optical knob encoder: HP/Agilent 33120A freq generator, tuning-knob failure

Its probably patentable so is there a patent outline for them somewhere, to show what the innards are?

I have not seen a failure on the 33XXX series of generators, but the part # is 0960-0892. Looking at the picture on the find-a-part website shows it as a mechanical encoder.

Which is obsolete and replaced by 0960-2545, also a mechanical one. It looks like its the same as used on the 65XX and 66XX power supplies. I do know that the older 65XX and 66XX do use the (obsolete) optical ones. It is possible to use the mechanical ones to fix the obsolete optical ones with a bit of hacking, but the mechanical ones need more turns to change the output of the supplies for a given amount.

Are you sure it's an optical encoder you're looking for?

Indeed.

Thanks, great info!

No, and as you say, the service manual identifies the mechanical one as the part, now that I bothered to look. Given the failures, perhaps being mechanical makes sense. I had just assumed that because HP long-ago developed and manufactured a fine optical part, they'd use it in their expensive premium instruments. Heck, I've long used their expensive optical encoder in my own designs!

The mechanical part looks much smaller than the optical one, which might not fit as an upgrade replacement. Hmm, their mechanical part looks a lot like the ones Spehro was selling as excess inventory.

BTW, as you can see at my Mouser link, the HRPG optical encoder is not obsolete (yet). Hah, it costs $34 instead of $4.23. Do we get what we pay for?

Hello, Win. I haven't seen you around for quite a while. :)

The only failed optical encoders I've seen either had a failed incandescent lamp (Very early type of encoder). The other was when they were dropped on the shaft, which shatters the code wheel. These use a thin glass disk that rotated through a pair of photo interrupters. The shock of impact hit the disks at both photo interrupters, making it unrepairable. A failed photo interrupter can be replaced in some units, but it takes more time than it's usually worth.

The mechanical encoders are a lot smaller than the optical, so some designs use them just to save space. I prefer optical encoders with a fairly heavy knob so you can give it a spin, and stop it with your finger when it gets close. :)

If you can get that part from Agilent for $4.23 I would get at least half a dozen while they re availible.

I bought a power supply E3647A and the encoder feels like very bad quality. My boss bought the same power supply six month ago and the encoder feels much better.

So I thought the quality of HP goes south and I bought the freq generator from Rigol. When HP is selling the quality of Rigol now, I dont like to pay the price for HP anymore!

Olaf

That part looks very familiar. I'll guess that it's a mechanical encoder made by CUI.

I don't know the coding or counts per turn, so I can't find an exact substitute. However, at $3.60 ea, you could probably buy an assortment.

CUI AZ16 data sheet:

Catalog page:

You're welcome.

Could be, but I've yet to see a mechanical one go south unless there was some abuse. Even then they are sometimes repairable. The bend over tabs that hold the encoder together come apart a bit when the shaft is hit at a right angle, which seems to happen all the time on the power supplies I mentioned. I've lost count on the number of times I've repaired them. After repair I solder two 24ga pieces of bus wire across the top from tab to tab which helps sturdy them up a bit.

If you *do* have an optical one, it may depend on whether it solders directly to a PCB, or is panel mounted. If the latter, it probably has a cable which attaches to a PCB, and you should be able to adapt the mechanical one. If you go that route, I've found that it is important to ground the case of the encoder. You'll probably have to fiddle around with the wires to get the order right. Trial and error...

And what a pain in the butt to get to on a 6034A! I wish I could find a "side firing" LED that would not only fit in that little slot, but also be the right wavelength for it to work correctly. I've experimented a bit with this, but never been successful.

I see mechanical ones go bad ALL THE TIME. I even keep a few of the more common ones in stock. I was going to mention this sooner, but it was said that there was a pushbutton function on the OP's unit which the encoders I see don't have, but one of the pics previously posted look like some I stock (and pay about 1.00 each for). They go bad from tarnished contacts, in my opinion, but my friend Arfa opines that it is the migration of silicon lubricant that is the root of the problem.

Mark Z.

I designed one into a marine radio and regretted it. If it moves, it breaks. After about 10 years, all the encoders have probably been replaced at least once. I also have a few ham radios (Icom IC-735) with genuinely horrible mechanical shaft encoders, but that respond well to cleaning.

The CUI data sheet shows versions with a push button option.

In my never humble opinion, y'er both right. It's both tarnishing and the silicon grease that causes problems.

It tends to absorb aromatic hydrocarbon solvents. It also tends to have residual acetic acid used in the manufacture. Both are very mild oxidizers that will eventually rot the surface of any metal contacts. Once pitted, the contact area is reduced. Silicon grease enters the voids, and since it's an insulator and incompressible, eventually ruins the electrical connection. Using soft metals, such as PCB copper, for contacts helps because the wiper scrapes away any surface pitting. That only works until the copper is worn away. I've seen encoders with grooves worn into the PCB traces.

In desperation, I found that cleaning off the grease from the copper with alcohol, and tinning the copper PCB traces, actually works fairly well. The problem was that when I lifted the soldering iron, there would always be a small solder points left behind, resulting in multiple bumps as the encoder rotated. I managed to file or press down the points, but the bumps are still obvious. One of my radios was repaired in this manner several years ago and is still functional (although the bumps are irritating).

I recently bought these 20 position with a push switch on Ebay for a project. I got 10 for $3.98 US. I'm not going to pay the $65 each for the nice optical encoders we used at Microdyne for a test fixture.

I don't know if its relevant with these encoders but a few times I've found with preset size pots that appear in audio mixers. The grease hardens and then lifts the very feeble wipers of the pots. No track wear at all as the wiper action is so slight , so why the grease ?

I think the silicon grease is there to provide the "feel" when you turn the controls.

Mark Z.

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I'd not thought of that as a reason but it makes some sort of sense. I suppose the first pot one ,I cleared the grease out with meths and reassembled dry, refitted, and returned to owner must be about 5 years ago. That one and subsequent ones, have not bounced back with worn tracks , so superfluous as far as electrical function is concerned , IWS. I suppose someone will now tell me I should have replaced like for like, not going against the "designer's" wishes.

Good question. You might need a microscope to see the groves.

My guess(tm) would be to prevent oxidation and prevent the small amounts of the resistance material from landing across contacts. I also recall reading that it reduces pot noise improves the "feel" by adding some drag, but that's mostly on the shaft, not the wiper.

This explains much:

Looks like they have different greases for different problems:

I've had good luck disassembling and cleaning these types of mechanical encoders that are often used as volume controls. They have to be disassembled. Using something like deoxit will work for a while, but it's not usually a lasting fix. I do normally replace the grease since I figure it was there for a reason (probably lubrication, and possibly to prevent oxidation of the contacts).

I recently repaired a 33120A, and the encoder was just held together with bent metal tabs, and should be easy to disassemble.

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Of course being a seller of lubricants there is no tabling of the long-term performance as far as immunity to hardening. That "velvet feel" becoming gumming up the works . And pot track lubrication being insufficient to cause "aquaplaning" and then with aging/ chemical reaction, hardening and forming a wedging under the wiper and so loosing contact for all or most of the track run.

A job for tomorrow is to take some measurements of one of those tiny preset size pots. The wiper arcs are something like 3 off 10mm long , and section .2x.4mm IIRC but I will try and measure the contact force , I'm guessing at this stage of order 10 gm. Once one wiper lifts I imagine its not long before all 3 lift at some part of their travel.

...

There's two optointerruptors out-of-phase with a toothed wheel (or somesuch, like in the old mice). Your symptom is of one blocked or dim lamp.