MC1377 Measurements on Mini Apple IIe Prototype video board

Out of context screenshots are out of context

On the Mini Apple IIe project, I am testing the composite/sync amplifier, which is an MC1377. The composite output looks great, until connecting the output cable to a receiver. It outputs about 2.6 Vpp until loaded with 75 ohms, then it drops to about 400 mVpp. We have replaced most of the passives on the output and have tried 3 different MC1377s. The measurements below are from a known good MC1377 removed from a working Apple IIgs.

Here is an MC1377 datasheet mirror link. The Figure references below match Page 8 of that datasheet. The biggest suspects are the Luma signals.

Not captured, but tested, none of the input signals change amplitude when the output (pin 9) is loaded with a 75 ohm load.

Problem: Update, SOLVED

Update: Turns out, R9 in the schematic above wasn’t fully soldered. After doing that, the computer now boots… at least, until it locks up.

These two screenshots are the same point, RCA Out Header in the schematic. On the left is the output when the node is left open. It is about 2.6Vpp with minimal DC offset. However, when the signal is terminated with a 75 ohm resistor (or a receiver circuit) it drops to about 300 mVpp!

00c-side-by-side uncropped

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DMM Basics Video Tutorial | Workbench Wednesdays

element14 Presents

A multimeter is the go-to-tool for electronics measurements. Knowing how to use it to measure voltage, current, diodes, and transistors can save time (and frustration) while troubleshooting. In this video, James explains how the multimeter works, so that you can understand them better. After watching, you will gain a solid DMM basics understanding.

Behind the scenes of wbw’s DMM Basics

The first Workbench Wednesdays episode covered a pen-style DMM. Back when we were first developing the show, the intent was to be more of a review show and less of a tutorial show. That episode became a mix of both. So we decided to do a straight-up tutorial on DMM basics. Measuring voltage usually has not tripped up too many people, especially on an auto-ranging meter. The main thing to remember with current is to move the red lead back to the voltage input immediately after making a measurement. I touched on continuity briefly but hope to come back and cover that more in the future.

After watching this episode, I would recommend moving to the bench power supply tutorial. After a DMM, the bench supply is one of the most used tools.

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4-Wire Resistance Tutorial | Workbench Wednesdays

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Bench DMMs have an extra set of banana jacks called “sense.” Known as a Kelvin or 4-wire resistance measurements, these inputs accurately measure small resistors. Like, milliohms small. This video shows how to make a 4-wire measurement, prove when it is accurate, and alternatives to 4-wire.  See below for an explanation of the alternative method shown.

Behind the scenes

Doing yet another DMM episode was a tough call for me. However, I am working on a project that requires me to characterize both a 1 ohm and a 100 milliohm resistor. The element14 community was kind enough to send me an MP720028 bench DMM. As shown in the picture, it has an extra set of banana jacks called “4-wire sense.” These connections make 4-wire resistance measurements. In this video, I show the same resistor measured with the traditional 2-wire and advanced 4-wire configuration. (Spoiler Alert! The 2-wire measurement was almost twice as big as the 4-wire!)

Alternative 4-Wire Resistance Measurement

In the video, I show one optional method of using a multimeter with 4-wire resistance capability. The trade-off is that it requires at least two multimeters. Since the 4-wire measurement is making two measurements at the same time: voltage and current, the alternative method does the same thing. A bench power supply, ideally with current limiting, applies a voltage to the resistor under test (RUT). One multimeter is used to measure the current through the resistor and, another measure the voltage across it.

In a pinch, you could use the bench supply’s ammeter for the current, however, its precision is probably much less than the average DMM. Once the voltage and current are known, a little bit of ohm’s law determines the RUT’s resistance!

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Digilent Analog Discovery 2 Review | Workbench Wednesdays

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The Analog Discovery 2 combines all the equipment found on a typical electronics workbench into one small package. It packs an oscilloscope, logic analyzer, power supplies, spectrum analyzer, and so much more. As impressive as the hardware is, the Analog Discovery 2’s software, called Waveforms, is fantastic as well. You can configure it for any measurement situation, and it has extensive scripting capability. See if you should be considering adding the Analog Discovery 2 to your electronics workbench.

My day job causes me to travel on a regular basis. Sometimes that means taking my circuits with me. I picked up an Analog Discovery 2 so that I could take test equipment with me. It’s a small enough box that fits nicely into my bag. The one significant trade-off is that it requires a PC to be functional. However, for travel, that works out great.

The key functions the Analog Discovery 2 provides are:

  • 2-Channel oscilloscope
  • 2-Channel function generator
  • Voltmeter (not DMM!)
  • 2-Channel Power Supply w/ negative voltage
  • 16 Digital Channels for Logic, Protocol, and General Purpose I/O
  • Network Analyzer (Bode Plots!)
  • Spectrum Analyzer
  • Impedance Analyzer

There are probably other functions, but that covers the major ones. Years ago, I reviewed the Virtual Bench from National Instruments. While the Virtual Bench has better specs than the Analog Discovery 2, the difference in price points is staggering. You can get the AD2 for $200-300.

Check out how some of the features work and my thoughts on this device in this Workbench Wednesdays review.

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Function Generator Basics | Workbench Wednesdays

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Generating signals with a waveform, function, or arbitrary generator lets you test all kinds of circuits. Learn how to get a function generator to output a signal, the 3 types of waveforms you can create, and which controls matter. James, the Bald Engineer, explains the difference between analog and digital generators.

The video introduction is my first attempt at doing an LGR style “Tech Tales” story. It is short with only a few images. However, it is something I would like to do more of in the future. My professional career started at Agilent right when they split off from Hewlett Packard. Most of my co-workers had been there while Bill and Dave still worked at the company. Even though the computer division took the name, Agilent was HP’s core: test and measurement.

Someday I hope to see an actual 200A. Their place in history has become legendary. Some people say that Disney’s Fantasia could not be produced without it. However, Bill (or Dave) set the record straight. Disney’s engineers could have used another piece of equipment, however, they did select the 200A.

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Logic Analyzer Basics | Workbench Wednesdays

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Logic analyzers capture digital signals and then display a waveform or list. Serial busses like I2C, SPI, or UART (Serial) can be decoded or triggered on when there are problems in your circuit. In this video, learn the basic controls you need to use (almost) any logic analyzer. You’ll learn how to set up a simple trigger, make measurements, and set things like sample rate or memory depth.

When I first graduated from college, I started as an Application Engineer for Agilent Technologies. Based in Austin, I supported computer companies like Dell, Compaq, HP, IBM, and Tandem. I helped their engineers set up million-dollar logic analyzer configurations to measure various PC busses.

I did embellish a tad during the introduction. Shown in the video is an HP 16500. While I did occasionally help program a trigger on these analyzers, my career really started with its successor, the HP 16700. Both were amazing analyzers for their time. It is mind-boggling to think that a small $150 USB-based device has very similar measurement capabilities to those old beasts.

Previously, I made a written Logic Analyzer introduction tutorial and benchmarked digitalWrite() with a Salee Logic 8. Contrary to what people have said, the LA2016 featured in this video does NOT use Salee’s excellent Logic software. However, the user interface is clearly copied from it.

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Spectrum Analyzer Basics | Workbench Wednesdays

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A spectrum analyzer displays signals in the frequency domain. To use one, you only need to know about four controls: reference level, center frequency, and resolution bandwidth. In this episode, see how to use a spectrum analyzer and determine the transmitting frequency of a device.

The example device is my microphone transmitter. It operates around 500 MHz with FM modulation. You’ll see how I step through the spectrum analyzer controls to find the exact frequency. At the end of the episode, I show some advanced measurements you can do with a modern spectrum analyzer. My favorite one is the demodulation. (more…)

Electronic Load Basics | Workbench Wednesdays

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During the Bench Power Supply basics video, I made use of my electronic load. An electronic load can sink current from power sources such as power supplies or batteries. Loads are useful to test a power supply’s design margin or verify a battery’s capacity. See how three different instrument options from ultra low-end to midrange to high-end compare. After this element14 Presents Workbench Wednesday video, you will understand how to use modes on an electronic load like constant current and constant resistance to make different measurements.

In the video, I compare three different types of tools: a $50 battery tester from eBay, BK Precision 8540 Electronic Load, and a high-end Rohde & Schwarz NGM202. They are all capable of acting as a constant current sink. In other words, you program the current and let the supply under test run. In the case of batteries, this mode is useful to test battery life. The 8540 and NGM202 both feature other capabilities like constant resistance which, as you can imagine, acts as an electronic resistor. The cool thing about using an electronic load as a power resistor is that some of them are capable of sinking up to 10s of amps.

After submitting the video to the producers, I realized I focused very heavily on batteries. More often, you use an electronic load to test power supply boards or modules. I wish I had shown those more, but it didn’t seem like it was missing until the end. So maybe in the future I can convert tools for measuring DC/DC power supplies? However, I did cover one topic significant to me. I show just how bad 9V batteries are, a topic which I like to come back to often. So I made sure to show off just how bad they work when driven past 100 mA. (It is terrible!)

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Bench Power Supply Basics | Workbench Wednesdays

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A bench power supply makes powering circuits easy and safe. Learn how to adjust basic controls like voltage. Finally, see how “current limiting” works (and why you should use it.) See how you can use built-in series or parallel tracking to increase a bench power supply’s voltage or current output. Last, if you are in the market for a power supply, do not forget to add some leads like mini-grabbers, alligator clips, and banana plugs.

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DCA Pro Transistor Tester Review | Workbench Wednesdays

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When it comes to transistors, there are only so many things a multimeter can measure. The DCA Pro from PEAK Electronics makes short work of testing parts like a transistor. This small device can determine pinout, component type, and essential parameters in a matter of seconds. Not only that, but it can be connected to a (Windows) PC and draw parameter curves.


Check out the video review to see how the device and software works. Then head over to the element14 page where you can download a zip file full of example parts I measured for you. Use the free DCA Pro software to open them.

You might also want to check out this MOSFET Curves post, which complements this video tutorial. Another resource you might find helpful on semiconductors, or transistors, is this post on MOSFET Myths.

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