Teardown VARIREF VF-5 Voltage/Current calibrator (Made in Sweden) - Page 1
Forum section: electronics / testgear

author: Micke
author_rank: Regular Contributor
timestamp: November 19, 2015, 09:57:15 pm
content: [My first teardown...]I bought this little voltage/current reference on a local auction site for approx 50 Euro, I assumed and hoped since it had stickers from an accredited lab it would not be total crap!It has 5 fixed/adjustable voltage/current ranges:Mode Fixed VariableA 0,1V 1mA 0-0,1V 0-1mAB 0,19V 1,9mA 0-1V 0-10mAC 1V 10mA 0-10V 0-100mAD 1,9V 19mA 0-5V 0-20mAE 10V 100mA 0-5V 4-20mAIt was exciting open it up to see what voltage reference they had used...Could it be like EDC/Krohn Hite units just a carefully hand picked and characterized zener, a LM399 or something else? I ruled out LTZ1000 since this unit looked to be produced before LTZ1000 was invented!From date codes of IC´s, it seems to have been produced in the early eighties.Surprisingly there seems to be no low ohmic low TC current shunt for the 100mA current range! lowest resistance value is 200R, which seems rather high. And it also turned out that especially the 100mA range is really bad! it starts with 99,92mA but quite quick it reach 99,93mA and keep on increasingThere is a reason the cal. lab specified the measured 100mA value with only 2 digits...On the backside there was a sticker from the calibration 1992, I brought it to work where we have a newly calibrated HP3458A, and compared with the values from 1992.The voltages were slightly higher, maximum deviation was 20ppm, which I consider OK after 23 years! hopefully it has stabilized now!(Hopefully I did the ppm calculations right...)Measured values compared to 1992 sticker:Nov 2015 Sep 1992 Diff ppm0,1000063V 0,100007V -70,1900186V 0,190017V 81,0000762V 1,00007V 61,900178V 1,90014V 209,999799V 9,99973V 70,99917 mA 1,0013mA -21001,89865 mA 1,9008mA -11009,99296 mA 9,9947mA -17018,9872 mA 18,988mA -4099,92 mA 99,92mA 0 unstable range...Current ranges were as expected much worse, ranging from 40-2100ppm deviation...Front board:2pcs 725CP "LM725" = OP-amp low noise, low drift, high open loop gain (obsolete)3pcs Wire wound resistors (probably TC 3ppm) 0,1% values = 200R, 250R, 1kMain board:1pc LM399H = Precision heated buried zener 6,95V voltage reference3pcs CD4066 = Quad CMOS Analog switch1pc UA308 = OP-amp1pc 4N33 = Opto coupler, photodarlington output1pc RC4195N = Fixed +/- 15V Dual tracking voltage regulator (obsolete)3pcs 725CP "LM725" = OP-amp low noise, low drift, high open loop gain (obsolete)8pcs Wire wound resistors (probably TC 3ppm) 0,1% values =900R, 1k, 8k, 9k, 9k, 20k, 30k, 50kOverview:Sticker telling last calibrated 93-11-15 by SWEDACGreen sticker on top telling "This instrument is calibrated according to new representations of Volt and Ohm effective 1990"Front board:Swiss quality rotary switch from ELMAFront board full size:Beckman potentiometer "Made in Japan" ... odd!?Main board:Mainboard full size:http://imgur.com/a/aZrTjIn total 13 multi turn cermet trimpots with unknown function, better not touch!The old trusty LM399H...V1 = BD378 Transistor PNP 75V 2A 25W (mounted on heatsink)Seems the blue relay is some kind of overload protection for voltage range, it clicks when short circuitCheap IC sockets, on a thing like this I would expect higher quality.Backside:Some extra resistors, perhaps the trim range of the trim pots was not enough?Well, thats all for now!

author: Vgkid
author_rank: Super Contributor
timestamp: November 19, 2015, 10:27:36 pm
content: Thanks for the teardown. Interesting that the UA308(lm308) is mounted in an acyrlic socket. That rectangular lump of metal on the main board has me intrigued.

author: Micke
author_rank: Regular Contributor
timestamp: November 20, 2015, 11:09:00 am
content: Quote from: Vgkid on November 19, 2015, 10:27:36 pmYes, it seems strange doing a custom heatsink! can not be cheaper than buying an off the shelf standard type...Not often you see those transparent acrylic IC sockets, when you see them it is in quite old equipment... I would like too see proper machined sockets instead.I guess changing the IC sockets is not a good idea, risk I will introduce some drift! as long as it works, don't touch it!I have considered re-capping it though.

author: nukie
author_rank: Frequent Contributor
timestamp: November 20, 2015, 11:22:22 am
content: Very interesting device, I notice some of the electronics age with blister on the solder mask. Is the solder mask breaking down? What causes blistering?

author: Gyro
author_rank: Super Contributor
timestamp: November 20, 2015, 12:35:26 pm
content: I've seen that sort of solder mask 'blistering' on many boards of that era. It's not lifting, just thick and wrinkly probably that way since manufacture.Edit: I think it occurred during wave soldering as the thick solder plating underneath melted.

author: anhtuanhorten
author_rank: Newbie
timestamp: November 07, 2017, 10:56:19 am
content: Hi,I would like one VARIREF VF-5.Would you please shhow me a dealer or company who still sell this instrument?Thank so much