Four more Study Groups left to scan in the RCA Radio & TV Course, 1958 Edition.
These should be a bit easier, as a couple of the Service Practices, which I actually like better than the theory bits, are missing – Service Practices 35 and 36.
I’m hoping to be done with these by mid-December. Then, perhaps some leisurely scanning of some Texas UIL Sliderule Tests from the 1970s.
I was able to discover the names of the missing booklets from items that were listed on eBay. Perhaps someone else has these and can scan them for me (or send them to me for scanning and return by U.S. Postal Mail). To match, I’m using 300dpi scanning, greyscale. I can crop or cleanup as needed and put into an Acrobat PDF file.
I found a great solution to old tape used in the Heathkit VTVMs for the RED pilot lamp. The lamp is just a #47 bulb, shining through a hole in the top of the meter face. The tape isn’t mentioned in the instructions, so perhaps it was pre-installed in the back of the meter face.
In any case, this tape sometimes falls off or is nearly falling off after 50 years. It also fades and loses it’s reddish glow. An excellent solution was found in the form of a red-neck repair from the auto parts aisle of Walmart: Tail Light Repair Tape, US$2.00.
Above: Below the roll of red, translucent tape, the old pilot lamp film and the new piece cut to replace it.
Left: Adhesive is sticky and the new piece goes over the hole in the meter through which the pilot lamp shines.
Zortch! Followed by (smolder). And a great and unholy stench was unleashed.
It seems I’d left the old selenium rectifier in-circuit. Big mistake. I’d discounted the many comments by “The Elders” on Antiqueradios.com regarding the failure mode of these old rectifiers. Never again. After using it for several months in the office, then occasionally at home, I can now say this: it may have been sitting in storage for 30 years, mean-time-to-failure (MTTF) is about a year.
And if you don’t know what burnt selenium rectifier smells like… you don’t want to.
Replaced the old selenium rectifier (which made a satisfactory ‘clunk’ in trash can) with a 1N4007 diode. Also replaced R21, a 33 ohm Fuse-Resistor which… had done it’s job by going not quite open, but to over 100k-ohms, with a 5 watt, 100 ohm resistor. This value put the DC input voltages at almost the exact levels indicated on the schematic.
Now I’m on a hard-target search for any remaining selenium rectifiers in any of my test gear or tube radios.
One of the disheartening things about buying an older (…well, they’re ALL older, now…) VTVM is opening it up and finding either an old, leaky battery, or evidence of one. This usually damages or destroys the battery contacts and sometimes also the circuit board. The battery, usually a standard 1.5V C-cell, is necessary for measuring resistances with the ‘ohms’ scale.
A great improvement has been suggested over the past couple of years to replace the battery entirely, using a modern voltage regulator, drawing power from the filament circuit. A version of this has been available on several Heathkit lists and I’ve used it in my V-7A and IM-18. This replaces the battery and removes the risk of leakage.
More recently, Peter Bertini, Pop Comm Magazine (and others) have pointed out that the circuit used an inefficient half-wave rectifier, probably adding stress to the (already old) VTVM’s transformer.
If, in ‘ohms’ mode, the probes are touched together (0.0 ohms resistance), the entire 1.55 Volts is placed across a 9.1Ω resistor, resulting in the maximum current draw of (I=V/R), or 1.55/9.1 = 170ma. This doesn’t count diode loss and heat loss in the voltage regulator.
The following circuit attempts to repair the inefficiencies and addresses ripple filtering in the regulator, so as to provide an efficient and accurate VTVM Battery Eliminator.
The input is from the 6.3VAC filament circuit. The output goes to the same locations as the original dry-cell battery.
The transformers in these VTVM’s are barely capable of lighting the two vacuum tubes. Is it possible that the addition of a ‘battery eliminator’ for the Ohms measurement could cause a problem?
While converting a Knight-Kit KG-620 for battery-free operation, I was able to measure the current draw. Instead of an LM317 (1.5A) regulator, I substituted an LM317L – 100mz, current-limited regulator.
Idle current is measured around 5 ma and peak current at around 95 ma with shorted terminals, and range switch to Rx1. Current limiting would probably curtail measurements on the low-end (below 100 ohms).
A second way to address current draw is as follows: the KG-620 has a Type #47 pilot lamp, drawing 150ma. Replacing this with a LED will reduce the lamp current by 10x from 150ma to around 15ma. This effectively ‘recovers’ any current used by the battery eliminator.
This would also work for the Heathkit V-7A and IM-18 (and variants) as the pilot lamp is also a 6.3V, Type #47 lamp across the filament supply. Added benefits: a) reduced heat, b) reduced transformer load, c) never have to replace the pilot lamp again.
This is probably the first project I’d done and it was when I worked at Mostek. I used a piece of scrap perfboard, some TTL ICs and sockets, and a few Old Style LEDs – big, old, current-consuming, 1982, RED LEDs. And some wire-wrap wire – it was all I could find, so that’s what I worked with.
The circuit is from Don Lancaster’s “TTL Cookbook”. It has worked for years and years, and uses almost any 6 to 9 volt Wall Wart. Unfortunatly, about 10 years ago a wire or two came loose from underneath (a rats-nest of misused wire-wrap). I finally got around to fixing it and actually replacing a rare burned-out LED.
I have no idea why I chose Wire-Wrap wire, other than it was something we had laying around or scrapped. It’s some of my earliest, gloppiest examples of soldering that I have. Wish I had that old Radio Shack P-Box that I build back when I was about 12.
Built one of the ubiquitous ‘Desktop Power Supply’ from a recycled, ATX-form, PC power Supply. Actually used it occasionally to power a 12V charger in the garage. Unfortunately, it fizzled an electrolytic capacitor and it’s really not worth reparing.
So, I rescued all my hardware and built a reusable adapter for ANY standard ATX power supply.
1 ATX Power extender – this is an extension to the wide plug that goes to the motherboard, usually about 9 inches or so (like this one: http://www.directron.com/atxextension.html).
All of the other hardware bits needed to alter an ATX supply for desktop use (binding posts, a switch, a 10 ohm / 15 watt resistor (I used a Dale, metal-cased).
And an enclosure – a wide, thin, Radio Shack enclosure I had on hand.
The 10 ohm 15 watt resistor goes from the +5V rail to Ground. This is needed so that the supply can sense a load – otherwise, it will not start.
Or, “Most Accurate” SB-630: Retro Style Desk Accessory, Updated on a Budget
A while back, I was given an SB-630 Station Console by an old timer acquaintance.
The SB-630 is a nice, but not especially necessary station accessory. Some hams built them just to have the complete SB-line. The console consists of a passive SWR meter, a phone patch, a motorized Digital Clock and the unique feature: a 10-minute Identification Timer. Better versions of the SWR Meter and Phone Patch were sold separately; the clock-timer was unique, so the SB-630 was merely an opportunity to wrap them all up in a single desktop accessory. Since there’s nothing unique about the SWR Meter or the Phone Patch, for my purposes, I shall focus on the clock-timer combination.
My plan was to build a new clock display, keeping some of the old style (albeit 1970’s style, not 60’s), and drive them with an Arduino micro controller. The real-time clock is provided by a Maxim DS1307 (formerly Dallas Semiconductor) 8-pin IC. The chip is tiny, uses very little current when it’s ‘on’, and is backed up (according to the datasheet for 10 years!) by a single CR2032 3-volt lithium battery.
All Tubes and the Plate-Filament transformer were removed (and saved, for future tube projects). The clock was given to a local collector who prefers to keep his Heathkits original. The functionality replaced – and enhanced – by adding a Real-Time Clock (RTC) chip, a WWVB receiver and Arduino code to interpret the 1950’s era clock signal.
The WWVB receiver reads each ‘pulse’ of the signal and interrupts the Arduino (INT1) to add the ‘tick’ to the buffer. Once the whole signal is received, it can be interpreted as a date and time. The RTC pulses (INT0) each second in order to drive the display clock.
In addition, an LM35 sensor provides the current room temperature.
Original WWVB decode source from http://duinolab.blogspot.com/2009/06/arduino-cmmr-6p-60-almost-accurate.html (Capt Tagon) and all others who’ve improved this code. Website seems abandoned, but the source code is good. My alteration is to remove the timer interrupt (1000 times a second) which operates the 1-second tick and replace it with the square wave output (SQWE) signal from the RTC chip, a Maxim DS1307.
The LCD is 16×2, and the layout needs to accommodate Date, Time, and Day, along with an ID Timer. A room temperature indicator is ‘extra’.
It’s a bit cramped, but I’m able to show everything I wanted to display, and I have plenty of Arduino pins left to trigger the ‘IDENTIFY’ lamps and ‘audio tone’, and a few pins left for future expansion.
The DS1307 has it’s own battery-backup, which retains the time on power-off.
The WWVB Receiver IC is no longer available. A better choice today would be a GPS receiver, which would provide the same (or better) accuracy and is in the same price range now, with some GPS modules selling for as little as US$14.
Finally, the ATMEGA168, LCD, ID Lamp Relay, Temperature Sensor, and the Tie-in with Original Switches.
Source Code contains classes for the DS1307 and the CMMR-6 WWVB receiver.