Simpson 250 needed help for repair - Page 1
Forum section: electronics / repair

author: NotisEf
author_rank: Newbie
timestamp: November 04, 2023, 06:31:57 pm
content: Hello to everyone,i like analog multimeters.I have a small collection of them Avo 8 mk7,Simspon 260 Xi,Hioki AF -105,Micronta 204-B range doubler,some kti and my late addition is a Simpson 250 from 1969.On the 10k ohm range the needle goes beyond zero n the zeroing potensiometer does not affect the needle move at all.On the Rx1 and on the Rx100 range with the probes shorted it works fine.Batteries are fresh and i have clean contacts with drops of wd 40 with a batonnete.All other ranges acv dcv m amps are nearly spotn on for his age.Every opinion n help is welcome.

author: floobydust
author_rank: Super Contributor
timestamp: November 04, 2023, 06:38:13 pm
content: What are you using for the Rx10k battery, the Eveready #411 15V? Is it 15V, not 22V.https://www.simpson260.com/250/simpson_250.htmSimpson 250L circa 1969 apparently uses 4xAA 6V battery instead for the Rx10k range.

author: NotisEf
author_rank: Newbie
timestamp: November 04, 2023, 07:42:54 pm
content: For the 10k range i use CR2032 lithium 3v x2 with adaptor with cables solder joint straight to 6v batterry contacts becuase one was destroyed from corossion instead of 4 AAA.

author: floobydust
author_rank: Super Contributor
timestamp: November 04, 2023, 08:27:41 pm
content: I see Rx10k uses all batteries in series, 6V+1.5VIf the total battery voltage is much higher than Simpson designed for, then you'd have to add a resistor in series with the 3V coin cells.AAA carbon-zincis still 1.5Vpc when newCR2023lithium should work at 2.9V each.So I don't see a reason for the meter to read so high due to battery differences.I would check the two resistors for the 10kΩ range - R4 117,700Ω and R3 21,850Ω have drifted high/low respectively.

author: NotisEf
author_rank: Newbie
timestamp: November 05, 2023, 07:19:34 am
content: I check this resistors floobydust.R3 was 21.853ohms but R4 was 109.856 ohms instead of 117.700ohms.

author: Stray Electron
author_rank: Super Contributor
timestamp: November 05, 2023, 01:53:05 pm
content: Does the S 250 use wire wound resistors? If so, they shouldn't have drifted that much so it sounds like someone connected it to a powered circuit while it was set the R10k mode.

author: floobydust
author_rank: Super Contributor
timestamp: November 05, 2023, 04:28:47 pm
content: Not sure the schematic matches what you have, but your R4 is -6.7% or 7,844Ω lower. It might have its value writing on it too. I'm assuming the battery voltages total around 7.5VAlso used for Rx10k is R1 1,138Ω and R16 11.2Ω so I would check them as well, but they would give problems with Rx100, Rx1 ranges you say are working.You can try measure Rx10k on a resistor around 7.8kΩ and see if the Ohms Zero behaves normal by adding that amount of the low value.This would confirm the resistor R4 is the issue. Even on 240VAC mains it would dissipate 1/2W, which does not seem like enough to damage it? If it's gone low then age/corrosion might or physical damage to it must have happened.

author: NotisEf
author_rank: Newbie
timestamp: November 07, 2023, 06:14:39 pm
content: Thank much guys for your answers,i leave a 180 kohm resistor in the probes and i could zero the range,then i clean with clean alcohol n a toothbrush the connections of the R4 in the board.Check again R4 117.878 ohms and now problem solved.R 10k range read n can be zero probes shorted.Thank you again for your time n advices.