HP 400H VTVM

Another interesting item from HP, capable of measuring true RMS AC voltages over a range of less than 1 mV to 300 V over a frequency range of 10Hz to 4MHz.The accuracy is specified as:

+/- 1% FSD, 50Hz to 500KHz

+/- 2% FSD, 20Hz to 1MHz

+/- 3% FSD, 20Hz to 2MHz

+/- 5% FSD, 10Hz to 4MHz

My unit after repair does meet this specification, it has a large mirror scale meter that is easy to read. I especially like the old-fashioned arched appearance of the meter, almost architectural!

The input impedances are given as 10 MΩ shunted by 15pF on ranges 1 V to 300 V and 25pF on ranges 1 mV to 300 mV. It uses a 4 stage closed loop amplifier and about 51dB of negative feedback to secure good stability and repeatability, the input stage (outside the loop) is a cathode follower. All the amplifier tubes including the CF are 6CB6 pentodes. The power supply is, of course, regulated. The regulator is a typical series triode with a pentode error amplifier and 85 V gas tube reference. The design incorporates feed-forward to the screen of the error amplifier to secure good line-regulation.

All the manuals I have found are for units having a serial number prefixed with 313 while mine is unit 5641. The overall topology is the same, however there are many significant detail differences. I do not know which came first, I suspect that my unit predates the 313 prefix. So to get things started, here is a picture showing it working:

The main difference is that the attenuator resistor chain forms the load for the cathode follower while the schematic in the manual shows a separate cathode resistor with the CF being coupled to the attenuator via a 5μF capacitor. The attenuator in my unit has a separate 51nF cap for each range, I cannot see any reason why it could not have been implemented using a single capacitor though. Clearly the front end was the subject of some extended development! Here is the schematic of the attenuator/front end of my unit:

And here is a picture of the attenuator and CF:

The wire boards are the precision attenuation resistors that being in series, are used as the CF load resistor.

As you may expect, I had to replace all the coupling capacitors in the attenuator. The other two capacitors that are critical and required replacement were the full-wave voltage doubler capacitors for the meter circuit. Any leakage will result in an unbalanced circuit and a residual deflection of the meter. Unbalance with the penultimate amplifier stage removed left the meter still showing an indication, replacement of the signal rectifier capacitors cured the issue. Other than that, the reservoir capacitor which is one section of a four section twistlock failed. I found a replacement on Ebay.

The main job was sorting the 6CB6 tubes according to microphony, they all have good transconductance. I did this by simply exchanging tubes with the first stage and tapping the unit to note the meter disturbance. Having found the least microphonic tube, I repeated the procedure with the rest of the tubes, still using the first stage location, until I had them ranked. I then installed them, least sensitive in the CF location, next in the first stage location and so on. At the end, I was rewarded with a unit that would show a perhaps 1/3rd scale flick instead of several wild full scale disturbances. As always, Deoxit is absolutely required on all tube pins and switch contacts. I also wrapped the CF and the first tube tightly with PTFE plumbers tape, just one width wide, I did not cover the whole tube, the heat has to radiate! Further, I similarly wrapped the first stage tube shield with PTFE tape (the first stage has the only shield) and slipped thin tubing over the conical spring (with considerable difficulty I note).

And here is the left side: