All posts by Martin Blustine

I studied physics and went on to work in infrared optics, millimeter wave and microwave engineering until retirement. My interests lie in teaching, music, radio astronomy, infrared systems and microwave and antenna engineering. I enjoy writing technical papers about ham radio topics. When I am not operating CW, I enjoy homebrewing ham gear and restoring vintage HP and Agilent test and measurement equipment.
Antennas, Featured

What’s Inside the Hy-Gain AV-640 Vertical Matching Unit Anyway?

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With the exception of a 2m handheld and a temporary end-fed half-wave vertical[1] for use on 20m in the IARU CW contest, I have been off the air since July 2020. After assessing the rocky soil in the backyard, I have come to the conclusion that ground radials won’t be gobbled up by the lawn as they were by the St. Augustine Grass in Florida[2]. That’s when I decided to install a compromise antenna – one that does not require radials. Since I am apt to hang half-wave antennas for the top bands, I settled on the Hy-Gain AV-640[3].

The AV-640 is an 8-band antenna that, in addition to the WARC bands, adds 6m. Since I have not operated on the “magic band” for many years, it’s a nice bonus.

The AV-640 arrived from the supplier in a box that was intact, but the first thing on the to-do list was to complete a parts inventory. That task was completed in, maybe, two hours. During that time, the parts, particularly small hardware, were separated into several Ziploc bags for easy identification later.

It turned out that there were a few pieces of stainless hardware and mounting brackets missing, and MFJ, with help from DX Engineering[4], replaced them in record time. Encouraged by the quick replacements, I decided to perform one last check before installing the antenna. Like any good homebrew tinkerer, I decided to open the Matching Network, Figure 1, to see what was inside and to make certain that nothing was broken. My curiosity was rewarded. I found that two wires had broken, thereby, separating them from the printed circuit board. There was also a Ty-Rap normally looped through the circuit board to anchor the toroid cores that had snapped. MFJ gave me the choice of repairing the unit myself which would have required complete disassembly, or a replacement assembly. I chose the latter.

Figure 1. Interior View of the Hy-Gain AV-640 Matching Unit. Please click on the image to enlarge it. A 1:1 current UNUN (right) for common mode rejection is followed by a 9T:20T autotransformer (left) for a turns ratio of 1:2.22. The black and white wires are connected in series. The black wire is the so-called “common winding”, while the white wire is the so-called “series winding”. The circuit board traces can be seen from the top. The matching unit arrived damaged with a black and a white wire detached from the PCB. The points of damage are circled in white. The broken black wire should be soldered to the PCB within the toroid. A Ty-Rap had also snapped. The 1:1 current UNUN is visible to the right.

Under “Theory of Operation” the AV-640 manual describes[5] the matching unit as a “broadband RF transformer” in one sentence and later on as a “4:1 toroidal transformer (voltage balun)”. Since the copper on the backside of the PCB is visible from the top, the wiring could be traced without removing the circuit card from the housing. What I saw was something that was a 1:1 stacked-core current UNUN[6] for common mode rejection followed by a stacked-core autotransformer having a 9:20 turns ratio (in Figure 1, the broken black wire should loop through the toroid and be soldered to the PCB within the core I.D.)

The schematic of the matching unit is shown in Figure 2. The autotransformer has a 9-turn (common) primary and a 11-turn (series) secondary. The black primary (common) winding of 9-turns is in series with the white 11-turn (series) secondary winding to form a 9:20 turn autotransformer. The voltage turns ratio is 1:2.22, whereas, the impedance transformation ratio goes as N2, or 1:4.93. So, the autotransformer transforms 50 ohms to 247 ohms. A shortened radial ground plane, lowers the impedance at the antenna base.

Figure 2. Hy-Gain AV-640 Matching Unit Schematic Diagram. Please click on the figure to enlarge it. A 1:1 current UNUN is followed by an autotransformer. Note that the left end of the UNUN is dotted. The coax shield is wound with the same sense as the center conductor to form a common mode choke. An autotransformer that follows transforms the impedance from 50 ohms to 247-ohms. Note that the left end of the autotransformer is dotted. The black and white windings are wound with the same sense. Point D is connected to point A to place the primary (common) winding in series with the secondary (series) winding. The antenna is placed at DC ground potential by an RF choke that serves to bleed static charge from the antenna. The autotransformer is AC-coupled to the antenna by a high voltage ceramic capacitor. The short (72″ long) ground plane radials, depicted, lower the impedance at the antenna feed point to one that is more easily accommodated by the autotransformer. Please note that the ground return for the autotransformer and ground plane radial combination is brought back to the input connector via the common mode choke coax shield that is wound around the ferrite core. The 247-ohm impedance match at the antenna feed point is a compromise match for the 8 bands. As a practical consideration, a remote antenna tuner should be located as close to the matching unit as is practical to remove standing waves from the transmission line.

We might also take a look at the voltages at the secondary of the autotransformer to see if they are reasonable. At 100W we expect to see 70.7 Vrms (100 Vpeak) under matched conditions at the current UNUN input. If we multiply this by the 1:2.22 voltage turns ratio, we have 157 Vrms (222 Vpeak) at the antenna terminal. These numbers increase somewhat for 1.5 kW to 274 Vrms (387 Vpeak) and 608 Vrms (860 Vpeak), respectively.

It has been shown previously[7] that these numbers may degrade by as much as the square root of the VSWR. Thus, for a VSWR of 3:1, we might expect these numbers to increase by a factor of 1.732, and so on.

The lossy ferrite used in the UNUN and in the autotransformer places limits on the continuous (key-down) operation of this antenna. This subject was discussed in other posts[8][9].

For these reasons, this antenna has been rated for operation on each band within their 2:1 VSWR bandwidths[10].

References

[1] Blustine, Martin, Temporary 20m EFHW Vertical Installation, N1FD post, July 2, 2023. https://www.n1fd.org/2023/07/02/20m-efhw-vertical/

[2] Blustine, Martin, A Flagpole Antenna Project for Residential Settings, N1FD post, May 23, 2022. https://www.n1fd.org/2022/05/23/flagpole-antenna/

[3] Hy-Gain AV-640, HF VERTICAL, 8 BANDS-40/30/20/17/15/12/10/6 M, MFJ Enterprises, Inc., 308 Industrial Park Rd, Starkville, MS 39759. https://mfjenterprises.com/products/av-640

[4] DX Engineering, 1200 Southeast Ave.Tallmadge, Ohio 44278. https://www.dxengineering.com/parts/hgn-av-640

[5] Hy-Gain AV-640 8-Band Vertical Antenna, Instruction & Assembly Manual, Revised 14 July 2023, pp. 3-4. Hy-Gain, 308 Industrial Park Road, Starkville, Mississippi 39759. https://static.dxengineering.com/global/images/instructions/hgn-av-640_co.pdf?_gl=1*em8g2o*_ga*Nzc2NjgwNDEyLjE2OTQxMTU0MjQ.*_ga_NZB590FMHY*MTY5NDExNTQyNC4xLjEuMTY5NDExNTg0MS41MC4wLjA.

[6] Blustine, Martin, Differential and Common Modes on Transmission Lines – Part II, N1FD post, September 14, 2022. https://www.n1fd.org/2022/09/14/differential-and-common-modes-on-transmission-lines-part-ii/

[7] Blustine, Martin, Worst Case Standing Wave Voltage on a Transmission Line, N1FD post, August 1, 2022. https://www.n1fd.org/2022/08/01/standing-wave-voltage/

[8] Blustine, Martin, Power Losses and Dissipation in Various Ferrite Devices – Part I, N1FD post, August 10, 2022. https://www.n1fd.org/2022/08/10/ferrite-device-losses/

[9] Blustine, Martin, Power Losses and Dissipation in Various Ferrite Devices – Part II, N1FD post, August 12, 2022. https://www.n1fd.org/2022/08/12/ferrite-loss-2/

[10] Hy-Gain AV-640 8-Band Vertical Antenna, Instruction & Assembly Manual, Op. Cit., p. 5. https://static.dxengineering.com/global/images/instructions/hgn-av-640_co.pdf?_gl=1*em8g2o*_ga*Nzc2NjgwNDEyLjE2OTQxMTU0MjQ.*_ga_NZB590FMHY*MTY5NDExNTQyNC4xLjEuMTY5NDExNTg0MS41MC4wLjA.

Featured, General, Station Equipment

Fun with the Clear Sky Institute HamClock

I haven’t had the occasion to use any programming languages since retirement. That’s why the addition of a Raspberry Pi 4 Model B to the shack was a welcome change. I like to think of the Raspberry Pi as just another computer – one that uses a different operating system. With the Raspberry Pi, I can browse the Internet, access email, and write and run programs.

When I began to assemble a shack, I reserved a space on the wall for a 32″ TV, Figure 1, which was purchased during a temporary rental stay. That TV has been unused for 3 years, but it was earmarked for a HamClock.

Figure 1. Unused 32″ TV Earmarked for HamClock. The TV was wall-mounted above the shack monitors. Please click on image to expand.

I searched the N1FD site to see if anyone had written about HamClock, but no articles were found. The first article for HamClock, written by Elwood Downey, WB0OEW, appeared in October 2017 QST[1]. In his article, he calls for the use of an Adafruit HUZZAH ESP8266 Wi-Fi system-on-chip. That device was fastened to the back of a 7″ TFT display.

The version of HamClock that I built for use with the 32″ HDTV employs the Raspberry Pi 4 Model B, Figure 2, with 2 GB memory[2]. The kit that I found on Amazon includes a 64 GB microSD card (with USB adapter) onto which the Raspberry Pi operating system had been preloaded. The kit also includes a plastic case with fan, little rubber feet, tiny screws to attach a camera, device heatsinks, a wall-wart power supply, a micro HDMI to HDMI cable, an instruction manual and various assembly instruction cards. The user has to provide their own USB mouse and keyboard. I already owned a wireless mouse and keyboard so I was able to use a single USB 2.0 port on the Pi for the wireless adapter.

If you already have a microSD memory card with USB adapter, power supply, mouse, keyboard and HDMI cable, you could get by with a Raspberry Pi Zero[3] at one-fourth the price.

Figure 2. Raspberry Pi 4 With Wireless Mouse and Keyboard. A single USB 2.0 port on the Pi is used for the wireless adapter leaving 1 x USB 2.0 and 2 x USB 3.0 ports unused. Power and HDMI cables are visible. Please click on image to expand.

If your Raspberry Pi does not come equipped with the operating system installed, you can download it from the official Raspberry Pi website and store it on a microSD card for installation provided that you have a USB to microSD card adapter. The Raspberry Pi site allows you to select an operating system and it allows you to specify where the operating system will be stored upon download:

https://www.raspberrypi.com/software/

Please, follow the directions to install the operating system on your device.

I noticed an ambiguity in the kit documentation regarding connection of the cooling fan to the Pi bus header. A tiny drawing, Figure 3, shows where the red and black leads should be connected, namely pins 1 and 14, respectively. The documentation should make it clear that the long header row that contains pin 1 contains all of the odd-numbered pins while the long header row that contains pin 14 contains all of the even-numbered pins. This makes it a bit easier to locate pin 14.Figure 3. Location of the Fan Voltage Pins. The long header row that contains pin 1 contains all of the odd-numbered pins while the long header row that contains pin 14 contains all of the even-numbered pins. Please click on image to expand.

The instructions advise that the HDMI port closest to the DC power supply input be employed if only one monitor will be used. I was confused about the power supply connector. It may be plugged into the Pi upside down. However, I found that a bright flashlight can be used to look inside the power supply connector and inside the Pi power supply input jack to ensure that the conductive contacts face one another. If the connector has been plugged in correctly, some LED status lights on the Pi circuit card should illuminate once the inline DC power cord switch, Figure 4, has been turned on.

This switch has to be used with caution. The operating system, if running, must be shut down prior to turning the DC power switch to the off position. Failure to do so could corrupt data stored in memory and on the microSD card. This is a minor weakness in the design.

Figure 4. The Inline DC Power Switch. The operating system must be shut down from the Raspberry Pi menu icon on the taskbar before turning the DC power switch to the off position. Please click on image to expand.

Two methods may be used to shut down the Raspberry Pi. There is a Raspberry icon that appears on the taskbar. One of the drop-down selections is to log out. Once selected, another dropdown appears that offers the choice to shutdown or reboot. There is a second method that permits shutdown from a terminal window, Figure 5, which may be opened from the taskbar. One need only enter the command:

sudo shutdown -h now

to close the operating system. A third option is to add a momentary switch to the Pi bus header. That may be used to assert a shutdown command for the operating system. I have not implemented that feature.

Figure 5. Terminal Window. A terminal window may be opened from the Pi taskbar. Please click on image to expand.

Once an unused HDMI input to the monitor or TV is selected, and the mouse, keyboard and HDMI cable are plugged in, the power switch in the supply power cord may be switched on. Within seconds the Raspberry Pi logo appears followed by a series of questions that include language and time zone. The operating system will also ask for access to WiFi. I found that WiFi was easier than connecting another Ethernet cable to the access point.

Once WiFi is connected, the operating system will update. Finally, a desktop appears. The taskbar was docked to the top of the screen, but I moved it to the bottom because that is where I am used to seeing it, Figure 6. That may be accomplished by selecting that feature from the Raspberry Pi icon on the taskbar. The operating system provides access to the Internet via a browser icon that appears on the taskbar. There are also icons for a terminal window and Bluetooth.

Figure 6. Taskbar Moved and Docked to the Bottom of the Screen. This may be selected from screen appearance under the Raspberry Pi icon on the taskbar. Wallpaper like this may be selected from a list. Please click on image to expand.

The Raspberry Pi icon, Figure 7, provides several selections for screen appearance, resolution and the usual accessories. It also provides a means for shutting down the operating system.

Figure 7. Menu Provided Under Raspberry Pi Accessible from Taskbar. Access to many options is provided here including Raspberry Pi screen resolution. This is not the same as the HamClock screen resolution setting, which will be selected separately. Please click on image to expand.

It was learned that the means for capturing screenshots within the Pi operating system is Print Screen (PrtSc) just as it is in Windows, so this method has been used to illustrate this article. The images are stored as PNG files in the home Raspberry Pi folder, not under Screenshots in the Photo folder where one would expect them to be.

After opening a terminal window, I followed the directions provided at the Clear Sky Institute[4] web page under the Desktop tab with some notable exceptions. First, I took the advice of KM4ACK[5] to circumvent error messages by executing two scripts before attempting anything else. These scripts may be found listed under the Desktop tab under the subsection titled “To install HamClock on other UNIX-like systems follow these steps”, paragraph 2, “If you get errors”[6]. These steps at Clear Sky are correct except for a syntax error pointed out by KM4ACK[7] in the script:

sudo apt-get -y curl install make g++ libx11-dev xserver-xorg raspberrypi-ui-mods libraspberrypi-dev linux-libc-dev lightdm lxsession openssl

This script must be corrected so that the word ‘install’ precedes the word “curl”. The corrected script should read:

sudo apt-get -y install curl make g++ libx11-dev xserver-xorg raspberrypi-ui-mods libraspberrypi-dev linux-libc-dev lightdm lxsession openssl

I don’t know why this has not been corrected on the Clear Sky Institute web page, but I am happy that KM4ACK pointed it out.

We may now follow the steps listed under “To install HamClock on a Raspberry Pi follow these steps”. It is suggested that the lines of code be executed line-by-line.

cd
curl -O http://www.clearskyinstitute.com/ham/HamClock/install-hc-rpi
chmod u+x install-hc-rpi
./install-hc-rpi

If you choose not to install a desktop icon, you may run this script from a terminal window to start HamClock:

hamclock &

The first time that HamClock is run, some entries are requested. HamClock will also ask if you would like to connect to WiFi, Figure 8. Since you may have entered these for the Raspberry Pi operating system, you may answer yes if you would like to connect. HamClock will enter your username and password for you.

Figure 8. Connect to WiFi Screen. You will enter your call sign on setup page 1. You may provide your network username and password after saying yes to WiFi. If you provided these to the Raspberry Pi operating system, they will auto-fill if you select, yes. You can enter your latitude, longitude and grid square here, or you can Geolocate on your IP address (not recommended). Please click on image to expand.

Once the HamClock screen appears, if the correct resolution has been chosen, the screen should be mostly filled from top to bottom, but there may be some black bars on either side of the window. There are numerous instructions online about how to deal with this. I didn’t bother. Once I had chosen the closest resolution to my screen, 1366 x 768, I left well enough alone. HamClock also asks if you would like full-screen view, Figure 9, which eliminates the HamClock title bar as well as the taskbar. I selected that view.

Figure 9. Where to Select Full-Screen View. Full-screen view selection is on setup page 5. Please click on image to expand.

To close HamClock, please note that there is a small padlock symbol, Figure 10, beneath UTC in the upper left-hand corner. If one left clicks and holds the lock for a few seconds, then releases, a script will appear that will ask if you would like to exit the program. If you also intend to shut down the Pi, please don’t forget the logout procedure that is found under the Pi logo in the taskbar.

Figure 10. Padlock Symbol Beneath the UTC Box. If you were to left click and hold on the lock for a few seconds and release, a script will appear that will ask if you would like to exit HamClock. Please click on image to expand.

If you would like HamClock to start automatically upon reboot, a script that will do it may be run anytime from a terminal window. The scripts are found under, ” To install HamClock on other UNIX-like systems follow these steps”, in paragraph 10:

cd ~/ESPHamClock
mkdir -p ~/.config/autostart
cp hamclock.desktop ~/.config/autostart

You may want to place a HamClock icon on your Raspberry Pi desktop. That may be accomplished by copying, pasting and executing the following script in a terminal window:

cd ~/ESPHamClock
mkdir -p ~/.hamclock
cp hamclock.png ~/.hamclock
cp -p hamclock.desktop ~/Desktop

After running the scripts, close the terminal window and reboot the machine from the Pi icon. HamClock should restart immediately. The size of the HamClock window may appear reduced after reboot but may be expanded just as one might expand any other window.

As soon as HamClock is up and running you may want to explore all of the options and items that are found under “Terrain” that is found in the upper left-hand corner of the map, Figure 11. An extensive dropdown menu will appear. Go ahead and press the radio buttons followed by “okay” to see what happens. Every new screen is a surprise. It’s not that hard to get back to the screen where you started, so please experiment a little bit.

Figure 11. Dropdown Menu for the Main Graphic View. You will want to explore all of the options available from this dropdown menu visible to the left of the Mercator projection. Other menus may be accessed by clicking on the titles that appear within the smaller graphics. For a complete description, please consult the User Guide. Please click on image to expand.

Additional views for the smaller graphics may be requested by clicking on the graphics titles, themselves. For example, you may want to display propagation paths from your locale as supplied by WSJT, Figure 12. There are also satellite and space station orbits.

To change the color of your call sign background, click to the right of your call sign. To change the color of your call sign, click on the letters.

Figure 12. Display of Propagation Paths From Our Locale. If WSJT-X was selected on Page 2 of the setup menu, this graphic will be displayed. My call sign color and background color has been changed for this view. Please click on image to expand.

A particularly interesting view of the aurora is shown in Figure 13. This is one more example of how much data is available for presentation in Hamclock.

Figure 13.  Display of the Aurora. This is another example of how much data is available for display within the HamClock application. Please click on image to expand.

A complete HamClock User Guide is available at the Clear Sky Institute website under the User Guide tab[8].

Please let me know if you build a HamClock of your own. It is nice to receive feedback.

References:

[1] Elwood Downey, WB0OEW, HamClock, QST, October 2017, pp. 42-44.

[2] Raspberry Pi 4, Model B, 2 GB.

https://www.amazon.com/dp/B07TMGBPFQ/ref=sspa_dk_detail_6?ie=UTF8&pd_rd_i=B07TKFKKMPp13NParams&s=electronics&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&th=1

[3] Raspberry Pi Zero (2017).

https://www.amazon.com/Raspberry-Pi-Zero-Wireless-model/dp/B06XFZC3BX/ref=sr_1_8?crid=2P2OQ2SF3LHR5&keywords=raspberry+pi&qid=1692662504&sprefix=Raspberry+pi%2Caps%2C151&sr=8-8

[4] Elwood Downey, WB0OEW, Clear Sky Institute. https://www.clearskyinstitute.com/ham/HamClock/

[5] Jason Oleham, KM4ACK, YouTube, 2:30. https://www.youtube.com/watch?v=2Cy5Swmk3gU

[6] Elwood Downey, WB0OEW, Clear Sky Institute, Op. Cit., Desktop tab.

[7] Jason Oleham, KM4ACK, YouTube, Op. Cit.

[8] Elwood Downey, WB0OEW, Clear Sky Institute, Op. Cit., User Guide tab.

 

 

 

Featured, General, Station Equipment

Restoration of an Agilent 53131A Frequency Counter

An enjoyable hobby is the restoration of vintage HP, Agilent, Tektronix and GenRad test equipment. In their day, these brands represented some of the finest U.S. engineering and manufacturing in the world, and two of them still do. What is common to all of them is the attention paid to ergonomics, product design and documentation.

Each time I restore one of these instruments, I gain insight into what the design engineers and the product designers had in mind.

Recently, I acquired an Agilent 53131A Frequency Counter found on eBay, one of my favorite sources for old test equipment. It is always a calculated risk because I never know for sure if whatever I have bought will arrive dead on arrival. Fortunately, eBay is very good about standing behind anything sold on their site that has been represented to be in good working order. They are unlike auction consolidators such as Allsurplus where everything is sold as is.

Naturally, when the counter arrived, I subjected it to the obligatory smoke test. The result is shown if Figure 1. The counter completed its self-test routine and rewarded me with a series of dashed lines in the display. That was a good sign, and it was off to a good start. When I purchased the unit, I made a best offer, and it was accepted. The reason that I did that is because I noticed some minor irregularities in the unit.

First of all, the display appeared to be dim in the photo, the likely result of running the counter for long durations for many years, maybe 30. The second thing that I noticed was the absence of the front and rear bumpers as well as a carrying handle. These, I am quite sure, were scavenged for resale on eBay. I have become quite used to that. Most of the instruments that I have restored were received minus their bottom and rear feet and in some cases their rack handles – a minor annoyance. I simply buy some and put them back on.

Most often these counters are purchased for HF use and do not feature prescaler options to extend the frequency range. This counter was no different. It was also shipped with the standard low-stability clock oscillator.

Figure 1. Agilent 53131A Frequency Counter, As Received. The display is dim but the unit passes self-test and enters ready mode. The unit was advertised and shipped without its front and rear bumpers as well as its carrying handle. The time base is a standard low-stability one, and there is no frequency extender option installed.

Upon seeing the condition of the display, I looked at the power supply schematic and found the test points for VFD display bias. The voltages checked okay. That test having been completed; I ordered a new display on eBay from a seller in China. The old display was marked Japan, but I could not find any other suitable displays for sale other than one in a non-working counter being sold for parts only.

I also looked for an inexpensive prescaler to install as an option. The one chosen works up to 3 GHz but others with higher division ratios were also available. This one provides accurate results for signals as low as -15 dBm, which is good enough for my purposes.

There were many sources for the front and rear bumpers and the carrying handle on eBay. I likely bought my own back.

All parts arrived in less than two weeks, which is about average for items ordered overseas that must pass through U.S. Customs.

An initial test was run on the counter to make sure that it would arm and run. I used whatever signal source was closest for this test, in this case, an HP 3314A Function Generator. The setup is shown in Figure 2. Both units are out of calibration so, it can’t be determined which is worse, the counter or the function generator.

Figure 2. Counter, As Received, Measuring 18 MHz. The most convenient signal source for initial testing was a bench-top HP 3314A Function Generator. Both the counter and the function generator are out of calibration. It’s difficult to know which is worse.

The first part to arrive was the frequency prescaler. The only disassembly required for installation was the instrument top cover.

I fastened the prescaler to the wall of the chassis on a pair of standoffs. The prescaler board was supplied with a coaxial cable for connection to a front panel SMA to BNC adapter and a ribbon cable for connection to the motherboard. The whole installation process was completed in an hour. The result is shown in Figure 3. The sensitivity of the prescaler was tested with a test signal of 2.55 GHz, as this particular synthesizer won’t tune much higher than that. The signal level was set to -10 dBm but the prescaler will provide stable counts for signals as low as -15 dBm.

Figure 3. Prescaler Test. The prescaler was tested with an HP 8663A Synthesized Signal Generator set to 2.55 GHz because it is the only microwave source available. A stable count is observed for power levels as low as -15 dBm. The front and rear bumpers as well as a carrying handle have been installed.

Once the vacuum fluorescent display arrived, I began to tear down the counter to the level required to remove the front panel. This also permitted cleaning 30 years of grime from the enclosure and circuit cards. Figure 4 shows the teardown.

Figure 4. Counter Teardown. The counter was disassembled using the assembly-level service guide. The power supply and motherboard remain in place. Only the front panel required removal.

Figure 5 provides a closer view of the front panel PCB and the old display. The new display is in the foreground. While the two displays are functionally the same, they have completely different layouts and are of different manufacture.

Figure 5. Close-up of the Front Panel and Vacuum Fluorescent Displays. While the displays look alike and possess the same pin-outs, they are entirely different designs.

The old display was removed from the front panel display printed circuit board that also contains the soft contacts for the front panel controls. Utmost care was taken in unsoldering and removing the old display. I wanted to preserve the old display if the new display was dead on arrival.

The unsoldering process was the one that I always use. The solder joints were painted with non-corrosive soldering flux. Next, each joint was resoldered with compatible solder. This is an important step because old solder joints tend to degrade with time. The newly resoldered joints are more easily heated and solder more easily removed with a solder puller like the one shown in Figure 6. This is an inexpensive tool that, when mastered, can remove all of the solder from the solder joints. Sometimes it takes three or even four passes with the solder puller on each joint to remove all of the solder from the holes. Once all of the solder has been removed, a needle nose pliers is used to wiggle each of the leads to ensure that it is free of solder and free from each hole. Another technique can be used provided that the replacement part is known to be good. Simply snip the leads of the old part and remove it. Then, there are still leads that have to be removed and holes that have to be cleaned. Some find this technique easier. Avoid the use of solder wick if at all possible. It really isn’t needed, and it usually ruins the circuit pads.

Figure 6. A Generic Solder Puller. This is a simple device that is just a spring-loaded plunger that, when released, can vacuum solder from solder joints. It pays to master the use of this device. Avoid the use of solder wick.

The old VFD was removed from the front circuit board. The old part and the new part are shown in Figure 7 for comparison. The two parts are of different provenance.

Figure 7. Old and New Vacuum Fluorescent Displays. The designs are very different: (top) the old part, (bottom) the new part. The pins on the old part are intact.

The display circuit card is shown in Figure 8. Non-corrosive soldering flux was painted on each solder joint and each joint was resoldered. This step is essential for old solder joints. Four passes were required to remove all of the solder with the solder puller. The board is undamaged as is the old display part.

Figure 8. Display Printed Circuit Board. The display was removed from the board with a solder puller. Four passes were required to remove all of the solder without damaging the pads. Flux residue was removed from the PCB with isopropanol.

The new display was soldered into the display circuit card and the PCB was cleaned with isopropanol to remove residual soldering flux and debris. The board was examined with a magnifier to ensure that the board was free of solder blobs and solder bridges. The counter disassembly procedure was reversed to reinstall the display circuit board into the front panel. Next, the front panel was fastened to the chassis, and the cover was replaced. The result is shown in Figure 9. The display brightness is just like new.

Figure 9. New Display Brightness. The new display works as advertised. A microwave signal of 2.55 GHz is displayed. The new prescaler BNC connector is marked Channel 3. The new front and rear bumpers and carrying handle are also visible. Please note that the difference in brightness across the display is an artifact that is due to the refresh rates of the counter and the camera.

One final modification will consist of the addition of a stable source to replace the standard one that drifts. Once complete, the instrument will be calibrated.

 

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