You – you lucky devil – will be learning about tube testing today. But before we do that, a background course in radio repair as a warm-up…
It was a quiet weekend on the electronics bench as Universe had presented a mildly interesting case: A super cheap (Under $100 on eBay) radio – classic Heathkit HW-100 late 60’s vintage, I seem to recall
Not a bad radio, really. Inexpensive, it was in the 100-watts output class and for thousands of hams, it was their soldering iron entry point to a career in electronics.
I didn’t have the time blocked to do a thorough work-up on this one – a more urgent radio was pending. But the initial pass through the radio revealed a non-functioning “loading control.”
This is nothing more than a simple variable capacitor – those things like look like “bread slicers” in the innards of older radio gear. What was unique about the Heathkit design was that the plate tuning and loading controls were on two concentric shafts. Tuning was the inner shaft and it was a solid rod from the front of the radio to the final amplifier compartment.
The loading was different. It was a co0ncentric sleeve that went to the vicinity of the outside of the final amp cage. And there, a large >1″ pulley was installed on the shaft. From here, the idea was that a soft rubber belt – scrounged from a plumbing supply joint – would run down an inch and a half to another pulley. And this pulley was on the front rod of the other capacitor – the one used for loading into the antenna.
There is a point to this part: My son rails at me constantly (when he’s not off working somewhere in the world) “Dad what are you doing with all these obscure parts?”
Whole thing was a 15-minute fix.
Yet today, there it was in the top drawer of the “Save me when I need it odd parts rollaround” – the precise and exact plumbing slip joint gasket that Heath engineers had used.
It was a grand morning, already, for the Radio Detective. I took a picture to put on the way: “This is why we hoard obscure but important radio shit around here…” reads a piece of masking tape (with three exclamation points following…).
I’m sure my younger sister (who is a professional hoarding fixer) would crack at such trivialities. But with 76 staring me in the face, I have no time to waste on shipping delays. Besides, my supply was bought at pennies on the dollar! Inflation’s a bitch – so when you pick a hobby, build in a Biden-Beater. Or a Fed F*ck plan. You’ll feel like a genius on the way out.
The SR-400 Case
Let’s begin with the evolution of Hallicrafters into Rockwell, first. Acquisition by Northrop Corporation: In 1966, Hallicrafters was acquired by Northrop Corporation, an aerospace and defense contractor. Sales slumped. Eventually, Northrop decided to divest or restructure parts of its operations, including Hallicrafters. They ended on the balance sheet of Rockwell International, another major aerospace and defense contractor. In the end, Hallicrafters died on the corporate vine as good people were offered other Rockwell jobs in automation or in Collins, a big name in avionics…
Now, with the backstory in mind, here’s what landed in the Radio ER this week:
The receiver in this is “hot as a $3-dollar pistol.” But the transmit side was non-existent. So it was time to start pulling the covers off.
The blue “A” is the rock solid VFO (variable frequency oscillator, the guts of tuning the radio). B is the amplifier section. Under the screwed on perf cover are two high power (relatively) transmitter output tubes. And the yellow arrow points to the black heat shield on the driver tube, a 12BY7. I’ve never (or more than 60-years of radio work) had very good luck with these tubes. Must be some kind of subtle astrological conflict.
IMPORTANT NOTE: Before opening radio, power supply is disconnected. Charged HV capacitors can kill. So all HV points in the now open radio are shorted to ground for safety! This is not a mistake you get do-overs on if you screw up. SHORT TO GROUND! Everything you want to touch. Anything you might inadvertently touch, too~
Regardless of reason, after having a quick “once-over” of the innards (looking for smoke traces and melted components) I was anxious to try the 12BY7 out on the tube tester.
Let’s explain the arrows. Top left, small yellow arrow points to the Sencore tester manual. When the clear plastic is slid to the down position, it holds the tube manual fairly flat and readable. Neat idea, really.
The lower larger yellow arrow points to the tube tester manual. It has two columns of information: In each, there is a tube number, which socket to test in, and settings that correspond to the four knobs which are to the right of our blue arrow above. Easy? The Major and I were doing this stuff at age 12. Except it was up at Owen’s Pharmacy up the hill; we we both poor to the extent that our parents didn’t provide suites full of test gear to “children.” (All has since been forgiven…)
Let’s focus next on the 12BY7: Here’s the socket number and settings dope:
The “normal short” is worth thinking about – lots of tubes have them. Here’s how the parts inside the 12BY7 are laid out:
We see that pin [3] is marked IS (internal short) which is internally connected to pin [9]. A lot of tubes so this because you can fit a component (to the suppressor grid, in this case) from either side of the tube which can make wiring more direct.
Also notice that for a 6.3 volt filament (heater) voltage pins 4 and 5 would be tied together and the 6.3 volts would be fed into pin 6. This allows (if you have a 12.6 volt filament winding) for one side to be fed to pin [4] and the other to [pin 5] while [pin 6] is left unconnected.
OK, so we put the settings into the tube tester’s A, B, C, and D positions, pop the tube itself into socket 12 and wait for the tube to warm up. Surprise! the 12BY7 is good!
If you ever find yourself wondering how long tubes should take to “light up?” The RCA Tube Handbook (depending on edition) specifies for (many/most) common receiver tubes a warm-up of 11.5 seconds generally, and 15 seconds being good.
Maybe my relationship with 12BY7’s has improved?
Meanwhile, Back in the OR
The Radio Detective considers his bench layout. The big yellow arrow left is the power supply for the radio. On the right is the radio itself. Between is the bench PC so I can eye the market now and then while working on gear…a small Win11 mini on the back of the monitor.
Looking at the clock? Time to go finish part 1 of this article, so the hand tools (so far) get tossed into the tool cases, the tube tester is stowed….
And the Detective takes a break to consider symptoms. Here’s the list for you to consider:
- There is plate voltage (draws >300 ma. when keyed so final tubes likely OK for now).
- 12 BY7 driver is good. We just did that.
- The plate tuning and loading controls are not changing plate current into a dummy load.
With a good driver, we look at the schematic and come up with a test plan: We will focus on the capacitors in the plate output circuit because a ground short on any of these (or the coupling capacitor C150 failing open) could all offer similar “no tuning outcomes.
That’s where will will pick it up next weekend.
Enjoy ham radio Field Day. See you on the low end of 40 M CW today and as the day warms up, onto the low end of 20-meters… first, a spin through 3806….
Write when you get rich,
George@ure.net ac7x
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