A Trip to the Museum of Communications

This morning, I woke up with an itch, an itch to see a switch.

Version 2

No, not that kind of switch, something bigger!

Battery Reserve Switch

Nah, that’s not a switch…

Panel Switch

That’s a switch! Well, part of one.

Museum of CommunicationsIt is part of a panel telephone switch, one of a number of operational telephone switches at The Herbert H Warrick Jr. Museum of Communications, a little known technological treasure trove in the Georgetown neighborhood of Seattle. The museum fills the top two floors of a Century Link central office building.

The museum’s collection includes all manner of equipment and memorabilia from over a century of telephone history. The presentation of the collection is a bit uneven. There are carefully dated and labeled exhibits of phones and other equipment, along with display cases stuffed with lineman’s tools, but to me, that’s all secondary.

The best part of the museum is that it houses multiple generations of telephone exchange switching equipment, the sort of stuff that used to fill small buildings and connect thousands of homes to the telephone network. Much of it of it is operational, and interconnected, and attended by a staff of volunteers, many of them technicians and engineers retired after long careers with Ma Bell and her successors. They answer questions, give tours, and maintain the equipment.

The automatic switching equipment spans almost a century. The oldest automatic switch is a panel switch that served the Rainier Valley. It was installed in the 1920s and served for over 50 years. Unlike that later automatic switches in the museum, which were made in a factory and then installed, the panel switch was assembled on site at the central office and moving it five decades required removing walls. They also have a #1 Crossbar Switch (#1XB) from the 1930s, and a #5 Crossbar (#5XB) from the 1950s. The panel and crossbar switches are all operational. Calls can be placed between lines serviced by the switches and you can hear the progress of the call set-up and tear-down sound across the racks as the various electromechanical parts do their thing. They also have a number of operational PBX systems, some old-time switchboards, some small Strowger step-by-step switches and a not yet operational #3ESS, a small variant of the first switches using transistorized logic for control.

The collection also includes inside plant, like power distribution equipment, outside plant, like cables, along with a variety of trunking and long-distance equipment, including equipment for carrying national network television broadcasts.

There is also test equipment spanning decades, and a nice cache of HAM radio equipment.

The museum was originally called the Vintage Telephone Equipment Museum when it was created in 1986 by Herbert H. Warrick Jr. Warrick was an engineering director at Pacific Northwest Bell, and started the museum with the company’s support to preserve generations of vintage telephone equipment that was being phased out in the transition to digital switching and transmission. More recently, it became affiliated with the Telecomunications History Group.

I’ve made many visits to the over the years, and each time, I learn something new. I recommend it to anyone with an affinity for technology, particularly communications and computing, but it should also interest anyone curious about industrial and economic history. I hope I’ve whetted your appetite.

 

Keithley 2000 & 2700 Data Logging in Python

ReadDMMs.py

This is a simple, braindead, python script to get measurements from a Keithley 2000 & 2700 DMMs using VXI-11 and store them in a simple timestamped SQLlite database.

It’s not a general tool, but it should be easy enough to tweak it to dump to a text file, take different measurements, use different ranges, different intervals, different communications transports.

It doesn’t have enough error checking, and bug fixing, but it mostly works well enough. It recovers from (some) malformed readings. It doesn’t recover from other errors (mostly communication related)

Requirements

  • OS X or Linux
    • It may work with Windows, but I haven’t tested it.
  • Python 2.7.x 
    • It may work with others, I haven’t tested it.
  • python-vxi11

How to use

The script takes voltage readings at ~10s intervals, on the 100v range, and stores them in a SQLlite3 DB called readings.sqlite3 in the current working directory. If you want different behavior, look through the source code and make the necessary changes.

You MUST edit the file to configure the name of your VXI-11 gateway and GPIB address(s) of the devices you want to poll.

Finally, run the script, ie:

python ReadDMMs.py

Tips

Since this script doesn’t do much error checking, it occasionally dies. In my experience, when it dies, it is because of a communcations timeout. In such situations, restarting the script is often enough for hours more logging.

I usually run it in a shell loop, so it restart automatically after a delay:

while true; do python ReadDMMs.py; sleep 150; done

If you modify this script to communicate with your Keithley 2000 DMM over RS-232 serial, be aware that a problems with hardware/firmware before ~2007 can result in frequent communication failures.

The simple workaround is to modify the RS-232 cable, or the DMM’s own RS-232 port, to ensure that the RTS pin (#7) is not connected.

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HP 6114a Precision Power Supply First Look

I already have more working electronic lab equipment than I need, and more broken equipment than I can fix in the next month or so, but I still check eBay daily, and occasionally, I see a deal that is too good to pass up. This time, it was an Hewlett Packard 6114a power supply on sale for $75 + $19 shipping. The supply had been listed for $150, and I figured that if the seller was willing to cut the price in half, they might be willing to accept even less. I offered $55.

I’m really not sure what I was thinking. Part of me felt like getting the supply for $75 would be a great deal. Part of me thought that not getting it at all would be smart, since I didn’t need it, and had resolved not to take on other projects. Part of me wanted to see if I could get it for even less. In the end, it looks like the compromise I made was to try and serve all three, because I picked a price that was low enough that it might not be accepted, but high enough that I might end up with another piece of equipment. And so I did.

The HP 6114a was introduced in in the early 1970s and produced until at least the early 1990s. They typically go for something over $100 + shipping, so getting one for just under $75 would rate as a good but not great deal. From the photos in the listing, I thought I could make out enough of the serial number to tell that this example was made in 1981, a fact I confirmed once it arrived. It had the base single-turn potentiometer for the current control, rather than a 10-turn pot with a turn-counting dial, but I thought I could upgrade it myself for $10-20 in parts.

Physically it seemed in fair shape. There were some bent fins on the heatsink, and the front panel trim wasn’t seated properly, and might need to be bent back into shape. Most of the finish seemed to be in good shape. The condition of the front panel was harder to judge. It was hard to tell what was sticker residue and what was scratches in the finish.

The unit arrived from Nevada about three business days after I ordered it. It was packed in a stout cardboard box, and heavier than I expected. Inside I found it wrapped in a few sheets of thin foam, nestled in a reasonable amount of packing peanuts. I think there were enough peanuts to protect the instrument as shifted in the box transport, but I’m not 100% sure, because there was some damage to the front panel and its hard to tell if it was pre-existing, or it occurred during shifting.

HP 6144a Front Panel

There weren’t any big surprises after I got it unwrapped. Next step was to start taking it apart so I could figure why the top trim on the front panel wasn’t seated properly, and what I could do about it.

Along the way, I also performed an initial inspection of the electronics to look for damaged components, PCBs and interconnect wires.

I quickly spotted some damage to some of the pvc tubing used for cable management (above left). It looked like it had been scorched by an errant soldering iron, suggesting a previous repair. It took my a while to figure out that the site of the repair was (probably) right there in front of me. That a big resistor (above right) is not like all the other big resistors. It’s epoxy packaged. The rest are ceramic.

 

My deeper inspection showed that displaced top trim wasn’t bent, as I feared. The aluminum extrusion was in good shape, other some chips and gouges in the finish, some of which had cut into the underlying aluminum. With the top trim removed, I could take a closer look at the other components of the front panel.

IMG_8562

The meter was in pretty good shape, but its grey plastic bezel, which also served to help retain the clear plastic lens piece, was broken in the lower left hand corner. I removed the bezel and glued the crack with some superglue. Once it was dry, and sanded it down and polished it with some nail files/polishers I got surplus from my wife. The crack is still visible if you look closely due to glue filling in fit is repaired and the profile and finish of the plastic is pretty close to what it originally was.

IMG_8565

During initial inspection, I noticed a rattle as I turned the instrument over. As opened the chassis up, I was attentive to the source of the noise, but couldn’t pinpoint it. In the process of removing the meter bezel, I realized the loose part was in the meter. I desoldered the meter leads so I could inspect the meter more easily. As I did, the source of the rattle quickly revealed itself to be small screw and copper lock washer. Once I had the meter free of its leads, I removed a clear plastic clip holding the meter lens to the back of its housing and worked the lens loose. With it free, I could see the source of the loose parts, they were one of the pair fastening the printed meter dial to the frame of the brass meter mechanism. They’d somehow worked themselves loose.

The copper washer dropped out as soon as I opened the meter housing, but I lost track of the screw. It wasn’t on my workbench, and gentle tapping of the meter housing didn’t shake it loose. I went in search of a suitable tool for removing the recessed nuts at the back of the housing that hold the mechanism to the meter. The socket wrenches I had were too thick-walled, but a fine set of needle nose pliers ended up doing the job. I still couldn’t find the screw though. I pulled the internal leads free of the pins that passed through the housing so I could take a closer look. Perhaps the screw had found a spot inside the works of the mechanism? After 15-20 minutes, I concluded that it must have dropped out as I carried it in search of a wrench. After searching around on the floor around my chair for 10 minutes, I broadened my search area and almost immediately spotted it camouflaged in a dust bunny on the other side of the table, along the path I took to the basement to look for tools.

HP 6144 Voltage Control Sub-panel

After reassembling the meter, I decided to remove the one screw holding the voltage control subpanel to the chassis and take a closer look at it. That doesn’t look right, does it? That bow isn’t lens distortion, that’s bent aluminum. I can’t tell from the photos in the listing if the damage was already done, or if it happened in transit. One way or another, it looks like the result of sliding or being pushed face-first against something.

In preparation for repair, I desoldered the two leads connecting the voltage control to the main PCB. One connected into a PCB on the sub assembly and was easy to remove. The other connected directly to a lead on the potentiometer shown in the photo. It was a bit fussy. The leads from the pot are basically ~22 gauge wire with a hairpin bend at the end. To get the wire free, I ended up removing most of the solder with some desoldering wick, and then alternated between teasing the hairpin open and working the wire free. All in all, it was fussier than I would have liked, and I ended up scorching some of the plastic potentiometer housing when I accidentally touched it wit the side of the soldering iron. In retrospect, i should have removed the knob and unscrewed the pot from the front panel before trying to desolder the lead.

With the assembly free, I removed the knob and retaining nut on the potentiometer, then I heated the aluminum carefully with a hair dryer so it was easier to pull the thinner front sheet with the labeling free of the the thicker extrusion so that I could get to the screws holding the decade switches.

HP 6114a Voltage Control Subpanel Repair

With the bent extrusion isolated, I used a hammer and a few hardwood blocks to carefully beat it back into some semblance of flat. I think I did a pretty good job.

IMG_8569

I also spent some time cleaning the stickers and adhesive off the remainder of the front panel. I started by peeling off what I could, which revealed some writing with permanent marker. Isopropyl Alcohol on some cotton balls and a little elbow-grease took care of all the adhesive and faded the permanent marker.

I used an old “drafting” eraser to rub out the last remnants of the magic marker and a few persistent spots. I think its looking pretty good. There are a few tiny scrapes in the white panel, and a few more in the grey strip at the bottom that I think I’ll leave be. Some of the lettering on the white panel is a little worn. I might try touching that up.

IMG_8576

The most glaring problem is the missing HP logo badge that is supposed to fit over those two holes in the upper left of the panel. Given the vintage of this supply, I think the original logo badge had charcoal and gray with a chrome border. Anyone happen to have any spares?

Next step, I think is to put it back together and check to see how it works.

 

HP 6177C DC Current Source Troubleshooting/Repair

I picked up a Hewlett Packard 6177C DC Current Source on ebay for less than $75 shipped. This is a precision constant-current source that can deliver 0-500mA at up to 50V.

IMG_7318The seller described the unit as used with responsive controls and indicators. When I received it, I could see that while in generally good physical shape the upper right portion of the front panel was more bent/buckled than I could make out in the eBay photos.

So, first thing I did was partially disassemble the unit to fix the front panel.

Once I got it back together, I did some quick functional tests and found that the current output was consistently 1/10th the expected value. In the 500mA range with the current pot set to maximum, it produces a max of 53mA of current, on the 50mA range, it produces 5.3mA, and on the 5mA range, 0.53mA. This behavior doesn’t vary noticeably between shorting the outputs and having a 30 Ohm load. With a suitably high resistance, the voltage will hit >50v, provided the current doesn’t exceed ~50mA.

So, next step was to look at the service manual and work through the troubleshooting steps.

First thing is to check some voltage rails.  These all checked out, though a few were out of spec on ripple.

Next is to go through the problem isolation procedure, which starts with checking the guard voltage to see if it varies between 0 and -1V. Nope! In each range it maxes out at… ~100mV, or 1/10th of the expected value. Notice a pattern forming?

I started to work through the guard supply troubleshooting instructions, but I got hung up. After disabling the main supply, as instructed and checking a few voltages, it wasn’t clear to me whether I should go immediately through the subsequent steps, or reverse the change and proceed from there. Subsequent instructions just raised more questions.

I asked for guidance in the EEVBlog forum, and while waiting for a response, worked to better acquaint myself with the schematic and theory of operation of the device.

I’m still not sure what to do, and rather than pushing forward, I realize that I already have other incomplete projects that need my attention, I’ve gathered everything up into a bin and put this one on the shelf, for now.