In Praise of Two Vintage Voltmeters HP8405A and HP427A

[Radio1950]

In Praise of Two Vintage Voltmeters HP8405A and HP427A.

HP8045A Vector Voltmeter

The first is the venerable HP8405A Vector Voltmeter which when released, revolutionised some types of VHF UHF RF measurements.

THE HP8405A is a two channel wide bandwidth RF voltmeter, which could display the amplitude and phase difference between two signals, once Channel “A” had locked up the auto AFC control.

The A channel could be used just as an RF voltmeter.

Coupled with its Accessory Kit and a dual directional coupler (eg HP778 inter alia), VHF RF measurements became relatively easy.

Prior to the release of this vintage VVM, and away from the lab, we had to use VHF bridges etc to adjust VHF RF devices, and to cut sections of coax line etc.
I remember using these bridges, and with some impetuosity of youth, I found them difficult to use in practice.

Along came the HP8405A, and what a revolution and a revelation!
We could now do most of these difficult RF jobs easily, and with accuracy and practicality, and away from the lab.

My only wish, looking back, is that we had available to us, ferrite or powdered iron core transformer type dual directional couplers for HF.

I don’t know if the HP8405A was the first of type in the world, I think it was, but it would be interesting to hear from other forum members if they are aware of any precedents.

I have a fondness for the HP8405A, using it for many years on the bench at work, and in the field, and even just beyond of its specified frequency range, eg constructing 90 degree coax sections at the SSR frequency of 1090 MHz.

Later HP digital models became available, and were better in some aspects, but the original HP8405A, with those large analogue meters, was very easy to use and I would expect that some units will be in use by techs for years to come, as it was repairable.
But you did have to buy a bagful of spare probe tips, which were tiny and always breaking for one reason or another.


I don’t own a HP8405A, but I do have a ....

HP427A Voltmeter

The HP427 is a small solid state bench or field VOM type meter (without current measurement), and superseded the older (very good) valve HP410 series VTVMs.

It’s large and easy to read meter scale, and sensible ranges, coupled with good accuracy and portability, made it a success.

Typical of HP quality, it is very well constructed.

I have had one of these on my bench for over 25 years, in regular use, without any fault.
The ZERO pot is sometimes a little scratchy, and I occasionally check the internal capacitors.
My meter is completely original.

I have the 01 option, designed for battery or mains, but I use it only on mains.

Highly recommended.

Some photos of the HP427 below.

Why chose these two voltmeters from many others?
They were released at the same time, and featured in the same HP Journal of May 1966.
HP, take a bow, and thank you.

http://hparchive.com/Journals/HPJ-1966-05.pdf

HP8405A Manual
http://users.monash.edu.au/~ralphk/H...e%20Manual.pdf

HP427A Manual
http://hparchive.com/Manuals/HP-427A-Manual.pdf

.

[Jac]

Just a few photos of my 427A.

Jac

[MrBungle]

I have a 427A as well. Very nice meters. I'm not using it at the moment due to a serious space problem and the convenience of some more modern meters so it is packed away in the cupboard.

Only photo I have is a poor action shot with it in the corner when I was playing with a super VXO, next to my 5315A (sadly departed and replaced with something I thought was going to be better but wasn't from TTi):



Mine is the older style one with the coloured posts and black knob with mains option. I don't use it on batteries in case they leak.

I might clean up my bench and make a space for it now I've seen this thread

[The Philpott]

I sometimes find that noisy potentiometers from this period (on equipment not used every day) can have their useful life extended if they are run through their full travel and back again before using them for fine zeroing adjustment.

Dave

[Radio Wrangler]

The VVM was an amazing tool. The remark about wide bandwidth can be misleading. It actually has very narrow bandwidth, but a wide tuning range. It's a down-converter into a dual narrow IFs and it phase locks its local oscillator onto the incoming signal plus an IF offset value. THat's why it has the frequency knob to do a coarse tune to give the phase lock search somewhere to start from.

Some of the people who designed the later digital replacement were rather good friends of mine. They stuck to the same architecture and extended the range to 2GHz, but without those big Simpson meters it lost something.

One interesting use is measuring the phase shift across a crystal in a fixture to measure the resonant frequency more precisely than amplitude alone gives

Another one is using attenuator accessories on the probes allows impedance measurements in transmitters at full power with live signals - something VNAs still can't do.

THe other little HP meter I'd add to your duo is the 432A The DC transfer standard RF power meter. Also a classic.

The HP journal articles and the manuals of this era are wonderful too.

DAvid

[Skywave]

When it comes to to instrumentation, I've always rated HP gear of this vintage very highly. I don't own a 427A, but I am on the look-out for one.
As for the 8405A, I remember it well. In the early 1970's, I used one frequently during the design of HF filters and BB amplifiers in the range 1 - 30 MHz. I'd have one today, but for two 'limiting factors': price and the amount of bench space it occupies!

Al.

[G0HZU_JMR]

Yes the HP8405A VVM is a very special piece of equipment. I have an early HP8405A here at home and I bought it to help with a research project about 5 years ago.

I bought it cheap on ebay for about £80 but sadly, it didn't work when it arrived. You can read about my repair saga with it here:

https://www.vintage-radio.net/forum/...d.php?t=102117

There were so many age related faults on it that I'm not sure I would recommend buying an early model of the HP8405A unless it is known to be fully working. Best to look out for a later one with the LED lock light.

Since I got this one running it has been completely reliable for about 5 years and I've used it for lots of things since the original research task. I also have the classic 778D directional coupler to use with it. In my first job there were loads of these VVMs in use in the RF labs and I have fond memories of using them.

The HP8405A is one of a select group of test gear that still gives me a tingle when I use it because it has such fabulous design integrity.

However, I'm not so keen on the HP427A as it doesn't measure true Vrms on its AC range. I think there are better (vintage) choices. The HP3400A is a classic Vrms meter but my favourite is the old Racal 9300. The 9300 is a compact 20MHz Vrms meter and is a reliable old friend and I've used lots of Racal 9300 meters throughout my career and I've also got one here at home.