DuMont 405 VTVM

I’ve been on the lookout for a DuMont 405 VTVM since Mike Kent posted about it on the Antique Radio Forum.

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In 1957, DuMont introduced their “400 Series” of test equipment, including the Type 405 VTVM. 

I hadn’t been aware that DuMont made any test equipment other than oscilloscopes, but even prior to the 400 Series, they at least sold rebranded test equipment …

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… such the DuMont 346 AC VTVM.

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The DuMont 356 is clearly a rebranded Waveforms, Inc Model 520-A AC VTVM. Was the 400 Series DuMont’s first foray into engineering their own test equipment? (Other than oscilloscopes, of course.)

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I managed to find a rack-mount version of the Type 405 on eBay and couldn’t resist buying it, though I feared it was likely missing the probes.

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These are eBay photos - I’ve received it, but I haven’t taken any photos of my own yet.

Mike Kent was looking for a manual for the Type 405. Steve Rosenfeld pointed me to the library at the InfoAge Museum. They will sell you a photocopy of their DuMont Type 405 Operating and Maintenance Manual. The sale of manuals provides income to support the library. Contact them at info@infoage.com for a price quote and payment options.

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The manual included some interesting photos. Though it’s hard to see in the scanned photo, the DC probe on the right is shown without a tip.

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DuMont provided a set of interchangeable probe tips that screw into the AC and DC probes. The manual says the socket for the tips is spring-loaded so that they will retract.

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They are similar to the Elwyn military test lead sets sold on eBay.

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On the non-rack-mount model, the DC probe is permanently attached, but the AC probe has a male BNC connector that mates with an RG-58A cable from the meter. It turns out that the DuMont Type 405 achieves its wide AC frequency and voltage range by using three different AC probes. 

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Here are the specs on them.  

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The probes use a DR313 semiconductor diode. I presume that’s a DuMont type number. I can’t find any specs for it. The VHF and UHF probes are rated for a max of 30 Volts AC, so that’s similar to a 1N34A. The UHF probe gets its higher frequency range by using a smaller input capacitor that has less inductance than the larger capacitors used in the VHF probe. They are otherwise identical.

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Philip Colston was kind enough to provide photos of the DuMont probes over in the ARF thread. I think I can make reasonable reproductions of them.

© Steve Byan 2011-2019