Oertling RB153 repair - Page 1
Forum section: electronics / testgear

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2015, 05:47:02 pm
content: Today I pulled out the Oertling RB153 massmeter, to use it to verify the cheap Ebay masspieces I bought.Plugged in for the first time in a year, and left to warm up for 4 hours. Drift was 7mg from that, about average for this non autozero tracking unit.Put the 205g of mass on the unit, and it agreed with them pretty much exactly, though they still have to go in to be verified by SANAS.As I was getting ready to return it to storage, there was some acrid smelle, a bit of buzzing and a curling waft of smoke coming out the side, followed by silence and no display.So, take it apart.....Power supply is pretty much standard for it's age, transformer and regulators, and a simple 8048 CPU driving it all.ADC is in it's own shielded section using a LM399H reference to keep it stable. No ADC in sight, it is a pure single slope converter, using CMOS analogue switches, good quality film capacitors and a LM311 comparator, and the opamps are Harris quad 741 units, and a TLO74. Not any ADC chip, all discrete. 1500g, with 1mg resolution.Load cell attaches here, with it's own amplifier.

author: PedroDaGr8
author_rank: Super Contributor
timestamp: July 20, 2015, 05:55:09 pm
content: Nice it has an LM399I have a taken apart a bunch of old high resolution analytical balances at work that died for one reason or another. Unfortunately, all have used their own insulated oven on a cheaper device. The ovens are huge and bulky with huge custom drive circuits. Basically, worthless for scavenging.

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2015, 06:09:03 pm
content: Here is the offending mains filter. Not Scaeffner, but Belling Lee Y21633 filter. Still cooked though, with a failed common mode choke, which is low resistance ( 12M with a DVM, but would be lower with 220VAC applied) from line to neutral. Will replace tomorrow with something that fits.The fuse on the input IEC connector stopped the fireworks after around 5 seconds, blowing the 500mA slow blow fuse. Open the fuse holder and there is a spare fuse there, or rather the 230VAC use fuse, as the spare is a 125mA unit, while the 500mA is the 110VAC use fuse. Oops, never changed by the importer.... Still, did it's job.Tried to depot the filter, and saw the cooked resin by the common mode choke, not failed film caps like the common Schaeffner fault, but still likely from not being plugged in to keep it toasty.

author: SeanB
author_rank: Super Contributor
timestamp: July 20, 2015, 06:10:21 pm
content: Quote from: PedroDaGr8 on July 20, 2015, 05:55:09 pmIt dates from 1990, so they did want the best stability with analogue, it does not have any zero tracking to hide drift.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 20, 2015, 07:18:49 pm
content: How is the balance working ? 6 Digit resolution is a lot for DMS. Is it using force balancing with a electromagnet ?The Converter does not need to be simple single slope. It could allready be a simple form or multi-slope charge balancing converter similar to modern DMMs.

author: SeanB
author_rank: Super Contributor
timestamp: July 21, 2015, 03:32:36 pm
content: Looked today a little when replacing the filter. I just used some 1R 1W fusible resistors off an old Tektronix phasor printer board, as I did not have any filters that fit, and I was not going to ask for vbudget to fix it, as it would then be scrapped.Took the stage cover off, and it looks like it is a force balance, but I am not taking it apart further, as then it will need a full calibration, not just a verify, Checked after the 3 hour warm up ( drifted 20 counts from cold to stable) and it was within 4 counts with the check masspieces. 100g out 3, 50g out 2 and my 1.0000g masspiece was showing 1.000g after settling down.Checked the cheap Chinese $7 masspieces I bought, and they are within 0.02g of nominal. They are adjusted by unscrewing the top and adding steel wire pieces to then to get them exact.