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Tech Forum

Sharing A Monitor With Two PCs Answered January 2018

I have two desktop computers and three 32" HP Pavillion 32Q monitors being used by two people at the same time. One of the monitors is assigned to each computer and depending on the applications being used, the third one goes to whomever benefits most by having two screens. I have to physically disconnect and reconnect the third screen when it needs to be moved to the other PC.

How can I do this without having to crawl uder the table to make the switch? I've looked at KVM switches, but I only need to share the third monitor, not the keyboard, mouse, etc. The monitors use the display port input and run at 2560 x 1440. I would just add another monitor to run two with each PC, but there isn't enough room in the work area.

Vicente Benedetti
Athens, TX


I have used a D-Link KVM to switch only the monitor. There is button on the front for switching. If you use the KVM to switch everything, you can use a keyboard hotkey for switching.

Bob Kottas
Omaha, NE

You can get displayport switch boxes that will do what you need, or you can simply use a standard KVM and only connect the video signals. The KVM doesn't care whether the keyboard and mouse signals are present.

James Sweet
via email

See a displayport switch at:

Also, search displayport switch at There are several listed.

Stan S
West Hills, CA

Converting S-Video to Composite Video Answered January 2018

I have a newer HD LED television that does NOT have an S-Video input jack! My old VCR and tapes are all S-VHS, so I need a schematic for building a converter for S-Video to Composite if I want to watch my old family movies. Please help!

Henry Vaden
Whitestone, NY


Henry, no need to build anything.  Just visit the following on-line retailers and purchase a ready-built convertor:

There are many, many more places that sell these convertors (do an internet search).  The above suggestions are My Preferred on-line A/V vendors.

These units plug directly into your S-video input (male plug), then use a composite cable from the RCA jack to your monitor's Yellow (composite) RCA input jack.

I've used these items before and they work very well! Do not expect "s-video" resolution from the composite input, but at least you'll be able to enjoy your S-VHS recordings.

Finally, consider getting a DVD Recorder like the Toshiba DVR430 (  This unit will DIRECTLY accept an S-Video input and stereo line-level inputs so you can transfer your S-VHS tapes to DVD (tapes eventually DO wear out! single-sided DVD record only!).  With the HDMI output, you can DIRECTLY connect it to an HDMI input of your LED TV and "pass through" your S-VHS tape programming to your TV through the recorder.  I have a similar unit (Toshiba DVR620KU - this unit also has a standard VHS recorder/player) and it works great for when I need to record broadcast programming.

Ken Simmons
Auburn, WA

VHS tapes deteriorate and fail. The machine is unlikely to be repairable due to parts availability and even replacing the machine is major dollars. For family videos, you really want to keep, I would suggest converting the tapes to a more modern format say DVD or a standard video file format.

Modern PCs have an HDMI port that connects to your TV. You can buy hardware you can use to convert them to PC format yourself or contract a service to do it for you. You can buy an S-Video to HDMI converter if you want to go that way. I will say that my HD TV had issues with playing the tapes due to slight variations in the timing signals from the tape which caused blackouts and breakups in the picture. You may have better luck with your TV.

East Hartford, Ct

You need a composite video, left audio and right audio to HDMI adapter. I believe S-video is composite video/audio basically as a digital TV of new would see it. Search eBay or Amazon for it. You’ll need cables with those names too.

Grand Rapids, MN

If you go to and look up S-Video, there is a simple schematic for an s-video to composit adapter.

Russell Kincaid
Milford, NH

S-video carries the luminance (B&W picture) and the chroma (color information) on separate wires as opposed to composite video which mixes them together to be carried by a single wire. There are fancier ways to combine the chroma and luma signals but often you can get acceptable results by simply tying them together, most of the inexpensive S-video to composite adapter cables do just that.

James Sweet
via email

No point in building at this price- As usual, eBay is your friend:

Larry Supremo
San Diego, CA

You can buy S-Video to Composite video adapters from,,,, and brick and mortar places like Fry's Electronics, Best Buy and others. I've even seen one at Target.

You can get them as small adapters, boxes you have to plug cables into or even built into a cable, with a S-Video DIN receptacle on one end and an RCA plug at the other. Plug "S-Video to Composite video adapters" into

Stan S
West Hills, CA

Gary Manigian
Martinsville, NJ

S-Video (also known as Y/C), refers to a video signal where the two components of the television signal (the sync and “luma” or “Y” which can be thought of as the black-and-white portion of the television signal, and “chroma” or “C” which includes the color information of the television signal) are carried on separate conductors inside a common cable. This is done to improve the quality of the television signal compared to a composite signal where the two components are combined and carried on a signal conductor.

Simple S-video to Composite adapters exist that combine the Y and C signals together with some capacitance and maybe resistance added for coupling and level-matching. Being that the required components are so small, they are often packaged together in an S-video to Composite coupling adapter or a short cable.

They are widely available and quite affordable. Two sources:

If you want to build your own S-video to composite converter, the following is a good place to start.
Y-ground—+ +— RCA/composite ground C-ground—+ Y—+ +— RCA/composite video C—||—+ 470pF
Note that going in the reverse direction is more difficult — using the adapter to convert composite video to S-video will result in a viewable picture, but the quality will be reduced due to the adapters inability to cleanly separate the “Y” and “C” signals.

Todd Efaw
Emerald Coast, FL

Found this on the web that will convert the S-video to HDMI with decent reviews. May be less hassle buying then building.

Steve Ghioto
Atlantic Beach, FL

The answer to your S-Video problem is one that is easy. Go to and look at these converters.

StarTech SVID2COMP 6-Inch S-Video to Composite Video Adapter Cable $2.06 each.

Electop 2 Pack S-Video Female to RCA Male Composite Video Adapter $6.59 (Prime) for a pair (very small device that I have used for years).

But your biggest worry is your S-Video machine. Start looking at the converters to put your tapes on your hard drive and a backup on a Thumb Drive for safe keeping, if your home movies are not replaceable.
Good Luck and a healthy New Year to you.

Dave Wreski
Conway, SC

Low-Cost 3D Printers Answered January 2018

I’ve been eyeing 3D printers for a while, and there are now basic units in the $100 range.

However, it appears that software can cost between free and several hundred dollars depending on features and capabilities. I like free. Is anyone using free software to create and print using one of these low-cost printers? If so, which software and printer are you using that you would recommend? Any caveats?

Brenda Spellman
Green Bay, WI


I am using Matter Control. By far this has been the easiest right out of the box. In most cases you do not need to go into settings, except to change resolution. When first installed you will need to enter type and print area of your printer. The only complaint I have ever had is there is no Emergency Stop. You can stop/pause the print, but in some cases you just have to kill the job. Good luck!

Todd Dragovich
Dothan, AL

I purchased my first 3d printer — the Tevo Tarantula — last February, and I have enjoyed every minute of it. The Tevo has a large support group on facebook (if you’re into that — I’m not) as well as on Thingiverse. I would also point you to Thingiverse to find dang near any model you could ever want — including a whole multitude of upgrades for the Tarantula. I myself have uploaded a few of my own modifications (my username there is ‘rebeltaz’).

The only drawback to low cost printers is that a lot of the controller boards are not able to properly handle the current required for a heated bed. You will want to get a MOSFET interface board — the cbd-3d being a popular model — to offload the current drain to a more suitable controller.

As for software, you are going to find diehard recruiters for all the available packages. Me personally, I prefer Sketchup for functional models, since I already had experience with that program. You will need to add a plugin called SolidInspector to ensure watertight (i.e. solid) models suitable for 3d printing. Another good free alternative I use is Blender. This program seems better suited for more organic models, although you can use it for functional designs.

If you lean towards “cloud based” software (I do not) there are various alternatives in that category as well — TinkerCAD being a popular one.

You will also need a slicer program whose job it is to generate the g-code understood by the printer from your 3d model. Simplify3D is a relative expensive example, but both Slic3r and Cura are excellent freely available alternatives. I personally prefer Cura due to it’s more user friendly interface.

Finally, (well, in closing anyway) I would also recommend that you think about getting a Raspberry Pi later on and load it with OctoPi. This will allow you to access and control your printer wirelessly from anywhere on your local network, or even around the world if you so desire.

If you ever have any questions or need any help/advice, I am always willing to help newcomers to this wonderful hobby. You can find me on Thingiverse. Hope to see you there!

Derek Tombrello
Shelby, AL

Chip Replacement January 2018

I have an old guitar “echo” pedal that is dead. After some troubleshooting, it appears the SAD1024 chip in the unit has given up the ghost and I have been unable to find a replacement chip.

Is there a substitute for it or a source for replacing it?

Mike Styles
Pennsauken, NJ

Solid-State Tube Replacement Answered December 2017

I’m refurbishing a tube-type Hallicrafters shortwave receiver. I was planning on using a solid-state plug-in replacement for the rectifier tube; mostly because I can’t find the tube.

I’ve been told that a solid-state rectifier could result in higher voltage, and may blow the filter capacitors and run the tubes at a non-linear part of their operational curves. Can someone confirm or explain if this is a good idea or not?

Matthew Stiefel
Steelville, MO


Vacuum tubes are resilient and can withstand over voltage better than solid state parts. Tube characteristics are dependent on the physical construction and don't change with voltage if the circuit is properly designed. If the radio has a power transformer, It certainly has a full wave rectifier. The 5Y3 has a tube drop of 60 volts so if it is replaced by a solid state rectifier, the DC output should be 60 volts higher. This is not significant when the nominal DC output is 300 volts. If you are worried about it, put a resistor in series.

Russell Kincaid
Milford, NH

The maximum voltage output from any rectifier is less than the peak voltage, so start from that end by measuring the RMS voltage of the AC power transformer output , and multiply that by 1.4 to get an average peak voltage. If it is less than the filter capacitor DC rating then you are safe to install any solid state rectifier that has a higher voltage and current rating. Anyway, replacing the filter capacitors is always a good idea due to aging.

Raymond Ramirez
Bayamon, PR

You might look at They have thousands of vacuum tubes.

David Goodsell
Apple Valley, CA

Tube guitar amps use solid state rectifiers all the time, check out Mesa Boogie. I suspect they are more efficient, and the supply voltage may increase a bit, but my guess is that it’s not so dramatic as to destroy everything, maybe 10-20% increase.

If you can disconnect the rest of the receiver you can always try it and measure the before and after supply voltage, see if it’s still in a safe range. Be careful doing that, of course, it’s a high voltage power supply.

Ralph Hipps

I think the filter capacitors are at a slight risk during the replacement procedure. But maybe they need replacement anyway.

First you need to know the existing rectifier output voltage, measured from a trial run or read from the instruction manual. Then you use silicon diodes to replace the tube rectifier. Re-measure the voltage and B+ current. Be careful these voltages can give you a memorable shock.

Then use Ohm’s law to calculate a resistor to put in series with the B+ line to return the B+ to near its original value. Don’t forget to calculate the power dropped by the resistor to make sure to get one of sufficient dissipation (watts) rating.

If you are uncomfortable with these steps, get a ham old-timer to help. The good side is you should improve reliability and save the power that went into the rectifier filament.

Chip Veres
Miami, FL

I would say it's a bad idea. Solid state rectifiers have a much lower internal resistance than tube rectifiers, resulting in significantly higher voltage. I'm surprised you can't find a replacement. What's the tube you need?

Steve Rohrer
Decatur, GA

A solid state replacement will have a lower forward voltage drop than a tube, typically less than a volt. So, the rest of your circuitry will see a few extra volts. This is not likely to be a problem, however I would have another concern.

Using a solid state rectifier means that it will begin rectifying immediately upon power up, the rest of the tubes will NOT yet be conducting and therefore the entire circuit will be drawing very little current. This means that your power supply caps etc. will probably be seeing close to PEAK voltage until the tubes begin to conduct and draw current.

When using a tube rectifier; it too comes up slowly as the rest of the tubes do, you do not get the PEAK voltage anywhere. I’d stick with the tube rectifier; eBay and lots of other on line suppliers can get you one.

Bob Morris
via Internet

There’s some good material on this subject at

I would also suggest that you look for your tube replacment at

By the way, the latter site also offers their T-SSR01 solid-state replacement for 5Y3, 5U4 and 5AR4 tube rectifier, but please note the cavaeat in the part description.

Peter A. Goodwin
Rockport, MA

In regards to your rectifier tube for the Hallicrafters Radio, I suggest you have a look at this website:

They have all kinds of older tubes, most are what is called New Old Stock. We could have provided more information as to use of a solid state rectifier replacement, but it would have helped to know the radio model number and the tube number you were replacing. Hope this helps you fix it.

Bruce Bubello

A solid state rectifier will have less than a voltage drop, a tube is higher depending upon the tube and the current level, etc. This will result in a somewhat higher voltage to the circuitry, but not an awful lot.

There is, however, another situation to consider. When you power up with all tubes, they all come on slowly. Tubes begin to conduct plate current as the rectifier begins to work, sort of a “soft” start. With a solid state rectifier, it will begin immediately upon application of power, while the tubes are not yet consuming any current as they warm up.

This means that since drain on rectified supply is quite low until the tubes come up to speed, the power supply caps will charge pretty much to peak levels, which may exceed their ratings, or certainly stress them more than if you were using a tube rectifier.

I’d suggest sticking with a tube rectifier. You should be able to get one via eBay, or many other on line sources.

Bob Morris
Pennellville, NY

Also, a solid state diode has no warm up period. It means that full power supply voltage instantly is present on a circuit that may not be ready for it yet. It can be done, but replacement of a tube with a solid state diode may need some additional precautions (a heavy resistor in line may be enough). I say may, since there are definitely sets which are capable of absorbing this difference.

Most Hallicrafters sets were not the most rugged out there, many without the safety of power transformers. I’d be careful. On the other hand, I find it hard to think you would not be able to find a suitable tube on FleaBay or some of several tube vendors on line (Try Antique Electronic Supply).

Bill van Dijk

You are correct in that there can be some issues to be considered when replacing a tube rectifier with a solid-state replacement. The primary issue is timing. A solid-state rectifier will provide the full B+ voltage almost immediately, while a tube rectifier needs to have the filament warm up, and the B+ voltage will come up much more slowly. Also, the other tubes which will use the B+ will need to warm up and be able to provide a suitable load for the B+. Without a load, the resistors in the supplies will not drop voltage as intended, and so the voltages at the plates of the consuming tubes will be much higher initially until those tubes warm up.

In addition, the tube rectifier has a “resistance” providing a voltage drop that the solid-state device will not. This means that the output of the power supply section will be at a higher voltage than that with a tube. The capacitors must be rated to handle the increased voltage, and the circuit will need to operate properly with the increased B+ voltage.

If the radio has not been operated in some time, it would be good to replace the filter capacitors at a minimum. Some solid-state replacements will include some internal resistors and thermistors (resistors that change value as they heat up) to approximate the characteristics of a tube rectifier. This type of replacement is usually best.

After installing the replacement, the B+ voltage should be checked against the normal expected voltage, and perhaps some of the resistance values in the power supply can be adjusted if needed.

George Kaczowka
York, ME

AMP Clamp Answered December 2017

Could someone explain in simple terms how an AMP clamp works? Does it have a transformer in it or Hall-effect sensors or similar?

Kevin Champion
Cleveland, OH


There are two major types of clamp on current meters. The AC type is a transformer; the wire in the clamp is the primary, the secondary is internal and is normally 1000 turns. If you are measuring 100 amps the secondary output will be 100 mA. The other type measures DC and could be calibrated for AC. The DC type uses a hall-effect or similar sensor. Amazon lists an AC/DC meter for $39.

Russell Kincaid
Milford, NH

Most clamp-on ammeters use a transformer located inside the body of the meter to step-up the measured current to a value which the unit can use.

Jeffrey Massey

I am no expert. But you are correct in your assumptions. I found this great article at Wikipedia on the subject:

Fernando Cordero

The common AC current meters use a transformer to measure the current. The “clamp” is the core for that transformer, and it is opened to go around the AC wire as it becomes the one-turn primary. There are simple components to scale the multi-turn secondary current down for the correct reading, either analog or digital. The digital part uses its own circuit to convert the measurement to its display.

A DC current meter cannot use the same transformer method, so it uses the Hall device that converts a magnetic field to a resistance, and the internal components convert that resistance to a reading, analog or digital.

Raymond Ramirez
Bayamon, PR

You are on the right track. The ones that measure AC use a pickup coil, and the ones that measure DC use a Hall effect sensor.

Chip Veres
Miami, FL

An AMP clamp is a transformer having a one-turn primary (the conductor carrying the current to be measured) and a multi-turn secondary (to feed the measuring instrument).

Being a transformer, it can only be used to measure alternating current. Be aware of its measurement rating, because too much current can saturate the clamp, resulting in inaccurate readings.

The clamp is rated for sinusoidal currents: measurements of non-sinusoidal current is best done with a true-RMS instrument connected to the clamp secondary winding.

Currents small relative to the capability of the clamp can be measured by passing the conductor through the clamp multiple times and dividing the measured value by the number of turns.

Peter A. Goodwin
Rockport, MA

AC amp clamps use a current transformer. The clamp is a laminated iron ring with a coil of wire wound around it. The alternating current in a powered AC line induces a magnetic flux in the clamp’s ring which the attached coil picks up via electromagnetic induction. That induced signal is fed to an amplifier who’s output is rectified and filtered and the resulting averaged (i.e. RMS) DC voltage is fed to the meter’s measuring circuit. The typical conversion is 1A AC = 1V DC on the meter.

DC clamps use Hall Effect sensors. As I understand them, the construction is similar to AC clamps, except an exciter coil is attached to the iron ring to pre-energize the ring with a specific frequency/voltage, which the Hall Effect circuitry sees as a zero level. DC current flowing through the clamp creates a magnetic field which alters that excite signal, which in turn changes the signal the Hall Effect sensor sees, which causes the Hall Effect detector circuit to output a voltage (positive or negative) proportionate to the current flowing in the measured conductor. Like the AC clamp, that detected signal is fed to an amplifier which outputs a DC level which is fed to the meter’s measuring circuit with a similar conversion scale (i.e. 1A DC = 1V DC on the meter).

As you’re aware, amp clamps are an EXTREMELY SAFE method of measuring high current (i.e. 10’s and 100’s of amps) in powered circuits because you don’t have to break any wires for making the measurement. HOWEVER, because their resolution is typically between 100 mA (0.1A) and 1A (due to the limitations of the magnetic detection circuitry), amp clamp’s use is limited to measuring power lines connected to large devices (i.e. pump motors) that naturally consume large amounts of current in operation.

Ken Simmons
Auburn, WA

VGA To LCD Answered December 2017

I would like to be able to drive a 4x20 line LCD with a VGA output from an old computer. Is there a simple interface to do this?

Evan Lee
Elizabethtown, IL


The short answer is no. VGA is an analog interface, LCDs use a digital interface. You would need a 3 channel ADC to start with, but even that’s not enough. It would be very difficult to extract alphanumeric characters for the LCD from that data. Plus the 4x20 LCD is so small compared to the VGA display that there’s no way it can hold all the information shown on VGA.

You would be FAR better off rigging up some kind of serial port interface to the PC with the VGA output and selectively sending whatever data is of interest to the LCD.

Ralph Hipps

In all honesty, I believe this would be a fruitless exercise. The interface would be far from simple, and the resulting image on the 4x20 LCD display would most likely not be recognizable from the original VGA image.

VGA has a resolution of 640 x 480 and the LCD display would be 100 x 28, assuming 5 x 7 matrix for each character. Keep in mind the LCD display consists of a 4 rows of 20 characters each. Each character is formed by a 5 x 7 dot matrix, and the characters are separated by approximately a pixel width and 0.2 pixel height. So you are going from an aspect ratio of 4:3 to 25:7 or 12.5: 3.5, a significant difference.

In addition the pixels in the VGA display can vary in color and brightness, where the LCD display is only black and white, and are not individually adjustable in brightness.

And finally, the electronic interfaces to drive each type of display are significantly different both in hardware and software, and would require a very complex interface to even come close.

Ira Wexler
Owings Mills, MD

VAX VMS Emulation Answered December 2017

Is there an emulator for a Digital Equipment Corporation’s VAX VMS 4.7 machine, either running on an SBC (Single Board Computer), or possibly an image that can run as a virtual machine in VMWare or something similar?

I want to create a four-node VAX cluster like the one I used to work on, and would love to see it sitting on my desk as a stack of Raspberry Pi boards.

Troy Thoele
Huntsville, TX


Ed Cernek
Phoenix, AZ

Consider looking at SimH. It has a network-capable VAX Simulator. It is available in source code form (say, if you wanted to run it under Linux on an SBC), and also as a windows executable (you would not have to have use a virtual machine). Visit

Jay Jaeger
Madison, WI

What you are looking for is SimH, a free VAX emulator that will run on the Raspberry Pi available at You can get VMS licenses and software at Check out this article: I think you will find everything you need.

Ronald Schubot
Kalamazoo, MI

I did a search for VAX VMS emulator and found the following: VAX/VMS on RaspberryPi at the RaspberryPi Forum -

Lance Corey
Santa Ana, CA

Troubleshooting Amp Problems Answered November 2017

I picked up a CrownDC150a power amp at a garage sale. When I tested it on my bench, one side is rather distorted but then clears up once the amp warms up, making it hard to trace. Is this a common problem with these amps? Any troubleshooting tips are appreciated.

Rich Wortman
Kankakee, IL


Crowns are generally great amps, very few issues. My guess is you have a cracked or weak solder joint someplace, perhaps in the power stage, that makes better contact once things get hot and expand. It might also be a bad cap somewhere, but that seems less likely to me.

Ralph Hipps

You mention the model is DC150A. Crown does not list that model but does have a D-150 and D-150AII listed.

Service info is here

The symptom you mention could be crossover distortion in the output stage due to an out of spec bias voltage (very low or non existent). As the amp warms up the bias voltage generator begins to function to some degree and the distortion becomes less. The bias voltage is designed to vary as the output stage temperature changes but should never fall to zero.

Connect an audio generator (1 KHz, variable 0 to about 2 V RMS sine wave) to the amps input and connect a suitable sized dummy load (8 ohm) resistor and oscilloscope to the output terminals. Power up the amp and observe the output sine wave for distortion at the zero crossing. Also look for a symmetrical sine wave amplitude. An offset step at the zero crossing is likely a bias problem.

A non symmetrical sine wave could be a power supply problem or output transistor problem. If the sine wave is clipped on the top (well before maximum output power is reached) but the zero crossing point seems ok then the bias may not be the issue. Rather, the power supply voltage could be too low or the distortion is being introduced at the input stage or possibly the VA (voltage amplifier) stage.

Erik von Seggern
Escondido, CA

One can often use a controlled burst of freeze spray to hit individual components once it has warmed up. Hitting resistors or capacitors, or even solder connections may reveal the culprit.

George Kaczowka
York, ME

Pump Torque Problem Answered November 2017

I have a submersible well water pump motor that is suspended down-hole on about 100 ft of plastic pipe. Whenever the pump starts, it torques the pipe and I fear it may eventually cause leaks/breaks. Is there a way to soft-start this motor? It's a three-phase 3HP unit.

Mario Rivera
Buda, TX


Motor soft-starters have become so common that there is even a Wikipedia article about them. Just look in the catalog of the nearest industrial electric supply and buy the size you need.

Chip Veres
Miami, FL

Yes, 3 phase motors can be soft started and such start controllers are readily available.
Two companies that offer them:

There are many distributors that sell soft start controllers online, just do a search for “3 phase soft start” in your favorite search engine.

Erik von Seggern
Escondido, CA

The solution here is a devise called a torque arrestor. Google "well pump torque arrestor" for more information.

Bill van Dijk

I recommend a commercial soft-start 3-phase motor controller such as one in the ABB Softstarter family. You can look through a catalog and technical information here: [url=][/url]. DO NOT try to build one yourself. Experimenting with three-phase line power can lead to problems that cost more than a commercial controller. Another suggestion: Connect a stainless-steel line to your pump to make it easy to hoist. If you try to lift it via the pipe or the electrical cord the pump could separate and leave you in a hole, literally. (I have no connection with ABB.)

Jon Titus
Herriman, UT

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