For much of the last twenty-plus years, I used simpler service grade generators, such as my trusty old EICO , but when working on a television restoration, I decided that I needed something better. Although the EICO can generate high frequencies, its direct-drive mechanical tuner is too crude to dial in a precise frequency in the tens or hundreds of megahertz. Enter the HP C, which has a numeric keypad and digital readout, and more than enough accuracy to meet my needs. Hewlett-Packard described this instrument as a "system" rather than a single device. The modulator can generate modulated signals internally or accept external signals.
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For much of the last twenty-plus years, I used simpler service grade generators, such as my trusty old EICO , but when working on a television restoration, I decided that I needed something better. Although the EICO can generate high frequencies, its direct-drive mechanical tuner is too crude to dial in a precise frequency in the tens or hundreds of megahertz.
Enter the HP C, which has a numeric keypad and digital readout, and more than enough accuracy to meet my needs. Hewlett-Packard described this instrument as a "system" rather than a single device. The modulator can generate modulated signals internally or accept external signals. The RF section controls the RF output level. The next photo shows the mainframe section at the left, with its big keypad and digital readout.
The smaller B modulator and A RF sections are located to the right, each with its own meter and controls. The C has a host of features and options. The customers for this device were obviously government and industry, not casual hobbyists. Today, however, you can find used C generators for sale in the range of hundreds of dollars, not tens of thousands.
This model was sold for twenty years, from , and many of them are still usable. First Look I bought my C on eBay, from a surplus seller who described it as nonworking. The cooling fan was missing, as the seller had disclosed beforehand.
Time to locate a service manual! I posted a query in the Test Equipment forum at antiqueradios. The case measures mm, a common fan size, but a CFM fan blows harder than your typical computer fan, which might deliver 50 CFM or less.
The local parts store had nothing appropriate. The results were mixed. The numeric display on the mainframe did not light up. However, the two plug-in modules did have power; their meters moved and responded to controls. Restoring Power to the Mainframe With some time on my hands, I started looking through the service manual to familiarize myself with this complex beast.
As you can see below, most of the internals are contained within shielded modules. The first photo shows the top and the second shows the bottom. After you loosen one screw, this large module the "reference section" can be swung up on hinges to provide access to boards in the mainframe: Near the back, I found one card that could easily be pulled, after removing only one screw.
This is the supply regulator card, A5: While I had the card out, I spot-tested a few of its electrolytics with my EDS capacitor analyzer , and they looked fine. The contacts on the card looked clean and bright, so I simply replaced it. To my surprise, the next time I tried powering the generator, the main numeric display lit up, showing the default readout of 1.
Installing the Cooling Fan On page of the service manual, I found the schematic for the line supply and fan card. Terminals 9 and 6 on the card supply power to the fan B1 : I connected new leads to those terminals and wired them to the fan: When I tried the generator again, the numeric display lit up but the fan refused to operate.
The fan is mounted inside a rear compartment that swings down from the main chassis. When I reopened the compartment and swung it down, I saw that three of the four leads from the main chassis to the fan card had broken. Every time you swing the compartment down, those four thin leads are flexed. I guess the stress of opening and closing the compartment several times was more than the old connections could stand.
The cable had just enough slack to allow me to strip the lead ends and reattach them. After adding a new mm fan guard from a local parts store, I secured the compartment one last time. The new fan worked perfectly and the displays lit up on the mainframe and both plug-in modules.
Basic Operation Checks At last, I could run the generator without worrying about an imminent meltdown. The checks confirmed that the keyboard and other controls were working, and that the display showed the expected output values. The keyboard operation reminded me of using my old HP scientific calculator a few decades earlier, when I was learning to write software in assembly language and various high-level computer languages. Just as with the calculator, the generator uses "reverse Polish" syntax.
First you enter a numeric value, and then you enter the commands operands that you want to use. For example, to set the center frequency CF to 3. I measured each of these points and I was heartened to find all four voltages right on the money.
Many of my test cables use BNC type connectors, so I bought a couple of adapters: Using real cables instead of clip leads, I was able to connect my oscilloscope in addition to the frequency counter. In this photo, the generator is set to How Accurate is Accurate Enough? My Grundig Yacht Boy is a modern shortwave radio with digital tuning, and I have confirmed many times that it correctly receives station WWV at its various frequencies.
WWV is operated by the National Institute of Standards and Technology and its broadcast frequencies are tightly controlled by atomic clocks.
When I set the generator to 10MHz KHz , the radio received that signal at exactly the same frequency, and few checks at other frequencies gave comparable results: The radio test tends to confirm the idea that my megabuck lab-grade C generator is more accurate than my not-very-expensive frequency counter. On the down side, this is a large and very heavy device. My little old EICO was easy to keep on a shelf and pull out whatever I needed it, but grunting this unit up onto my workbench is a bit of an ordeal.
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HP Agilent Keysight, 8660D
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HP 8660D PDF
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