Hewlett-Packard HP-400A AC Vacuum Tube Volt Meter

After repairing both a Hewlett-Packard HP-400D and a Hewlett-Packard HP-400C VTVM, I decided to tackle the oldest model of the series, the 400A. The 400A is the same size as the later 400C but uses all octal tubes and has an upper working limit of 1 MegaHertz. It's lowest level range is 0.03 volt full-scale. Its circuit consists of a 6SQ7 as cathode-follower input amp, two stages of 6AC7 amplifiers and a 6H6 as meter rectifier. Designed by David Packard in 1942, it is listed in the first HP catalog in 1943.

HP-400A measuring the no-load AC voltage of a wall-mounted transformer.

When compared to the 400C and 400D models, it lacks external connections as a wide-band amplifier and does not use DC for amplifier filaments. DC is necessary in the other two models for their lower ranges (down to 1 millivolt).Like the other two models, the 400A power supply is voltage regulated. The 400A uses a 5Y3 rectifier, 0D3 regulator, 6SQ7 error amp, and 6V6 as pass tube.

This particular example was first owned by Emerson Radio and Phonograph Corp. according to the property tag bolted to the side of the case.

HP400A property tag showing Emerson Radio as first owner.

I was able to locate a schematic for the 400A on the web at the HP Memory site.

Early HP catalogs can be found at the HP Archive site The earliest HP catalog is 1943. It includes the 400A

HP400A in 1943 HP catalog.

"AC Voltages as small as .005 and as high as 300 volts can be simply and directly measured without any precautions over a frequency range of 10 cycles to 1,000,000 cycles. Accuracy of readings is assured because the high 1 megohm input impedance does not disturb the usual circuit under test. Furthermore, the calibration error of the instrument under all conditions is less than 3 % to 100 Kc and less than 5 % to one megacycle."

Price was $185.

Repairs
Before ever plugging in the 400A, I replaced the rotted power cord, a leaky 1 MFD capacitor, and the fuse holder for which the rivets had failed due to metal fatigue. After those repairs, I did a slow power up to reform the electrolytics. The meter went to full scale without any input. I pulled one side of the 0.1 MFD coupling cap between the two 6AC7 amp tubes. That allowed the meter to stay at zero. Although the original cap was a bit leaky, replacing it did not solve the problem. Even with the first 6AC7 pulled out, the meter still read fullscale. At this point, I suspected that the meter was responding to AC ripple in the power supply. Sure enough, a quick check with the oscilloscope confirmed a high ripple in the B+. Replacing the three section 10 MFD electrolytic input cap solved the problem.

The 400A schematic shows a neon bulb used as the voltage regulator reference. Apparently, a change was made early in the run. This example, serial number 5828, uses the 0D3 gas regulator instead.

HP-400A chassis. The 0D3 is lighted and visible as the tallest tube.

The HP-400A has very limited cooling with just a few louvers in the back. There are 8 tubes in the relatively small box. Apparently this did not cause too many problems since these old-timers keep on working with relatively minimal repairs.