Monday, January 18, 2021

SB-301 Rebuild Progress - Wrap-Up -

WWV on 15MHz with a few feet of wire. "S-9" signal, and the antenna is several feet below ground level.

 

 I'm just about to call this one a wrap. As with all the equipment like this I rebuild, I'll let it sit for a couple of days, and then go through it again before I put it back in the case. Sometimes my eye/brain combo needs a rest, and I might just pick up on something I missed the first few times through.

And speaking of things I missed......I forgot to clean the two switches on the front panel while I was busy scrubbing away at the three under the chassis. The "Function" switch was the first one to get cleaned, and then the "AGC" switch. The AGC (Automatic Gain Control) circuit is very important in the radio, as problems in the circuit can make the radio either "deaf" (no gain, or amplification), or overly sensitive (too much gain), which will cause distortion on strong signals. I'd noticed some oddness to the way the AGC circuit was operating, and after I cleaned the switch contacts, I started resoldering the connections to the switch. The first one I went to resolder popped off the terminal as soon as I touched the iron to it! This one wasn't a case of the lead just being stuck through the terminal and soldered. It was just resting on the terminal (like a "Lap Joint"), and was very poorly soldered to it. It looked like the kind of soldered connection that would get you flunked in a soldering class, and between the dirty contacts and bad soldering, was definitely causing problems in the AGC circuit, which now works "As Advertised".

The Function switch also turns on the Crystal Calibrator by grounding the cathode of the tube used in the circuit, and cleaning the contacts on that wafer resulted in the Crystal Calibrator turning on properly when switched into the "CAL" position. 

Cleaning these two switches, and resoldering the connections to them, pretty much cleared up the last few issues I was having with the set behaving oddly, so I ran the calibration procedure again, which went smoothly. I was able to get the dial mechanical zero properly adjusted, and by tuning in WWV on 15MHz, I was able to "Zero Beat" the Crystal Calibrator to WWV, meaning the calibrator is calibrated.

The last thing I did was to run the alignment procedure again, but this time with the Main Tuning set for the middle of the band. Sometimes this allows you to pick up some sensitivity in the section of the band you most frequently operate, and it made a very slight (1/2 S-Unit) improvement.

 

So, I'll let this sit a day or two, and then move on to the SB-401 Transmitter. Before I tear into the transmitter, though, I'm going to go through these two items:


I have Dummy Loads, and I have Wattmeters. These two are self-contained, and one of those "Handy-To-Have" items. I know the Yaesu one on the left works. I suspect the Heathkit one on the right also works. However, I have no idea of the calibration status of either. Again, I suspect the Yeasu is pretty close, but I have no idea about the Heathkit unit. Since it's a Heathkit, purchased on eBay, it's most prudent to assume it DOESN'T work. The Heath unit requires a 9-Volt battery for the "Hi Temp" warning light, so I have to open it up for that, and I might as well check it and calibrate it. Since I have an accurate RF Probe, it's a simple matter to use one of my transmitters to generate the 100 Watts of RF, which corresponds to 70.7 Volts across the load resistor in these, and then set the calibration controls so the meter reads 100 Watts.

The Yaesu unit is fan-cooled and rated for 150 Watts continuous, and the Heathkit unit is not fan-cooled, but still rated for 175 Watts continuous duty, and 1000 Watts for 4 minutes.


After I've checked/calibrated these units I'll move on to the SB-401 transmitter, which I'm guessing needs as much work as the receiver did.....


12 comments:

  1. Back in the dark ages I held a 3rd Class Radiotelephone license. As part of that process the school made us build a radio transceiver from a kit. Mine functioned, barely. It isn't the "smarts" that is a problem for many of us. Rather, the fine motor skills. Those of us from the, Don't force it, get a bigger hammer", have problems.

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    1. Hah! Yeah, it can get real tedious and nit-picky in confined areas, like the switch where the wire popped off. After I cleaned all the old solder off, it took a good 15 minutes for me to maneuver the wire INTO the terminal slot and bend it over. To just "stick it in there and solder it" would have been a minute or two.

      More than little patience involved, too.....

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    2. I've been tested for patience and found to be free of it.

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    3. Than I guess nobody can try your patience?

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  2. Great progress. Looking forward to the watt meters and the SB-401 refurbs.

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    1. Started on the Heathkit watt meter/dummy load and found some shipping damage of the hidden kind. I've fixed all of it except for replacing a 7.7pF 1kV disc that got ripped in half when the load resistor got loose on one end.

      Just ordered a dozen of them, and when they get here this one will get finished. The cap is only needed if you want to self-calibrate the meter using a transmitter on 40 Meters. Since I'm actually going to measure the voltage across the load, I can probably calibrate the meter more accurately than using that Heathkit method.

      Still, I need that cap replaced so it passes final test! IOW, I want it "As Manufactured".

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  3. I enjoyed the learning and I will do a better soldering job in the future because of your lessons.

    I didn't know the silver cleaning thing.

    And your earlier mention of the contact cleaner may have caused a world wide shortage in Amazon's supply chain because they are taking forever to get a can to me.

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    1. I should probably do a mini-tutorial on How To Solder.

      This is the first set of switches I've ever had that were difficult to clean. I don't usually use Tarn-X on switch contacts, but these were filthy.

      Usually the DeoxIT breaks through the crud and restores conductivity, but not this time.

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  4. Nicely done, and you're going to have a complete set of Heathkit equipment...

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    1. Thanks, Old_NFO!

      I have the receiver and matching speaker, the transmitter with the Heathkit sold microphone, a "Station Console" with a clock, 10-minute ID timer, SWR meter, and a phone patch, remember those? And now I'm rebuilding the dummy load/wattmeter, so my collection of "Green Machines" is growing...

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  5. Is WWV the time keeping government station? Forgive my ignorance...

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    1. Yes, and they've been doing it for over 100 years now. They started out just broadcasting on extremely accurate frequencies, and sending standard tones that Radio Engineers could use as calibration sources. Now they send digital time coding on 60kHz, which is used for all those "Atomic Clocks" you can buy for $10.

      https://en.wikipedia.org/wiki/WWV_(radio_station)

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Keep it civil, please....