Tuesday, October 18, 2016

More Junk Box Rigs

FPM20 Rebirth of the hallicrafters FPM300
11/09/2016 ~ More progress.
 Today I installed a few more controls and am closing in on having the complete front panel in working order.

Pete N6QW

11/08/2016 ~ More on the Front Panel
 Since we already voted there is no guilt in working on the rig. The Tuning dial has been added and some more video of the rig.


73's Pete N6QW

11/07/2016 ~ Building the front panel.

Today I began work on the front panel. With my manual 3 axis mill (yes I have both a manual and 3 axis CNC mill) I made the cut outs for the display and S Meter. There is also a short movie. The final size will be 4 inches high by 10 inches wide by 12 inches deep.
FPM 20 Front Panel

Pete N6QW
11/5/2016 ~ More work and headed toward an enclosure! 
Today I did some work on the power supply board and that is now complete. It seems like the original designer's utilized several unique voltages on the main board such as 12.6 VDC, 3.6 VDC. 12 VDC and 6 VDC. I have a built a power supply board that now outputs these several voltages.
I have more or less decided on a box size (large it is) which will be about 4 inches high, 12 inches deep and 8 inches wide. There will be some blue paint involved. For those checking it is slightly less than 1/4 cubic foot in size.
I continue to make contacts and since have corrected the LSB BFO by moving it 50 Hz. I now can tune dead on 7.2 MHz and it sounds right!
Pete N6QW

11/2/2016 ~ The original FPM300

I have received a couple of inquiries wanting to know what did the FPM300 look like as a radio (before I got to it)? Wait no more. The photo below shows it was quite an attractive radio with a top cover that had toggle snaps on the side to enable quick access to the "innards". Once the snaps were released the cover simply rotated over and above the radio. It had a somewhat military appearance. [In the photo look along the lower right hand side of the case and you can see the toggle mechanism.]
Too bad it had some problems such as using a rubber band (I exaggerate it was a splined rubber band) for the band switching mechanism to gang control the band switch. Over time the rubber band failed and the band switch became inoperative. The VFO drifted and some of the suggested cures for several of the internal spur problems was the use of aluminum foil to provide shielding.
There were several variants such as the last gasp fix called the Mark III. These units tend to be overpriced on the auction sites. Today there are a couple of such listings on eBay and the one below has a buy it now price of nearly $250. If you can find one in a "garage sale" for $5 snap it up.
Pete N6QW
The FPM300 Mark I



10/31/2016 ~ Serious DX Action in the CQ World Wide Contest

No I am not a contester! BUT contests are a great time to make DX contacts which might not be available on a regular basis. To that end running full power (about 600 watts for the QRP aficionados) I made several Caribbean contacts, three with JA stations and one with a station down in ZL Land. [Keep in mind this was on 40 Meters from Southern California.] Admittedly I got 5X9 reports from all stations; BUT what is telling is that usually I made the contact on the 1st call. No my rig does not have DSP, nor does it have a noise blanker, and there are no funny equalizer  presence settings or dual watch; but it does have a very stable signal and is spot on in frequency. Did mention it does have a color TFT display?
What is significant is that the mainboard of the radio was built in the 1960's and after some horse trading cost me about $20. The rest of the hardware came from the junk box and it value is maybe around $50. So here I am with a $70 radio making serious DX contacts using nothing more than a droopy dipole whose center is 26 feet high. Just imagine what I could do with a pair of phased extended double bazooka antennas at 70 feet?
Now my mainboard does have speech compression and VOX so it does have a few refinements which I intend to fully explore. So OK cosmetically it now looks like a hunk of junk but in time I hope to run it through the "Juliano Blue" process and make it respectable. So my advice to any one reading this --keep your eyes out for the sow's ear --it just may turn out to be a silk purse.
Any one out there have a mainboard from a hallicrafters FPM300 that they would like to have move from their garage or shack? Email me and let me know. BTW there are many radios lurking out there with blown finals that otherwise are functional --you can give those jewels the same treatment and have a reborn radio.
Pete N6QW
10/28/2016 ~ New Power Amp Stage
 I have somewhat changed direction and added a 4 Watt Linear Amplifier Stage which uses a device I found in the junk box --a 2SC2075.  Had a board layout in my computer so about 10 minutes worth of work on the CNC mill and I had a new board. This is shown below. With 4 watts I can drive thru an intermediate amplifier the SB200 to about 600 Watts out. This is a signal that can be heard on 40 Meters all the way to the right coast from the left coast.
Schematic ~ the 2SC2075 is substituted for the 2SC2166 with no other circuit changes

The "As Built" amplifier ~ The metal case is the heatsink!

Looks like crap works like Hell!
You too can build rigs like this!
Pete N6QW


10/24/2016 ~ More refinements -- improving the power output.
Initially I set the carrier oscillator frequencies in the Arduino sketch to the values indicated in the schematics. The USB frequency seemed way off. A quick and dirty test is to set the mode to USB and normally speak into the microphone. The output was much lower than LSB. A second part of the test is to whistle into the microphone (the rig was connected to a dummy load) and the output noted. On LSB the whistle test produced about 1/2 the output on LSB as compared to speaking into the microphone. On USB the whistle test produced about 1/10 the output as compared to the LSB with voice.
Using a separate external oscillator for the carrier input I adjusted the BFO frequencies until the whistle test on LSB and USB was at the same power level and these in turn matched the output using speech. These values were much different from the schematic values. But then on closer examination of the circuit schematic for the BFO  both crystals had trimmer plus fixed caps connected across the crystals -- so the actual oscillating frequency was modified in production and different from the marked frequencies. In my rig the BFO is supplied by the Si5351 and the values are set in software. I suspect that I am really close but may be off by about 30 Hz on LSB, which I will further test to verify that hunch. 
These changes have resulted in greater output!  I switched the linear amp following the driver stage to the brick liberated from an Atlas 210X. Now I am easily seeing about 50 watts output and driving the SB200 produces about 450 watts on peaks. So we are cooking. This evening I had a QSO with a station in New York where I was running 50 watts and he had a KX3 cranked down to 1 watt. So with a modest power level you can have coast to coast contacts on 40M.
I am really excited about this rig! There are some plans in work to make this a two band rig covering 20 and 40 meters.
So far I have had about two dozen contacts with stations in California, Washington, Nevada, Utah, New Mexico, Arizona, Texas, Nebraska and New York.
Pete N6QW
FPM20 now working at 50 watts with an external amplifier!
10/23/2016 ~ The FPM20 is "Alive" and making QSO's
The rig went "live" on 10/22 and so far I have made about a dozen contacts. Some of the contacts were made using just the driver stage at 350 Milliwatts (this is the EMRFD variant with a 2N2222 and BD139). Several were made using the outboard SS amp running about 25 watts. One piece of DX was a QSO with a station in Nebraska. (KD7YUW) where I was running 25 watts.
This is how it looks but I desperately need to tidy things up.
This is the rig "as is" as of 10/23

This is the 350 MW driver stage and low pass filter.

Pete N6QW


Building a Junk Box Rig from a Commercial Transceiver Board.

In the photo below is a "breadboard" version of the FPM20. Essentially the heart of this rig is the main board from a hallicrafters FPM300 SSB/CW transceiver and the S Meter. I have added a 2N3904 Receiver RF amp stage, a band pass filter comprised of two 42IF123 10.7 MHz IF transformers padded to 7.0 MHz, a Si53551 providing the LO and BFO signals and a power supply board (the FPM used weird voltgaes0. Rounding this out is the SBL-1 that acts as a receive and transmit mixer stage. The audio output stage uses an LM-386-3.
In the video above the receiver was working but I got no output on transmit -- that is until I realized on the output chain (the final device on the board) consisted of a 40841 Dual Gate MOSFET. that has ALC applied to Gate 2. Upon close examination of another of the circuit diagrams I saw a note about ALC. On receive the voltage is "0" but on transmit with no signal applied the voltage was 6 volts supplied from the ALC circuit -- this was a negative ALC. As more signal (such as from over driving) is developed the ALC becomes less. I had applied no voltage to Pin 17 on the board so essentially the circuit was cut-off! I added a 5 volt regulator to Pin 17 on transmit and the circuit now works on transmit --and CW. The FPM 300 used a keyed tone for CW generation.
A new board will be built using my standard 2N3904 bi-directional circuit which acts as the RF amp on receive and the transmit pre-driver on transmit. There is sufficient output from the 2N3904 to fully drive the 20 Watt brick sold by CCI. This is why the rig is dubbed the FPM20. We are on our way.

Pete N6QW

Saturday, October 8, 2016

That's All Folks!

This is the latest and last posting on this blog. There will be no further blog entries!

Keep on melting solder.

Pete N6QW

Saturday, October 1, 2016

What is a VFO (variable frequency oscillator)?

The Variable Frequency Oscillator ~ aka "Grief in a Box".

Whether you have an appliance radio (store bought) or one that you made with your own hands somewhere buried in the innards is a variable frequency oscillator.  However if you are running a single channel fixed frequency device such as a crystal controlled transmitter or receiver then maybe that is not the case. The acid test is if you move a knob or mouse pointer and you are able to change the frequency, then you have a VFO. A special case may be the VXO which is a variable crystal oscillator which uses the properties of a quartz crystal to shift its frequency of oscillation over a small range. But the VXO usually has a knob adjusting a variable capacitor or a pot controlling a voltage variable capacitor.
But I really want to focus on the knob turning, mouse pointing variety of variable frequency oscillator. There are various ways to generate a variable frequency starting with the very early approach involving analog circuits where an inductor and capacitor formed the very heart of the VFO. When an inductor and capacitor are linked up, either in series with each other or in parallel, a resonant circuit is formed. If this combination is placed in an amplifier circuit and certain conditions are met with respect to internally generated feedback the circuit will oscillate at a frequency determined mostly by the  inductor and capacitor values. Other factors that impact the actual frequency of oscillation include stray capacitance and/or inductance as a result of component lead lengths. For these reasons most VFO construction tutorials stress short direct connections and isolation from other circuit elements.

There are many types of analog VFO circuits with names like Hartley, Colpitts, Clapp, Vacker, Seiler, Franklin, ECO. The difference among these types basically is how the L and C are combined (series or parallel), the method of achieving feedback, where the load is placed and finally the coupling to the load. Some of these circuits are very load sensitive and you will frequently see some sort of follow on buffer amplifier to isolate the actual load (such as a mixer stage or convertor stage) from the oscillator itself. Below are two examples of oscillators with the first being a Hartley and the second called a Vackar. [The naming typically follows the person who invented the circuit.]

The Basic Hartley circuit (parallel L and C)

Vackar Circuit with L and Co in Series
For those who have an insatiable desire to know where every nut and bolt is hiding there are many factors concealed in the bushes as to why the circuit oscillates. Some factors have to do with the physics of inductors and their response to  being charged and collapsing electric fields while others have to do with capacitance discharge rates and flywheel effects. I do not plan to cover these other than to recognize that certain conditions have to be met for an analog circuit to oscillate.
Here is part of why I used the term "grief in the box "-- getting a circuit to oscillate is but a slice of the pie as then the builder is confronted with keeping the circuit in the oscillating state over the desired range. When you twiddle the knob or move the mouse pointer the objective is to keep it oscillating over the entire tuning range. That does not always happen.
Another pie slice is oscillator stability which has several subsets. There is the "drift factor' manifest as either "initial turn on drift" where as the circuit elements heat up from a cold start the frequency will drift. This can be a very large amount in the range of several hundred hertz to kilohertz. Capacitors especially are subject to temperature drift and the inductor as well will respond to temperature changes.
Another drift factor is "long term drift" where after the initial "turn on drift", over time the frequency will shift albeit usually a smaller amount. Turn on drift as stated may be in order of magnitude of kilohertz whereas the long term may be 100 Hertz. Fifty years ago a few kilohertz drift was common. Today the Flex Radio Operators on 40 Meters will scream at you if your rig moves 10 Hertz -- so with progress there are some penalties.

Yet another form of drift is the result of poor mechanical construction of the VFO. If your inductor flops around --the VFO will change frequency. If you move your hand near the inductor it will change frequency and even blowing cold or hot air on the inductor will change the VFO frequency. You get the drift (pun guys).
Other issues include "FMming" a term often applied to certain older boat anchor inexpensive commercial appliance radios that lacked good voltage regulation. Typically these vintage radios featured 500 watt sweep tubes and with a marginal power supply on voice peaks the VFO regulation suffered. The result is the SSB signal frequency that varied with voice peaks --thus frequency modulation. Before we leave VFO voltage regulation that is an issue in itself as the lack of adequate voltage regulation results in frequency drift.
A similar problem was when the VFO was built  "al fresco" with no shielding such that the output RF signal was being picked up in the VFO circuitry and this results in distortion. 
Hopefully now you understand why I call it "Grief in a Box"! But in examining all of these maladies there is a general approach that cures many of these problems and most of the cures are physical in nature. So for those contemplating building an analog VFO behind the schematic is a host of factors that if properly addressed  will assure a success. Lets us examine some of the physical factors.
  1. The components themselves are one factor. Use NPO temperature coefficient capacitors and use multiple caps in parallel. Capacitors have AC current passing through them which cause a heating of the cap. If you have say a 100 PF cap in parallel with a 50 PF variable cap to set the band range then use ten 10 PF caps in parallel as then each caps is drawing 1/10 the current and the heating of each individual cap is dramatically reduced. While you are at it the 50 PF variable must be a double bearing type (supported at both ends). This keeps the cap linear and not subject to vibration. Old style brass capacitors work the best but may not be readily available.
  2. Inductors are very subject to temperature and mechanical impacts. Air wound inductors seems to be the best BUT how you mount them may negate their otherwise excellent properties. Frequently the inductors are wound on a grooved ceramic core or on a cardboard form that has been varnished. Keeping an inductor away from the chassis or walls of an enclosure is an art. Ceramic pillars are often used for this chore. One other approach is to super glue the air wound inductor to a 1/4 inch thick piece of plexiglass which is then mounted on the pillars.
  3. Physical isolation is another factor. High quality VFO's are built in shielded enclosures and power is fed to the VFO via "feedthrough" capacitors. The signal output is best done utilizing SMA connectors. Real die hard VFO builders will put the Inductor and Capacitor in one box and the electronics in another box where the two boxes are interconnected with a short length of RG174/U 52 Ohm coax. This approach isolates any heat from the frequency determining elements.
  4. The build itself should utilize short direct connections over a common ground plane. This is where a Manhattan style build could result in an extremely "solid" VFO. For best results do not utilize Zener regulators but instead a three terminal type such as the 78L08. That is another point use the  minimum amount of voltage to sustain reliable operation. This reduces  circuit heating thereby impacting the drift in the least amount possible. Further amplification of the VFO signal can be done externally.
  5. Mechanical factors must be addressed wherein any vibrations or mechanical movement will cause movement of  components in the VFO itself. One the earliest frequency agile ham transmitters was the self-excited Hartley oscillator which frequently used a type 45 tube. Essentially this jewel was a keyed VFO built on a wooden breadboard. This was a pure example of "al fresco construction" . Literature of the time cautioned that the operator must place the transmitter on a shelf above the operating table as the mere act of striking the Morse key would transmit vibrations to the transmitter and the signal would vary in frequency based purely on the movement of the key. The physical isolation solved that problem. The second caution was to insure the antenna was taut --- if the antenna moved with a breeze it would present a variable load to the oscillator and the frequency would shift. A hefty mechanically sound enclosure serves two purposes: one is to shield the VFO and secondly to reduce the impact of mechanical vibrations.
  6. Voltage regulation is key as mentioned earlier. Resist using Zener diodes and resort to stout three terminal regulators. You ask why? -- Most of the three terminal regulators feature internal temperature compensation to keep the output constant --Zener diodes do not. Very sophisticated regulation circuits will keep the output constant even when subjected to wild swings in the input side. Lack of voltage regulation and heating of the circuits (especially with tube type vfo's) frequently resulted in a chirpy signal especially if one were keying a VFO for CW operation. The message here is to use the lowest voltage practical to sustain oscillation as this facilitates maintaining the applied voltage and reduces the device dissipation which in turn generates less heat.
  7. Once you have the VFO built typically some sort of gear reduction drive mechanism coupled with a mechanical readout like a circular dial is fixed to the capacitor shaft. Careful alignment is required so that no stresses are introduced into the capacitor shaft (ie binding) such that the assembly lacks smooth tuning. This takes time to get it right!
  8. Finally --don't be greedy! Select a reasonable tuning range like 500 kHz for your VFO as this satisfies several factors one of which is stability and another is linearity. The ultimate goal is to have the same degree of movement at one end of the VFO range produce the exact same increment of change at the other end. Did I mention that a smaller range facilitates the use of reduction drives to give you "fine tuning". 
The above cover much of the well known cautions and tribal knowledge in analog VFO construction and if you are looking for a simplification in building an stable analog VFO --there is none! There is much effort involved if you want to have a stable VFO. Translate this to hours and days of work versus a hour with some wire wrap tools and 30 Minutes writing some code for an Arduino. Unless time is well spent on analog VFO construction you will have grief in a box!

Above all feel free to add your additional comments on how to avoid "Grief in a Box".

Pete N6QW

New Technology for 2020 ~ Improvise, Adapt and Overcome

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