Ebay 3457A meter repair

[martink]

Hi,
Purchased 3457A meter  from ebay as working but in real life with errors.

When I start the meter it starts with err indicator blinking
ACV and ACI ranges are way off.
blue key, test--->SELF TEST  OK
blue key, error--->0 ACAL REQ`D (after pressing backspace err indicator goes away)
autocal (ALL)--->err indicator goes back ON
blue key, error---> 1 CAL. FAILED

Reading 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.2

Best regards
 

[picburner]

I didn't find any difference between your file and the one attached.

[martink]

Thank you picburner, firmware is not the problem.

I did some more tests but I did not get further,
After powering up = ERR? 1024
Autocal AC:2  = ERR? 1024
Autocal OHMS:3 = ERR? 1026
Blue key+test= ERR? 1024

AUXERR? always =0

All ranges are looking ok except ACV, ACI
With inputs shorted:
00.1073mVAC
001.052mVAC
0.01070VAC
00.1052VAC
001.051VAC

00.1050mAAC
001.042mAAC
0.01046AAC

Is it possible that the cause of the problem is wrong calibration?

[tppc]

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.
 

[martink]

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.
 

Hi,
Serial: 2538A01424
Cpu board: 03457-66511 REV B ("11" written with marker by hand)
A/D controller: 03457-66502 REV D
Yes, it is possible that I have meter with wrong firmware

Making  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

[Kleinstein]

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.

[martink]

Tested with 100mV RMS sine wave 200HZ (AC RMS double checked with TEK scope)
OVLD
100.222mV
0.10106VAC
00.1462VAC
001.062VAC

[martink]

3V RMS @200 HZ

3.00048VAC
03.0028VAC
003.192VAC

One 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

[Kleinstein]

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.

[martink]

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.

Hi,
Battery voltage 3.399V, yellow Panasonic

Will test TP302, TP303 when I get home from job.
Thanks

[martink]

Inputs shorted:
TP302     TP303
-9mV       9mV

30mA 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 ranges
TP302     TP303           Meter reading
2.692V     2.687V           30.0329mAAC
270mv     269mV           030.018mAAC
29mV       28mV             0.03186AAC

300mV AC
TP302     TP303           Meter reading
12V !!!       6.85V            OVLD
2.707V      2.709V          302.515mVAC
270mV      276mV          0.30267VAC
28mV        28mV            00.3203VAC

TP302 waveform:

[Kleinstein]

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).

[martink]

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).

Thank You Kleinstein,
Will make tests/ repairs and let you know.

[martink]

Hi,
Does 37mV on TP301 look OK with inputs shorted?

On the U302:
pin2        pin3 
1.381V   1.382V
Measuring pin2 to pin3 I have 1.1mV

Measured with Fluke 175, sorry, do not have better meter

Regards

[Kleinstein]

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).

[martink]

Hi,

U307 pin1: 1,477V  same voltage on W303 pin14 (DAC)
U307 pin7: 23mV
I attached waveforms when doing ACAL:ALL.

When I change the mode to ACDCV:
TP301 37mV
TP302 1.2mV
TP303 2.7mV
U307 pin1 1.477V
U307 pin7  -1.3mV
Reading on meter 00.0244mVAC
The offset in ACDCV mode is much less than in ACV mode
What do you think, maybe I order and try to change the U308?

[Kleinstein]

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.

[martink]

Yes Kleinstein, bullseye!

U308 pin2: -0.7mV
U308 pin6: -20mv
HFAC makes a big difference, when HFAC is active TP303 goes to 0.8mV

Desoldering the Q309 made the offset 10x less
Now the  U308 values are:
U308 pin2: -0.7mV
U308 pin6: 8.4mv

Still the same errors on meter

[Kleinstein]

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.

[martink]

Did not find JFET at home, need to order them
Will 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 attachment
What 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 Johnny10
https://www.eevblog.com/forum/repair/hp-3457a-multimeter-repair-advice-needed/?action=dlattach;attach=286053
Will make some more tests...

Regards
 

[Kleinstein]

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.

[martink]

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`D
ERR? 1026 = 1024+2
ERR? 2 = CAL or ACAL failed
 
So I hope when the AC and GOHMs problems are solved the errors will clear.

Best regards

[Kleinstein]

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.

[martink]

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:
2M
   2W                  4W          Fluke                   
002.092M     001.996M       1.995M
02.0751M     01.9958M
2.07507M     1.99575M

1M
  2W                   4W          Fluke
001.043M     000.999M       0.999M
01.0384M     00.9989M
1.03842M     0.99870M
 
0.5M
   2W                   4W          Fluke
000.520M     000.499M       0.4989M
00.5188M     00.4990M
0.51878M     0.49894M

Did some tests with higher values:
30M
   2W           JM101                         4W      JM101                       Fluke
OVLD           13.12V                 29.9595M   10.15V                   29.91M
035.391M     2.541V                 029.990M    2.541V

10M
 
 2W           JM101                         4W      JM101                       Fluke
10.3871M     3.383V                 09.9901M   3.383V                   9.98M
010.818M     1.693V                 009.993M   1.693V

The 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

[Kleinstein]

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.