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Found in the Attic: RF Test Twofer

Posted by Tom Vernon on Dec 28, 2015 2:19:05 PM

Millen 90651 Grid-Dip Meter & Heathkit HD-1250It's hard to believe in this day of digital technology and broadband RF strips, but at one time a solid understanding of tuned RF circuits was an essential part of electronics troubleshooting, as well as to getting your FCC First Class License. Today, the grid-dip meter is all but forgotten, but it was, and still is, an essential item in the RF engineer's toolbox. This Found in the Attic is another twofer, and looks at both the Millen 90651 Grid-Dip Meter and Heathkit HD-1250 solid-state dip meter.

The Millen 90651 was introduced in 1949, and was manufactured for a number of years. The Heathkit HD-1250 was introduced in 1975, and was listed in the Heath catalog until 1991. It replaced the Tunnel Dipper HM-10, and in 1962, the HM-10A, which was in production until 1970.

The circuitry of the grid-dip meter (or simply the dip meter when solid state devices are used) is simplicity itself. In fact, grid-dip meters make good construction projects for those new to RF. Countless plans have been available over the years in the Radio Amateur's Handbook asMillen_meter well as magazines such as Popular Electronics.

In the case of solid-state meters such as the Heathkit, the components are organized on two boards for the oscillator and detector circuits.

In the injection mode, the oscillator generates a signal which is injected into the device under test. The detector circuit detects changes in the impedance that is reflected back from the device under test, and displays these changes on the meter. In the absorption mode, the oscillator is used as a Q-multiplier. The detector detects the signal that is already present in the device under test.

The HD-1250 uses a balanced Colpitts RF oscillator. The transistor is a UHF bipolar type, operating Class A. A variable resistor controls the power supply voltage to the oscillator, and thus the level of regeneration.

In the detector, the voltage impressed on the oscillator tank and is then amplified by a broadband MOSFET amplifier. In a balanced phase-splitter configuration, the transistor develops two output voltages across a HD1250_-_casepair of resistors. These out-of-phase signals are rectified by hot-carrier diodes. These detected signals are combined and filtered by capacitors in order to develop an average positive-peak DC voltage, which is indicated by the meter.

There are several ways a grid-dip meter can be used. Perhaps the most common is as an oscillating frequency meter to determine the resonant frequency of de-energized RF circuits. When a tuned circuit which is resonant at the oscillator's frequency is coupled to the probe, it absorbs energy, and this is indicated by a dip in the grid current reading. The point of minimal grid current, as read on the calibrated dial is the resonant frequency.

A popular application in broadcasting is tweaking transmitters for best efficiency. Most manuals have tuning charts which show capacitor values and coil taps for different parts of the band, typically three or four. These values will get the resonant circuits of the transmitter in the neighborhood and meeting published efficiency specs. Going through those same circuits with a grid-dip meter will usually yield a significant improvement in efficiency.

The grid-dip meter can also be used as an oscillating detector, for determining the resonant frequency of energized RF circuits. The application is identical to that for the oscillating frequency meter described above; only a pair of headphones is inserted in the phone jack. The calibrated dial is tuned while listening to beat notes.

This instrument may also be used as a basic signal generator. You won't be able to apply modulation, sweep or set a precise RF level. However, you're good to go for applications where you just need a stable source of RF.

As a tuned RF diode or non-oscillating detector, this instrument becomes an absorption-type frequency meter. No plate voltage is applied, and the tube functions as a diode. TheMillen_meter__tubes grid meter is then in the diode load circuit, and will read upscale when the instrument is tuned to a source of RF emissions.

Maintenance and calibration of grid-dip meters is pretty simple. Unless the meter is dropped, tubes last forever, due largely to the low battery voltages used. In the early days, calibration was checked by zero-beating the meter against WWV, and adjusting its dial if necessary. More recently, the meter is loosely coupled to a frequency counter and checked that way.

The frequency range of our Millen and Heathkit meters stops just above the broadcast band. In the case of the Millen, three additional coils could be purchased which would extend the range down to 220 kHz. For the Heathkit, you would need to fabricate your own coils.

Boston and its surrounding suburbs were a hotbed of electronics manufacturing from the early days till after the war, due in no small part to their proximity to MIT. Malden Mass is remembered by radio amateurs as the home to both the National Radio Company and the James Millen Manufacturing Company. Young Jim Millen started his career at National, taking the company, in just 13 years, from a toy manufacturer to the leader in ham and commercial radio. In an early example of corporate greed however, National's financial backers tried to squeeze Millen out of the profits by raising their own salaries, thus reducing the amount of profit to be split. Millen left and started his own company. The bitter experience with National also led him to learn more about finance, and he became a banker as well as electronics entrepreneur.

The May 1938 issue of QST carried an ad announcing the new James Millen Company. The products listed were coil forms, insulators and every form of RF hardware needed to fabricate radios and ham gear. Later, RF test gear such as absorption wavemeters, secondary frequency standards, SWR bridges and grid-dip meters were added.

While RF hardware and test equipment are most often associated with the Millen logo, they were not the company's biggest money maker.

Subcontracting for larger firms such as Raytheon, GE and RCA was what really paid the bills. It often accounted for three quarters of the company's total output, and frequently subsidized the amateur radio products.

HD1250_sideCompanies often found it was cheaper for Millen to make equipment that to set up their own production lines. A few products came to the company ready to go into production, but more often, all that was supplied was a schematic and a more-or-less working prototype. From there, Millen would design the mechanical fabrication themselves, and then go into production. Solving the mechanical issues often entailed fabricating new RF components and hardware, and these items in turn, would be added to the amateur radio component product line.

In the early days of electronics, most of the engineers and managers knew each other personally, and most relationships between companies were ethical and cooperative. An example is when RCA approached Millen to produce their service oscilloscopes.

To set up a production line, Millen needed $100,000. Getting that kind of loan through the banks at the time would have been very difficult. So RCA agreed to sell CRTs to Millen, who would install them in the scopes and sell them back to RCA at the same price. Millen's invoice to RCA was paid within 10 days, while RCA was allowed six months to pay their invoice. The result was a $100,000 interest-free loan from RCA to Millen.

The company continued with subcontracting and selling RF gear for another 38 years, but times changed. The outdated Malden factory stood in the way of developers and urban renewal, and had to go. The cost of building a new OSHA-compliant factory would have been prohibitive. Around this time, Jim Millen also suffered a heart attack. The combination of these two led him to the decision to close the factory in 1977. He retired to his farm and continued with his banking interests.

The Millen 90651 is in like-new condition, and was acquired about 20 years ago. It was a gift from a radio amateur friend who is long gone. It is still in the original box, and has seen very little use. The Heathkit HD-1250 is in the toolbox, and gets regular use. It is often used to check the tank circuits in FM transmitters, where a surprising number have been tinkered with to the point of being grossly inefficient, or not working at all.

Topics: Vintage Radio Technology