3517 RALEIGH AVENUE, ST. LOUIS PARK, MN 55416
Allen Lein lives in NW Minnesota and builds wonderful test equipment. He designs all of his own circuits, winds his own transformers, and fabricates his own aluminum cabinets from scratch in his garage workshop.
Our first exposure to Lein's unique abilities was his little frequency counter (shown below).
Dial up your radio's intermediate frequency on the mechanical thumbwheel on the top of the device, then couple the output of your radio's local oscillator into the counter.
In "NORM" you'll see the oscillator frequency. If you're tuned to 1500 kHz and your IF is 455, the counter will read 1955 kHz. Switch to the "DIFF" position and you should see the frequency on the radio's tuning dial, in this case, 1500 kHz.
The “SUM” reads the sum of the thumbwheel switches and the input signal. Some shortwave receivers will operate the oscillator below the incoming signal on the highest band. So you would use "SUM" to read the dial frequency.
You can also use “SUM” to check for image rejection. For instance, if you tune the radio to 590, the oscillator is at 1045 and the image is 1500. On a poor radio you might hear 1500 weakly at 590 on the dial. With “SUM” selected, tune for 1500 to look for the image and the dial will be at 590.
On 1920's superheterodynes the oscillator is tuned separately, resulting in ”two spot” tuning. You can use "SUM" or "DIFF" to find the station.
Lein frequency counter
Al's next project was an AM receiver that used the front end and variable IF section of a wrecked Philco 37-116. He rebuilt a Clough-Brengle Wobbulator to align the IFs, then decided to build a modern "sweeper." This led to a series of sweep generators that Al used to test the IF response of a variety of classic radios.
Philco 116 sweeps
Wide (10.2 kHz) and narrow (3.1 kHz)
Lein 37-16 Philco
Here's Al's first sweeper using the XR-2206 function generator chip, The XR2206 is no longer available from Mouser or DigiKey but Jameco has them for $7.95. Here's a link to the schematics.
Al's next major project was a three-inch oscilloscope with a built-in sweep generator. It uses a vintage 3BP1A CRT and a MAX038 function generator. It's shown below connected to a variable IF coil that Allen built to illustrate overcoupling technique.
|Mock-up of variable IF coil on left; Lein's three-inch scope on the right|
After a couple more versions of the MAX038 Sweeper Lein came out with a minimalist version. This one uses four transistors and a 12-volt zener as a varicap diode. It uses the sawtooth sweep signal from a scope to modulate a fixed frequency oscillator and mixes it with another tunable oscillator, similar to the old Clough Brengle. That way, once the sweep width pot is calibrated to the scope, labels can be put on the panel and, like the old Clough Brengle, the sweep width will be constant at all output frequencies.
|Front and rear views of Lein's minimalist sweeper |
Here's a link to the schematic and bottom view
Al never sells any of his equipment; it's done simply for the joy of creation. After the challenge of sweeping IFs was overcome, Al was on to the next project; creating response curves for vacuum tubes. The device below was the star of the 2013 Radio Workshop.
The tracer is perched atop the scope on the left that displays the four grid-bias voltage steps applied to the tube under test. Grid voltages represented, from top to bottom, are 0, -5, -10, and -15 volts. The actual curves are on the Tek 455 at the right. The horizontal axis represents plate voltage (50 volts per division), while the vertical axis represents plate current (10 mA per division). Both halves of the dual triode are displayed at once, making it easy to see that the two are badly matched. The curves with the solid lines are pretty good, but the dashed lines show that half of this tube is a little tired. A perfect tube would show only four curves, since both the dashed and the solid curves would be identical.
The foundation of the curve tracer is shown on the right. The variable grid and plate voltage power supplies produce plate current changes in the tube under test. The tracer starts with a fixed grid voltage and zero plate volts, then logs the increase in plate current as plate voltage is increased. Each plate curve shows the increase in plate current for a specific grid voltage. This would be a tedious process to graph manually.
Lein’s tracer solves the problem by electronically producing the sequence of events shown below. It has the option of creating either four or eight curves; we’re showing four.
Special thanks to Bob Schreiner for converting all of Lein's drawings and schematics into great-looking graphics
Just in case you want to build your own tracer, Al has graciously provided a schematic. This is not a project for the faint of heart. Al even wound some of his own transformers.
Lein's new "Dual Tracer" presented at the 2014 Radio Workshop, operates much like his last one. It requires two scopes, one to look at the grid voltage steps and the other, with a Z-axis input, to look at the actual curves. But unlike Lein's earlier design, this one can generate and display curves for both halves of two dual triodes at a time.
Lein's new Dual Tracer
In 2016 Al sent us a couple new testers that should really save time,
an Output Transformer Impedance Tester and a Loudspeaker Impedance Tester.
|Lein's Output Transformer Impedance Tester & Loudspeaker Impedance Tester|
To see complete circuits and layout details go to LeinXfmr.html or LeinSpeaker.html
Both devices use Al's 400Hz oscillator circuit.
We asked Al if he could put together a simple, low-distortion audio generator that would give us
400 Hz and 1,000 Hz. Here's what he came up with:
It's the same Wien Bridge circuit he used for the Output Transformer Tester and Loudspeaker Tester (above).
It puts out approximately 1.25 VRMS into 600 ohms with 0.04% distortion; just fine for testing vacuum tube circuits.
Here's a description of how it works and directions how to build one yourself.