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 Post subject: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Mon 25, 2019 6:50 pm 
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Joined: Jul Tue 23, 2019 2:43 pm
Posts: 25
Location: San Francisco 94103
Hello,

I ran across a fairly weathered, non-working H-P DC VACUUM TUBE VOLTMETER at a recycling center yesterday. Appears to be an 412A?

After opening and taking a quick look-see, all of the parts seem to be in place sans the model identifier badge on the front panel, it's dusty and neglected, but for $5 I thought why not take a chance as a possible restoration project.

I've downloaded the 412A service manual, but before going into the rabbit hole any deeper I was wondering if anyone has any comments, caveats or cautions about this unit that I may want to consider before starting?

My interest here is mostly as a challenge and as learning experience. Not expecting to turn this for a profit.

Thanks in advance!

Greg


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Mon 25, 2019 8:56 pm 
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Joined: Jul Sun 21, 2019 3:32 pm
Posts: 306
Location: Upstate NY by the big lake!
I just got my RCA Senior fixed and calibrated. I put a lot more time than I expected figuring out how it worked, and replacing parts.

I wanted the same thing as you, to learn. And I certainly learned a lot.
Advice... figure out what is working, what is not and break the circuit down into manageable bites.

The people here were great, as was the website Funwithtubes.com.

Good luck and be patient.


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Mon 25, 2019 10:05 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 10009
Location: Long Island NY
That's a 412A. Interesting and still usable instrument, but it's a chopper stabilized AC amplifier type of instrument, considerably more complicated than a "two tuber" like a Heathkit V-7A or RCA WV-98. Sensitivity will be quite a lot more on DC volts, ohms, and current, allowing a wider range of measurements, but no AC ranges.

First thing you need to do is make sure you have the right version of the manual. The 412A was in the catalog a long time and there were changes over the years. The serial number should be on the chassis, visible from the back. The manual should match the serial number range of the instrument. If not, look for a later version of the manual. Later HP manuals usually have back-dating pages for earlier instruments, but obviously an older manual won't necessarily be correct for a newer instrument.

The trick to these is the chopper which is a spinning disk that interrupts four lights shining on four photocells. If it is not spinning at the proper RPM the circuit won't work because they filter around the expected chop frequency. One or more bad chopper bulbs will stop it from working; if you change one change them all because differences in brightness may throw things off enough to cause erratic performance.

HP went to a lot of trouble to use the best parts they could and it's best not to replace anything that you don't absolutely have to. I test before changing parts and ones that are still in working order are left alone.

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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Mon 25, 2019 10:24 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8234
Location: Sunbury, Ohio 43074
It's a great learning instrument, but don't be upset if something, like the chopper, proves to be unrepairable. But even if so you will still learn a lot. As a bench VTVM I might opt for something a little newer, but this is a cool looking one, and possibly a decent place to start. Check around the other HP series of this era and slightly newer, and you may find one you'll eventually like better..... then you'll have two. ;-)

this is how the hoarding starts........

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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Mon 25, 2019 11:15 pm 
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Joined: Aug Tue 28, 2018 9:22 pm
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Location: Sanford Fla 32771 (USA)
Here is a good explanation of chopper stabilized amplifiers.
https://hackaday.com/2018/02/27/chopper ... bout-then/

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Paul of Florida ….I had my patience tested. I’m negative.
https://paulsironhorse.smugmug.com/


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Mon 25, 2019 11:26 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1672
The HP 412A isn't an ordinary VTVM. For example:

1. It is totally isolated so that you can put the black lead anywhere you like in your circuit without a problem.
2. It has a resistance range that goes lower than most meters, with a center scale value of one Ohm, and it's a four wire resistance measurement, at least up to the test clips.
3. There are no ac ranges.
4. The current range goes down to very low values, useful for measuring leakages of diodes and capacitors.
5. Its accuracy specification is better than most VTVMs.

I keep one on the bench all the time, finding it of value in more than just ordinary measurements.

Bob


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Thu 28, 2019 7:16 am 
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Location: Lincoln City, OR 97367
Greetings to Greg and the Forum:

If it is one of the mechanical chopper-stabilized meters, don't waste your time... part it out. I have yet to run into an HP meter with the mechanical chopper that wasn't already useless due to chopper failure or suffered a chopper failure soon after I got it. If they do fail, they are un-repairable. They use factory selected and matched CdS photocells that are unobtanium.

HP was a good company.... but everyone is entitled to one mistake. That chopper-stabilized amplifier was HP's.

Regards,

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Jim T.
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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Thu 28, 2019 7:37 am 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 1672
Not mechanical. It's optical. The only moving part is a motor, almost identical to the ones in the old electric clocks. I have spares but seriously doubt I will need them.

It uses #12 lamps and polystyrene tubes. It must be well thought out, since the main failure modes of these meters have nothing to do with the chopper. They had problems with the range switch material deteriorating. Mine is okay.

Download the manual and see how they do it. Nice meter.


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Thu 28, 2019 8:05 am 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to Bob and the Forum:

Sorry, but I must beg to differ. If it has a motor and a spinning disc (which these do), it is a mechanical chopper... as opposed to an electronic chopper using solid-state switches such as FET's.

Every one I have ever come in contact with has failed due to degradation of the CdS photocells. I understand that there is someone in the HP group over on Yahoo that has designed a FET chopper that replaces the mechanical one. However, I have no interest in getting a Yahoo account and one cannot communicate with that group without one, so I don't know any details. IMHO, none of the instruments that use that circuit are sufficiently unique or useful to be worth the trouble.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Thu 28, 2019 8:17 am 
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Joined: Jan Tue 10, 2012 8:39 am
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The main thing is that it does not use mechanical contacts, which is what I meant when I said it was an electronic chopper. Yes it has a moving part. I have heard stories of failed photosensors but never had one go out on me.

Meanwhile the meter works as new and is rather useful on the bench. The high sensitivity current ranges are nice. And 1 millivolt full scale is nice too.

This meter is great for setting idling current in audio amplifiers, where you need a floating meter with high sensitivity. It's also good for measuring diode leakage.

It's been a few years since I got mine working so I guess it's reliable enough. And I have two parts donor units.


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Thu 28, 2019 4:00 pm 
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Location: Sanford Fla 32771 (USA)
Quote:
it is a mechanical chopper

I would call it an optical chopper.

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Paul of Florida ….I had my patience tested. I’m negative.
https://paulsironhorse.smugmug.com/


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 Post subject: Re: Hewlett-Packard 412A Restoration. Cancel or allow?
PostPosted: Nov Sat 30, 2019 7:42 am 
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Joined: Dec Thu 03, 2015 11:33 pm
Posts: 119
I have a 412A and a box of spare parts. Let me know what you need.


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