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Old 7th Apr 2023, 4:16 am   #1
Ingo_Seidel
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Default Any HP 410C VTVM experts around?

I bought a HP 410C multimeter from the sixties in fairly good condition.
DC V and A and Ohms are very good but I am not able to calibrate the AC V ranges.
The supply voltages are all inside their specified range.
The first issue is that I am hardly able to do the AC zero alignment, the coarse adjustment pot A3R31is almost at the cw limit, I had to bypass the A1R7 resistor to have any headroom for adjustment.
The 0.5V, 1.5V, 5V and 15+V trimmers are at their ccw limit to just or not quite get a correct reading.
Is anyone here familiar with this specific piece of equipment?
First I suspected the AC probe to have somehow deteriorated, but even when feeding DC voltage into the AC socket, I am not able to do an alignment.
Furthermore I noticed that the Orange Drop capacitor A3C1 is quite sensitive to temperature changes - do those fail this way?
Any help would be appreciated.
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Old 7th Apr 2023, 9:54 am   #2
Ingo_Seidel
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Default Re: Any HP 410C VTVM experts around?

The capacitor seems to be fine, isolation resistance beyond measurement and capacitance excellent
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Old 7th Apr 2023, 10:34 am   #3
frsimen
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Default Re: Any HP 410C VTVM experts around?

I've never worked on one of these meters but I think you are saying that the DC and ohms ranges can all be brought into calibration correctly, while the AC won't adjust.

Applying DC to the AC input jack isn't going to work, because there is an offset introduced by the AC zero circuit at the top of the diagram. This is intended compensate for an error caused by the valve in the AC probe. DC won't need this compensation, so the readings will be in error.

As everything else is working properly, the problem is most likely going to be in the probe itself, perhaps the diode valve has low emission, for example. Have you tried changing that, or has someone substituted a silicon or germanium diode?
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Old 7th Apr 2023, 11:00 am   #4
Ingo_Seidel
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Default Re: Any HP 410C VTVM experts around?

Yes, I understand that offset but considered it not relevant for the higher ranges.
The calibration pots for the higher ranges stay at the limit nevertheless.
I got to think this through…
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Old 7th Apr 2023, 11:07 am   #5
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Default Re: Any HP 410C VTVM experts around?

Substitution by a silicon diode is an undesirable option.
I have some 400V ultrafast diodes, rated 75pF compared to 0.5pF of the vacuum rectifier.
Though I never might need an opportunity to measure 700MHz with several V of amplitude, I just would like to completely restore the multimeter..
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Old 7th Apr 2023, 12:34 pm   #6
factory
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Default Re: Any HP 410C VTVM experts around?

Quote:
Originally Posted by Ingo_Seidel View Post
The capacitor seems to be fine, isolation resistance beyond measurement and capacitance excellent
And you checked it at different temperatures? as you mentioned temperature change to be an issue.

I'm still puzzled as to why a 1960s 410C has orange drop capacitors, mine has these polystyrene radial cans, with one of those old carbon resistors on the back.

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Wonder if some of the issues could be drifted resistors, some high value ones in there, for the older 410B the probe cable is a known issue, insulation can break down causing shorts.

P.S. this is also being discussed on eevblog here; https://www.eevblog.com/forum/repair...alists-around/

David
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Old 7th Apr 2023, 1:56 pm   #7
Ingo_Seidel
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Default Re: Any HP 410C VTVM experts around?

This is my parallel thread 😆
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Old 7th Apr 2023, 7:59 pm   #8
factory
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Default Re: Any HP 410C VTVM experts around?

I've actually got another one of these, it's a bit later (1970s build) 954 prefix and made in Germany, these usually have differences that will not be covered by the US manual, the correct change sheets are required.

It contains a Rev E A3 board, with orange capacitors, still has the valve fitted, later A3 boards were solid state.

David
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Old 8th Apr 2023, 7:36 am   #9
Ingo_Seidel
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Default Re: Any HP 410C VTVM experts around?

Thanks for the hint, David.
Finally I found some date codes that indicate a production date in ‘77.
So the unit is younger than expected but still equipped with the valve.
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Old 8th Apr 2023, 9:43 am   #10
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Default Re: Any HP 410C VTVM experts around?

It looks like one of HP's nickel-plated boards. Soldering for these could be difficult. The production line wave solder machine used an acid flux that needed neutralising afterwards. Hand soldering and repair was difficult. Have a look for bad joints where repairs might have been made.

It's likely to have been made in the HP plant at Boeblingen.

David
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Old 8th Apr 2023, 10:03 am   #11
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Default Re: Any HP 410C VTVM experts around?

Mine is gold plated and has stampings „made in USA“ on every board.
It bears an inventory number of the Bundeswehr (German Army) but I don’t know who else owned it.
Inside it looks very good, only little dust, the rotary switches have been cleaned.
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Old 8th Apr 2023, 11:17 am   #12
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Default Re: Any HP 410C VTVM experts around?

One observation, it has a diode switched into the feedback path on AC ranges to offset the threshold voltage of the diode in the probe. If this was bad, it would offset the AC zero setting quite badly.

Other than that, it's a DC meter with an AC detector in the external probe.

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Old 8th Apr 2023, 1:39 pm   #13
factory
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Default Re: Any HP 410C VTVM experts around?

Quote:
Originally Posted by Radio Wrangler View Post
It looks like one of HP's nickel-plated boards. Soldering for these could be difficult. The production line wave solder machine used an acid flux that needed neutralising afterwards. Hand soldering and repair was difficult. Have a look for bad joints where repairs might have been made.

It's likely to have been made in the HP plant at Boeblingen.

David
The board pictures are from a prefix 550 410C made in the US, the meter face is starting to peel, there are lots of signs of rework in this one, I can't remember if it worked when I got it, it's ex MOD.

The other one I have is the German made one (prefix 954), this currently has no bottom cover, or fuse holder cap fitted and all the test leads are perished, meter is good in this one. Not taken any pictures yet, needs a good clean, lots of dust inside from the perforated covers.

Sorry to add to the confusion, I have to agree the boards of this era are tricky to work on, the valve sockets can be a pain to resolder too.

David
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Old 8th Apr 2023, 2:44 pm   #14
Ingo_Seidel
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Default Re: Any HP 410C VTVM experts around?

A6CR1? It tests fine.
Does the needle jump around on your meter when you touch the AC probe as well?
0.5V AC range and probe tip shorted to GND clip.
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