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HP 3410A AC Microvoltmeter overheats - power supply mods
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enut11:
I am in the process of refurbishing an HP 3410A AC Microvoltmeter.
This meter is unique unusual 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
enut11:
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 [also] need some changes to resistor values in the power supply.

Suggestions anyone?
enut11
vk6zgo:

--- Quote from: enut11 on December 07, 2019, 06:19:37 am ---I am in the process of refurbishing an HP 3410A AC Microvoltmeter.
This meter is unique 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

--- End quote ---

It is not "unique" many companies produced selective millivoltmeters & microvoltmeters.
floobydust:
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
Kleinstein:
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.
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