell which lead is which). Is there any opto-encoder that you'll recommend for this case?

Thanks.

Cheers, Hoi

Reply to
Hoi Wong

On Tue, 12 Aug 2008 11:47:29 -0700, "Hoi Wong" put finger to keyboard and composed:

Page 121 of the Model 23 manual shows that pin 3 of SW1 is grounded and pins 1 and 2 are switched to ground. The switched pins connect to IN1-0 and IN1-2 of the uProc/Synthesizer PCB (page 126) via pins 1 and

2 of J13/J12. These inputs are pulled up to +5V via R21 and R22.

- Franc Zabkar

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Franc Zabkar

On Wed, 13 Aug 2008 07:42:14 +1000, Franc Zabkar put finger to keyboard and composed:

How about this 2-bit, 64 pulses/rev rotary encoder:

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?name=EM14C0D-E28-L064N-ND

Datasheet:

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Digikey have 2 other suitable encoders in that same series that I can see.

The above encoders are optical types, so you will need to wire up a 5V DC supply.

- Franc Zabkar

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Franc Zabkar

On Wed, 13 Aug 2008 08:00:00 +1000, Franc Zabkar put finger to keyboard and composed:

I notice that there are three unused wires in the J12/J13 harness in the Model 23. If the Model 75 also has unused conductors, then maybe you could utilise them for bussing the +5V supply to the optical encoder.

- Franc Zabkar

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Reply to
Franc Zabkar

Thanks a lot Franc. I found a convenient +5 from CR2 (left part of battery test schematic on p.126) to steal for optoencoders too. By the way, am I supposed to wire the switch terminals 1,2 to the ground in this case?

Cheers, Hoi

Reply to
Hoi Wong

shows 549 encoders to chose from.

the mecanical encoders use pullup resistors, and the switch pulls the

+5 VDC lines low as the shaft is turned. The interface may need to be modified to use the electronic encoders. The electronic encoders I've used had a direction output, and a pulse output which will not work with that Wavetek design.

The scan from the manual indicates that the part was customized for Wavtek. We used a similar design in a product, 20 years ago. I have no idea how many steps the detent has, per turn. The damaged encoder has the Panasonic logo, but that body style is no longer made.

shows the current body styles, and something might fit, if you can determine the shaft length, style and number of detents,

Personaly, I would see if any of the CTS encoders would fit. They are a lot higher quality, and were used in some of our military grade designs. The only bad ones I saw were damaged in shipping, when some idiot returned equipment in a wood crate, and no packing material. The equipment had over $2000 physical damage after being shipped half way around the world so it was no wonder the shaft was pushed back through the encoder's body.

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Michael A. Terrell

In article , Michael A. Terrell wrote: X-Newsreader: trn 4.0-test74 (May 26, 2000)

Interesting... those would be ones with a decoder IC on board. You're right, those would not be appropriate for repairing the Wavetek.

The rotary optical encoders I've used myself have a direct two-bit quadrature output, and use open-collector phototransistor pulldowns rather than electrical-contact pulldowns. They don't have on-board direction/pulse decoders - they were/are designed to be almost direct replacements for similar mechanical encoders.

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Dave Platt

On Tue, 12 Aug 2008 16:47:40 -0700, "Hoi Wong" put finger to keyboard and composed:

AIUI, if you use one of the Bourns EM14 series optical encoders, then you would wire the original 1 & 2 terminals of your mechanical encoder to the Channel A and Channel B output pins of your optical encoder. If you get the direction wrong, then you just swap the A & B terminals.

As for the 1 & 2 switch terminals of your optical encoder, I would think that you would leave them unconnected.

- Franc Zabkar

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Franc Zabkar

On Tue, 12 Aug 2008 16:47:40 -0700, "Hoi Wong" put finger to keyboard and composed:

I notice that the optical encoder's output is rated to sink about

20mA. However, the pullup resistors are 221K which would result in a current draw of only 20uA. Perhaps it would be advisable to change the pullups to 2.2K in order to bring the collector current into the 2mA range ???

- Franc Zabkar

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Franc Zabkar

given the costs of an optical encoder, and the complexity (changing the pullups), I might try out the grayhill mechanical encoders first (if dialing becomes unbearable, I'll open the AWG up again for another surgery) then. Thanks for your advice.

Cheers, Hoi

Reply to
Hoi Wong

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