HP 3410A AC  Microvoltmeter overheats - power supply mods - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: December 07, 2019, 06:19:37 am
content: I am in the process of refurbishing an HP 3410A AC Microvoltmeter.This meter isuniqueunusual as it tunes-in the signal to be measured thereby allowing much lower levels of measurements in the presence of noise.Indeed, the lowest ACV range is 3 microvolts full scale!I have replaced all the PS electros and the instrument is now behaving.However, I am not happy with how hot it gets after only 10min of use.enut11

author: enut11
author_rank: Super Contributor
timestamp: December 07, 2019, 06:39:06 am
content: For some reason, HP 3410A engineers chose to run the voltage regulators from very high inputs.Eg, the +45vDC regulator has an input of +74vDC and the -25vDC regulator has an input of -49vDC.Using a variac, I reduced the regulator inputs to 10v more than the outputs. This greatly reduced heating but caused instability and loss of regulation.So, it appears that to reduce heat [by reducing mains AC input] might  need some changes to resistor values in the power supply.Suggestions anyone?enut11

author: vk6zgo
author_rank: Super Contributor
timestamp: December 07, 2019, 08:13:58 am
content: Quote from: enut11 on December 07, 2019, 06:19:37 amIt is not"unique"many companies produced selective millivoltmeters & microvoltmeters.

author: floobydust
author_rank: Super Contributor
timestamp: December 07, 2019, 09:06:43 am
content: You're up against the laws of physics for a linear regulator, they'll make more heat with higher input voltage- regardless of any circuit mods.The schematic shows 72V, you have 75V surely from mains being higher than 230VAC.It's from an era where electrolytic capacitors were big and expensive so a very lean design. They also were conservatively rated, the old Spragues came +50% at least and could be run at rated voltage.The only reason I could see for it to go "unstable" with lower input voltage would perhaps be mains ripple showing up and modulating the output.What did you use for replacements on the filter caps, C2 80uF 75V, C10 65uF 60V +100%/-10%? You kept the same critical GND point "J" for the cap's (-)?I find I have to round up as modern electrolytics are low value i.e. 82µF ±20% measure 70µF, whereas old electrolytics were on the other end i.e 95µF. I would double their value and then try it with lower mains input.Otherwise, I would say it's meant to run hot.https://www.eevblog.com/forum/testgear/hp-3410a-ac-microvoltmeter-restoration

author: Kleinstein
author_rank: Super Contributor
timestamp: December 07, 2019, 09:26:33 am
content: It is running hot by design. If there is space, one could in theory add a series resistor or inductor (power factor choke) and increase the filter caps a little. this is little like reducing the voltage with the additional small advantage of higher power factor and less transformer loss. The larger filter would allow it to regulate even with the reduced voltage.

author: enut11
author_rank: Super Contributor
timestamp: December 07, 2019, 07:50:24 pm
content: Quote from: floobydust on December 07, 2019, 09:06:43 amYes, supply here in Australia is 240vAC.I have further experimented with running the meter on 205vAC using an auto-transformer. This gives a ~20vDC margin. It is stable and does run somewhat cooler than standard.Wrt the caps, I used close to double the original value.

author: duak
author_rank: Super Contributor
timestamp: December 07, 2019, 08:54:25 pm
content: I rescued a Wavetek 23 synthesized function generator with a bad power transformer from the tip. The transformer was custom with a number of secondaries of various voltages and currents and was a certain size and shape. I managed to find an off the shelf replacement that could be made to work albeit with greater regulator dissipation. Wavetek designed the mains input circuit to accept various voltages by moving jumpers around. I added a couple of 2 W, 20 odd ohm resistors in series with the transformer primaries using the existing terminals. These resistors reduced the raw DC voltage quite nicely, but could be jumpered out if I ever had to run the unit on a lower voltage.I wonder how close to magnetic saturation the transformer in the 3410A is running? 240 VAC at 50 Hz is the most demanding corner of its operating area and a slight reduction in voltage could reduce dissipation quite a bit because the magnetizing current is reduced. I think that if there is room, one or two resistors in series with the primary windings could work quite well.

author: TimFox
author_rank: Super Contributor
timestamp: December 08, 2019, 03:18:28 pm
content: Do you have space to add a small filament transformer to "buck" the 240 V down to the desired voltage?

author: enut11
author_rank: Super Contributor
timestamp: December 10, 2019, 03:25:36 am
content: Thanks for the suggestion @TimFox. I am using an external auto-transformer 240vAC to 205vAC which achieves a similar thing.I used a 240vAC/35vAC transformer and wired the 35v secondary in series with the 240v primary.I fed mains (240vAC) into the combined primary plus secondary winding and took the output (205vAC) across the original primary winding.In the sketch below N2 is the original primary winding and N1 is the original secondary winding.The lot was mounted in an earthed metal box with fuse and AC output socket.enut11