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Author Topic: Useful test equipment for homebrewing  (Read 5626 times)

Posts: 6

« on: May 20, 2001, 08:31:08 AM »

Do you have any suggestions for a beginning builder like myself on the kinds of test equipment I will need?  I would like to start out with some kit building and eventually work up to more complex projects.  What would be the "bare minimum", what would be some "nice additions", and what would you have
in your "dream shop"?

Posts: 21764

« Reply #1 on: May 21, 2001, 11:13:41 AM »

Depends on the kinds of kits you might be building.  I have more of a "homebrew it from scratch" kind of shop which provides sufficient equipment to build almost any kind of amateur gear.

It would be pointless to provide you (or anyone) with makes and model number of equipment to look for, since the best deals in test equipment are used/surplus/refurbished and you never know what you'll find.  Since I build everything from teeny-weeny accessories to husky kilowatt amplifiers, I've assembled test gear over a period of several years and occasionally still add something.

I'd recommend anybody serious about building have at least the following items, any of which can be purchased inexpensively surplus.  One or two of the items, asterisked, are so inexpensive they can be purchased brand new without making much of a dent in the piggybank:

-VOM, portable, 20KOhms/V min sensitivity, multirange* (If possible, a used/surplus Simpson 260 or equivalent is a great deal, and a better deal than a "brand new" $39 VOM from Radio Shack -- reason is, it has higher current and higher voltage measurement capability...typically 5000Vdc and 10Adc, that won't be found on the cheapie meters.)

-Frequency counter, portable, upper frequency limit of at least as high a frequency you might be working with (typically 500-600 MHz)*

-VTVM or other high-impedance input voltmeter (most are benchtop units, AC powered, available used/surplus for $30-$50).  This is often a vital instrument in addition to a VOM, because a VTVM won't load down higher impedance circuits and are thousands of times "more sensitive" than VOMs.

-Signal generator, laboratory-quality, for low-frequency work (up to 500 kHz) such as a used/surplus HP 200CD or equivalent.  Unbelievably valuable, and used/surplus prices run under $100 quite often.

-Signal generator, laboratory-quality, for RF work (10 MHz - 480 MHz or so), such as used/surplus HP 608C, D, E or F series or equivalent.  This is a large and heavy (65 lbs) instrument and is AC powered, but again, incredibly valuable and used/surplus prices are astonishgly low...I've seen them, fully functional and recently calibrated, for $200.  If not damaged, this instrument has calibrated output of 1V down to 0.1 microvolts and can be trusted, for testing receiver sensitivity and other functions impossible to test with lesser grade instruments.

-Oscilloscope, laboratory-quality, good to at least 200 MHz.  This is very common nowadays and they are no longer expensive.  New "hobby grade" 200-300 MHz scopes can be found for <$500, but I prefer used/surplus Tektronix stuff, often found for $100 or so in someone's garage, and typically better grade instruments than the inexpensive new ones.

-RF Millivoltmeter with 50 Ohm adapter as well as high impedance test point probe, such as Boonton Electronics (dozens of models).  Extremely handy when you're measuring RF circuits and need an accurate indication of voltage or power (the Boonton instruments will provide either one, depending on the probe used) down in the small-signal range, too weak to measure with a passive instrument like a Wattmeter.  Used/surplus, see them all the time for less than $100, and worth their weight in Unicorn dung.  A good bench scope can provide some of the same functionality, but the Boonton's broad dynamic range (70 dB, in 10-dB increments at the push of a button!) and extended frequency response (good to at least 1200 MHz -- some models extend that to 6 GHz) make it irreplaceable.

-An MFJ 259B antenna analyzer or equivalent is a very useful multipurpose instrument that contains a small-signal RF generator and detector, a digital frequency counter, and the ability to add coupling loops externally to make it a dip oscillator, to check the resonant frequency of a tuned circuit.  Not bad for a little hand-held instrument that also makes antenna tuning a breeze!*

If you are interested in VHF-UHF work, especially if you're seriously interested, there are some unique but not expensive instruments that help a great deal when tinkering up there; things like slotted lines and stub tuners, maybe a G-R Admittance Bridge.  Readily found surplus, these items are not costly and are invaluable when working in the VHF-UHF-SHF-EHF range, but not terribly interesting for someone restricting their activities to below 50 MHz.

If you are tinkering with transmitters, of course a high-quality 50 Ohm dummy load is a great accessory.  While these can be homebrewed, or purchased new as an oil-filled "Cantenna" type product from MFJ for about $60, I'd recommend simply picking up a lab-quality surplus unit made by Bird.  I have two large oil-filled Bird dummy loads, a 600W unit and a 1200W unit, which are 50.00 Ohms from DC to microwaves and really handle the full rated power continuously, picked up used/surplus for a song.  When you can buy a $1200 load for $75 at a Swap Meet, why homebrew it?

While there are several surplus test equipment vendors known to the amateur realm, and some even advertise in the ham magazines, their prices tend to be a bit high compared with what you can find out there.  e-bay tends not to be a particularly good deal, either...I've seen used stuff sell there for higher than the "new" prices!  Amazing.  If you can attend a good Swap Meet or two, or search the "Pennysaver" type mailers that come in the mail, the "good stuff" is out there, very inexpensively.  I bought my Tektronix 475 scope from an ad in the local "Pennysaver" (freebie, comes every week in the mail), for $100 in perfect condition with all accessories and the original manual and shipping case.  The seller was a retired engineer who worked for Hughes Aircraft Company and bought it from them at an employee auction.  Such deals abound.

73 & happy building!

Steve WB2WIK/6

Posts: 7

« Reply #2 on: May 24, 2001, 08:57:37 AM »

OW! What a very detailed, useful post - and expensive, too.....<grin>

While I do NOT disagree with the previous poster that all this equipment is very useful and really handy if one really wants to be serious as far as rig design and building, i think that the list is more than a bit overkill for someone describing himself as a beginning kit builder.

Someone starting out really doesn't need a rack full of old tube test gear to be successful, especially if there is any possibility that they can find an elmer who might help them out with the really thorny problems (can you say local ham club?)

Were I starting out again, I think I'd likely go with the following:

DVM of good quality (I've alway been fold of Fluke DVMs myself) with built-in capacitor measurment, AF/low RF frequency counting and the like. This beast kind of serves as an all-arounder - it doesn't do anything really well but it does it all. Most if not all o fthese I've worked with over the years have very high input resistance and don't overly load the tested circuit - not unlike a VTVM...

A signal generator in the AF frequencies is handy as is one in the RF frequencies (though that can be breadboarded up pretty easily), and a frequency counter (even assembled as a kit) can take the place of an oscilloscope for a lot of measurement and alignment uses.

Don't get me wrong - a scope is a wonderful thing to have and I own two - but it isn't a death-or-glory essentiall when you start out.

A cheapish SWR bridge for antenna alignment wouldn't hurt but again can be shared if one can find other kindred souls in the neighborhood. I don't own a wattmeter and never have - don't do a lot of high-power stuff so never felt the need.

The VOM would be a nice addition if a used Simpson or the like wanders your way - an analog pointer can tell you a lot that a digital reading won't.

More than that, good hand tools are a prerequisite. Don't buy import junk - good nutdrivers, pliers and the like aren't all that expensive and can keep things from getting frustrating. Ditto the soldering iron - don't buy the RatShack 3.00 special - get a good Weller or the like and a stand for it. Personally I use an industrial soldring station I've had for many years and it's served me well (bought used/surplus, of course).

Add the fancy stuff as you go along - then you'll know if you want to.

One last point - mostly an example of what I'm talking about re: simplicity:

A friend had bought a Ramsey single-band receiver kit based around the NE602. Said friend had never been able to get the rig to produce more than hiss no matter how antenna'ed or tuned and had lost the documentation.

Frustrated - he heaved it at me for parts and/or repair.

I sat down at my desk one evening and hooked the silly thing up - he was right. Hiss and more hiss  but no signals. Even the hiss didn't sound right.

Rather than drag this down to the shop I decided to play with what I had at hand - which was:

Hand tools
Soldering iron and soldering tools
Cassette player and a cut-off patch cord
Random bits of wire, clips, jumper leads and the like.

The first thing I did was source documentation on the chips on the silly  thing - no big deal in today's digital age. The kit was basically a couple of demo circuits strung together - no biggie to figure out the interconnections that weren't on the manufacturer sheets.

Then it was a game of building blocks - sort the receiver into its sections and test them individually. I usually do this by taking an AF source and feeding it into the end of each stage, working my way out from the speaker/headphones. A cheap diode in line with the cassette player's cut off patch cord made an AF signal injector - and a random music tape was the signal source (I hate Barry Manilow....but all the good stuff was on CD).

So, tracing it this way led me all the way to the AF output of the NE602 tuner section. Following the sample circuit diagram I had it was evident that this was a simple varactor-tuned circuit that, if the chip was good, had little between it and functioning.

With the DVM on and the power off I check ed the values of the components in the circuit - simple continuity stuff - and looked to see if anything was grounded or the like. It all looked good, so I flipped the supply back on and checked to see that power was getting where it needed to be. Again, more-or-less all well and good.

Hmmmm...y'know, if it's more-or-less functioning (and keying up my HT near it proved the thing could receive after a fashion - can you say splatter? <grin>), the the lack of receive can be either:
a) blown chip won't tune anywhere good
b) Dead oscillator
c) Fragged varactor causing it to not tune
d) fragged tuning  circuit

A I couldn't fix and wouldn't have bothered for entertainment value. B was easy to check by seeing if varying the tuning of the oscillator (vary the slug in the tuned circuit) to see if it did anything.Varying the oscillator in this circuit is like varying the input tuning - you get it to "look" at a different spot in the spectrum....

WHOA! Houston, we have signals.....

OK, this tells us that the problem is in C or D. I didn't have a swap-out for C, so I decided to check D as an eliminator. Rather than the voltage on the tuning pot varying smoothly from 0 to supply it only dipped about a volt.

Popped out the tuning pot and lo and behold - a bad 5K pot was actually 50K across the resistive element. New pot in and the receiver worked like it was supposed to...can you say birdies? <grin>.

To put a short end on this long drivel of mine knowledge and logical thinking with simple test equipment is as good as if not better than a room full of gear and scattershot techniques. Mind you, the gear makes it a LOT easier - but the gear itself takes experience to use also.

                            73, N1TWY, ex-WA1YHY....

Posts: 196


« Reply #3 on: June 02, 2001, 03:54:15 AM »

Just an additional thought.... Do not throw away your old analogue voltmeter.... It can be very useful to measure when a digital meter is too slow....

Posts: 29

« Reply #4 on: January 15, 2002, 10:38:08 PM »

Know I am a little late but the analog vm that was previously mentioned is really great for the ocassional problem with a power supply or rf circut. Most digitals do not show power or rf changes unless:
1 they are large
2 they last
But the analog you can watch the needle move to any change present in the circut.

Posts: 1071

« Reply #5 on: January 19, 2002, 10:40:50 AM »

After more than 30 years of ham radio and home brewing, I've recently built a fairly simple and yet very satisfying project. It's an off-air frequency standard that uses straightforward circuitry and no special parts. Cost around $50 including new parts and case (depends on how fancy your case is...)

The signal frequency is 60 KHz, here in the UK there is a strong time and frequency signal from "MSF". I understand that WWV also transmits on 60 KHz in some parts of the USA.

The output from the standard is 10 MHz and 1 MHz. Obviously other outputs can be included, such as 100 KHz if you simply wanted a high accuracy "calibrator". I use it as an external standard for a frequency counter and also to calibrate the same counter.

Odd how a simple project like this could have passed me by for all these years.

If anyone wants details, drop me an e-mail off-line to:

g4aon at astromag dot co dot uk

Dave, G4AON
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