[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" <d.garrido at me.com>
> To: <volt-nuts at febo.com>
> 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
>> _______________________________________________
>> volt-nuts mailing list -- volt-nuts at febo.com
>> To unsubscribe, go to
> https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
>> and follow the instructions there.
> _______________________________________________
> volt-nuts mailing list -- volt-nuts at febo.com
> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
> and follow the instructions there.
>