Everyone needs a remote VFO from 1955
The 1955 remote VFO was unique because it had a built-in power supply. It's also interesting that its base oscillation frequency is in the 160m band. Using harmonics from the base frequency means it doubles for each subsequent band (x2 for 80m, x4 for 40m, etc.) That doubling means it also multiplies the drift. Specified drift is 300Hz an hour. That doesn't sound too bad, but multiply that by x6 up in the 10m band and holy-smokes, it's drifting 1800Hz an hour.
Note to self: Never browse eBay just before you go to sleep
That's gonna make operating CW like a game of chase, or hide and seek after every exchange.
This is gonna be fun.
|Surprisingly the big dial is actually operating the variable cap through a reduction gear and it's very smooth|
|Uses 4 tubes. Power supply up top, VFO circuits in the bottom to minimize impact of heat from the PS.|
|old electrolytic power filter cap must be replaced|
|10k 7watt resistor had failed|
Replacement bitsThe 450v electrolytic cap must be replaced for safety reasons. All the other components measured within 10% of specifications except for the 10k 7w resistor connected to the OA2 tube. It had gone up into mega ohms of resistance, which is likely when the VFO was taken out of use.
Handwritten notes inside the chassis indicated the VFO tubes had been replaced in 1977. Until I get the replacement parts for the power supply I won't know the condition of the tubes.
Surprisingly, it outputs 10 volts of signal, so I may also build an output filter and use it as a QRPp transmitter on its own.
The possibilities are endless.
RepairedThe replacement parts arrived from Mouser... A 500v 47uF electrolytic capacitor and a 10k Ohm 7-watt resistor. The new high wattage resistor is tiny compared to the giant, defunct resistor that was in there before, and of course the capacitor was about 1/3rd the size of the original. I used some spaghetti on the capacitor leads since the lead lengths were so much longer with the replacement cap. So the power supply section was now repaired.
I also replaced the 2-blade, non-polarized, ungrounded, un-fused 1950's power cord with a 3-pin grounded plug and added a 1-amp/250v inline, replaceable fuse. So hopefully there's a reduced risk of death or fire now. Electrical safety didn't seem to be foremost on the minds of kit builders 60 years ago. The size of the 3 wire power cable and it's much thicker insulation didn't fit the opening in the back of the VFO as both the power cable and the VFO output come through the same hole, so I had to remove the insulation and use heat shrink to get things to fit. Additionally the large in-line fuse holder didn't fit well inside the VFO housing so the wiring is quite a bit more cramped in there than it was before.
After the components were replaced and the wiring was complete I plugged it in... no-smoke. Then I flipped the repaired switch (the phenolic disc for the on-off switch was broken in half when I received it), and wallah! The indicator light lit up through its pretty little blue jewel eye. So I knew the transformer was supplying 6.3v for filament. I heard a low hum from the little transformer and then the tubes began to glow. The OA2 was glowing it's pretty violet color, and no bad smells were emanating. I was ready to button it up and begin calibration.
The sparse instructions directed me to back out the tuning slug for the 80m band nearly to the end and screw in the slug for the other bands all the way, so I did so. I set the trimmer caps C1 and C2 to their fully engaged positions.
I carefully re-installed the front face holding the VFO and PS sections it in its heavy-duty case, taking care to get all the new power cord/fuse wiring inside the VFO section from binding up on the sharp edges of the case as it went in. In screwed in the plentiful 10 screws that holds it together and Bob's your uncle. Well, maybe Bob isn't your uncle but I just wanted to say that.
I had already attached an RG-58 coax to the output inside the VFO and run it out the hole with the new power cord, so I then installed a BNC connector on the end of the RG-58 to make hookup easy. I like BNC connectors because they are secure and I have lots of adapters for different connectors. I then connected the VFO output to my Elecraft CP1 RF coupler and terminated the other end with my ugly dummy load. I connected the RF coupled cable to the Oscilloscope and turned everything back on.
CalibrationI let it warm up 20 minutes or so. The cabinet does not get very warm, just about 15 degrees above room temperature. That's actually a good thing, from what I've read. If the VFO is at room temperature then it's more susceptible to the variations of that room temperature. Having the case stabilize above room temp can make the VFO more stable.
I had my frequency counter attached to an output from the oscilloscope. In the 80m band setting with the VFO dial set to 3.5 Mhz the freq-counter was reading around 1.75'ish. The VFO primary oscillation roams around the 160m band and generates the first harmonic in the 80m band. The freq counter had trouble tracking due to all the harmonics, and the output on the oscilloscope was not very pretty because it was showing the primary frequency with the first harmonic interfering.
I was unable to properly calibrate the VFO using a frequency counter, due to the interference from the harmonics, so I turned on my SDRPlay, software defined radio. It can display up to 10 mHz bandwidth but for this test I was displaying 2 mHz bandwidth so that I could easily see the harmonic for the band I was calibrating.
That made quick work of calibration. I adjusted the variable capacitor C1 (near the bottom left hand side of the VFO) for the 80m band and adjusted the one above it for the remainder of the bands. I was able to verify that adjusting the VFO dial in the CW portions of the bands was extremely accurate with regard to frequency.
It appears to work like a charm. I hooked up a key and even sent some test messages and listened to them on the SDR. I'd been advised to not key the VFO directly because it tended to chirp but frequency stability was much better than I expected. Over the course of an hour that I was calibrating I saw very little drift after the initial warm-up.
Here's a little video demonstrating the completed calibration...
That's all for now .
So, warm up your Tubes and spray some RF into the air.
Superstore for Vacuum Tubes
So if you've been looking for a good source for all your classic vacuum tube needs visit...
Tektronix 475 Oscilloscope and Android Signal Generator App
|Watching a capacitor charge 250 times a second|
The lines are a bit wide because the signal source was noisy
Tek 475 Specs
- Bandwidth -- 200 MHz (475), AC cutoff 10 Hz, switchable BW limit 20 MHz
- Rise time -- 1.75 ns (475)
- Deflection -- 2 mV/Div to 5 V/Div, 1-2-5
- 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
An oscilloscope needs a function generator
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
Only the beginning
That's all for now
Sow lower your power and sample it with a scope
Ionization is a beautiful thing
|OA2 VR tube glowing beautifully in back corner|
|Tbis 6EA8 should not be glowing violet|
Lots of flatulent tubes
Solved a bit of a mystery tonight
|Tube shield needed to be grounded to the socket spring|
High voltage indeed
|OA2 in action at the right rear of the Audio board, notice the glowing gas inside|
|Look at the pretty violet glowing gas in the OA2|
The OA 2 is actually not a vacuum tube it contains a gas that ionizes and in the process of ionization acts as a voltage regulator. In a vacuum tube, if you see glowing gas inside the tube that means it's leaking and has become "gassy", that's a bad thing. In the case of a gas filled tube like an OA2 you expect to see glowing gas and if you do not, then there's a problem.
In this case The OA2 regulates voltage to 150 volts and current is limited by the two high-wattage resistors connected in circuit to that tube (seen beside the OA2 in the photo above). Those resistors are dissipating a lot of heat. I measured over 300F degrees with my IR temperature gauge (ouch).
Tube Testing without a tube tester?
The lack of a tube tester is not a limitation. You can tell a lot about a tube test by looking at the voltages in the circuit. If the filament goes out, the tube is dark and does not conduct. That one is pretty obvious. If the tube cathode loses it's ability to emit electrons, the voltages dropped across the cathode resistor and any resistors in the plate or screen grid circuits will change (the voltage drops across those resistances will drop). A good tube tester will measure the ability of the tube to vary the plate current for changes in grid voltage, and you can see this with a scope by looking at the AC voltages on those elements, at least for the audio circuitry. Probably not so much for the RF circuits as you are affecting the circuit when you probe it. The best way to check for a suspect tube is to replace it with another. I have 0A2's new in the box. I may have the other tubes pulled from radios but it takes longer to look through those as sometimes the numbers are hard to read. Hint. to bring out a faded number, rub it on the hair on the back of your neck-- better if it's oily.A lot of resistors in old tube-type radios were 20% tolerance so don't expect the voltages to be that precise. Tube rigs had much more variability than modern solid state circuits. For the unregulated voltages, the precise voltage will depend component tolerances, tube health, and line voltage. The line voltage here is about 125 when where I lived in Raleigh it was around 110. You can do the percentages but that alone will account for some of the variation you might see. I don't think that any of your tubes were damaged by overvoltage. If they are drawing too much current, they will get hot, and in severe cases, the plates will glow red. That's a concern. Much less than that, the life might be reduced but I don't think you will see any short term effect. When I've run tubes too hot, you might see signs of the tube going gassy, which is indicated by a blue low in the space inside the tube that is supposed to be a vacuum.If you want to see the plates of a tube glow red, operate the final mistuned so it is drawing too much current. They will glow red, haha.The voltages out of the 0A2 are different -- they should be held pretty closely to 150V. I don't know the exact tolerance. It might be a few percent. So in your radio, 150V supply and any that are derived from it are the only ones that would have a tighter tolerance. The 0A2 has an operating current range of 5 to 30ma, and an operating voltage of 150V. The supply voltage should be at least 185V to get the tube to "fire" but once "on", the voltage feeding will drop to the 150V level and be regulated there by the action of the tube which causes the voltage drop across to change (like zener diode regulators). The regulation comes from the fact that a very small increase in the voltage across the tube results in a significant increase in the current so the resistor values are chosen so that the current through the tube remain in the 5 to 30ma range as the input voltage and the current draw of the regulated circuits varies for whatever reason. Remember the current draw when you first turn on the rig will be low because the tubes don't conduct until the cathode gets heated so the designer must account for that in the selection of the input resistors to that circuit.
More to learn about hollow state
As a young-ish ham I certainly have a lot to learn about old tube radios but I'm enjoying the journey. Just the thought of transistors operating by thermionic emission, tossing their electrons across empty space, being attracted to a plate with more positive voltage is fascinating to contemplate.
That's all for now
So lower your power... or at least regulate it with a OA2... and raise your expectations
Get a remote signal report via WebSDRI've been making modifications / upgrades to my old Heathkit HW-101.
I've had a good number of CW QSOs and a couple of phone contacts with it so far and received good reports. One concern I had, regarded losing the first DIT of a letter on initial relay closure. There is an upgrade article for the HW-101 with a section describing a fix for losing the first DIT of a letter on the initial close of the relay. The modification involve replacing a resistor with a lower value that triggers the VOX. It's a resistor change from 470k to 1k which seems kind of extreme to me.
According to the article you will hear the initial DIT in the sidetone but above 20wpm that DIT is not transmitted due to a delay of the relay closure because it's driven by a VOX circuit.
This test is difficult to describe to a remote CW operator.
I recalled that webSDR stations usually offer a remote recording facility. I found a webSDR station that was within propagation on 40m today and sent my CQ with my call while recording from that remote station. My call, of course, begins with the letter A, which begins with a DIT. I paused for the VOX delay to timeout between sending my call-sign, thus causing AA4OO to be sent with the relay closing at the beginning each time. I started the recording before my call and stopped when I was finished and downloaded it.
|remote webSDR station recorder|
I confirmed, that although the initial DIT was shortened a bit above 20wpm, it was still being heard and didn't affect copy of words that began with a DIT on relay closure. So I've decided to not do that modification at this time. I previously used DEOXIT on the both sets of relay contacts in the HW-101, because there seemed to be some corrosion present that was affecting receive at times, and that has cleaned up the performance of the relay considerably.
So if you're wondering how to get a remote signal report when no one is answering your call the facilities of a remote webSDR station may help.
NOTE: The HW-101 doesn't have a precise VFO readout, even after calibrating it with the built-in crystal calibrator. I use the reverse beacon network to spot me and take the frequency reported by remote beacon to enter into the frequency of the webSDR station to find my signal.