Nuts and Volts - February 2018

February 2018 61

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 okay, 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 Escondido, CA

#3 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 CA

#12171 - December 2017]

VAX VMS Emulation

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.

#1 I did a search for VAX VMS emulator and found the following: VAX/VMS on RaspberryPi at the RaspberryPi Forum - https://www. raspberrypi.org/forums/viewtopic. php?t=7552&p=93217.

Lance Corey Santa Ana, CA

#2 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 use a virtual machine). Visit http:// simh.trailing-edge.com/.

Jay Jaeger Madison, WI

#3 What you are looking for is SimH: a free VAX emulator that will run on the Raspberry Pi available at http://simh.trailing-edge.com. You can get VMS licenses and software at www.openvmshobbyist.com. Check out this article: https://www.rs-online.com/designspark/a-raspberry-pi-vax-cluster. I think you will find everything you need.

Ronald Schubot Kalamazoo, MI

[#12172 - December 2017] AMP Clamp

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

#1 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 whose output is rectified and filtered; 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., 10s and 100s of amps) in powered circuits because you don’t have to break any wires for making the measurement. HOWEVER, because their resolution