Tektronix 475 Oscilloscope and Android Signal Generator App
When I was debugging problems with my Ten-Tec Century/21, and especially my problematic one-watter kit, I needed to see more than DC voltages. I carried my problem stuff to my friend Paul to see what his scope and signal generator revealed.
Why would a ham need a scope? Audio and RF are both AC (alternating current) and a voltmeter alone doesn't offer much insight into that world of voltage across time and phase.
I almost bought an inexpensive digital scope last year, then thought better of it. Then I almost bought a featured digital scope and checked my wallet and thought better of it. A good digital scope in the 100 Mhz and up range from reliable sources costs upwards of $500. On the other hand, older professional scopes that have been well maintained and kept in calibration are excellent choices and will last a lifetime. You do give up handy on screen cursors for measurements, so you have to count divisions by hand and do the math. You also don't have digital storage in a digital scope, but smart phone cameras and video can make up for that.
When I saw this recently calibrated Tektronix 475 listed in the classifieds on eHam.net for a nice price, I decided it was time to step into the world of visualized AC.
Watching a capacitor charge 250 times a second
The lines are a bit wide because the signal source was noisy
Tek 475 Specs
The Tektronix 475 is a portable (30 lbs), dual-trace oscilloscope with dual time-bases similar to the 465, but with 200 MHz bandwidth and a maximum vertical sensitivity of 2 mV/Div. It is all solid-state except for the CRT. It was introduced in November 1972.
This scope cost $3,000 when it was new. Now you can find them in good condition for less than $200.
Cascaded mode -- 400 μV/Div, 50 MHz with CH1 input connected to CH2 VERT SIG OUT
Time base -- 10 ns/Div to 500 ms/Div, 1-2-5, and ×10 magnifier
Input impedance -- 1 MΩ // 20 pF
Triggering -- 0.3 Div (int) or 50 mV (ext) to 40 MHz, increasing to 1.5 Div/250 mV at 200 MHz; AC coupling >60 Hz; LF REJ >50 kHz, HF REJ <50 khz="" li="">
X bandwidth -- 3 MHz
Z axis input -- 5 Vp-p, 50 MHz
Calibrator -- 1 kHz, 30 mA / 300 mV square wave
Outputs -- CH2 Vert Signal Out, 20 mV/Div into 1 MΩ or 10 mV/Div into 50 Ω; A and B +GATE OUT, +5 V; Probe power jack
CRT -- 8 × 10 cm², P31 phosphor (P11 opt.)
Power -- 110, 115, 120, 220, 230 or 240 VAC ±10%, 48-440 Hz, max. 100 W
Real knobs and switches
One advantage of an analog scope is that there is a labeled switch or knob for every function. No need to dig through menus to figure out how to do something. To me this is the a true advantage to finding a well calibrated, analog scope.
An oscilloscope needs a function generator
An scope let's you visualize AC within a circuit, but when you testing something you often need to inject AC into that circuit. That's the role played by a function generator. Function generators allow you to choose a frequency and a wave type (sine, triangle, square, etc.), or sweep across frequencies.
In general, the higher the frequency they support the more they cost.
If you have a mobile device you can get one that uses your headphone jack as output up to 22 kHz for free...
For a free app it is very nice. It outputs sine waves very well, triangle waves are a bit soft pointed and square waves are for entertainment purposes only. But it is free so I won't complain. In the image below you can see the oscillations as it tries to generate a square wave but the audio amplifier of the mobile device just doesn't have that kind of control.
Frequency Generator App set to 1 kHz
Square Wave?
Square waves are not
Reduce the time base to zoom in
Yea, square wave.... not so much
The square wave is bad but sine and triangle waves look good until the frequency get's near the top of the range or the amplitude is raised too high.
Sine Waves look good
Triangle waves are on as well until you go up in frequency
The free app is inadequate for bench testing
While I appreciate that this would be a useful, portable signal generator for testing audio circuits, I'll be ordering a purpose-built function generator because generating clean square waves is an important test signal to be clean. I also will need a generator that works above audio frequencies, hopefully up the the IF frequencies of the some of the equipment I'm testing.
Only the beginning
Having an oscilloscope is a new adventure for me. I have another 1-watter kit ready to build that I've been holding off on because I wanted a scope for troubleshooting. In the meantime I'm using the scope to watch transistors trigger and measure the timing circuits I'm building and learning how to control the scope. The Tektronix 475 is a feature-rich analog scope. If you plan to fix your own equipment or do some homebrew electronics work a scope can come in handy.
The Universal 1Watter (also called the 1H2O) is a full featured little superhet radio transceiver that you can build for about $50. It doesn't come with an enclosure, a tuning pot, speed pot or an on/off switch so that will cost extra unless you already have some in the junk bin.
Some of the features include;
1 mighty watt of output
Good selectivity from the 3 crystal filters
A VCXO tuned frequency range for the 40m band from approximately 7,020 kHz through 7,039 kHz
A built-in full functioned keyer with provision for adding a speed pot and messages
Included command button accesses the functions of the electronic keyer
Natural sounding sidetone (nicer than my Ten-Tec Century/21)
The Build
The kit is delivered in a box and inside are a couple of brown paper bags stapled together. Inside one of the bags are a couple of plastic bags with the components. The other bag contained the header kit. The ferrite toroid mix types are separated in different unmarked plastic bags so don't mix them up (the instructions tell you which bag has each mix). If anything is missing the kit supplier (Diz, W8DIZ) is very responsive.
The kit includes both SMT caps and through hole caps. I tried to solder one of the SMTs but I didn't have the right kind of tweezers to hold it in position for soldering so I used the through hole caps.
SMT and through hole caps are supplied
This is the 3rd revision of the Universal 1Watter board and I was the first to build the 40m version.
While the schematic was correct, some of the instructions weren't sorted out properly for the 40m kit. I related issues as I found them to the designer and he promptly updated the online documentation.
I soldered the components and wound toroids as I had time over a few evenings and the initial voltage tests went well.
using through hole capacitors rather than the SMTs
some of the bits and bobs
build is progressing
close up
XTAL filters give it good selectivity
Everything except the final transistor
AGC circuit
Debugging
When the build was completed I connected the rig to an antenna and heard nothing.
The keying circuit and transmitter worked fine and I verified those functions but the receiver was deaf as a stump.
Thus began a number of days of investigation. Diz (the creator of the board) guided me through a number of debugging steps.
The first recommendation was to examine and rewind the binocular toroid balun that transformed the impedance from the xtal filters to the input of the U5 oscillator. He believed that I may had wound it incorrectly. I desoldered it and rewound it but that did not resolve the issue.
He then guided me through determining if one of the filter crystals or filter capacitors was bad. I desoldered a few components as a tests but that did not resolve the issue.
There are 3 identical mixer chips on the board. I swapped them around as there was a suggestion that there were some faulty chips in one of Diz's shipments.
I then took the board to my Elmer Paul Stroud AA4XX. He had a signal generator, Oscilloscope and RF detector. He traced the RF and all looked well but we still were unable to obtain any signal through the U5 mixer. Lastly we tried disconnecting the AGC transistor to see if it was clamping it and that didn't resolve it either.
Diz asked me to return the radio to him so he could take a look. After a couple weeks he emailed me saying he thought the BFO xtal might have a problem. But he later discovered that the oscillator in U5 was not starting up. Apparently the circuit design had a low Q and needed more current to get the oscillator working. He modified the design, adding a 16k resistor to the bottom of the board on U5 to get the oscillator going. After that all was well and he shipped the board back to me.
The FIX for all those problems required an extra resistor connected across U5
Learning from problems
Being the first person to build a particular version of a kit brings its own set of challenges, especially when you're as new to kit building debugging RF problems as I am. However I'm actually glad the kit didn't work right at the initial build. The process of debugging the board, was a great learning process. I studied the schematics and learned, as best I could, the function of each circuit so that I could better understand how to test it. During the debugging process Diz instructed me that although RF signal generators and scopes are useful you can tell a lot by touching a RF component with an inductive metal object and listening for a buzz or hum from the BFO. So all-in-all, even though the bug in the board was not due to a error on my part, I'm glad it occurred. I understand more about superhet radio design than I did before and more than if the kit had worked right off the bat.
On the air
After receiving the board back, I hooked up the frequency XCO potentiometer, paddle, command button, audio and output potentiometer and an external speaker. I then connected a 12v battery and heard the 1H2O keyer chip announce itself at power up in Morse "1 W".
Frequency control pot on the left
Volume control, output jack, cmd pot and paddle input
You can change speeds and modify settings via the command button which I have not reviewed yet. I also plan to add the speed pot so that I can easily change keyer speed without entering the command menu. For this first on-air excursion I was using it at the default startup 15wpm keyer speed. You can default the speed higher with a different resistor value. I have a resistor shrink wrapped and connected in-line to the blue-white wire coiling above the radio connecting to the speed pot terminal. In essence fixing the speed at 15wpm until I add the speed pot.
Ready to transmit
On the air... I was using my paddle out of the photo to the right of the Bug
First On Air QSO
I tuned around and found a strong station at the end of a QSO near 7030 kHz.
When he sent the final dit-dit I called and WD4AXJ answered my first call. He was in TN near Knoxville, and I received a 559. We chatted for about 10 minutes. Sorry about the blurry video. I thought I'd focused.
After I recorded this video I found an open frequency and sent out my call. Very shortly thereafter KD2FSHanswered my call and reported me as 599!
Whoo - hoo. 599 for my little 1Watter 40m.
I was transmitting using my 40m attic antenna. So deed restricted HAMs take note. You can build a one-watt radio and make contacts using your attic antenna. Haha.
You'll hear in the video there is some weirdness going on with the audio derived AGC. It is clamping down sometimes and is worse when I don't have the volume turned up very loud. When I began calling it clamped after every semi-break-in but didn't do it much after that. I'll have to look into that.
The AGC clamping may be a side effect of the increased gain Diz added to the BFO oscillator. I'll ask the forum.
Other than the AGC issue I'm super pleased with the little board. I touched the heat sync a couple of times after transmitting my side of the qso and it was warm but not really hot. It seems as though as long as you have a reasonable match to the antenna the power transistor should be happy.
My next steps are to get it in an enclosure and get it out to the Excalibur antenna site to hook onto that nice 40m doublet we put up a couple weekends ago. I plan to use my efficient little BLT tuner for that purpose. I will do a further review of the feature set on the keyer and record some more qsos for a later review.
Summary
The band was fairly busy and the little 1Watter did a fine job with stations on nearby frequencies. You can hear some getting around the passband but it is not bad at all. I'll do some tests to further define it's selectivity but at first glance it is far better than my old Ten-Tec Century/21.
My calls were answered quickly and I received good signal reports. It didn't sound as though the transmitter was drifting at all during the QSO. That's one advantage of using VCXO in the design. The disadvantage of using a crystal controlled oscillator for the frequency control is limited tuning range. The transmitter only has about a 18 kHz tuning range around 7030 kHz and I don't find many of the SKCC folks around that frequency but it is the QRP watering hole for 40m.
It is possible to shift the frequency with some capacitance changes but I think I'll leave it as is for a time and see how many states I can work.
Just imagine. This little $50 single band kit has good selectivity, a nice built-in keyer with a natural sounding sidetone, and lest we forget... You get a MIGHTY 1 WATT of OUTPUT. What more could a QRP ham need.
That one-watt of output was sufficient for all the QSOs I attempted tonight.
So lower your power and raise your expectations
72/73
Richard, AA4OO
UPDATE: 04/01/2016
I
am still having the AGC pumping issue and others on the list have
reported similar issues but only on receive. It happens to me when I
key unless I turn up the volume very high. I did get it installed in a
case but I still need to wire up a real power connector rather than
using alligator clips.
!Watter installed in a case
UPDATE: 04/05/2016
After doing quite a bit of reading I learned that the LM386 op-amp used in the 1watter
is rather notorious for audio oscillations. There are a number of
suggested fixes. I went with a 4.7uf cap connecting Pin 7 on U6 (the
LM386) to ground. That hasn't totally resolved the issue but it's much
less pronounced now.
cap fix for LM386 oscillations
I
have it in the case with all the proper plugs now (see below) so I'm
happy. I've been making QSOs every day with it and it continues to
amaze me and the stations that work me. It is stable as a rock with
regard to frequency and the large knob with the single turn 10k pot
seems to work well for tuning. I have enough control to vary the
frequency slightly without having to turn it too much. The tuning range
is only about 20kHz so just 3 frequency markers are plenty to let me
know what frequency I'm near. The selectivity is just fantastic for
such a simple little radio. Diz has created an inexpensive winner.
1Watter in enclosure with all the proper connectors for the case Information on case and knobs:
I've been considering ordering a 1 watter kit but I was concerned that it wouldn't be useful in the downward sunspot cycle.
1Watter board with initial 100nf caps installed and starting inventory of other parts. Not shown are the myriad toroids (fun, fun), transistors and pots.
So I spent some time this afternoon making 1 watt contacts on 20m and 40m with my Elecraft KX3 just to see what staying limited to that power would be like. I've made casual 1w contacts in the past but staying at 1watt was a good test for me.
It turned out that I had a plenty of answers to my 1 watt CQ calls and received reports that were only one or two points down from my report to the calling stations. The last station I worked dropped down to 300 milliwatts and I still copied him fine.
So I ordered a 1 watter kit for 40m from Kits and Parts today.
The 1 watter kit is significantly more involved to build than the Elecraft kits I've assembled to date. I hope to learn a lot from the build and alignment and I just might finally have a reason to buy an oscilloscope (hopefully my wife doesn't read this).
Update 2/20/2016: I received the kit yesterday. It includes 21 100nf SMT capacitors that the builder "highly recommends using". Optionally he supplies through-hole capacitors. I tried figuring out hold to hold the SMT part in place to solder for about 30 minutes and gave up. I don't have 3 hands. I'd have to have someone show me how to solder an SMT part. Even in the linked video to building the kit the builder used the through hole components so I don't know why the SMTs were supplied or were highly recommended. So I used the through hole capacitors.
Through hole versus SMT capacitors
I couldn't figure out how to solder the tiny SMT capacitors
1watter board with the 21 through hole 100nf caps installed
Inventory of resistors
Next step was installing resistors. There are six 1/8w resistors missing in the kit:
R3,9,21,22 (470)
R12 (1.2K)
R14 (4R7)
I haven't built enough kits to have spares like this so I sent them a note and will see how long it takes to get the resistors. I haven't inventoried the rest of the parts yet but so far I'm a bit disappointed. I have built 4 kits from Elecraft and one from 4-State QRP and all of those kits had all the parts. Although in the case of 4-State QRP one of the plastic knobs was broken in shipment. They sent me another in 3 days.
