# Test Equipment



## DrGeoff (Dec 27, 2007)

I often get asked about which bits of test equipment should the new DIY constructor invest in.
It's always worth investing in the best you can afford, and there are some good quality items that are quite reasonably priced.

1. DMM (Digital Multi Meter)
Forget the cheap ones as they will not give a reading that you can be confident in, which is very important, particularly when calibrating. Go for good brands like Fluke, who make some really good meters for quite reasonable prices. Don't necessarily go for the all-singing, all-dancing meter with LC ranges, transistor testing etc. You will want accurate resistance and voltage measurements. In many cases you may need 2 meters (one to measure current and one to measure voltage), however start with a good one if your budget is limited. If you can get one that will measure dBu for audio frequencies then this will be very useful for testing and calibration.

2. DSO (Digital Storage Oscilloscope)
Digital scopes have really come down in price recently. I found a Rigol DSO (50MHz) available out of China for about $500 landed. It was recently discovered that with a serial cable and a few commands this 50MHz scope can be turned into their 100MHz version (http://www.eevblog.com/2010/03/31/e...l-ds1052e-oscilloscope-into-a-100mhz-ds1102e/). A DSO is handy for measuring things like compressor attack and release times, power supply overshoot etc.

3. Analogue oscilloscope
These can be found for quite reasonable prices second-hand these days. A 20MHz dual-trace is a good starting point (I still have my old Trio scope from around 1980). Look to see that the input amplifiers are still working correctly and that the beam intensity is still OK.

4. Oscillator
Some people use sound cards and DAW software to create audio tones, but this can be inconvenient at times so a reasonable since wave oscillator is a good investment. You can usually find kits to build your own Wein Bridge oscillator (which gives a very low distortion since wave) as a starting point. Look out for second-hand HP and such like oscillators as they can also be had for a bargain these days and are built to last for decades.
Of course if you are wanting to conduct frequency response tests (sweep) for equipment you can use the Room EQ Wizard software and push the signal through your equipment (rather than speaker and microphone).

5. Bench power supply
A good quality linear adjustable bench power supply is useful for testing and troubleshooting. These can be found almost anywhere now and usually have a voltage range of 0-30V up to 2A or so output. If you are building circuits that require dual PSU rails (+/-18V etc) then you'll need either two supples or a dual supply with tracking (whcih just means that both the + and - output can be tracked together with a single knob).

6. Capacitor ESR meter
Something for those who are re-capping equipment or servicing faulty power supplies. Measuring capacitance of electrolytic caps is not good enough to see if they are good or bad, you need to measure the 'equivalent series resistance (ESR)' of the capacitor. As electo's die this value gets higher and higher, and the cap can no longer store the charge it used to. I'd recommend Bob Parker's ESR meter kit if you can find it. It does a brilliant job of identifying bad caps and is reasonably priced and easy to build as a DIY piece of test gear (http://members.ozemail.com.au/~bobpar/esrmeter.htm).

7. Micro Current Adaptor
Something useful as an add-on gadget for your DMM if you need to measure very low currents (mA, uA,nA) as all DMM's have a quite high burden voltage across their shunts and the accuracy of the current measurements is not too good. These adaptors are active current amplifiers with very little burden voltage and are used in conjunction with your DMM voltage range (which is usually very accurate).

8. Audio millivoltmeter
Another DIY project that can be very useful. If your DMM frequency response isn't that good, or it's not sensitive enough for those low voltage audio signals, try building your own audio millivoltmeter. Using a standard 100uA DC analogue voltmeter and a handful of components I built one that is very sensitive (down to -50dB/3mV FSD).

There's probably a few things I've forgotten, which is why I started this thread. 
Feel free to add and comment!


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## fractile (Mar 15, 2009)

What do you think about an Impedance Bridge for measuring input and output impedances for various types of equipment?


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## DrGeoff (Dec 27, 2007)

fractile said:


> What do you think about an Impedance Bridge for measuring input and output impedances for various types of equipment?


Can't say I've ever found it necessary. I use a scope and signal generator to do this, or calculate it from the circuit.


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## fractile (Mar 15, 2009)

I picked up some classic standard test eqpt for about $60 or less: a BK 2120 20MHz analog 'scope and a Wavetek model 20 waveform generator. Also in that group is an EG&G [PARC} model 113 preamp; it's a scientific instrument: gain 10k; LF rolloff DC- 1kHz, HF rolloff 3-300kHz; 2 inputs with gain control into 1 output.


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## fractile (Mar 15, 2009)

fractile said:


> I picked up some classic standard test eqpt for about $60 or less: a BK 2120 20MHz analog 'scope and a Wavetek model 20 waveform generator. Also in that group is an EG&G [PARC} model 113 preamp; it's a scientific instrument: gain 10k; LF rolloff DC- 1kHz, HF rolloff 3-300kHz; 2 inputs with gain control into 1 output.


There are BNC to RCA and RCA to unbalanced TS phone jack adapters for this. I got a good deal on RCA patchcords of all length; Tascam.


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## trifidmaster (Nov 18, 2006)

gswan said:


> 3. Analogue oscilloscope
> These can be found for quite reasonable prices second-hand these days. A 20MHz dual-trace is a good starting point (I still have my old Trio scope from around 1980). Look to see that the input amplifiers are still working correctly and that the beam intensity is still OK.


I like to chime in. For audio work analog oscilloscope is still *very valuable*.
Digital scope will not show you what analog can, unless you buy a very high end digital scope.
And one can find rather cheap analog scopes on ebay, for example: Tektronix 561 or similar.

Stefan.


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## KYBOSH (Sep 16, 2011)

Just ordered the The Blue ESR/Low Ohms Meter kit and will solder it up myself.
This should be fun and good practice!


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## KYBOSH (Sep 16, 2011)

I just finished chatting with a guy about ESR and after mentioning that I had the Blue ESR he sent the following response. 

I'd like to get feedback from this forum about what he said:



> The BlueESR is more of a hobbiest tinker toy project than a reliable ESR meter to be used daily on a tech bench. It has been around for nearly 2 decades in one form or another. It lacks any sort of auto calibration and only tests ESR at 1Khz. That is a BIG problem as testing at only one reference frequency will miss about 80-90% of the true problem caps where fringe ESR is an issue. Also unlike more expensive ESR meters that recalibrate themselves at every measurement the BlueESR relies on a reference chart to calculate a derrivation constant to get the real ESR from the reference value displayed on the LED screen. Most who buy/build the BlueESR miss that point from the manual in their haste to start using the BlueESR. You can calibrate the display of the BlueESR to provide a reliable refernce at 1Khz but it is a PITA and is only temporary as it must be occasionaly checked as the interal reference components age. In addition the BlueESR only measures ESR. For only a few bucks more you can get a fully assembled Peak ESR70 which unlike the BlueESR has a microprocessor controlled programmable crystal that automatically provides high speed testing everytime at 1hz to 100Khz in 10hz increments. That's 10K individual measurements in only a few seconds! Plus unlike the BluESR the ESR70 also concurrently measures capacitance too!. I have used both and the BlueESR is not even in the same league as the ESR70. I personally have seen a "calibrated" BlueESR give readings off more than 1 ohm in ESR when compared to the ESR70 and our HP4278A reading the same capacitor. The ESR70 and HP4278A are usually with a couple of hundredths of ohms on readings. With the potential of a > 1 ohm ESR read error what is the point in measuring ESR at all?


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## planetnine (Mar 3, 2009)

I'd recommend getting a cable tester.

As an example, the Behringer CT-100 tester will check minijack, jack, 5-pin-DIN, RCA and XLR, and any converter leads between the various types, including an intermittent connection test. Saves you getting the DMM out when you're in the middle of something, and easier too.


I use a function generator for my test signals. I can generate arbitrary signals with it (like sin with a amplitude marker) to test signal polarity, phase or audio delay path, as well as the usual sin, triangle and square wavs.

It also can generate audio sin sweeps -great for checking crossover frequencies, response of sealed-front processors, etc -ten oscilloscope graticules for the ten octaves of 20Hz to 20kHz...


>


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## Starliner (Jan 9, 2010)

Two things I have found useful (especially working on older gear) are an isolation transformer and a variac. The variac has been useful when testing older equipment with questionable power supply electrolytic capacitors. You can slowly increase the voltage with the variac and this will often prevent a sudden short in an older electrolytic, which can happen when full line voltage is suddenly applied. Of course, if the caps are really old, they should be replaced (due to electrolyte drying out over time), but this will allow some further testing first. The isolation transformer comes in handy if you have to test a piece of gear with one side of the AC line grounded to the chassis. Note that a variac is not an isolation transformer... 

Starliner


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## lcaillo (May 2, 2006)

With all of the repair shops going out of business there are lots of old scopes and meters available pretty cheap these days. I personally have three analog scopes if anyone needs one. The problem is the cost of shipping, but I would virtually give away a couple of them.


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## hyghwayman (Jan 15, 2008)

lcaillo said:


> I personally have three analog scopes if anyone needs one. The problem is the cost of shipping, but I would virtually give away a couple of them.


^^^^^ +1 ^^^^^


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## Starliner (Jan 9, 2010)

I am interested in getting an analog scope myself. I lost a dual-channel HP with a fresh calibration in a fire in late 2011, along with several other pieces of test gear, none of which I have replaced yet. 

What brand / models do you have?

Also, I am moving to FL soon, so it might be possible to meet near Gainesville.

Starliner


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