Ebay 3457A meter repair  - Page 1
Forum section: electronics / repair

author: martink
author_rank: Contributor
timestamp: June 16, 2019, 10:16:12 am
content: Hi,Purchased 3457A meter from ebay as working but in real life with errors.When I start the meter it starts with err indicator blinkingACV and ACI ranges are way off.blue key, test--->SELF TEST OKblue key, error--->0 ACAL REQ`D (after pressing backspace err indicator goes away)autocal (ALL)--->err indicator goes back ONblue key, error---> 1 CAL. FAILEDReading ERR? register =1024 ( manual says: U602 ROM checksum error)I read the orginal ROM (TMS27PC512) and write it in blank 27c512, I was hoping that my programmer reads it better as HP meter, makes no difference with the 27c512 installed.Could someone please compare my bin file with good one or send it to me?Version 03457-88810 rev.2Best regards

author: picburner
author_rank: Frequent Contributor
timestamp: June 16, 2019, 04:55:52 pm
content: I didn't find any difference between your file and the one attached.

author: martink
author_rank: Contributor
timestamp: June 16, 2019, 09:00:21 pm
content: Thank you picburner, firmware is not the problem.I did some more tests but I did not get further,After powering up = ERR? 1024Autocal AC:2 = ERR? 1024Autocal OHMS:3 = ERR? 1026Blue key+test= ERR? 1024AUXERR? always =0All ranges are looking ok except ACV, ACIWith inputs shorted:00.1073mVAC001.052mVAC0.01070VAC00.1052VAC001.051VAC00.1050mAAC001.042mAAC0.01046AACIs it possible that the cause of the problem is wrong calibration?

author: tppc
author_rank: Regular Contributor
timestamp: June 18, 2019, 01:31:39 pm
content: Hi there,- What is the version of your meter (first 4 numbers on the SERIAL sticker)?- What is the version of the CPU board (the one with the backup battery)?- Is it possible that someone did replace the EPROM with a wrong firmware version?- As per the manual, Current offset calibration is to be done with input terminals open, not shorted.(I noticed that the reading is actually different with current input terminals shorted).Regards.

author: martink
author_rank: Contributor
timestamp: June 21, 2019, 07:15:50 pm
content: Quote from: tppc on June 18, 2019, 01:31:39 pmHi,Serial: 2538A01424Cpu board: 03457-66511 REV B ("11" written with marker by hand)A/D controller: 03457-66502 REV DYes, it is possible that I have meter with wrong firmwareMaking ACAL with shorted or open terminals makes just minor difference for me.I did not do any calibration, I still hope that I can keep the original calibrations as I do not have proper AC voltage source nor AC current source.Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: June 21, 2019, 07:51:07 pm
content: The AC values look like some offset in the AC part. This could be some drift / defect in the hardware, or just an error in the cal data. There likely is some raw offset that is subtracted as a cal constant.Are the AC ranges other wise working at least roughly ?The amps part used simple passive shunts - so there should be very little difference between open and shorted terminals for amps.

author: martink
author_rank: Contributor
timestamp: June 21, 2019, 09:09:32 pm
content: Tested with 100mV RMS sine wave 200HZ (AC RMS double checked with TEK scope)OVLD100.222mV0.10106VAC00.1462VAC001.062VAC

author: martink
author_rank: Contributor
timestamp: June 21, 2019, 09:13:22 pm
content: 3V RMS @200 HZ3.00048VAC03.0028VAC003.192VACOne more problem with OHMs mode,When I clear the ACAL error the error is not present in ohms mode until I go up in to Gohm range

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2019, 07:15:00 am
content: The AC ranges seem to work, except for the offset, which could be a problem with a cal constant.Is the backup battery for the cal ram still OK ?The error with the GOhms range could also be something like an corrupted cal constant.Besides a bad constant this could also be a slight hardware problem that would cause cal values that are outside the permitted range. So something like too much offset from the AC converter and too much leakage (or a drifted resistor) in the ohms ranges that causes the GOhms range to be off too much.I would consider the AC part simpler here: with a shorted input or high range AC current one could check the RMS converter circuit, e.g. at TP302 and TP303.

author: martink
author_rank: Contributor
timestamp: June 22, 2019, 09:32:15 am
content: Quote from: Kleinstein on June 22, 2019, 07:15:00 amHi,Battery voltage 3.399V, yellow PanasonicWill test TP302, TP303 when I get home from job.Thanks

author: martink
author_rank: Contributor
timestamp: June 22, 2019, 05:30:23 pm
content: Inputs shorted:TP302 TP303-9mV 9mV30mA AC , when I change ranges the load changes for about 5 mA , like from 30 to 35mA, I readjusted power source to 30mA when changing rangesTP302 TP303 Meter reading2.692V 2.687V 30.0329mAAC270mv 269mV 030.018mAAC29mV 28mV 0.03186AAC300mV ACTP302 TP303 Meter reading12V !!! 6.85V OVLD2.707V 2.709V 302.515mVAC270mV 276mV 0.30267VAC28mV 28mV 00.3203VACTP302 waveform:

author: Kleinstein
author_rank: Super Contributor
timestamp: June 22, 2019, 07:02:04 pm
content: The 9 mV offset measured at TP 302 and TP303 about matches the observed offset for the AC readings.The offset is rather high for the OP U308 (=LM308A), which suggests that there maybe something wrong in this area. This could also be way too much offset from Q307 (AC amplifier) so that U308 or U307b reaches saturation.Another possibility may be a wrong correction signal from U307a (DAC signal - likely mainly for the ACDCV mode).

author: martink
author_rank: Contributor
timestamp: June 22, 2019, 07:45:18 pm
content: Quote from: Kleinstein on June 22, 2019, 07:02:04 pmThank You Kleinstein,Will make tests/ repairs and let you know.

author: martink
author_rank: Contributor
timestamp: June 23, 2019, 04:06:17 pm
content: Hi,Does 37mV on TP301 look OK with inputs shorted?On the U302:pin2 pin31.381V 1.382VMeasuring pin2 to pin3 I have 1.1mVMeasured with Fluke 175, sorry, do not have better meterRegards

author: Kleinstein
author_rank: Super Contributor
timestamp: June 23, 2019, 04:44:47 pm
content: 37 mV at TP 301 looks acceptable to me. This corresponds to an offset of some 3.7 mV for Q307, which I consider quite good ( something like < 5 mV is usually the highest grade available for JFET pairs).The input of U302 are at the same level - no surprise as the amplifier is kind of working.The interesting voltages would be the outputs of U307 (pins 1 and 7). It they are very high (like > +-10 V) this would indicate a fault. If pin 1 is high than it would be something with the DAC. If only pin 7 is high it would be more like the DC correction part (e.g. U308, Q309, U303).

author: martink
author_rank: Contributor
timestamp: June 24, 2019, 12:30:22 pm
content: Hi,U307 pin1: 1,477V same voltage on W303 pin14 (DAC)U307 pin7: 23mVI attached waveforms when doing ACAL:ALL.When I change the mode to ACDCV:TP301 37mVTP302 1.2mVTP303 2.7mVU307 pin1 1.477VU307 pin7 -1.3mVReading on meter 00.0244mVACThe offset in ACDCV mode is much less than in ACV modeWhat do you think, maybe I order and try to change the U308?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 01:21:22 pm
content: The voltages at U307 look OK. In AC+DC mode the DAC does offset compensation and this seems to work correct.A defect of U308 is a very real possibility. One last point to measure could be the voltage at pin2 of U308.Leakage through Q309 could be another possibility. In this case it should make a difference of the HFAC signal is active.For U308 one may not have to use exactly the same type (LM308). An OP07 could be good enough, at least for a 1 st test.

author: martink
author_rank: Contributor
timestamp: June 24, 2019, 02:29:05 pm
content: Yes Kleinstein, bullseye!U308 pin2: -0.7mVU308 pin6: -20mvHFAC makes a big difference, when HFAC is active TP303 goes to 0.8mVDesoldering the Q309 made the offset 10x lessNow the U308 values are:U308 pin2: -0.7mVU308 pin6: 8.4mvStill the same errors on meter

author: Kleinstein
author_rank: Super Contributor
timestamp: June 24, 2019, 03:52:48 pm
content: A replacement for Q309 should not be so difficult. Most TO92 case JFETs could do the job. If TO92 FETs are hard to get one could add wires to a sot23 one.For fine adjust of the offset one could add an zero adjustment pot to U308. Some 0.8 mV like in HFAC mode sound very reasonable. There may be a small offset range that can be compensated by correction of the result by software. So one may not need to bring the offset all the way down.It may take an ACAL call before some errors disappear.With only 0.8 mV offset I would consider the AC problem essentially fixed from the HW side.For the check sum error - have you checked the 5 V supply. Maybe there is something wrong / borderline with the EPROM part, like ripply on the supply that makes the checksum fail for the fast test. It is still off to have a checksum error and still essentially running program.For the Ohms part a first test would be checking the test currents, e.g. with another meter. Testing the highest range may be tricky though, as very small currents are involved. The Gohms range uses 10 M in parallel and thus would be quite sensitive to cal errors / drift.

author: martink
author_rank: Contributor
timestamp: June 24, 2019, 09:03:52 pm
content: Did not find JFET at home, need to order themWill order LM308 too because I made some hot air test and when I hit U308 the measurement drifts a lot.Tested the voltages, worst noise I could find was on CPU board at U603, Voltage 5.06V. Please see attachmentWhat do you think, its to much?For GOHMs range did some measurements at U101(hybrid)Voltages are very close as discribed in this post from Johnny10https://www.eevblog.com/forum/repair/hp-3457a-multimeter-repair-advice-needed/?action=dlattach;attach=286053Will make some more tests...Regards

author: Kleinstein
author_rank: Super Contributor
timestamp: June 25, 2019, 07:31:22 am
content: The LM308 is not readily available anymore. The OP is not that critical - the main part would be low offset for a 200 K impedance and getting one in a DIP package. Something like LT1012 could be a good placement, an OP07 or OP177 could be sufficient. Modern replacement parts (e.g. OPA202) are often not available in DIP.Hitting it with the hot air gun could raise the temperature a lot, so that exponential rising input current could come into play. There is a chance the offset drift (and offset itself) could be improved with an bodged in additional Zero adjust pot.The 5 V signal looks acceptable, though not very good. A checksum error is odd, if the memory reads OK and the program seems to run fine. There is a small chance that the upper address line on the board could have a problem and the program only very rarely (and not in the test used so far) uses this.

author: martink
author_rank: Contributor
timestamp: June 26, 2019, 07:44:00 pm
content: Hi,I ordered some parts, will take some time until they will get here.With the checksum, I made a mistake, after reading the USER manual its clear now for me, Its ERR? not AUXERR?ERR? 1024 = ACAL REQ`DERR? 1026 = 1024+2ERR? 2 = CAL or ACAL failedSo I hope when the AC and GOHMs problems are solved the errors will clear.Best regards

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2019, 08:11:35 pm
content: The GOhms reading showing overflow could be just an ACAL problem.It is essentially using the 10 M range with the divider in parallel.As the 10 M range still seems to work one could check do some more checks on this range: e.g. check zero (probably OK) and linearity (test a few 1 M resistors separate and in series).Another point to check could be the input current in the DC mode. The critical point should be near 1 V.

author: martink
author_rank: Contributor
timestamp: July 05, 2019, 05:02:52 pm
content: Hi,After waiting a week for precision resistors, today I got information that they are not even shipped out.To get this process running again I got 1% SMD resistors and soldered them in series.Results:2M2W 4W Fluke002.092M 001.996M 1.995M02.0751M 01.9958M2.07507M 1.99575M1M2W 4W Fluke001.043M 000.999M 0.999M01.0384M 00.9989M1.03842M 0.99870M0.5M2W 4W Fluke000.520M 000.499M 0.4989M00.5188M 00.4990M0.51878M 0.49894MDid some tests with higher values:30M2W JM101 4W JM101 FlukeOVLD 13.12V 29.9595M 10.15V 29.91M035.391M 2.541V 029.990M 2.541V10M2W JM101 4W JM101 Fluke10.3871M 3.383V 09.9901M 3.383V 9.98M010.818M 1.693V 009.993M 1.693VThe 4W mode has some changes when I change ranges, the 2W is way off.What do you think, could the cause be wrong calibration?Best regards

author: Kleinstein
author_rank: Super Contributor
timestamp: July 05, 2019, 05:29:37 pm
content: The error in the 2 wire mode looks like an calibration error. There usually is a very small offset added for the leads - but here it looks like some 20-40 K. The error may be even more obvious at samller resistors (e.g. 1 K / 10 K).The 2 Wire ohms offset can usually be adjusted separate from a full calibration.The 4 wire ohms results look resonable.What is the JM101 voltage ? It looks suspicious to have the high voltage with 30 M in 2 wire mode.

author: martink
author_rank: Contributor
timestamp: July 05, 2019, 08:11:34 pm
content: With inputs shorted JM101 120mv 00.0120OHMI have a resistor decade but only till 100K, some tests:2WR Meter JM1011K 01.0354K 0.339V1K 0.99996K 3.386V10K 10.3574K 3.386V10K 010.395K 0.339V20K 20.7136K 6.77V20K 020.789K 0.678V50K 051.979K 1.694V100K 103.956K 3.387V4W values are looking spot on, voltage on JM101 same as 2W

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2019, 06:55:46 am
content: The wrong readings in 2 W ohms mode look a lot like a calibration issue. There should be an extra zeroadjustment for 2 wire ohms that could be way off. Still odd that the error is different in different ranges. The lowest ranges may be even OK 0.012 Ohms is a reasonable offset for a long time after adjustment.For JM101 my question is about the position in the circuit. I could not find it as a test point.

author: martink
author_rank: Contributor
timestamp: July 06, 2019, 08:10:28 am
content: Hi,The JM101 is just before TP5 (hard to read in schematic).What do you think, should I try to calibrate the 2W mode?I can compare the resistance that I am using with 4W mode to get accuracy...

author: Kleinstein
author_rank: Super Contributor
timestamp: July 06, 2019, 08:44:39 am
content: I would do the zero adjustment for 2 Wire ohms. This is 2 Wire ohms offset calibration and only needs a short (e.g. copper wire).Currently I see no need to do anything about the Ohms gain factors, unless they are really off. Slightly off gain factors should not cause an error message. Way off offsets could be a problem.I have used a 3457 quite a lot - but that is long ago, and no 2 wire ohms.

author: martink
author_rank: Contributor
timestamp: July 12, 2019, 10:47:07 am
content: Hi,After measuring all the circuits, parts for days I cold not find the cause of the Ohms problem.I decided to touch the 30M gain factor (it was 5M off), before doing anything I made a call to our local meteorology department- they confirmed that they are able to do calibrations on 3457A meter.So I made the 30M calibration, now meter has no errors, passes selftest and AcalI name it partial success, because I am not still happy with the AC readings:1)I changed the Q309 to 2N5460, had minor impact of the offset with inputs shorted.2)Soldered in socket for U308 and ordered used LM308AN 5pcs. from ebay, tested each of them and used the best one.Before error was 00.1073mVAC now 00.0350mVACWill work with the AC problem and keep you updated.Thank you Kleinstein, I will look for problems around U308, U307, if I will not find problems there, I will do as you suggested with the zero adjust pot.Best regards

author: Teichhermelin
author_rank: Regular Contributor
timestamp: July 14, 2019, 10:26:50 am
content: many Thanks!