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The Triplett 650 Vacuum Tube Voltmeter or VTVM looks like the famous 630 Volt Ohm Milliammeter or VOM for which it probably was based on. This VTVM differs from most service bench VTVM's in that the RF probe uses a vacuum tube diode and is able to measure RF signals up to 150 MHz. These type of VTVM's are rare and this capability is often most sought after by users and collectors. The Hewlett Packard 410B VTVM is the most well known VTVM capable of measuring RF signals up to 1 GHz. Most RF VTVM's don't come with the RF probe which in turn drives the price down. This sections shows how a restoration entails not only repair but fabricating an RF probe.
Original Accessories and Specs
Repair and Restore:
1. Power transformer a. Unmount the power transformer and was able to locate broken lead out wires from the primary winding. Carefully straighten and prep free ends so extensions can be soldered on.b. Checked resistance and it reads 159 ohms.c. Applied spray on adhesive over exposed windings and extensions, then placed protective paper to cover exposed windings.d. Reconnect 6.3 V line to 6AL5 and checked resistances of transformer secondaries with values of 1 ohm and 330 ohms. 2. Securing power transformer a. Secure one side to minimize movement to repair broken bakelite transformer b. Epoxy broken part and completely mount the piece to the main body.
Other Upgrades:
3. Install new neon NE2 lamp and resistor.4. The 20K ohm +/- 5% plate resistor was out of tolerance and was replaced.5. Decided to replace the 50 uF 250V capacitor with a 33 uF 160 V capacitor.
RF Probe Fabrication:
Fortunately there is a schematic of a Triplett 650 RF probe in the operations manual. The probe will have a metal box to house the 6AL5 tube plus a few components. The cable from the probe box to VTVM needs to be a shield cable to take up to 1000 VAC. The only thing was easy to get that parameter would be a coax cable with the center conductor having a non-foam insulator. Foam is only take 600 V. Also had to find a screw on two pin microphone made by Amphenol and is now obsolete. Able to find a source that sells an aftermarket Amphenol equivalent and got one.
Completed Probes
Above: AC/RF Probe
Above: AC/RF Probe
Conceptual Idea
Conceptual Wiring of AC/RF and DC Probe.
DC/Resistance Probe
Restored showing new probes
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