I’ve been on the lookout for a DuMont 405 VTVM since Mike Kent posted about it on the Antique Radio Forum.
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 …
… such the DuMont 346 AC VTVM.
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.)
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.
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.
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.
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.
They are similar to the Elwyn military test lead sets sold on eBay.
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.
Here are the specs on them.
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.
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.