Repairing VTVM Pilot Light

Lurid Red of the new Pilot Lamp
Lurid Red of the New Pilot Lamp

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.



A Cheap Fix
Also Useful for Repairing Antique Pilot Lamp Film


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.

Tape replaces old, crinkly, peeling tape from this spot.




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.

Cosmic Stink

Last night, I turned on the Hallicrafters S-120 to catch some C&W music on AM.

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 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.

An Improved VTVM Battery Eliminator

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.

An Improved 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. In the case of a Heathkit V-7A, this is 0.0 to VTVM ground, and the +1.55VDC to the ‘free end’ of the 9.1Ω resistor (see a schematic at More detail and a (somewhat better) solution for Heathkit’s grounded filament supply available here.

My First Project

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.

1982 TTL Dual Dice - Top View
1982 TTL Dual Dice - Top View

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.

A Rats Nest of Wire-wrap Wire - not the best choice, but it's what I had.
A Rats Nest of Wire-wrap Wire

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.

ATX Desktop Supply Adapter

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.

Parts List:
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:
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.

Excellent how-to’s everywhere on the net, but this one is great:

Great current ratings here: The +5 and +12 rails can supply a good deal of power and these ATX switcher supplies are very efficient.

This adapter will provide desktop power supply access for a recycled ATX Supply.
ATX Power Supply Adapter

“Most Accurate” SB-630 Update

Updated (and hopefully, finished) the SB-630 update.

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 recieved, 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 (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.

Vacuum Tube Inventory Online

Added this to the website, under ‘Other Projects‘.

Mostly for personal use, this is in response to a posting on the ARF ( Forums) regarding ‘how do you track what tubes you have’.

The inventory system keeps each user’s inventory separate, and draws from a common database of tube data (still building that common list, currently at around 450 tubes).

Some features include an ability to import from Excel (actually CSV exports from Excel), export back to Excel, print an inventory, keep (and print) a separate ‘shopping list’, and some searching facilities.

Garage Parking Lights Translation to Arduino

I really liked the Nuts & Volts Magazine project “Garage Parking Assistant” in January 2010, but I don’t ‘do’ Basic Stamp – no reason really, just not my favored platform and N&V seems to base a lot of projects on that platform.

So, I translated it to Arduino and replaced the Basic Stamp with a minimalist Arduino (Atmega 168, a clock resonator, a couple of resistors and capacitors, and an LM7805 voltage regulator). From there, code translation (BASIC to  “C++”) was easy. I’ve written up a project listing and I make no originality claims and I’ve only provided a code translation.

a personal website