Yet another Fluke 8050 - Page 1
Forum section: electronics / repair

author: IconicPCB
author_rank: Super Contributor
timestamp: April 11, 2016, 11:06:11 pm
content: At long last I tackled the power transformer issue in my New Old Stock 8050.Bought new on Ebay after careful consideration of powersupply circuit schematic to make sure it would operate on 240V.WRONG!Voltae options are not obtained bychanging transformer taps as sugested on the schematic.Yesterday I obtained an 18V center tapped 5VA transformer and nudged it into position.8050 wakes up however with a small offset.I checked the power supplies and lo and behold the 13V is 12.8V and -10V is -12.whatever.Here is the problem the schematic shows nominally +13 and -10V between the outputs from a single brige across the center tapped transformer.I would have expected a symmetrical output as I am seeing, NOT asymmetric positive and negative voltages as annotated on the schematic.Please comment on the power supply situation.Should I look into ensuring new transformer in fact does provide only -10V unregulated ?Is the original transformer regulation so poor as to result in intentionally asymetric outputs?

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 12, 2016, 12:21:39 am
content: Quote from: IconicPCB on April 11, 2016, 11:06:11 pmProbably so. Current draw from the positive side is 22mA, the negative side is -61mA on a non-battery unit I have here. The CMOS logic is powered between ground and the regulated -5V, which probably accounts for the asymmetric loading. The transformer is pretty wimpy-looking.I have several units here, mostly with the battery option, and as I recall, the unregulated supplies vary all over the place from 10 to 15V. The meter doesn't seem to care. Indeed, this particular one is +15.3V and -13.6V.

author: IconicPCB
author_rank: Super Contributor
timestamp: April 12, 2016, 12:32:06 am
content: Modem,Thanks for the comments. There is room enough for a gruntier transformer .. but then again... economies of scale and all of that..I shall probably have to go about with a shotgun replacing electrolytics. It might scare it into proper operation

author: IconicPCB
author_rank: Super Contributor
timestamp: April 12, 2016, 04:56:46 am
content: So farthe meter has been switched on for a few hours.ACV range shows significant offset with open circuited input.200 mV range fluctuates between 27.8 to 28.5 mVas the ranges are switched in further up the scale the last two digits eventually stop at 1.4VDCV range is free of this problem. Inspection of the circuit shows three electrolytics around the true RMS converter assembly.Two on the input and one which seems to be a part of the converter circuit. Will replace them in the hope noise is eliminated.

author: Shock
author_rank: Super Contributor
timestamp: April 12, 2016, 05:09:34 am
content: You do know that the multimeter will fluctuate on AC without a load right?Short the leads and check. Then measure something like a signal generator or other test equipment, something that has a sinusoidal signal within the multimeters spec.

author: IconicPCB
author_rank: Super Contributor
timestamp: April 12, 2016, 05:20:11 am
content: Yes You are quite right.The old 8020 also shows a reading in similar circumstances.Shorting the input reduces the reading to 0.16mV

author: Shock
author_rank: Super Contributor
timestamp: April 12, 2016, 10:46:18 am
content: If it looks like you did a decent job you should post the part numbers, the model spec of the transformer and a photo or something. Will help others doing the same thing.I almost brought one of these myself a while back thinking it was just a case of swapping the taps.

author: ModemHead
author_rank: Frequent Contributor
timestamp: April 12, 2016, 12:18:51 pm
content: Yes, your 8050A sounds normal. The service manual (Table 4-9) indicates any reading of less than 40 digits in ACV range with inputs shorted, is acceptable for the RMS converter offset. Section 2-98 also discusses this offset and it's neglible effect on readings above 5% of range.Concerning the transformer, the manual (4-22) states that the AC-only version requires different transformers while the battery-powered option has taps. However the schematic incorrectly shows a transformer with taps on both the AC-only and battery-powered supplies. Very confusing.

author: IconicPCB
author_rank: Super Contributor
timestamp: April 12, 2016, 09:45:30 pm
content: Modem,Thanks for confirming.Shock,I got hold of Altronics ( Australian components and bitsa distributor) PCB mounted transformer Altronics stock number M7058Ahttp://www.altronics.com.au/p/m7058a-powertran-5va-9+9v-pcb-transformer/the image shows 15V centre tap model. Be sure to get the 18V centre tapped model M7058A.The transformer consists of two secondary winding and a primary side thermal fuse in series with the primary winding.Itis the primary side of the transformer whihc needs some slight reforming of the niddle pin ( the pin connected to the the point where fuse and coil meet).This way should the thermal fuse blow the transformer is still recoverable ( yes I know...but buggers cant be choosers).The alternative is snip the pin off and the transformer slips into place like the proverbial in a sleeve.On the secondary you will have to install a link between adjacent middle pins. In doing this make sure the linkis as close to the PCB as practicable. Make the link from a tinned bit of wire since You will need a dollop of solder to join the link to the hole where the centre tap of the old transformer used to be.Thats it. HOWEVER... before soldering the transformer in place use a cable tie, position the transformer in place and tie it down making sure it is held in place by a cable tie.Only after You have secured it down with the cable tie apply solder and subsequently the shorting link.