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Purple Streetlights are Raising Awareness of an Important Issue

Whether you spotted one in your own neighborhood or saw one while driving into town, you’ve probably noticed the purple streetlights that are cropping up all over America. Street lights vary in color depending what kind of bulb is used. Most street lights are yellow, but long-lasting orange colored sodium lights are also popular. Whiter LED lights are now available, but why purple? It turns out the purple lights raise awareness of a very important issue.

You’ve probably read in the news how very wealthy individuals avoid paying income tax through questionable tax arrangements, offshore accounts or even outright fraud, but did you know that the rich manage to avoid property taxes too? A study of property taxes nationwide found that properties valued in the lowest 10% in a community are typically taxed at a rate 33% higher than properties valued in the top 10%.

How is this possible? The answer is property tax appeals. Wealthy landowners can typically afford to hire law firms to challenge the valuation of their properties. The tax assessor then has a choice: they can put up a fight against a wealthy, well-connected campaign donor, or they can give up and get another campaign donation for their reelection. Even when the tax assessor isn’t elected, they report to someone who is. Unsurprisingly, the tax assessor usually gives the rich what they want. It’s not hard to make an appeals system that is hard for ordinary citizens to navigate but easy for a well-funded lawyer, so that’s usually how they do it. Poor landowners simply don’t have the time to challenge their assessments, so they pay too much while the rich pay too little.

What does this have to do with purple lights? Well, cities across America are under-funded due to the rich not paying their property taxes. Cities have pinched pennies everywhere they can, including by buying the cheapest streetlight bulbs available. Many of these bulbs are defective and turn purple after a few thousand hours of service. So the next time you see a purple streetlight, remember that rich people in your town don’t pay the taxes they’re assessed.

Worst. Graph. Ever.

I don’t have any idea what this is meant to convey, but I laugh each time I see it.

Bad Vibrations

As the previous post hinted, I recently needed to fix my Phillips PM3217 oscilloscope. The problem? Bad vibrations. Or, rather, oscillations on the trace that just shouldn’t be there. Most all oscilloscopes have a test point on the front that is marked “cal” and has a hole to which you clip your oscilloscope probe. This test point provides a square-wave oscillation at a fixed frequency that you can use to adjust the compensation on the probe. I was doing just that when I noticed ripples on the square wave.

Not a great shot, but you can make out the ripples.

My first thought? Worn-out electrolytic capacitors in the power supply. Why? Probably because that would be the easiest problem to fix. Really, though, it’s a very common problem for electronics of every era. The first thing you should check is always the power supply.

Looking at the service manual, I discovered that the switch-mode power supply oscillates at about 18 kHz. That’s pretty close to the unwanted ripple, so that made me even more sure it was the power supply. Amazingly, my kid could hear the oscillation. I’m old enough that I can’t hear those frequencies, but young ears can!

So I opened up the case to take a look. The case opens up with two captive screws. That was refreshing. It’s like they expected you to fix your own equipment, so they made it easy to open. The power supply was easy to find. It’s in the back. A visual inspection turned up no leaking electrolyte, but that doesn’t mean the capacitors aren’t bad. I tested the ESR of each capacitor, and they were almost uniformly too high.

Thankfully I didn’t need to replace that huge cap at the top right. That cap filters the 120 Hz rectified AC input. Those usually aren’t the ones that fail first. It’s the smaller axial caps that are filtering the 18 kHz output. I replaced them with new axial caps, which cost a bit. Axial caps just aren’t popular anymore, so you have to pay a bit for new ones. I got them all from Vishay, from their industrial temperature, low ESR lines.

So did it fix the problem? You bet! Always check the power supply first!

That’s better, and a better picture! Turn down the intensity when taking CRT scope pictures.

Not Today, RIFA Cap!

Whenever buying or restoring an old piece of equipment, it is best to start with a good visual inspection. Burnt diodes or resistors, leaking capacitors, or melted-down tubes can mean that a component has failed, that the device was not operated as intended, or that it has been modified in an unsafe manner. But there are also pieces of equipment that are stock, in good condition, but which still have nasty surprises inside.

One such nasty surprise are the radio frequency interference (RFI) suppression capacitors made by RIFA circa the 1980’s. These capacitors may be working fine when powered on, but they are ticking time bombs. The problem is the plastic enclosure: it develops cracks over time that allow moisture into the capacitor. The capacitor fails short, exactly what an X rated capacitor is not supposed to do, and causes some fantastic smoke to pour out of the equipment. The video below shows a successful attempt to document such a failure.

RIFA cap failure on a TRS 80 Model II, captured by Peter Cetinski

I found one of these RIFA caps inside of a Philips oscilloscope that I have had, and used, for years. I had no idea it was there until I saw it mentioned in a forum post about this particular oscilloscope. Rather than being on a circuit board, it is soldered directly to the back of the mains voltage selection knob at the back of the unit and hidden under a cover. If I hadn’t been looking for information about replacing the capacitors in the power supply section, I might not have found it until it failed.

Smells Bells

Hola, amigos. I know it’s been a long time since I rapped at ya, but I kind of had to pack up and move. Moving is a pain, but our new place is big enough to hold all our stuff. That means I’m encountering unfinished projects that have been in storage.

Back in Iowa, spring is the time for yard sales. While looking for hand tools for the maker space, I found a man selling his late wife’s record collection. Looking through the titles, it was a great collection of 70’s and 80’s rock. I bought the whole thing for what I thought was a good price. As soon as I got home, I realized it was a bad decision.

I know what you’re thinking: PJ, you didn’t own a single BTO album before you bought this collection, and now you have all their greatest hits! How could this be a bad decision? Well, as soon as I popped the trunk to unload my haul, I was overwhelmed with the smell of cigarette smoke. I could smell that it was kind of smoky when I went to the man’s basement to look at the records, but it really didn’t seem too powerful while I was down there. I think being in a space that wasn’t smoky at all made the smell of the records really stand out.

So what to do? A record store wasn’t likely to buy the records that smoky, so I decided to see if I could get the smell out. I put the records in garbage bags with charcoal filters and then put them into boxes. Those went into the storage unit for maybe a year and a half, until the move. And when we got to Florida? Well, let’s say I could tell which boxes had records because I could smell them right through the plastic and cardboard.

So I decided on a more active remediation: I planned to blow filtered air over the records for a long time to see if I could get the smell out. My theory was that filtered air would be better able to dissolve and carry away the smoke particles in the records. My wife made the excellent recommendation of using the Blueair air purifier we already owned rather than building something new. So I bought a new particle and carbon filter for the air purifier, put a box on top with a wire file holder to keep the records upright and let the fan do the work.

How effective was it? After 24 hours of blowing on any particular record, it would smell well enough to have in the house. For a while. What I found was that after a few days of blowing on other records, the smell would return to processed records. I interpreted this as the smell working its way out from deep in the paper. So I put them back through another 24 hours of blowing. At that point the smell seemed to be more stable.

I did find that it was a good idea to pull the inner sleeve and replace it if it was generic. If it was a printed inner sleeve that I wanted to keep, I would air it out separate from the outer sleeve.

All Sense of Proportion

I made a little breakout board for a chip I wanted to try out. It’s a current sense amplifier, so I put a spot for a shunt resistor to monitor. Can you spot the problem with my layout?

No? How about now:

Even if you had Lincoln’s tiny eyes you might have trouble making that out because text that size is at the limit of the silkscreen’s (or, probably, the inkjet’s) resolution. The software will zoom in with infinite resolution when you’re laying out the board, so there’s no warning when things get small. You have to keep track of the units. In this case, the text is 16 thousands of an inch.

WP-702A PSU Upgrade

I was working on an op amp circuit when it started to oscillate unexpectedly. I threw in more capacitance, took out capacitance, changed resistors, switched op amps, all to no avail. But then if changing your circuit doesn’t fix the problem then maybe the circuit isn’t the problem. Sure enough, it was the power supply that was oscillating. It looked just fine until you put a load on it. The other half of the dual supply failed at the same time, but it would just go dead as soon as there was a load. Maybe there was a spike on the AC line that fried them. Maybe it was the weather.

When I opened it up I was struck by how old the construction was and how crusty it had all become. The supply had been given to me by my grandfather years ago. I had assumed it was 80’s vintage but the dates on the meters were January 1972. It’s an RCA WP-702A, which appears to have continued production under the VIZ brand for many years. The design uses just 3 transistors and a few zener diodes per supply. The capacitors were clearly gone.

So it was time to learn about power supply design. I wasn’t going to throw this thing out, it’s just too handsome. So I decided to keep the transformer and front panel just as they were and design a drop-in replacement for the regulator boards, but with a slightly more modern design. I sketched out how the original circuit worked, mostly so I could get a handle on how the overload lights were switched on.

As far as I got with the existing circuit.

As you can see, turning on the overload light is pretty darn simple. They selected an incandescent bulb that begins to light at a current of 200 mA and wired it ahead of the pass transistor. All the current that comes out of the supply goes through the bulb first, and it lights itself at the maximum rated current.

So what about the new design? I decided to upgrade to the great granddaddy of regulator IC’s: the uA723. Created in 1967 by Bob Widler, it existed when this unit was manufactured, if not when it was designed.

This design includes ideas I got from others’ schematics, but I’m not sure all of them are good. The data sheet indicates, for example, that pre-regulating the voltage to the 723 improves ripple rejection, so I made a pre-regulator from a 24 V zener (D6) I already had and used one of the power transistors (Q1) as an emitter-follower. Is this a good place to put a capacitor to make it a capacitor-multiplier as well? I would suppose so, but I’m not sure how to calculate what value should be used. It would seem to need a big one to reject 120 Hz, and it reduces the ripple pretty darn well without it.

A bigger issue, perhaps, is how to get good regulation down to 0V. There are three resistors (R12, R13, R14) in the feedback loop that try to map the nominal 0 to 20 V output range onto the 2 to 7 V input range of the error amplifier. It works well enough with a load, but the voltage creeps up when the supply is unloaded. So I put some load resistors on both the 723’s output (R9) and the output of the external pass transistor (R15). I imagine giving the chip a negative supply helps with this by keeping 0 V in its normal operating range.

One idea that was definitely good was replacing the original one-turn pots on the front panel with ten-turn pots. A 20 V range on a one-turn pot is just too much. Also, I added a fuse to the primary side of the transformer. That combined with the over-current protection of the 723 (which I set at 250 mA) makes this thing a lot safer to use.

The PCBs returned perfect and purple, but I clearly made some goofs. Some of the footprints are just too big. (I intentionally left the second filter cap unpopulated.)

All in all, I think this project turned out pretty well. The supply works, and that’s what’s most important. If it ever fails again, I’ll get to learn more about power supplies!

Measure Up

“Be the reference you want to see on the internet.” – Franz Bibfeldt

(No) Dummy

For a recent power supply project (more on that later) I needed a dummy load for testing. So I reached for my favorite Ohmite rheostat: 75 ohms with an approximate log taper and an off position. This is one I picked up at a hamfest real cheap. To test at the max voltage and current, I needed 100 ohms, however, so I added in a 25 ohm fixed wire-wound resistor for those tests. They worked really well, except they were just loose on the bench connected with gator clips that would sometimes slip off. So I built an enclosure for them.

The enclosure isn’t fully enclosed, as you can see. It consists of just two laser-cut pieces of acrylic with four 3″ 8-32 standoffs. The open sides allow the resistors some ventilation. It doesn’t have feet yet, but I plan to add non-marking rubber feet held on by the lower screws. I was pleased to find some short scraps of brightly colored solid wire in my junk bin.

I decided to add a scale for the rheostat. I did this by sketching up most of the layout in GeoGebra and then printing it out on paper. I poked the shaft of the rheostat through the paper, added a knob, and then used an ohm meter to find the correct angles. If I was laser-cutting it over, though, I would make the numbers bigger and not so deep. They’re readable, but could be clearer.

I like the layout of the connectors. To add the 25 ohm fixed resistor to the rheostat you can just short the black terminals at the right and then use the red terminals for a 100 ohm to 25 ohm range. The black terminal at left will rarely be used, but is available if I need to use the rheostat as a voltage divider.

A word about GeoGebra: I was at least 40 times faster sketching this up with GeoGebra than I would have been with the more commonly used Inkscape. The fact is, Inkscape isn’t made for technical drawings. Getting precision out of it takes work, including a lot of calculation on the part of the user. GeoGebra makes it trivial to put every circle, arc, and segment exactly where you want it with infinite precision.

Lightning Fail

If you don’t hate Apple’s lightning connector yet, it’s almost sure that you will. Should you snap off the tip in your device you will find it impossible to remove. (Don’t take it to the apple store, they will just offer to sell you a new device.) The cables tend to break if kinked or pulled. Even if you’re consistently gentle, you could develop corroded contacts that prevent the device from charging.

Another failure I have encountered: pocket lint. It seems the female lightning connector on the bottom of the iPhone is just the perfect size for getting lint stuck in it. Because the opening is so thin, it takes a very small tool to get the lint out. In my case, a pair of static-safe tweezers I use for placing SMD components was small enough.

iPhone, Lint, and Tweezers — This was the first of three chunks removed.

I have never had this problem with any other connector. That includes USB mini, USB micro and Apple’s 30-pin connector, all of which have traveled in my pockets for years without incident. I haven’t had those others snap off in the device or become corroded, either, but both have happened to me with the lightning connector. Even the name is bad, since Apple also uses the thunderbolt interface, producing confusion. Sadly, I don’t think we can expect a change in connector for new iPhones. With Apple’s revenues missing expectations, they’ll need to sell lots of replacement cables and phones.