Tektronix PWS4721 power supply teardown / diagnostics / repair

I broke my PWS4721 power supply by connecting it to a battery and driving current back into it. (I don’t remember if I reversed the polarity, or just had the output voltage lower than the battery voltage.) The end result was that the power supply had it’s output shorted internally, so that the output voltage was always near zero, and the current was right at the maximum current limit but no power was coming out of the front (or back) connections.

The broken component turned out to be Diode D408 on the main circuit board right in front of the power output header. It appears to be a reverse polarity protection diode, so it’s likely that I accidentally reversed the leads when connecting to the battery (all I remember is the spark). This diode is a 1N5408 (general purpose 1000 volt 3 amp diode) which I was able to replace for $0.40 (Although my total cost was closer to $10, as I ended up buying 10 of them just to have a few more sitting around if needed, and shipping cost me $4.33 from Digikey.) The only specialized tool I needed was a Trox-10 (t-10) screwdriver to remove the security (star) screws from the factory maintenance port so I could remove the back panel.

Of course, I had to disassemble the entire unit to get to the bottom of this circuit board to make de-soldering and diagnosis easier.  The output power rails were shorted before I removed the diode, and were NOT shorted after I removed it, and the diode had failed shorted, conducting in both directions.

In retrospect, I could have probably desoldered the diode in place from the top of the circuit board (the long lead would have been easy, and since the diode was already ruined, I could have heated up the body and pulled the whole thing out from the top and then replaced it without removing the entire circuit board. But, at the time, I wanted access to the rest of the circuit board just in case the diode wasn’t the (only) problem.

You know it’s a high quality piece of equipment because in addition to checking that it worked, they let it burn in to find out if any parts were going to fail quickly, and then calibrated it!

If you want to see how to tear down a PWS4721 and what is inside, here is the video:

 

And a few photos of the main board with heatsync and top logic board:

 

My Glowforge Lid Handle Fell off!

I went to use my Glowforge laser cutter today, and when I tried to open the lid the handle fell right off onto the floor!  This wasn’t the first failure I was expecting from this device! (I figured the laser tube would die, or something to do with the electronics would fail…but no, the glue used to attach this metal and plastic handle piece to the safety glass that makes up the lid just let go!)

Unfortunately, the lid handle has some wires running to it (for the “lid closed” sensor I assume) and now my glowforge thinks the lid is open all the time and won’t do anything. The lid handle has two wires on each side that normally plug into sockets on the end of the LED light bars on the left and right side of the lid. The one on the left nicely unplugged itself when the lid fell, but the one on the right pulled the socket off of the lightbar, and would require at a minimum some soldering to reattach the socket to the light bar PCB.

As my warranty has expired, Glowforge offered to charge me $200 for round trip shipping of the unit to and from their repair location, and after they receive the unit they will tell me how much a repair would cost (if a repair is possible.) The customer service email said that they couldn’t repair the part in the field as some of the parts that would need to be replaced require calibration which they can’t do in the field. (I assume they were speaking about the camera mounted on the underside of the lid. They may have mistakenly thought that the lid had separated from the hinge in the back, but I guess if they replace the lid entirely instead of just attaching the handle again they would need to re-calibrate that camera to the rest of the laser cutter.)

Since a brand new cheap Chinese K40 laser cutter only costs $400-500, I decided to attempt this repair on my own. After laying out a moving blanket to protect the laser tube from errant solder drops, I was able to solder the socket back in place with only a few scorch marks on the shiny white PCB.

 

I plugged the lid closed sensors back into the sockets, held the handle onto the lid while closing it (the weight of the lid keeps the handle in place when closed) and my Glowforge operated as normal!  The only thing remaining is to determine the right type of adhesive to permanently re-attach the handle to the lid. Glowforge support understandably didn’t want to go on record with a specific recommendation for this unauthorized DIY repair, so I went with JB Weld (Clear) 2 part 5 minute epoxy. So far, the handle remains attached to the underside of the glass lid.

 

Mechatronic system for an art project

A local artist asked me if I could build a mechatronic system that would cause “something” to emerge from a piece of art “every so often”. (I’m being vague as it’s not my idea to share.)

I suggested the addition of an PIR (passive infrared)  motion sensor so that the “something” could also react to the approach of people.

I am using a SparkFun Arduino compatible Micro-Pro board, due to its small size and built in USB port which can be used for both programming and power.  In the video and photos, I’m using a small 9g hobby servo, but I have also tested the same circuit with a standard 3003 size servo.  I am powering the SERVO directly from the RAW (red) and GND (black) pins on the MicroPro board (so basically, right from the USB power), and the control line (white) is wired to pin 3 on the board.

The PIR module has three pins. +5V, ground and signal. You provide it with power, and when it detects motion it raises the signal line (it even has a built in pull down resistor on the signal line).  I connected it’s (yellow) signal line to pin 5, and ran it’s black line to GND and red line to the VCC connection on the Micro-pro. (Which is actually tied directly to the USB power supply and doesn’t make use of the onboard regulator when the pro-micro is powered via the USB port.)

The code is a small arduino sketch that will sweep the servo when it detects motion, or every X seconds if it doesn’t detect motion, with a configurable “cool down” period after activating. You can download it here:  mechatronic.ino.zip

The entire circuit can be powered by a USB cell phone charger. I wanted to be able to make it battery powered simply by plugging in a USB powerbank, but unfortunately, the circuit doesn’t draw enough current when not moving the servo to keep most USB Powerbanks from shutting down due to their “low current auto power off” features.
There are a few USB powerbanks on the market which have an “always on” feature for use with low current devices such as webcams, but you can’t just power it with any USB powerbank.

 

Running our 240 volt Well Pump in a power outage

The last hurricane knocked out our electricity for 3.5 days. (Other people in our neighborhood were without power for 6+ days). I have a 120 volt inverter that I used to power our fridge, microwave and internet, but our well pump requires 240 volts.  It was really annoying to have to transfer water from our tub to the toilet tank to flush, and not be able to wash our hands at the sink or get water from the tap when we wanted to.

My wife has given me a $1,000 budget to expand our backup system so that we can run the well pump in power outages, and the first step is to figure out the actual power draw of the well pump.


The pump is powered off of a 20 amp circuit, so the maximum draw is 240v * 20A = 4,800 watts. I measured the actual continuous draw when the pump is running at just under 10 amps, or 10 * 240 = 2,400 watts continuous.  I will probably need to purchase an inverter with a surge rating of twice that to support the inductive surge the pump motor is likely to draw when it first turns on.

I hooked up an energy monitor to the pump circuit and monitored it for a few weeks. The energy monitor estimates that the well pump uses between 6-8 kWh of power over the course of a month, so I will only need 1-2 kWh of battery capacity to be able to run it for several days.

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Megatree Ball Topper

For Christmas 2017, I built a computer controlled RGB Pixel LED megatree around a pine tree in my yard. For Christmas 2018, I built a 48″ outside diameter “ball” topper for the tree. (It looks better when it’s pulled 20′ up the tree, and lit up at night…)

This was the first “large scale” project that made use of my Maslow CNC router to cut out a large number of parts (which took many hours) that bolt together in such a way that each individual part can fit into a 27 gallon tote for storage.

I think it actually took longer to cut out all of the parts than to paint them, but painting them with exterior house paint for some weather protection also took a bit of effort.

If you want my OpenSCAD files to make your own version, you can find it in this zip file:
topperOpenscad

And here is a video that shows off some of the animations I sequenced for 2018:

ILLuminArt 2018 Tornado

This is “Florida Weather” (a.k.a. The Tornado) an entry to the 2018 ILLuminArt show by Florida Sculpture Guild members Amy Wieck, Louise Buhrmann and Cathy Farrar. My only contribution was the computer controlled lights to add “Lightning” which can be seen in the video below:

They won both 2nd place and the People’s Choice Award.

8×8 floating dock section

 

I bought a 8×16 floating dock for $100 delivered. The main reason it was so cheap was that they had originally used Melamine covered particle board for the decking, with predictable results.

But the pressure treated 2×6’s and foam floats were in relatively good shape, so I bought 4 brand new 2×6’s to have nice new lumber on the outside, and built my own 8×8 floating dock.

I used 7 2×6’s total (4 new ones and the 3 interior ones salvaged from the original dock) to build the frame around the foam floats, wheeled it down to the lake, and then screwed in 19 composite deck boards.

And here is the finished product (before adding hardware to fix it in place with pipe floats, bumpers, solar lights, etc…)

 

The total cost was about a dollar per pound of dock. The composite deck boards were the heaviest and most expensive single item, although the hardware to hold the dock on pipes and attach it to other dock sections will also add significantly to the cost.

Item Price Weight
Old Dock parts w/ Delivery 50 100
4 (new) + 3 (used) 2×6 -ground contact PT 23.52 117
3x tubes of black calk 14.34 2
deck screws 3.5” 8.05 1
19 composite Deck Boards 252.7 307
2 lb deck screws (1 5/8″) 16.08 2
2x TommyDocks floating dock pipe guide (with pipe & augers, not included in the weight) 111.9 4
4x Solar Pathway Lights 57.95 1
Totals (Cost / Weight) $534.32 534lb

Pokeball Decoration (MDF, Paint, Resin)

I built this for my son. It was also a learning project for my new Maslow CNC Router (and using tinted casting resin to fill in pockets for a mixed-media project).

Videos of the process:
1 – How to design the digital file

2 – How to convert the SVG file into Gcode using the Makercam.com webapp

3 – Running the Maslow CNC Router and cutting out the part.

4 – Hand finishing, spray-paint and colored resin pouring to finish the piece.

 

You can download a zip file including the SVG and gcode (.nc) files here: pokeball_files

Maslow CNC “hanging router” review

I have completed a few projects using my Maslow CNC “hanging router”. Although I’m not yet an expert on its use, I feel like I have enough experience for a general review. The bottom line is that it provides excellent value for the cost for a hobbyist, but will not replace a professional gantry style CNC router for professional use.

 

The source of the Maslow’s sub $500 cost is its unique motion system, which relies on gravity working against two variable length chains to position the router sled, which must slide on a flat work piece. Because you provide your own router, build the frame yourself, and cut out the final round sled using a temporary sled that you cut by hand, the electro-mechanical parts of the Maslow can ship in a large USPS priority mail box.

My Maslow is the 2nd generation that includes a ring for two chain carriages to roll along. The rolling chain carriages allow the two support chains to virtually “end” at the center of the sled where the router bit is positioned. This mostly eliminates negative effects of sled rotation and simplifies the kinematics of the machine. Earlier versions tried to model and account for the sled rotation with chains anchored off-center, or used a mechanical linkage system to achieve a similar effect. In my opinion, the ring and carriages is the best solution.

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Maslow CNC setup

After building the frame for my Maslow CNC machine, the rest of the setup was just a matter of assembling all of the pieces.  I used 1/4-20 “superstrut” nuts and 1/4-20 machine screws to mount the motor brackets. The slots in the brackets are almost, but not quite wide enough to let 1/4-20 screws go through them, so I had to drill them just slightly larger in the two spots I mounted the screws.

   

I made use of one of the four motor mounting holes to place an extra long (60mm) M4 screw holding a plastic idler that keeps the chain wrapped around the sprocket to avoid chain slips under tension.  I’m currently using a binder clip to keep the plastic idler from “crawling up” the screw and eventually letting the chain fall onto the screw.

I considered buying a shorter screw to keep the plastic idler from crawling up the screw, but if I ever want to manually adjust the chain position on the sprocket all I need to do is remove the binder clip and slip the plastic idler up out of the way, so I’m leaving it as-is for now.  I also paid Lowes an outrageous $5 for a set of two blue plastic end caps to make the end of my superstrut look nice.

 

 

You can see that I also hung the far end of the chain from the idler mounting screw, and adjustable tension is placed on the slack side of the chain with an idler sprocket weighted down with a few pounds of water. (So far I just put a few inches of water in each jug, and haven’t needed to add significant weight.)

I found that I could balance a small level directly on top of the chain and use the bubble to get a tooth of the sprocket aligned vertically within a 10th of a degree. (At least, there was a small but visible difference between the bubble between presses of the 0.1 deg button in the software…you’ll probably have to click the photos to zoom in before you can see it…)

 

I built a temporary sled out of a piece of plywood that was left over from covering a window during Hurricane Irma. Instead of bothering to countersink the heads of the provided brick mounting bolts, I just used deck screws to mount the temporary bricks.  I also was able to use the (too short) screws originally meant for the clear router base to mount the router to the plywood by abusing the heck out of a large countersink bit to REALLY countersink the screws so the short length was no longer an issue.

After calibrating the machine using the foam waste board, I used the temporary sled to cut a fancy round sled out of some MDF I had laying around. I don’t like the super fine sawdust that MDF generates, but it is more slick than regular plywood, which I figure is a good property for a router sled to have. (Plus, already had it laying around….)