[volt-nuts] Fluke differential meters

Wed Aug 7 16:10:21 EDT 2013

Hi Robert

Many thanks for the clarification re the reference, I wasn't intending to  
be nitpicky it was just that your comment about the Weston cell was  what 
started me on the struggle to retrieve the meter from where it was  nesting, 
so I was paying it a bit more attention that I might have done  
otherwise......pain is quite good for focussing the attention:-)

Thanks too for the warning re the switch wafers, that was something I  
hadn't considered.

Fortunately the wafers themselves were ok in this instance  and the jammed 
switch was down to the grease in the mounting bush drying out and  becoming 
"gummy".
A couple of very small drops of three-in-one from the end of a  cocktail 
stick between the front of the bush and the circlip on the shaft, plus  a bit 
of patience, and all was fine again.

Regards

Nigel
GM8PZR

In a message dated 07/08/2013 12:55:40 GMT Daylight Time,  
robert8rpi at yahoo.co.uk writes:

Hi  Nigel,
I just re-read my post and I wan't very clear. I was trying to say  that 
the 803B has a Weston cell reference. Jammed switches are often caused by  the 
readout drums shrinking and seizing on the shaft :-( . Watch out for  the 
switch wafers, at least on the higher accuracy units they were impregnated  
with silicone oil to reduce leakage. Standard waferes may not be up to  
specification. 

Regards,
robert  G8RPI.

________________________________
From:  "GandalfG8 at aol.com" <GandalfG8 at aol.com>
To: volt-nuts at febo.com  
Sent: Wednesday, 7 August 2013, 11:10
Subject: Re: [volt-nuts] Fluke  differential meters

Hi Robert

Your comments reminded me that  some years ago I bought a Fluke 
differential 
voltmeter and a hernia  inducing struggle through other equipment to reach  
it down from a  shelf just confirmed it's an 893A.

I must admit I was more interested  in the case when I bought it, sacrilege 

I know but at least it  looked too nice to take apart so did remain intact, 

and I'm much  more inclined now to leave it that way:-)

Component markings suggest  this one dates from around 1984 and it  uses 
the 
same style blue  plastic switch wafers in the Kelvin-Varley divider  that 
later became  notorious for failing on the Redifon 551 HF receiver.
There are, or were,  some references on line to repairing the switches  in 
the 551 and I  have seen the wafers offered for sale from time to time, I 
seem 
to recall  they were also used in one version of the RS components   
"make-a-switch" kit.
As far as I can tell from the manual though and  unless I'm missing  
something, which isn't entirely unkown:-), the  voltage reference  consists 
of a 
pair of zener diodes rather than a  Weston cell.

Either way, I've just discovered one of the switches is  jammed, the wafers 

look ok and I don't recall it being jammed when I  bought it so not sure 
why 
yet, but there goes what I already had planned  for today:-)

Regards

Nigel
GM8PZR

In a message  dated 07/08/2013 07:37:44 GMT Daylight Time,  
robert8rpi at yahoo.co.uk  writes:

Hi  Joe,
The 893 is worth having as a working  instrument. The older ones are  
collectibles (possibly) or a source  of parts. The Voltage reference is a  
Weston 
cell which will be well  past it's expected life. In particular  these 
meters 
have a Kelvin  Varley voltage divider. This can be used  without the meter 
section  working, possibly recased. The plastic switch  dials/drums do tend 
to  
crack or warp due to srinkage of the plastic though. If  theK-V  dividers 
switches are worn out, they are still a good source for   precision matched 
resistors. Perhaps not spot on value but all the   resistors in a decade 
will be 
very close in value. These can be used  for  voltage dividers and Hamon 
resistor units (google Hamon resistor  and see  
http://conradhoffman.com/HamonResistor.html). They may not  look much  but 
are high performance. There are 
other precision  resitors in these  units Take care when removing them, 
don't 
apply  any force or stress to  the resistor body as this can affect
their  performance.

Robert   G8RPI.

________________________________
From: Joseph Gray   <jgray at zianet.com>
To: Discussion of precise voltage  measurement  <volt-nuts at febo.com> 
Sent: Wednesday, 7 August  2013,  6:00
Subject: [volt-nuts] Fluke differential  meters

The local  surplus guy has at least three different  differential meters 
that
have been  collecting dust for decades. I  have borrowed two of them (so 
far).

The  first one is a Fluke  893A and is solid state. Although it appears to
work,  it fails the  very first of the performance tests. Now that I have a
full  manual,  I'm going to look at this a bit closer.

The second one is a  Fluke  803B (I think. There is no label on it). It has
tubes. I am slowly   bringing it up on a variac before messing with it. I
plan to open it  up  later and take a good look.

I'm not sure what the third model  is, but  it looks like it may be an even
older Fluke. It is on a high  shelf and I  haven't gone over there to lift 
it
down yet. I'll  probably do that later  this week.

At the moment, I'm just playing  around with this gear and  learning a few
things. In the long run, are  these things worth  having?

Joe   Gray
W5JG
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