Tuesday, October 4, 2016

SMD Beak

An SMD Beak is used to hold a surface mount device in place as you solder it, leaving both of your hands free to hold your iron and solder. The idea came from Vpapanik. The beak must be sharp in order to grip the device you are soldering, and have enough mass so as not to be moved easily. My design is adjustable and folds flat after use.

3mm mild steel was cut to 10mm strips.
Base is tack welded in place.
Holes drilled to fasten the arm with a wing nut.
Beak arm sharpened to a point.

Tools for the Electronics Tradesman

As with any trade, purchasing the right tools for a job in the electronics industry is essential. Here I have listed the tools that I believe are necessary for an apprentice starting work in the electronics industry.

The tools for the electronics tradesman will vary between disciplines. The majority of work I do is amplifiers repairs, with some fabrication of enclosures and the like. An embedded design engineer would have tools specific to that area, but many of the tools listed here would still be relevant.

You will not need to purchase all of these tools at once, rather you will add each tool as it is needed over the course of an apprenticeship. A soldering station will likely be supplied by your employer. A nice pair of side cutters and a set of insulated screwdrivers are essential. As soon as you can afford it, purchase a large rolling tool chest with sliding drawers.

As you accumulate more tools, careful attention should be paid to organisation. It saves a lot of time if tools used for particular tasks are grouped together.

Toolbox Top: large frequently used tools.
Probes + Clip leads for multimeters.
Two Multimeters; Measure voltage and current at the same time or monitor dual supply rails.
Discharge resistor 25W 40R, used to discharge supply capacitors before working on a PCB.
Helping hands for holding connectors when terminating.
SMD beak; a small pointed arm that holds SMDs in place while soldering.
Small socket set, Hacksaw and safety glasses.
Stanley security driver set.
IR thermometer; check heatsink thermal protect circuits are working correctly.
Small top drawers: Small tools.
Combination spanner set.
Punches, deburring tool.
Small screwdriver set.
Screwdriver bits.
IDC removal tool, Tweezers, Prying tools.
Torch, Otoscope for inspecting small devices.

Second drawer: Most frequently used tools.
Stubby screwdrivers.
Insulated screwdriver set.
Bullnose pliers, Wire strippers, Long nose pliers.
Screw removal pliers.
Small long nose pliers.
Side cutters *2; keep an old pair for cutting steel wire.
Third drawer: Measuring tools.
Vernier calipers.
Square and 12" Ruler.
Caliper set.
Screwdriver extension bit.
Microphone for testing audio inputs.
Fourth drawer: Fabrication tools.
Files + wire brush or file card.
Tin snips.
Large shifter.
Ball peen hammer.
25mm scraper.
Drill bits.
Fifth drawer: Infrequently used tools.
Analog multimeter.
Cable tester.
Wooden mallet.
Large socket set.
Stanley knife blades.
Hacksaw blades.
Above workbench: Test equipment
Curve tracer for power-off in-circuit testing.
Mains current limiter.
Speaker impedance tester.
Dual rail lab supply.
Above workbench: 
Device programmers, Connectors, Test CDs.
Scope probes and Differential Scope Probes
Alligator leads.
Solder, solder wick, solder sucker.
Containers for holding screws.
A good quality temperature controlled soldering iron is essential. I use the JBC CD2BB, as it has easily removable tips and heats up very quickly.

A PC modified with a power button on the rear of the case allows easy access to all ports.
I also find it necessary to wear a tool belt, so that I always have these items on hand:
Markers and pen.
Small note pad.
Tape measure (for measuring freight boxes).
Folding stanley knife.

A lot can be learned from tools of other trades; Tom from OX Tools produced a great video on tools for the apprentice machinist.

Thursday, April 21, 2016

Speaker Impedance Measurement

Comparing two speakers side by side is the best way diagnose a fault, but it's likely you will not always be able to do this. Speaker impedance measurement software is a useful tool for recording the characteristics of a woofer or speaker box for future comparison.

Different from a frequency response graph, an impedance graph indicates load on the amplifier vs frequency. 
Software produces a sine sweep (or pink noise). This signal is taken from the PC output and amplified. Current (voltage across a known resistance) and voltage of the output is measured, and fed back into the PC. Software then calculates Impedance and compares it to the source signal.

Here is a comparison of a speaker before (gray) and after repairing a fault in the crossover (black). Note the significant difference in impedance at 4kHz.

LIMP by Artalabs is the software used in this project.
It has great calibration tools, and a thorough tutorial.

My example; Speakon connects to woofer  / speaker box to be tested, 3.5mm TRS are used for simple interfacing with PC.

Constructed from several other PCB projects (TDA7294 50W amplifier, and Differential Scope Probe circuits), with point to point wired linear supply for simplicity. These were just things I had on hand at the time of construction.
Output of the amplifier passes through a 1R 10W sense resistor. 
Sense outputs are voltage dividers, and PC mic level inputs are buffered.

Wednesday, April 13, 2016

Octopus Tester / Component Curve Tracer

The Octopus Tester  is an accessory for an oscilloscope which permits a method of power-off, in-circuit testing (ie; it is used to test damaged equipment without powering it up, and without removing components). Touching the probes to a device will produce a voltage-vs-current characteristic diagram on the scope screen, so a component under test can be quickly compared to a known working component simply by comparison of the image on screen.
One channel of a stereo amplifier is not working. You can use the Octopus Tester to compare suspect transistors on the blown channel to the same transistors on the working channel. Each time you find a damaged device, remove it, and test all components to which it was immediately connected (Resistors should be checked with a multimeter).

The mains transformer produces a 12V 50Hz sine wave with a floating output. This signal sweeps the Device Under Test, and an image is produced on an oscilloscope in X-Y mode. The voltage across the DUT is shown on the X axis, and the current through the DUT (measured as voltage drop across a known resistance) is shown on the Y axis.

Shown here is a diode. Voltage across the diode increases until the diode turns on at 0.6V. Current through the diode is then at maximum.

When testing components in circuit, there will typically be resistive, and reactive elements to the image on screen. You can first familiarise yourself by testing components out of circuit.

After using the device for a short time, you will begin to associate the images on screen with the devices you are testing, and the surrounding circuitry. It then becomes simple to quickly troubleshoot faulty circuits, based on knowledge of the devices you are probing.

The octopus circuit is housed in a small plastic enclosure. BNC connect to scope X and Y inputs, banana sockets are for regular multimeter probes.
Due to the simplicity of the circuit, point to point wiring is the quickest method of construction. Be sure to ground your unit correctly.

Setting up the octupus tester.
Ensure that your scope is set to DC coupling, and that the circuit you are testing has no other path to ground, ie; disconnect all power / audio / data cables to the unit.

It's also necessary to discharge any large caps before testing (You cannot damage anything, but your trace will disappear from the screen).

Some scopes come with a curve tracing function build in. Set-up and use of the component test function of the Hung Chang scope is the same as most other brands I have used.
I have two scopes on my test bench, the Hung Chang pictured is used solely for this function, such is the frequency of it's use.

This is a schematic of the Hung-Chang 3502C scope front end, from which I copied the design of my device. The component testing circuit is highlighted.

Thursday, April 7, 2016

Mains Current Limiting Bulbs

When working on a mains electricity powered device, you will need to safely power up the unit by limiting mains current.

This is achieved here by placing a non linear load in series with the unit. The resistance of the incandescent globes increases as the current through them increases, so under normal conditions (where the device under test does not draw a large current) the globes will not light up.

Under a fault condition, the device under test will draw a large current, and the globes will light up, dropping voltage to the device under test, preventing further damage to the device.

Maximum current (at short circuit) can be calculated:

For one 100W bulb
I = P/V = 100W/240V
I = 0.4167A

The unit can be constructed from several batten fixtures.
4* 100W incandescent globes is enough to provide idle current for a very large amplifier.
If a lower current limit is required, simply unscrew a few globes.

Current limiting bulbs installed on test bench.

Example of fault in device under test
(here provided by a short circuit).
Alternatively, this device can be constructed as a rack unit.
Switches are wired such that each bulb can be taken out of circuit in order to provide the correct current limit.
A meter displays output voltage and current.
Bulbs are mounted in lamp-stand fittings.

Please note, this method cannot be employed on some "universal" switch mode power supplies, as the supply may enter a mode designed for a different voltage mains supply (ie; a 240V supply might think it's receiving 110V). I have found this only rarely to be problematic.

Monday, February 8, 2016

Differential Scope Probes

A differential scope probe is used when a signal cannot be analysed relative to ground. An example of this is the output of a bridged amplifier; neither of the output terminals can be connected to the scope ground, and reading each separately may not be an accurate representation of the signal.

This scope probe is DC coupled, with an input impedance over 1M, and has a relatively low noise floor. A single quad op-amp chip provides input buffers, differential, and has provision to split a single floating supply for dual rail operation. My example is a x100 probe using a 12V plug pack. Input voltage divider resistor values and rail voltages can be changed to suit your needs.


Thursday, December 11, 2014

Soundcard Scope Interface

Soundcard Interface
A computer soundcard can be used as an oscilloscope for testing frequencies below 20kHz. This circuit is a simple protection and control interface, with BNC connectors for oscilloscope probes.

This device is useful for generating a sine tone into audio equipment, and testing the output. The amplitude control knob of the input circuit allows testing of line level audio (1V), and up to speaker level audio (100V).

This circuit is designed to be used on a work site where a full size scope is impractical. It contains only passive components (therefore has a very low input impedance), and as such should be used only for rough troubleshooting.

Circuit Diagram
The protection circuit ensures voltage at the soundcard input is clipped at 1.2Vp-p.
Veroboard layout
Circuit will mount nicely on the back of a 16mm potentiometer.
Probe Input Circuit
Mounted on a small piece of veroboard.

Completed Unit
BNC connectors are used to connect test probes.
6.5mm socket for use with audio test leads.
Banana socket to connect ground lead.

Visual Analyser (Sillanumsoft) is the software I have chosen to use in this project.
It is available for free download here: http://www.sillanumsoft.org/download.htm

Visual Analyser uses your soundcard to display oscilloscope traces, as well as frequency spectrum. However, the soundcard must first be set up correctly.

Right click on your volume control icon in the task bar.
Select "Recording Devices".
Choose the soundcard input
Select "Properties".
Select the "Custom" tab.
Un-check AGC (automated gain control).
Select "Levels" tab.
Increase microphone to 100%.
Visual Analyser is now ready to use.


Always connect the ground clip of the device to the chassis of the equipment to be tested. Using the ground clip as a probe will short live circuits to ground through your test device and PC (This goes for any oscilloscope that is not isolated from ground).