Re: [volt-nuts] Fluke 720a self calibration repair

[Charles Black]

Hi David,

My 720A's trimpots were original. I have them somewhere but they are a 
bit scarce right now. Anyway they were 20 turn wire wound trimpots. I 
suggest that you use wire wound ones and if 20 turn pots are not within 
your budget then 10 turn will not be too much of an issue. On one or two 
of my trimpots were very sensitive to adjustment so 20 turn is best. 
Wire wound trimpots are far more reliable than the others. If they do 
have a problem with the contact resistance going up it is usually fixed 
by just giving the adjustment screw a little turn. Properly fixed my 
720A tracks my 3458A very closely.


The circuit board was cleaned and clear coated after manufacture. Be 
careful about creating leakage paths. Never spray anything on the 
circuitry. I used to use Krylon to paint repaired circuit boards but 
decided to just clean each connection with alcohol swab and call it a 
day. I forgot to mention that all the decades need many revolutions to 
condition the contacts before calibration and before each important use.


Charlie

On 9/29/2015 12:08 AM, Bill Gold wrote:

David:

 I just looked inside my 720A at the 20 turn 5K trimpots.  They are
Bourns 3005P-502 printed on them, cermets, standard 3/4", tempco 50 ppm.  So
the replacement is available directly from stock and nothing fancy.  Mine
are date code 8412 so my unit was built somewhere in the mid '80s.  Not too
sure where the idea of a wirewound pot came from here.  They are more than a
little pricey at $13.98 ea from Allied.  Looks like you need 24 to replace
them all in the "A" and "B" decades.  So that would be $335.52 unless you
buy 25 or more at which time the price drops to $12.51 which totals $312.75.
They come in tubes of 25 each which accounts for the price drop.

 Digikey is $14.35 and Mouser is $14.35.  Wow!  Expensive little devils.
On the other hand (left) Digikey shows a unit from "TT Electronics/BI
Technologies" for $1.40 ea.  Interesting.  Why such a massive difference in
price?  These are 100 ppm but as I have noted in another posting the tempco
isn't very important.  It's the resistors in the oil filled can that set the
overall stability.

Bill

- Original Message -
From: "David Garrido" 
To: 
Sent: Saturday, September 26, 2015 5:05 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair



Thank you for all of the input on this folks.

Does anyone have a copy of the datasheet for the original Ramo-Eltra P/N

3800P-502?

The reason I ask is that the price of these parts is driven in part by the

tempco and I found info that indicated 20ppm/C on one website, but that was
not a DS, just a listing of the info.  If they can be 50ppm/C or even
100ppm/C, then the price drops exponentially.

I will do whatever is necessary to maintain or (LOL) improve the 720’s

legendary performance.

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[David Garrido]

Thank you Bill.  I think I found the exact same thing.  I have to add them to 
the "need to order” list.

I am very hopeful that I can get this whole analog ac/dc calibration nailed by 
the first of the year.  Going at it from a mostly independent lab standards 
point, it is not an easy thing to accomplish for less than a WHOLE bunch-o-time 
and a little cash at the right moment.  It is a great learning experience so 
far!

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[Charles Steinmetz]

Bill wrote:


I just looked inside my 720A at the 20 turn 5K trimpots.  They are
Bourns 3005P-502 printed on them, cermets, standard 3/4", tempco 50 ppm.
 *   *   *   Not too sure where the idea of a wirewound pot 
came from here.


Perhaps from the Bourns 3005 DATASHEET I posted a couple of days ago.

See 


Digikey is $14.35 and Mouser is $14.35.  Wow!  Expensive little devils.
On the other hand (left) Digikey shows a unit from "TT Electronics/BI
Technologies" for $1.40 ea.  Interesting.  Why such a massive difference in
price?


Cermet ($1.40) vs wirewound ($14).


Charles


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Re: [volt-nuts] Fluke 720a self calibration repair

[Tony]

Stan,

The Farnell prices don't include the 20% VAT. We get used to 1:1 £/$ 
exchange rates but Farnell often seem to charge >= double the Newark 
prices. Unfortunately ordering from Newark doesn't always help because 
you then get hit with expensive shipping costs and the outrageous, and 
seemingly random, customs handling fees by the courier.


Tony H

On 27/09/2015 22:56, Stan Katz wrote:

Tony,
I'm a US citizen, but I do know that the UK has a 20% VAT on each level of
manufacture and/or distribution. That can add up quickly in the end
purchase price.
Or am I missing something here?

No VAT in the USnot yet anyway;-)

On Sun, Sep 27, 2015 at 2:15 PM, Tony  wrote:


Apologies - my mistake - I wasn't paying sufficient attention when Google
decided that this was a perfect match when searching for 300SP-1-502:


http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=3400s&catalogId=15001&categoryId=70005471&langId=44&storeId=10151

But £180.27 each whereas Newark, the US arm of Farnell only want $120 or
approx £80 for the same part! Are there really that many Brits who put up
with being ripped-off to these levels?

Tony H

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Re: [volt-nuts] Fluke 720a self calibration repair

[Bill Gold]

Charles:

Mia Culpa!  I had the datasheet also but got caught by the description
that Allied used when I looked them up.  It showed "Cermet" but obviously
Allied & Mouser has these shown incorrectly.  Digikey shows these as
Wirewound.  How interesting.

I stand corrected.

Bill

- Original Message - 
From: "Charles Steinmetz" 
To: "Discussion of precise voltage measurement" 
Sent: Tuesday, September 29, 2015 8:46 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair


> Bill wrote:
>
> >I just looked inside my 720A at the 20 turn 5K trimpots.  They are
> >Bourns 3005P-502 printed on them, cermets, standard 3/4", tempco 50 ppm.
> >  *   *   *   Not too sure where the idea of a wirewound pot
> > came from here.
>
> Perhaps from the Bourns 3005 DATASHEET I posted a couple of days ago.
>
> See <https://www.febo.com/pipermail/volt-nuts/2015-September/004504.html>
>
> >Digikey is $14.35 and Mouser is $14.35.  Wow!  Expensive little devils.
> >On the other hand (left) Digikey shows a unit from "TT Electronics/BI
> >Technologies" for $1.40 ea.  Interesting.  Why such a massive difference
in
> >price?
>
> Cermet ($1.40) vs wirewound ($14).
>
>
> Charles
>
>
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Re: [volt-nuts] Fluke 720a self calibration repair

[Bill Gold]

David:

I just looked inside my 720A at the 20 turn 5K trimpots.  They are
Bourns 3005P-502 printed on them, cermets, standard 3/4", tempco 50 ppm.  So
the replacement is available directly from stock and nothing fancy.  Mine
are date code 8412 so my unit was built somewhere in the mid '80s.  Not too
sure where the idea of a wirewound pot came from here.  They are more than a
little pricey at $13.98 ea from Allied.  Looks like you need 24 to replace
them all in the "A" and "B" decades.  So that would be $335.52 unless you
buy 25 or more at which time the price drops to $12.51 which totals $312.75.
They come in tubes of 25 each which accounts for the price drop.

Digikey is $14.35 and Mouser is $14.35.  Wow!  Expensive little devils.
On the other hand (left) Digikey shows a unit from "TT Electronics/BI
Technologies" for $1.40 ea.  Interesting.  Why such a massive difference in
price?  These are 100 ppm but as I have noted in another posting the tempco
isn't very important.  It's the resistors in the oil filled can that set the
overall stability.

Bill

- Original Message - 
From: "David Garrido" 
To: 
Sent: Saturday, September 26, 2015 5:05 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair


> Thank you for all of the input on this folks.
>
> Does anyone have a copy of the datasheet for the original Ramo-Eltra P/N
3800P-502?
>
> The reason I ask is that the price of these parts is driven in part by the
tempco and I found info that indicated 20ppm/C on one website, but that was
not a DS, just a listing of the info.  If they can be 50ppm/C or even
100ppm/C, then the price drops exponentially.
>
> I will do whatever is necessary to maintain or (LOL) improve the 720’s
legendary performance.
>
> Cheers,
>
> David
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Re: [volt-nuts] Fluke 720a self calibration repair

[David C. Partridge]

> I do know that the UK has a 20% VAT on each level of manufacture and/or 
> distribution. That can add up quickly in the end purchase price.

That's not quite how VAT works.  If I buy a widget with a notional value of a 
pound, I pay £1.20.  If I am registered for VAT (typically the case for most 
businesses), I can then sell it on at £2 plus tax (IOW £2.40), I offset the 
input tax (20p) against the output tax (40p) and end up owing the govt. 20p.

Cheers, Dave

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Re: [volt-nuts] Fluke 720a self calibration repair

[Stan Katz]

Tony,
I'm a US citizen, but I do know that the UK has a 20% VAT on each level of
manufacture and/or distribution. That can add up quickly in the end
purchase price.
Or am I missing something here?

No VAT in the USnot yet anyway;-)

On Sun, Sep 27, 2015 at 2:15 PM, Tony  wrote:

> Apologies - my mistake - I wasn't paying sufficient attention when Google
> decided that this was a perfect match when searching for 300SP-1-502:
>
>
> http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=3400s&catalogId=15001&categoryId=70005471&langId=44&storeId=10151
>
> But £180.27 each whereas Newark, the US arm of Farnell only want $120 or
> approx £80 for the same part! Are there really that many Brits who put up
> with being ripped-off to these levels?
>
> Tony H
>
>
>
> On 26/09/2015 01:27, Charles Steinmetz wrote:
>
>> Charles wrote:
>>
>> I had to look it up. The company was Arrow (800 833 3557). The pots were
 manufactured by Bournes with a manufacturer part number of 300SP-1-502.
 Unit cost $7.73 in 2011. I bought 26 so my cost was about $200. I wanted a
 "Manufactured in America" part but can't remember if that was too pricey or
 not.

>>>
>> Tony wrote:
>>
>> They are now $92.55 from Arrow now and Farnell want over $200 each if you
>>> buy 25 or more There don't seem to be any obvious cheap alternatives
>>> for 10 turn 20ppm/C 5W pots.
>>>
>>
>> The original pots were Bunker Ramo-Eltra (later, Bunker Ramo-Amphenol)
>> P/N 3800P-502.  These were 20-turn, 5k, 10%, 1W, +/-50 ppm/C wirewound
>> parts in the 34b rectangular package.  The Bourns 3005P-1-502 matches these
>> specifications (I suspect "300SP" in Charles's message was a typo).  I do
>> not know if the pin layout is the same as the 3800P-502.  Verical has the
>> lowest price on the 3005P-1-502 that I could find in a quick search, $7.68
>> each. Others charge from $9 to $14.
>>
>> Tony -- what part number were you quoting specs and prices for?  I found
>> nothing searching for "300SP-1-502."
>>
>> Best regards,
>>
>> Charles
>>
>>
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>>
>>
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Re: [volt-nuts] Fluke 720a self calibration repair

[Dave M]

Rob
Thanks for the offer.  I received an offer of a (nearly) complete unit from 
another list member.  If that turns out well, I should have all the spares 
that I'll ever need.


If it doesn't work out, I'll surely be back in touch with you.

Dave M



Rob Klein wrote:

Dave,

How many of those pots do you need? I've had the innards of an
88(something) lying around for ages and your welcome to a few, if
you're willing to pay shipping from the Netherlands. With only the
trimmers in a small box, that should not break the bank.


Met vriendelijke groet,
Rob Klein.

Verstuurd vanaf mijn tablet

Dave M schreef:


I tried to find this style trimmer pot with a wirewound resistance
element, but found that all I could find was the Cermet type.  It
appears that wirewound pots have all been replaced by Cermet. the
wirewound pots had a 1W power rating, whereas the Cermet pots arte
only rated at 3/4W.  In the 720A, the reduced power rating won't be
an issue, but in some vintage equipment, it might be.

I've also tried, to no avail, to find a source for the metal-cased
wirewound trimmer pots used on the K-V divider in some older Fluke
differential voltmeters, such as the models 883A, 885A and 887A.
Does anyone know of a source for replacements for those pots?   I
have an 887A on the shelf that I'd like to get back to original
condition if I could find a couple of that style trimmer pot.
They're only 2 ohms, making the search even more difficult.  At this
point, my only recourse is to find a defunct donor unit at
reasonable cost, and salvage the parts. Does anyone have a hangar
queen that I could get for shipping charges?  I'd be really happy to
get mine back on the road to health.

Cheers,
Dave M

Tony wrote:

Apologies - my mistake - I wasn't paying sufficient attention when
Google decided that this was a perfect match when searching for
300SP-1-502:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=3400s&catalogId=15001&categoryId=70005471&langId=44&storeId=10151

But £180.27 each whereas Newark, the US arm of Farnell only want
$120 or approx £80 for the same part! Are there really that many
Brits who put up with being ripped-off to these levels?

Tony H


On 26/09/2015 01:27, Charles Steinmetz wrote:

Charles wrote:


I had to look it up. The company was Arrow (800 833 3557). The
pots were manufactured by Bournes with a manufacturer part
number of 300SP-1-502. Unit cost $7.73 in 2011. I bought 26 so
my cost was about $200. I wanted a "Manufactured in America"
part but can't remember if that was too pricey or not.


Tony wrote:


They are now $92.55 from Arrow now and Farnell want over $200 each
if you buy 25 or more There don't seem to be any obvious cheap
alternatives for 10 turn 20ppm/C 5W pots.


The original pots were Bunker Ramo-Eltra (later, Bunker
Ramo-Amphenol) P/N 3800P-502.  These were 20-turn, 5k, 10%, 1W,
+/-50 ppm/C wirewound parts in the 34b rectangular package.  The
Bourns 3005P-1-502 matches these specifications (I suspect "300SP"
in Charles's message was a typo).  I do not know if the pin layout
is the same as the 3800P-502. Verical has the lowest price on the
3005P-1-502 that I could find in a quick search, $7.68 each. Others
charge from $9 to $14. Tony -- what part number were you quoting
specs and prices for?  I
found nothing searching for "300SP-1-502."

Best regards,

Charles



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Re: [volt-nuts] Fluke 720a self calibration repair

[Dave M]

Yes, I see.
Please excuse my stupidity.

Dave M

Charles Steinmetz wrote:

Dave wrote:


It appears that wirewound pots have all been replaced by Cermet.


Horse puckey.  See the current Bourns 3005 datasheet (below).  As I
posted previously, it is WW.

Charles




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Re: [volt-nuts] Fluke 720a self calibration repair

[Charles Steinmetz]

Dave wrote:


It appears that wirewound pots have all been replaced by Cermet.


Horse puckey.  See the current Bourns 3005 datasheet (below).  As I 
posted previously, it is WW.


Charles

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Re: [volt-nuts] Fluke 720a self calibration repair

[Rob Klein]

Dave,

How many of those pots do you need? I've had the innards of an 88(something) 
lying around for ages and your welcome to a few, if you're willing to pay 
shipping from the Netherlands. With only the trimmers in a small box, that 
should not break the bank.


Met vriendelijke groet,
Rob Klein.

Verstuurd vanaf mijn tablet

Dave M schreef:

>I tried to find this style trimmer pot with a wirewound resistance element, 
>but found that all I could find was the Cermet type.  It appears that 
>wirewound pots have all been replaced by Cermet. the wirewound pots had a 1W 
>power rating, whereas the Cermet pots arte only rated at 3/4W.  In the 720A, 
>the reduced power rating won't be an issue, but in some vintage equipment, 
>it might be.
>
>I've also tried, to no avail, to find a source for the metal-cased wirewound 
>trimmer pots used on the K-V divider in some older Fluke differential 
>voltmeters, such as the models 883A, 885A and 887A.   Does anyone know of a 
>source for replacements for those pots?   I have an 887A on the shelf that 
>I'd like to get back to original condition if I could find a couple of that 
>style trimmer pot.  They're only 2 ohms, making the search even more 
>difficult.  At this point, my only recourse is to find a defunct donor unit 
>at reasonable cost, and salvage the parts. Does anyone have a hangar queen 
>that I could get for shipping charges?  I'd be really happy to get mine back 
>on the road to health.
>
>Cheers,
>Dave M
>
>Tony wrote:
>> Apologies - my mistake - I wasn't paying sufficient attention when
>> Google decided that this was a perfect match when searching for
>> 300SP-1-502:
>> http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=3400s&catalogId=15001&categoryId=70005471&langId=44&storeId=10151
>>
>> But £180.27 each whereas Newark, the US arm of Farnell only want $120
>> or approx £80 for the same part! Are there really that many Brits who
>> put up with being ripped-off to these levels?
>>
>> Tony H
>>
>>
>> On 26/09/2015 01:27, Charles Steinmetz wrote:
>>> Charles wrote:
>>>
> I had to look it up. The company was Arrow (800 833 3557). The pots
> were manufactured by Bournes with a manufacturer part number of
> 300SP-1-502. Unit cost $7.73 in 2011. I bought 26 so my cost was
> about $200. I wanted a "Manufactured in America" part but can't
> remember if that was too pricey or not.
>>>
>>> Tony wrote:
>>>
 They are now $92.55 from Arrow now and Farnell want over $200 each
 if you buy 25 or more There don't seem to be any obvious cheap
 alternatives for 10 turn 20ppm/C 5W pots.
>>>
>>> The original pots were Bunker Ramo-Eltra (later, Bunker
>>> Ramo-Amphenol) P/N 3800P-502.  These were 20-turn, 5k, 10%, 1W,
>>> +/-50 ppm/C wirewound parts in the 34b rectangular package.  The
>>> Bourns 3005P-1-502 matches these specifications (I suspect "300SP"
>>> in Charles's message was a typo).  I do not know if the pin layout
>>> is the same as the 3800P-502. Verical has the lowest price on the
>>> 3005P-1-502 that I could find in a quick search, $7.68 each. Others
>>> charge from $9 to $14. Tony -- what part number were you quoting specs 
>>> and prices for?  I
>>> found nothing searching for "300SP-1-502."
>>>
>>> Best regards,
>>>
>>> Charles
>
>
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Re: [volt-nuts] Fluke 720a self calibration repair

[Dave M]

I tried to find this style trimmer pot with a wirewound resistance element, 
but found that all I could find was the Cermet type.  It appears that 
wirewound pots have all been replaced by Cermet. the wirewound pots had a 1W 
power rating, whereas the Cermet pots arte only rated at 3/4W.  In the 720A, 
the reduced power rating won't be an issue, but in some vintage equipment, 
it might be.


I've also tried, to no avail, to find a source for the metal-cased wirewound 
trimmer pots used on the K-V divider in some older Fluke differential 
voltmeters, such as the models 883A, 885A and 887A.   Does anyone know of a 
source for replacements for those pots?   I have an 887A on the shelf that 
I'd like to get back to original condition if I could find a couple of that 
style trimmer pot.  They're only 2 ohms, making the search even more 
difficult.  At this point, my only recourse is to find a defunct donor unit 
at reasonable cost, and salvage the parts. Does anyone have a hangar queen 
that I could get for shipping charges?  I'd be really happy to get mine back 
on the road to health.


Cheers,
Dave M

Tony wrote:

Apologies - my mistake - I wasn't paying sufficient attention when
Google decided that this was a perfect match when searching for
300SP-1-502:
http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=3400s&catalogId=15001&categoryId=70005471&langId=44&storeId=10151

But £180.27 each whereas Newark, the US arm of Farnell only want $120
or approx £80 for the same part! Are there really that many Brits who
put up with being ripped-off to these levels?

Tony H


On 26/09/2015 01:27, Charles Steinmetz wrote:

Charles wrote:


I had to look it up. The company was Arrow (800 833 3557). The pots
were manufactured by Bournes with a manufacturer part number of
300SP-1-502. Unit cost $7.73 in 2011. I bought 26 so my cost was
about $200. I wanted a "Manufactured in America" part but can't
remember if that was too pricey or not.


Tony wrote:


They are now $92.55 from Arrow now and Farnell want over $200 each
if you buy 25 or more There don't seem to be any obvious cheap
alternatives for 10 turn 20ppm/C 5W pots.


The original pots were Bunker Ramo-Eltra (later, Bunker
Ramo-Amphenol) P/N 3800P-502.  These were 20-turn, 5k, 10%, 1W,
+/-50 ppm/C wirewound parts in the 34b rectangular package.  The
Bourns 3005P-1-502 matches these specifications (I suspect "300SP"
in Charles's message was a typo).  I do not know if the pin layout
is the same as the 3800P-502. Verical has the lowest price on the
3005P-1-502 that I could find in a quick search, $7.68 each. Others
charge from $9 to $14. Tony -- what part number were you quoting specs 
and prices for?  I

found nothing searching for "300SP-1-502."

Best regards,

Charles



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Re: [volt-nuts] Fluke 720a self calibration repair

[Tony]

Apologies - my mistake - I wasn't paying sufficient attention when 
Google decided that this was a perfect match when searching for 300SP-1-502:


http://uk.farnell.com/webapp/wcs/stores/servlet/Search?st=3400s&catalogId=15001&categoryId=70005471&langId=44&storeId=10151

But £180.27 each whereas Newark, the US arm of Farnell only want $120 or 
approx £80 for the same part! Are there really that many Brits who put 
up with being ripped-off to these levels?


Tony H


On 26/09/2015 01:27, Charles Steinmetz wrote:

Charles wrote:

I had to look it up. The company was Arrow (800 833 3557). The pots 
were manufactured by Bournes with a manufacturer part number of 
300SP-1-502. Unit cost $7.73 in 2011. I bought 26 so my cost was 
about $200. I wanted a "Manufactured in America" part but can't 
remember if that was too pricey or not.


Tony wrote:

They are now $92.55 from Arrow now and Farnell want over $200 each if 
you buy 25 or more There don't seem to be any obvious cheap 
alternatives for 10 turn 20ppm/C 5W pots.


The original pots were Bunker Ramo-Eltra (later, Bunker Ramo-Amphenol) 
P/N 3800P-502.  These were 20-turn, 5k, 10%, 1W, +/-50 ppm/C wirewound 
parts in the 34b rectangular package.  The Bourns 3005P-1-502 matches 
these specifications (I suspect "300SP" in Charles's message was a 
typo).  I do not know if the pin layout is the same as the 3800P-502.  
Verical has the lowest price on the 3005P-1-502 that I could find in a 
quick search, $7.68 each. Others charge from $9 to $14.


Tony -- what part number were you quoting specs and prices for?  I 
found nothing searching for "300SP-1-502."


Best regards,

Charles


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Re: [volt-nuts] Fluke 720a self calibration repair

[Bill Gold]

David:

Run the numbers.  Assuming that the 5K-20 turn pot R1005 is set in the
middle of it's travel (2.5K), that all of the other resistors, in series and
parallel ( R1006,R1007,R1008 and R302), that make up one step of the "A"
decade have a "0" TC, only R1005 has a maximum of +/- 100 ppm per degree C,
then the maximum change on the total resistance between step 0.9 and 1.0 of
the "A" decade would only be 0.00410 % change over the entire +/- TC of
R1005.  That is 0.00410 ppm.  The "Absolute Linearity" of the "A" decade is
specified at 0.1 ppm at +/- 1 degree C, without a temp correction chart.  Of
course all of the other resistors mentioned have a TC also, which could be +
or - to the total nominal of 10,000.  Therefore, unless I am mistaken, the
less expensive +/- 100ppm pots would work just fine and you could not
observe or measure the changes.  This is also why I didn't worry about my
"trimming" R1008, the "factory selected" resistor, slightly to get the 20
turn pots to adjust to the middle of their ranges, when necessary.

IMHO

Bill

- Original Message - 
From: "David Garrido" 
To: 
Sent: Saturday, September 26, 2015 5:05 AM
Subject: Re: [volt-nuts] Fluke 720a self calibration repair


> Thank you for all of the input on this folks.
>
> Does anyone have a copy of the datasheet for the original Ramo-Eltra P/N
3800P-502?
>
> The reason I ask is that the price of these parts is driven in part by the
tempco and I found info that indicated 20ppm/C on one website, but that was
not a DS, just a listing of the info.  If they can be 50ppm/C or even
100ppm/C, then the price drops exponentially.
>
> I will do whatever is necessary to maintain or (LOL) improve the 720’s
legendary performance.
>
> Cheers,
>
> David
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Re: [volt-nuts] Fluke 720a self calibration repair

[Todd Micallef]

I noticed that some of the vendors listed overall end-end tcr and at least
one that listed the wiper tcr too. Hopefully you can get the datasheets
with both.

Todd

On Saturday, September 26, 2015, David Garrido  wrote:

> Thank you for all of the input on this folks.
>
> Does anyone have a copy of the datasheet for the original Ramo-Eltra P/N
> 3800P-502?
>
> The reason I ask is that the price of these parts is driven in part by the
> tempco and I found info that indicated 20ppm/C on one website, but that was
> not a DS, just a listing of the info.  If they can be 50ppm/C or even
> 100ppm/C, then the price drops exponentially.
>
> I will do whatever is necessary to maintain or (LOL) improve the 720’s
> legendary performance.
>
> Cheers,
>
> David
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>
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Re: [volt-nuts] Fluke 720a self calibration repair

[David Garrido]

Thank you for all of the input on this folks.

Does anyone have a copy of the datasheet for the original Ramo-Eltra P/N 
3800P-502?

The reason I ask is that the price of these parts is driven in part by the 
tempco and I found info that indicated 20ppm/C on one website, but that was not 
a DS, just a listing of the info.  If they can be 50ppm/C or even 100ppm/C, 
then the price drops exponentially.  

I will do whatever is necessary to maintain or (LOL) improve the 720’s 
legendary performance.

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[Charles Steinmetz]

Charles wrote:

I had to look it up. The company was Arrow (800 833 3557). The pots 
were manufactured by Bournes with a manufacturer part number of 
300SP-1-502. Unit cost $7.73 in 2011. I bought 26 so my cost was 
about $200. I wanted a "Manufactured in America" part but can't 
remember if that was too pricey or not.


Tony wrote:

They are now $92.55 from Arrow now and Farnell want over $200 each 
if you buy 25 or more There don't seem to be any obvious cheap 
alternatives for 10 turn 20ppm/C 5W pots.


The original pots were Bunker Ramo-Eltra (later, Bunker 
Ramo-Amphenol) P/N 3800P-502.  These were 20-turn, 5k, 10%, 1W, +/-50 
ppm/C wirewound parts in the 34b rectangular package.  The Bourns 
3005P-1-502 matches these specifications (I suspect "300SP" in 
Charles's message was a typo).  I do not know if the pin layout is 
the same as the 3800P-502.  Verical has the lowest price on the 
3005P-1-502 that I could find in a quick search, $7.68 each.  Others 
charge from $9 to $14.


Tony -- what part number were you quoting specs and prices for?  I 
found nothing searching for "300SP-1-502."


Best regards,

Charles


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Re: [volt-nuts] Fluke 720a self calibration repair

[Tony]

They are now $92.55 from Arrow now and Farnell want over $200 each if 
you buy 25 or more There don't seem to be any obvious cheap 
alternatives for 10 turn 20ppm/C 5W pots.


Do they need to be 5W? Is that to minimize self heating?

The TT Electronics 7286 series are 10 turn, 2W, 20ppm/C, but only $10.66 
for 25+. Or you could use 2 10K ohm 7270 series in parallel at $8.45 
each for 50+ if you could fit them in somehow!



On 25/09/2015 20:10, Charles Black wrote:

Hi David,

I had to look it up. The company was Arrow (800 833 3557). The pots 
were manufactured by Bournes with a manufacturer part number of 
300SP-1-502. Unit cost $7.73 in 2011. I bought 26 so my cost was about 
$200. I wanted a "Manufactured in America" part but can't remember if 
that was too pricey or not.


Charlie

On 9/25/2015 9:45 AM, David Garrido wrote:
I like that Idea.  Do you recall the part number of the resistors you 
found?


Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[Charles Black]

Hi David,

I had to look it up. The company was Arrow (800 833 3557). The pots were 
manufactured by Bournes with a manufacturer part number of 300SP-1-502. 
Unit cost $7.73 in 2011. I bought 26 so my cost was about $200. I wanted 
a "Manufactured in America" part but can't remember if that was too 
pricey or not.


Charlie

On 9/25/2015 9:45 AM, David Garrido wrote:

I like that Idea.  Do you recall the part number of the resistors you found?

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[David Garrido]

I like that Idea.  Do you recall the part number of the resistors you found?

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[Charles Black]

Hi David,

I had the same problem with my Fluke 720A, among other problems. The 
problem was the plastic for the adjustment pots had disintegrated with 
age and a couple of them had quit working. I ended up fixing the unit 
but removal of the bad pots made it obvious that the other "working" 
pots were just hanging on. As I removed the bad pots they kind of fell 
apart. I have been there before so I found a good price on 30 new pots 
and replaced them all at once. It really made a new instrument out of my 
720A. I was amazed how well it agreed with my Agilent 3458A, to better 
than the spec! It was nice to have confirmation of the linearity my 
3458A since it has not been out for calibration since it was almost new.


Charlie

On 9/24/2015 9:38 AM, David Garrido wrote:

Hello All,

I am trying to track down what I think is a small issue in the calibration of 
my new to me 720a.  I am dreadfully paranoid of jacking something up in the 
process and need a little hand holding.

I was going through the Self Calibration steps and every thing was going well up to the 
adjustment of the "A 1.0" decade switch position.  All of the variable 
resistors up to that point needed to be turned CW in order to null the appropriate switch 
position.  The “A 1.0” adjustment pot does not null the switch position and is not 
getting any closer than about 2.5uV.  I verified the meter balance between every other 
step.  I have checked, in circuit, R302 and R303 and I am getting a VERY SOLID reading of 
9.896 on all of those.  Of course, checking R1006 and R1010 give me significantly 
different readings than the stipulated 8.45k.

I cannot seem to be able to find a CCW or CW limit to this resistor and I think 
I read somewhere that there are supposed to be run from end to end several 
times if an issue arises.  Do these var resistors have limits?  I need a little 
help moving forward.

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[David Garrido]

Bill,

After having spent the last couple of hours getting to know the switching, 
schematic, circuit, and layout I had a chance to lift the leg of R1006.  
Measured spot on as did R1005 and R1007.  I was able to measure those resistors 
in the tank as well.  Having done all of this I was thinking about the VERY 
same thought to find the center as I did have to turn all of the pots in the 
“A” decade in the same CW direction to bring them into spec.  If I remember 
correctly as I was checking the R1005 (20 turn pot), I recall it bringing the 
resistance out of R1005 down, as I was turning the pot CW and that was 
lowering, so opposite.  I am going to try this and if all the rest falls into 
place after the adjustments, I will call it a success.

BTW……….so far after initial inspection and testing, the 335D, 845AR, and the 
721a all seem like they just needed cleaning and adjustment.  This is the only 
one left from that 7105a Calibrator and I have a 750a on the way to finish it 
all out.  The 5200a is all clean and adjusted and I am still working on a small 
issue in the 5205a.  Other than that, I just need a 540b and a rented 3458a to 
calibrate all of the calibrators.

LOL!

Thank you so much for allowing the bandwidth.

Cheers,

David
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Re: [volt-nuts] Fluke 720a self calibration repair

[Bill Gold]

David:

I have a 720A that I purchase around 1995 from a local surplus dealer.
The price was right at around $250.  When I did the "Self Calibration
Procedure" from 2-24 I found that several of the "A" decade pots could not
be adjusted within 0.5 microvolts, and I think at least one of the "B"
decade pots also.  After trying all of the other "Calibration" adjustments
from 4-19 and seeing that nothing had improved I decided that the Precision
Resistors in the "oil tank" had probably drifted over time.

After looking at the schematic I saw that there were "trim" resistors
( i.e R1008, R1012 and so on ) that were "factory selected".  I didn't have
the equipment to measure the resistors in the "oil tank" to sufficent
accuracy to determine exactly what there values were and if the match was
still working.  So I decided to change or "trim" the "factory selected"
resistor on those settings which wouldn't adjust to the +/- 0.5 uv spec.  I
think I started at the lower end of the Ratio, ".0", and then worked my way
up to the top, checking the adjustment range of each adjustable multi-turn
pot ( I think they are 20 turns ).

I added or subtracted a little resistance, as necessary, to the "factory
selected" resistors.  If they needed more resistance then I added in series
whatever was necessary to bring the multi-turn pot to the approximate center
of it's range.  If they needed less resistance then I added in parallel
whatever was necessary to bring the multi-turn pot to the approximate center
of it's range.  This required several passes in the procedure as each decade
depends upon the total resistance of all of the lower resistors.  I used the
best tolerance resistors in the series/parallel adjustments that I had on
hand or could find in the local surplus stores ( usually at least .01% ),
assuming that they would have the best temperature stability also.  When I
got done I had the whole thing working correctly and it has been easily
"Self Calibrated" for ratio for all of these years.  The overall temperature
stability seems excellent for room temperature changes.

Of course you can send the 720A back to FLUKE and get charged an "arm
and a leg as well as your first born" for this repair.

    Just one suggestion, good luck!

Bill

- Original Message - 
From: "David Garrido" 
To: 
Sent: Thursday, September 24, 2015 9:38 AM
Subject: [volt-nuts] Fluke 720a self calibration repair


> Hello All,
>
> I am trying to track down what I think is a small issue in the calibration
of my new to me 720a.  I am dreadfully paranoid of jacking something up in
the process and need a little hand holding.
>
> I was going through the Self Calibration steps and every thing was going
well up to the adjustment of the "A 1.0" decade switch position.  All of the
variable resistors up to that point needed to be turned CW in order to null
the appropriate switch position.  The “A 1.0” adjustment pot does not null
the switch position and is not getting any closer than about 2.5uV.  I
verified the meter balance between every other step.  I have checked, in
circuit, R302 and R303 and I am getting a VERY SOLID reading of 9.896 on all
of those.  Of course, checking R1006 and R1010 give me significantly
different readings than the stipulated 8.45k.
>
> I cannot seem to be able to find a CCW or CW limit to this resistor and I
think I read somewhere that there are supposed to be run from end to end
several times if an issue arises.  Do these var resistors have limits?  I
need a little help moving forward.
>
> Cheers,
>
> David
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[volt-nuts] Fluke 720a self calibration repair

[David Garrido]

Hello All,

I am trying to track down what I think is a small issue in the calibration of 
my new to me 720a.  I am dreadfully paranoid of jacking something up in the 
process and need a little hand holding.

I was going through the Self Calibration steps and every thing was going well 
up to the adjustment of the "A 1.0" decade switch position.  All of the 
variable resistors up to that point needed to be turned CW in order to null the 
appropriate switch position.  The “A 1.0” adjustment pot does not null the 
switch position and is not getting any closer than about 2.5uV.  I verified the 
meter balance between every other step.  I have checked, in circuit, R302 and 
R303 and I am getting a VERY SOLID reading of 9.896 on all of those.  Of 
course, checking R1006 and R1010 give me significantly different readings than 
the stipulated 8.45k.

I cannot seem to be able to find a CCW or CW limit to this resistor and I think 
I read somewhere that there are supposed to be run from end to end several 
times if an issue arises.  Do these var resistors have limits?  I need a little 
help moving forward.

Cheers,

David
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