HP 3457A Multimeter No Ohms Measurements

[Johnny10]

Yesterday I purchased a non-working HP3457A .

Unit was missing Front Current fuse and has the Current Measurement post taped off.
After some investigation I found a dislodged chip which after fixing a mangled pin and adding two screws brought back DCV & ACV however,  no ohms and no current measurement.

Diagnostic test error reads  AUXERR? 96  Test 5 ( Current Source Gain Select) and 6 Failure (Amps Function Error)
Something I have to work on this weekend.

The taped off Current jack makes me think this Amps measurement was an ongoing problem.
Anyone fix this multimeter lately?

[Johnny10]

U101 ( brown & gold chip) was the chip floating in the bottom of unit with bent pin holding it at angle to board.
No screws just floating with one pin attached ?

[TAMHAN]

I have had a similar problem with a HP 34401A, where someone  had removed a key inductor from the unit.

Please check whether the unit emits a current when in Ohms mode. Also, use a second Ohmmeter to measure the internal impendance in Current mode. This gives us more info.

[Kleinstein]

There is a service manual with schematics available.

U101 is the input hybrid. This chip includes selecting the signal source (DCV, DCI, ACV and so on) and also switching the test current for the ohms measurement. If this chip is broken, a full repair can become expensive or impossible. As the chip was already removed / loose, this is well possible.

Checking for the test current in the ohms ranges is a good idea.

For the current ranges one could check if the shunt voltage gets up to the chip - only one switching chip in between.

[Johnny10]

OK,  current measurements are now working took it to 1 amp of the 1.5 pk no problems.
Now for the Ohms measurements.
I get no output current in Ohms mode.
Guess this is next circuit to work on.

VDC compared to Fluke

[Johnny10]

Page 8-59 of the Service Manual

I went back and looked at the photo of the U101 chip and wondered what the damaged pin (#21) connected to...

That would be Ohms Current Source.
Gonna have to find a way to get in there with probe.

[GreyWoolfe]

Johnny10, you got that at Planet Surplus, didn't you.  I was the fat guy talking to you about it.  Good luck with getting it working.

[Johnny10]

Hey!
Don't tell everyone about Planet Surplus I won't get any more good deals!!!
Unit was in pretty bad shape.

DCV, ACV, Frequency, Current all work now.

Just the ohms current circuit I am working on.

I am going back today to look for some JFETs.


Hope to see you again.

[TAMHAN]

Hello,
does that store also have an online shop, so I can participate.

Re the current source: in many cases, it can be a pilfered component (the Gowanda inductors seem to find many friends). Check the path from the IC to the output, step by step. Its boring, I know...but it helps. If I had done it, I had saved myself a lot of effort on my 34401A.

Tam

[Johnny10]

It would seem to me that if the Amps section is working and the Ohms and Amps measurements use the same current source then
I should be able to find this problem easily . Except... this circuit is at the bottom of the case under 2 other boards and a shield making easy probing impossible.
And as you said a lot of time to get there and then the parts are select grade.

I will get it ! 

Planet Surplus does have an eBay store.
http://www.ebay.com/sch/planet-surplus/m.html?_nkw=&_armrs=1&_ipg=&_from=

And I am in the physical store a few times a week 

Great people interesting products.

[TAMHAN]

Hello,
if you ever find something cool, please hit me up by all means .

Re the taking apart: welcome to the club. Just hope the component which was pilfered is replaceable - in my 34401A, I had to substitute a lesser part as Gowanda has no one who sells one piece of their product.

[Johnny10]

Time for an update on my 3457 repair.
I looked over the forum for information on this multimeter and a forum member whose posts came up quite often was cncjerry. I decided to try a PM to the member to see if he had any insights.
Fortunately he was more than willing to look over my shoulder, so to speak, which gave me the confidence to dig right in to this repair.

I got down to the A2 board much easier than anticipated and started measuring and comparing voltages that cncjerry suggested.
Johnny10,  Get the schematic page 8-77/78 and check the voltages on the input hybrid.  There are a number of them called out:

From Schematic                                    My Measurements
Pin 4    10V                                         10.19
Pin 8     8V/5.6V                                    3.16
Pin 21   12V/8.4V                                -0.430
Pin 17    8.4V                                        0
Pin 18    8.4V                                        1.905
Pin 31    +5                                           5.05
Pin 22    -15                                        -15.04

My measurements were being brought down by something in that constant current circuit.
I then went back to schematic on Pg 8/69-8-70
it shows:
U212 Pin 7 Should be +15v    I have  -0.359 .
I did have correct supply voltages on U211
+15, -15.
U213 correct supply voltage.

I seemed to have proper voltages at other locations except around U212 and Q211

I unsoldered the U212 chip.
and got 5.6V on Pin 8.  Good sign!

I then did a trick that Ronald over at Dos4ever taught me .
Threw in a 741 OPamp to see if I could get something on Ohms range.
Everything worked!

Ohms function began displaying for the first time.

I still get  errors :Hardware Error:  1 Calibration error.

Ohms measures 20% low but I decided before I calibrate unit I will order and replace the proper 1826-1381 chip.

Will keep you informed of progress once proper U212 chip is installed.

cncjerry was most gracious and helpful and gave me the confidence to tackle this repair. Thank you  Jerry!  A truly great forum member.

[cncjerry]

Johnny10 was in the right area and all I did was point him to the simplest circuit that wasn't working, a voltage divider in the hybrid.  I figured once he corrected that issue the real problem would be obvious.

Glad we saved another great meter from the dumpster!

[Kleinstein]

There are lists to show what hides behind the HP part number. In this case U212 could be something like an LT1012, OP177 or similar.

[Johnny10]

Yes, that is correct... U211 and U212 are originally 1826-1381 ( LT1012CN8 )
and as Jerry noted, there was an update to 1826-1133 (LM11CN) Precision opAmps.
I have been looking for that service note.
I am wondering if they were chip for chip replacements or if that 40K resistor R211 needs to be changed also?

[cncjerry]

I checked the other components in that area and don't see any other applicable updates.