[volt-nuts] HP-3455A calibration

[Jerry Hancock]

I have five 3455 meters and love to work on them.  I have a 3455 and 3456 in my garage I was sent to repair and haven’t had time to get to them yet (sorry).  

I’ve never been able to trim the reference back using the resistor network.  Usually the ref has drifted too far and had to be replaced.  You can try by just clipping and solder bridging them back.  I also remember there being a zener on the buffer opamp, and this depends on which of the few refs you have, that drifts a little.  Lastly, you should setup a test bench for the references themselves to make sure you don’t have a problem somewhere else in the meter.  You should be able to set the 10V output by feeding voltage to the ref using jumpers and a few power supplies.  I took the header out of my parts unit and used that to connect the power supplies but you can find places to hook them up to the ref itself.  You should check the lettering on the header on the motherboard to make sure you have the pins numbered and lettered correctly.  I remember the power supplies hook to like pins M and 11?  which are on the top/bottom of the connector and adjacent.  I think the grounds are all bridged so you don’t have to worry about them.  

I made the mistake and purchased 10 of the LM399 refs from our Asian pals and though they looked ok, the leads broke right off when I was trying to just put them through the holes.  Check that zener voltage on both units and you might find one is lower than the other and can swap that in.

Regards,

Jerry

> On Nov 19, 2017, at 11:54 AM, Ludger.Lenzen at gmx.de wrote:
> 
> I did it time ago.
> Warmed up ~3 hours / Measure value with my 3548A.
> Next trimmed to mid of trimmable range (pot to center).
> Than trimmed (by shorting step by step) as close as possible to 10.000V.
> /PeLuLe
> 
> 
>> Gesendet: Sonntag, 19. November 2017 um 06:51 Uhr
>> Von: "Jeremy Nichols" <jn6wfo at gmail.com>
>> An: volt-nuts at febo.com
>> Betreff: [volt-nuts] HP-3455A calibration
>> 
>> My new-to-me 3455A DVM (s/n 1622A06699) does not want to calibrate to the 10 Volt level although 1 Volt is fine. The 10V adjustment pot in the 11177B reference assembly simply doesn't adjust far enough—I can get the reading down to 10.00020 Volts but no lower.
>> 
>> Just for the heck of it, I picked up another reference assembly from an eBay seller. It's an "A" assembly rather than the "B" the 3455 came with but it works fine and I am able to calibrate both 1V and 10V. 
>> 
>> The reference assemblies have a set of jumpers for "coarse adjust" so it should be possible to tweak the "B" version back into calibration. Does anyone have experience doing this? The service manual just says "return to the factory." It would be nice to hear from someone who has worked with these things before I start messing with the jumpers.
>> 
>> Jeremy
>> 
>> Sent from my iPad 4
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