Re: [volt-nuts] Update on 720A

2017-08-10 Thread Charles Steinmetz
Chuck wrote: The part that bothers me is the lowering of resistance. I don't believe that the contacts to the wire improved with overload, so that leaves shorts between turns, and changes in the bulk resistance of the resistance wire... maybe the difference between hard and annealed wire?

Re: [volt-nuts] Update on 720A

2017-08-10 Thread Chuck Harris
Interesting. The R302 and R311 that have lowered in value, are each comprised of 4x 2.474K wire wound resistors, and are each in the middle of a series string of the same sized resistors. I suppose one could over dissipate them, but being in oil, that would be pretty hard. The part that bothers

Re: [volt-nuts] Update on 720A

2017-08-10 Thread Charles Steinmetz
Chuck wrote: I wouldn't expect physical trauma to be the issue, more something like a metal flash due to an arc, or tin whiskers... Something like that. The damage to the resistors in the unit was almost certainly caused by an electrical overload (the "trauma" to which I referred). Or, more

Re: [volt-nuts] Update on 720A

2017-08-10 Thread Chuck Harris
I really doubt that contamination of the oil is an issue. My top candidates are: 1) there is a unexpected (by David) parallel resistance across the affected resistors. It could be caused by a strap, or connection on the output terminals, or a missed circuit when reading the schematic.

Re: [volt-nuts] Update on 720A

2017-08-10 Thread Chuck Harris
I wouldn't expect physical trauma to be the issue, more something like a metal flash due to an arc, or tin whiskers... Something like that. If the feedthroughs were capacitors, I would suspect a metal migration similar to what is being seen on many older silver mica capacitors. I am pretty sure

Re: [volt-nuts] Update on 720A

2017-08-09 Thread David C. Partridge
I'm not going to open up oil-bath for a PM (unless someone wants to buy me a replacement 720A). D. ___ volt-nuts mailing list -- volt-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the

Re: [volt-nuts] Update on 720A

2017-08-09 Thread David C. Partridge
> I am thinking that the resistor leads have to leave the oil tank some way. > Is it possible that the insulator on whatever sort of feedthru has been > compromised? They are glass (or very similar) feedthroughs, so if that were compromised, I think I'd see oil ... There is no visual sign

Re: [volt-nuts] Update on 720A

2017-08-09 Thread Dan Kemppainen
On 8/9/2017 12:00 PM, volt-nuts-requ...@febo.com wrote: I have trouble believing that the precision WW resistors in the oil bath did go low. I wouldn't presume to argue that you didn't measure them right, but there has to be something other than their value drifting low. WW resistors usually

Re: [volt-nuts] Update on 720A

2017-08-09 Thread Chuck Harris
Hi David, It is amazing what gets done to equipment in the name of repairing it... I have trouble believing that the precision WW resistors in the oil bath did go low. I wouldn't presume to argue that you didn't measure them right, but there has to be something other than their value drifting

Re: [volt-nuts] Update on 720A

2017-08-09 Thread Didier Juges
Maybe a quality Bourns wirewound 10 turns pot would work as a replacement? On Aug 9, 2017 9:22 AM, "David C. Partridge" wrote: > > The bridge balance pot may not be so lucky, though. I don't think it > needs to be anything very special. > > As far as I can tell

Re: [volt-nuts] Update on 720A

2017-08-09 Thread Chuck Harris
Ah, ok. I went for a quick look through the manual, and couldn't find it... It could always be replaced with a concentric shaft pot that has two appropriate sections wired in series. That would lose the "nifty" factor, though. -Chuck David C. Partridge wrote: >> The bridge balance pot may not

Re: [volt-nuts] Update on 720A

2017-08-09 Thread David C. Partridge
> The bridge balance pot may not be so lucky, though. I don't think it needs > to be anything very special. As far as I can tell it is a very special pot wired with two resistance tracks. The coarse track has with two wipers connecting to the fine track which has just one wiper, all driven

Re: [volt-nuts] Update on 720A

2017-08-08 Thread David C. Partridge
> My concern is you seemed to be going at this repair like you were working on > an oscilloscope, rather than a super precision instrument. More a case of: Oh my goodness how did this end up in this condition, and is there any chance I can bring it back from the edge. No, I didn't use super

Re: [volt-nuts] Update on 720A

2017-08-08 Thread Charles Steinmetz
David wrote: When I got it, two oil bath resistors were *way* off value (202 and 100 ohms low respectively), and the S2 shunt resistors were about 1.3 ohms high which was far enough off to prevent S2 calibration. There were were also sundry other problems like two open circuits in the

Re: [volt-nuts] Update on 720A

2017-08-08 Thread Chuck Harris
Hi David, It isn't your soldering quality or ability that will degrade the instrument, it is rather the quality of the parts you add, or replace, and what you do to fudge things into sort of working, that will damage/degrade the instrument. Devices like the 720A are firmly in the category of

Re: [volt-nuts] Update on 720A

2017-08-08 Thread David C. Partridge
I can't replace the bad resistors in the A decade - they are in the oil bath and the manual says that in this case you sent the unit back to Fluke for a re-build. These days, I suspect they won't even do re-builds at all (or only at *silly* money), but make you buy a new one, and may well not

Re: [volt-nuts] Update on 720A

2017-08-08 Thread Bill Gold
tridge" <david.partri...@perdrix.co.uk> To: "'Discussion of precise voltage measurement'" <volt-nuts@febo.com> Sent: Tuesday, August 08, 2017 7:39 AM Subject: Re: [volt-nuts] Update on 720A > Chuck, > > I totally get your point that ideally you should use any KVD i

Re: [volt-nuts] Update on 720A

2017-08-08 Thread Chuck Harris
e KVD "Bridge Balance" pot as that's a bit > noisy but > shudder to think what that might cost if I could even find one. > > Cheers Dave -Original Message- From: volt-nuts > [mailto:volt-nuts-boun...@febo.com] On Behalf Of Chuck Harris Sent: 08 August > 2017

Re: [volt-nuts] Update on 720A

2017-08-08 Thread David C. Partridge
ssage- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Chuck Harris Sent: 08 August 2017 13:34 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Update on 720A Hi Dave, Fundamentally, any input impedance from your 3458 is going to throw your 72

Re: [volt-nuts] Update on 720A

2017-08-08 Thread David C. Partridge
Update: After changing the resistor I added in series with R1008 (A decade position 1.0) from 204.8 to 202.4 Ohms (I wonder if this means that R302 is slowly increasing in value back towards nominal) and re-calibrating again, the A decade is now within 1ppm of linear (using the 10V range and

Re: [volt-nuts] Update on 720A

2017-08-07 Thread David C. Partridge
Thanks Dave -Original Message- From: volt-nuts [mailto:volt-nuts-boun...@febo.com] On Behalf Of Bill Gold Sent: 07 August 2017 19:22 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Update on 720A David: Ignore my previous post. In thinking about this while

Re: [volt-nuts] Update on 720A

2017-08-07 Thread David C. Partridge
:19 To: Discussion of precise voltage measurement Subject: Re: [volt-nuts] Update on 720A Hi Dave, I think you need to review how to calibrate the 720A and Kelvin Varley dividers in general. The loading you need to worry about is the load you put on the output terminals, irrespective of the particu

Re: [volt-nuts] Update on 720A

2017-08-07 Thread Bill Gold
David: Ignore my previous post. In thinking about this while out shopping this morning I know the answer. The output resistance of the 720A is about 66k ohms. Your voltmeter has an input resistance of 10 megohms. You are simply loading the output of the 720A. The reason that the 0.1

Re: [volt-nuts] Update on 720A

2017-08-07 Thread Bill Gold
David: First suggestion would be to see what the output measures, with the input still set to 100.0014 volts (same as before), when setting the dials to: 0.9X(equivalent to setting the "A" decade to "0.1") 1.9X(equivalent to setting the "A" decade to

Re: [volt-nuts] Update on 720A

2017-08-07 Thread wb6bnq
Hi Dave, I think you need to review how to calibrate the 720A and Kelvin Varley dividers in general. The loading you need to worry about is the load you put on the output terminals, irrespective of the particular decade. I do not know how you are measuring the output, but it would appear,