Datron Wavetek 1271 and 1281 Self Cal circuits woes and repair Pt 1A - Page 1
Forum section: electronics / testgear

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: November 30, 2020, 12:36:19 am
content: Datron 1281 and 1271 self cal circuitI have attached a two-part guide to the Datron Wavetek 1271 and 1281 self cal circuit. This was borne out of a 2342 failure (AC Pre amp) on my 1281 that in fact turned out to be a failure in the self cal circuit itself. Description of the actual failure and use of the pathway commands follow the circuit description in Part 2.The waveforms and voltages show a Datron 1281 self cal circuit. It is basically the same circuit in the 1271 except for some pinouts and the deletion of two reference resistors on the 1271. The photos show the WF and voltages into T701/T601 transformer and the WF and voltages on the transformer outputs and through the filter.The waveforms were measured with the scope powered by an isolation transformer and a “cheater” AC plug for extra measure. The RMS measurements were done with a Fluke 179. While not a “true” RMS reading of the AC voltage because of the square wave it is repeatable and is a quick measurement without probing with a scope.The reference points were the Black lead for the primary voltages and the green/yellow wire for the secondary.The signal to the transformer is an approximate 1 kHz square wave with a 50% duty cycle. The use of a transformer eliminates any drift issues.Taking a very quick look at the circuits in the 1281 versus the 1271, it seems that they are identical except for R614 and R615 in the 1281. The transformers have the same p/n 350001 for the 1281 and 1271.The voltages (measured below) and the waveform photos, are keyed to the test point callouts on the diagram below. I was unable to find all the Pathway commands to route the signals to TP103, I resorted to pulling the DG508 and manually jumpering the voltages to Pin 8. For these tests I pulled PL121 off to ensure no damaging voltage levels were present on the INT_SIG_BUS line going off the DC board.Test location Datron 1281 Signal Voltages @ TP10, 11, 12, TP103 INT_SIG_BUS, PL121-2 – PL121-1TP1 T601 Primary 24.3Vpp, 10.75VAC RMS N/ATP2 Orange +9V93 20.4Vpp, 9.91VAC RMS +10.00VDCTP3 Yellow +7V23 14.6Vpp, 7.16VAC RMS +7.23VDCTP4 Green +1V83 3.84Vpp, 1.808VAC RMS +1.895VDCTP5 Blue +1V00 1.99Vpp, 0.951VAC RMS +0.995VDCTP6 Violet +0V19 386mVpp, 183.2mVAC RMS +190mVDCTP7 White -7V23 14.6Vpp, 7.16VAC RMSTP8 Pink -9V93 20.4Vpp, 9.91VAC RMSM609 Pin 6 -5.480VDC , M609 Pin 7 -7.23VDC This is with P206 enabled.I am assuming that if anotherPathway command was enabled that needed + feedback to the error amp that polarity would be the opposite due to the phase relationships on the transformer windings.M609 Pin 11 +5.480VDC , M609 Pin 10 +7.23VDCThe M611 DG508 MUX selected signal goes off the DC board as INT_SIG_BUS at PL121-2 with PL121-1 as reference.The fault lay in the Violet +0V19 T601 winding, it was putting out 1/2 of what it should have. Instead of a +2.0 to +1.7VDC test signal for 100mV AC Preamp test (giving a P206 reading of -2.0 to -1.7VDC) the test signal was +0.86VDC giving a P206 reading of -0.86XXVDCUnfortunately Datron provided no voltage levels, the indicated levels were taken from the 1271 schematics where they are marked beside the transformer T701.Fortunately TheSteve lives in town and was kind enough to measure the +0V19 Viloet winding and confirm my diagnosis.Since a new/used transformer was not available I built a SMD non inverting LM358 2x amplifier and used it to boost the output to it's proper level.It's all in the attached documents, Pt 1A, 1B and 2.

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: November 30, 2020, 12:38:42 am
content: Final part, the fix.............

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: November 30, 2020, 01:27:43 am
content: Part 1B of the tale.........

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: November 30, 2020, 01:30:15 am
content: Pt 1C of the tale..

author: coromonadalix
author_rank: Super Contributor
timestamp: November 30, 2020, 02:13:05 am
content: starting multi threaded posts is a nightmare to follow and manage why did you do that ?? .... sorry but its not a good way to keep an eye on this.

author: EEVblog
author_rank: Unknown
timestamp: November 30, 2020, 02:45:51 am
content: Quote from: coromonadalix on November 30, 2020, 02:13:05 amAll threads now merged.

author: Kleinstein
author_rank: Super Contributor
timestamp: November 30, 2020, 11:04:52 am
content: It is really odd fault to have a transformer winding to give a totally wrong voltage, especially if the other voltages still seem to be OK.The possible fault at the transformer would be something like 1 shorted winding, but this would also effect the rest of the transformer, so the ratios would be no longer valid.It is more like there may be a production fault (e.g. using 2 instead of 4 turns). AFAIK the actual numbers are written to the cal memory and these numbers are used. So the smaller voltage could be OK, if correctly noted in the memory. It could still be boarder-line for the AC test. So is sure the memory and transformer belong together ?

author: doktor pyta
author_rank: Frequent Contributor
timestamp: November 30, 2020, 12:59:10 pm
content: Thanks for the post, but I have strange feeling that You only swept the real cause under the carpet.Please consider that differences in cal trafo output voltage may be related to serial number.Does TheSteve have 1281 with white or brown colour front panel?I'd rather think that some cal memory bits were changed than trafo have changed it's turns ratio.Shorted turn would cause more mess.BR

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: November 30, 2020, 03:05:40 pm
content: Unfortunately the cal memory was dumped with no backup, so no ability to check any previous numbers.I have checked with a couple of people I know that service the Datron meters and a cal transformer going bad does happen. I do agree that I am curious that it only affected the one winding. The part number of the transformer is the same in the 1281 and 1271 manuals and these people also confirmed that it was the same transformer........The proof would be for some one to check a brown Datron 1281, my ser# is 19287-3 and see what the Violet winding puts out on a P206 test.One thing of note that I may not have mentioned is that even though it was giving the error during the tests, inputting a 100mV AC signal was close to what it should be given that it is out of cal.

author: TheSteve
author_rank: Unknown
timestamp: November 30, 2020, 05:18:36 pm
content: As it was asked my 1281 is newer with a white face and the newest firmware revision.Ken's problem is very curious. I also didn't think only a single winding in the transformer would fail in such a way. It seemed more likely to me that the transformer was wound that way from the factory. I don't know how it would pass a self test that way though. The voltage it puts out is exactly half of what it should be - which is also suspicious.Might just be one of those Datron mysteries. Being the meter operated just fine and that it was only the self test that had an issue his fix seems just fine.

author: volvo_nut_v70
author_rank: Regular Contributor
timestamp: November 30, 2020, 06:09:10 pm
content: I had upgraded the firmware to 3.12. I did try the original firmware 2.03 back into it, it made no difference.The AC Preamp (looking at the expected self test input) is an inverting 10x amplifier (at least for the DC signal) ~+0.19 V in, DC 1V range selected at beginning of the P206 test, -2.0 to -1.7V DC reading.I also pulled the DG508 MUX to ensure that nothing was dragging it down.Anyone who has an old 1281 and cares to take a quick look at it that'd be great!!Thanks!!