Sunday, January 17, 2021

SB-301 Rebuild Progress - Part 7 - Updated

 Spent yesterday cleaning all the switch contacts on all of the rotary switches in the receiver.

As an example, here's a set of contacts before cleaning:


And here's after cleaning:

 The contacts consist a silver-plated ring that rotates when the switch shaft is rotated, and a fixed contact that touches the rotating ring. I clean them with Tarn-X and a cotton swab, and then flush them with Isopropyl Alcohol to remove any remaining Tarn-X, and then treat them with DeoxIT, which helps prevent the tarnish from returning.

As you can see, I had a lot of contacts to clean on just the band switch, and there are contacts on both sides of several of the "wafers" making up each section.


And here's the mode switch after it's bath:

And along the way I found some more iffy soldered connections, so I wicked the old old solder off, bent the "excess lead length" around the terminal, and resoldered the connection.

The last thing to do was replace all the old hard, brittle RG-188 coaxial cables with new RG-316 Teflon insulated cables.

Each one of the ten such cables in the radio was unsoldered, removed, and used as a length guide for each new cable. The RG-188 cable is more flexible than the the RG-316, so in some cases I had to make the cable longer so I could snake it in, and in other cases I had to shorten it a bit.

I cleaned each terminal after I removed the old cable, wicking off the old solder and tinning the terminal with fresh solder. Then each end of the cable was soldered into place on the terminal, jack, or circuit board pad.

Tedious work, but I got into a rhythm of remove, fabricate, replace, and finally finished up around 0030. Some of the new cables may look a little "tight", but there's slack in them, it just doesn't show well in the picture.

So the test gear is warming up down in the workshop, and I'll start the alignment procedure again. If cleaning the switch contacts and replacing the ancient coax doesn't clear up the issues I found on Friday, I'll have to start poking around in there on a point-to-point basis, starting at the antenna connector, and moving along to the speaker.

This was sometimes referred to as "Hand-to-Hand Chassis Combat" by the Old Timers who taught me a lot of this stuff.



I'll do a "wrap-up" post on this tomorrow, but it appears to be fully functional, and seems to meet specs for sensitivity. When I first started aligning it, it took 30 MILLIvolts to get an "S-6" signal. Now that it's been aligned, a 30 MICROvolt input gives me an "S-9" reading meaning the receiver is now 1000 times more sensitive than it was when I started. Dirty switches, poor soldering, degraded cable, lots of little things.


  1. This is fun to watch. If ever I can get the space I have some restorations to do. Reading what you have done teaches me a few things I didn't know.

    1. Glad to help, Bill!

      It's pretty much alive right now, after finding and fixing a couple of more workmanship issues.

      Had a really funny "crunchy" background static that was driving me nuts, and then I turned off my big LED magnifying lamp.

      BAM! Noise floor dropped from S-9 to zero!

      That's a GOTCHA on me....

  2. All this newer solid state takes the fun out things for some. For those of us with no talent, the choice is clear.

    1. I can fix certain solid-state items, depending on what's failed in them

      My Big Bruiser Yaesu radio is solid-state, and I had to dive into that. The kicker is the parts on these new radios some use "bespoke", or proprietary parts, and when those dry up from the factory, like they are, you're reduced to looking for a parts radio, or the module you need from a radio breaker.

      This old tube stuff is fairly easy if you know the basics well.

  3. Was this a kit assembled by someone or a factory radio?

    1. This was a Heathkit, built by someone in the late 60's/early 70's. I built an identical pair of these when I was in eight grade.

      This radio most likely worked fine when it was newly built. Time was not good to it, and the deficiencies in numerous soldered connections and the less-than-stellar workmanship made by the original builder have come home to roost.

      I don't know when it was last powered on, or how it was treated in storage, so it's getting everything looked over. The matching transmitter has some obvious issues with the dial mechanism, and I expect I'll find the same level of workmanship in it, as they were apparently built by the same guy. The notes, pen/pencil type, and handwriting in both manuals match, and these were extremely popular to be sold as pairs, like my Father bought for me back in late 1964.

      Yep, lots of deja vu moments working on these!

  4. A 60 dB improvement from before to after (63? from S6 at 30mV to S9 at 30 uV) is night and day. It's the difference between a usable rig and a display piece.

    Did you notice a decrease in the audible noise while turning those switches? I'm just wondering how much difference the silver de-tarnishing made.

    1. Yeah, cleaning everything, replacing the mini-coax, and resoldering all those connections really woke it up. I can now receive WWV at S-9 on a 12" whip down in the basement. Yes, they're only 9 miles away and run a lot of power, but a 12" whip isn't much of an antenna at 15MHz, and the radio is 6' below ground level, so I *think* it's working now.

      I'm guessing your question has to do with the conductivity of Silver Oxide vs Silver. I've always heard not to worry about tarnish on your silver-plated RF connectors as the Silver Oxide is conductive. And I have other radios where the switches are tarnished and they appear to work just fine, so I honestly don't know.

      BUT....these were VERY tarnished and had a "sheen" to them that almost made them look oily. Usually you put the Tarn-X on a silver part and the black coloring goes away pretty fast. These I had to scrub with the cotton swab to break up the oxide coating, so they might have had some other contamination on them.

      Usually just the DeoxIT is enough to clean them enough to get them back to working, but not this time.

      And I don't know if Silver Oxide has rectifying properties like Copper Oxide, which could cause birdies to be generated in the radio in the presence of strong signals.

  5. "30 MICROvolt input gives me an "S-9" reading" Not being a radio guy but have some basic understanding, how does this translate to receiving? An (%) increase in hearing more distant, fainter signals?

    1. Yeah, pretty much. A 30 microvolt signal is one thousand times weaker than a 30 millivolt signal, so you could say the work I did made the receiver a thousand times more sensitive.

      As SiG pointed out, that much difference means I now have a working, usable radio, as opposed to one that just sits on a shelf and looks pretty.

  6. Replies
    1. Damn near! I just came out of turn four, headed down the Main Straight, and I can see the checkered flag waving.

      I'll take a break on this for a few days and then go back and run the alignment again. It only takes about an hour (once everything is warmed-up) to run the procedure, but I'm almost finished, and just about ready to put it back in the case with new rubber feet.


Keep it civil, please....