[volt-nuts] Fluke 720a self calibration repair

Dave M dgminala at mediacombb.net
Sun Sep 27 14:55:41 EDT 2015

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wirewound pots have all been replaced by Cermet.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.

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=700000005471&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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