#67: HP-400D: A first look

[@mranalogman3525]

I have one of these and look forward to watching the rest of this series. Thanks for all your hard work putting this together. 4 years later but still valuable!

[@TheZippydoda123]

Amazing! I worked on these series of voltmeters for HP, always quality manufacturing and had Dave and Bill (founders) tour the production line. I purchased the components for these great machines!

[@transconductor888]

Your appreciation for older, quality test equipment is obvious. I share your fascination and look forward to seeing how this one functions once completed.

[@materialsguy2002]

The unit appears to be in very good as received condition. I’m anticipating the restoration process. Thanks.

[@robertcalkjr.8325]

Nice score!! Can't wait to see the next video!

[@oturgator]

Great start to a restoration series, looking forward to next videos (I have a similar (400H) voltmeter that I want to restore)

[@old64goat]

I just picked up an HP 400H for $5 at a flea market, someone added an RCA jack at the input.

[@dougmcartin3881]

AKA Bumble Bomb capacitors. Nice meters and well worth restoring. Good luck with it. ATB Doug.

[@Andrewausfa]

Gorgeous construction! Calibration I imagine demands outlandishly rare and costly instrumentation like decade voltage references etc Good luck with this.

[@estudiolatostadora8771]

Hi thanks for video, i found 3 o this voltimeter. But. Its possible use meter for use with audio, balanced signal? Thanks

[@tedrobinson372]

The reference for 1 milliwatt across 600 ohms is 0 dBm and the scale was used extensively in the early days for telecommunications and broadcasting. For example Broadcasters used the dBm reference for setting audio level. The earliest broadcasters used reference of 8dBm = vu meter 0. Broadcasters later standardized to 4dBm = vu meter 0. The BBC PPM meter set pointer 4 to 0dBm. These all depended upon the 600 ohm load. In the 1980's broadcasters began moving away from dBm measurement and reverted to purely voltage level audio measurement. It meant that the voltage established with 1 milliwatt across 600 ohms was retained but dispensing with the 600ohms and 1 milliwatt power reference. The 0.771v was deemed 0dBv.

[@jedisgarage4775]

I have got a 400H version. Problem is that it wont zero... I tried to adjust bias (R119) but it would not do much. Any ideas what could be wrong?

[@dhpbear2]

14:27 - That's a selenium rectifier. I DO recommend replacing it! I f they are overloaded, they can release poisonous gas! Their characteristics are lousy to boot!