HP410B Vacuum Tube Voltmeter (VTVM) Repair

[@greengrayradio1394]

Often ferrous metal shavings, if allowed to enter, stick to the magnet gap and block the meter movement from sweeping across the scale. I have cleaned several with this "disease" with strong light and strips of sticky tape after finding the offending piece. Great patience is needed.. A clean work surface is a must for this

[@peteroneill404]

Yes, this can cause meter movements to stick. In my case I carefully inspected the movement under a strong light and 10x magnification and couldn't see anything obvious. As the instrument was generally very dusty, I decided just to try cleaning the movement and this approach worked.

[@greengrayradio1394]

@@peteroneill404 Maybe the set screws for the jewel bearings are too tight?

[@InssiAjaton]

After cleaning, I would think some (like 1/4 drop) of very low viscosity oil at each jewel bearing would be recommended. The kind jewelers use for clocks or wrist watches. Then, I remember years ago in an HP Journal they showed how the movements of analog meters were typically non linear. In fact in this video it also is pretty clear, different ranges exhibit the same physical differences at the same locations ( 2 - 4 - 6 - 8 ). As I have understood, the vintage ultra precise analog meters had some sort of magnetic field trimming yokes for trimming their linearity. HP did later on custom scale printing, like for each HP 4000 true RMS meters. That was what their Journal article demonstrated. They showed two scales printed superimposed, for two nominally same meter movements. I guess they found it cheaper to do, than tweaking any magnetic field compensation yokes.

[@peteroneill404]

Yes, it is well known that analog meter movements are nonlinear, its particularly bad in the lowest 10% of deflection, so best practice is to make measurements with 30-100% of full scale deflection, that's one reason for the 1, 3, 10, 30... range scaling. And yes, the scale can also be used to correct for some of the nonlinearity. In addition to the meter movement this meter like most VTVMs has a nonlinear scale for ac measurement, to compensate for the diode in the ac probe, which is also nonlinear, worst on the most sensitive ranges.

[@greengrayradio1394]

Jewel bearings in meter movements like these shall not be lubricated, but left dry

[@EvLoutonian]

Ooh! I have the same unit... I shall pull it out and power on at some stage... see how she fares! Loving your videos Peter! Cheers Evan

[@peteroneill404]

More to come soon. The Mod/Demod boards are nearly ready to go into the Type 4 limiter.

[@EvLoutonian]

@@peteroneill404 most excellent!!!!

[@SpinStar1956]

In all honesty, there should not be that much error in the meter's linearity. I have one of these and it is much more linear--typically within a needle's-width. The HP-story goes that each movement was characterized, then a meterface was dynamically printed to match the actual movement's dynamics. It is possible that it may require lubrication but you're getting into an area that I'm not an expert on. Great video and 73...

[@peteroneill404]

I'm also not an expert in analogue meter movements, so I would assume that lubricants used by watchmakers would be suitable, but I haven't tried this, and I may be wrong. Do you have a link to the HP-story you cite? I would have thought dynamically printed meter scales would have only been used for instruments with greater precision, the 410B doesn't even have a mirrored scale to assist in reducing parallax error.

[@itsonlyme9938]

I had a issue with a moving meter and used lighter fuel and application with a watch maker oiler with a fine tip, when it dried it left a fine trace of oil. I think over time the bearings become corroded.

[@2tallB]

This was fun and you know your stuff, but I was cringing at 9:15 when you had the weight of the unit leaning on the tube, yikes! . ;-O

[@peteroneill404]

Well spotted, when filming I didn't notice this, it was only later while editing I saw it, then I checked the tube and fortunately no damage was done.

[@nathanokun8801]

Hewlett-Packard had a manufacturing code of "CAQI" in the US Navy during the latter half of the 20th Century. LOTS of CAQI electronic test equipment was on US warships at that time. LOTS!!