Fluke 8600A with a single problem in Amps AC

[kblue]

Hello there!

I just stumbled upon a problem with my Fluke 8600A in Amps AC Mode (which i rarely use). It's always showing a value with the last 3 digits changing between 174 +/- in >all< ranges. It does not zero properly So its always be like 0,174 / 1,74 / 17,4 etc depending on range.

Thing is: All other functions seems to work just fine with a quick test. DC Amps is spot on, DC Volts, AC Volts and Ohms - everything is fine. The "area isolation chart" in the manual does not have a specific "AC Amps" section for troubleshooting / isolating the problem.

So i marked "V" as green, because VAC IS working just fine. Only "I" red (Amps AC) is not. Voltages are okay too i assume: +15,083 / -15,045 / +4,995

Any suggestions? Thanks in advance.

/edit: I guess the problem could be within the "AC Conv PCB" which just works fine for AC Volts but not AC Amps?
/edit2: The unit is NOT powered from 230V, rather from externally 5V Supply like you find here: https://www.eevblog.com/forum/testgear/fluke-8600a-battery-power-supply-conversion/msg629290/#msg629290

[kblue]

Checked again and the wrong messurement/numbers are also visible in the 200 mV AC range. I'm sure it was once zeroed there but maybe i'm wrong. So everything kinda points to the AC Conv Board.

Shorted the Input
AC V selected
200 mV range selected

C of Q7 jumps around between 32 - 38 mV (DC). The manual wants with no input 0V or even more mV jumping around in higher ranges.
B of Q7 (measured between LO and B) shows -7,694 VDC
E of Q7 (measured between LO and E) shows -8,380 VDC

[Harry_22]

Hi!

The AC section is the same for AC(V) and AC(mA) measurements.
You told that AC(V) is Ok. Thus the AC section is also Ok.

Could you push AC(mA) button and check the resistance between current inputs:

200uA range - 1k
   2mA - 100 Ohm
  20mA -  10 Ohm
200mA -   1 Ohm
   2A -    0.1 Ohm

[kblue]

No longer sure if ACV is 100% okay because the numbers shown in AC V (200mV) range is round about the same as in all AC (mA) ranges. In higher ACV ranges its just rounded off and not visible anymore. Mayhaps this video might help a little bit. In short: whatever "value" is shown just in 200 mV AC range is shown in all AC mA Ranges. At first i switch between AC mA after that AC V. So i guess, as long as it does not show zero in 200 mV AC it wont in mA AC.

In the beginning of the video i pointed to ACV but ACmA is the selected mode. As seen its now around 195 in all AC (mA) ranges and 195 in 200 mV AC but rounded off in higher ranges.



Shunts seem to be okay (with a bit of lead resistance and stuff):

200 µA = spot on 1k
2 mA = 100,10 Ohm
20 mA = 10,10 Ohm
200 mA = 1,05 Ohm
2 A = 0,15 Ohm

[Harry_22]

Very good.
As a result voltmeter "sees" at ACV Input of AC Converter 2mV at every range.

Otherwords the result is shifted by 2mV.
Could you measure any AC voltage below 20V at 20V range while monitoring with another voltmeter to prove this conclusion.

[J-R]

Have to ask, did you clean all the selector switches?
And double-check to make sure all the vertical PCBs are plugged in correctly?
I have three of these in various states and one specifically upon disassembly had two pins on the AC board bent by some previous monkey.  I admit I've only fixed one of the three units so far, so I'm clearly not a great resource though...

[kblue]

Switches are clean, yes. To be sure i measured (with REL) every contact and its 0,04 or less Ohms. Same for the PCB mounting points - they fit, none is missing or bent. Resistance also less than 0,10 Ohms.

Not sure if i did, what you want, Harry but here are some results:

Fluke 8060A (which i can trust / its tested with a cal source) VS 8600A both in ACV mode

200 mV range
shorted input in 200 mV range:

8060A: 0,01 mV
8600A: 1,98 mV

200 mV range
100,00 mV sin 50 Hz input:

8060A: 100,00 mV
8600A: 100,05 mV

---

2 V range
shorted input in 2 V range

8060A: 0,0000 V
8600A: 0,0020 V

2 V range
1 V sin 50 Hz input:

8060A: 1,0000 V
8600A: 0,9970 V

I'm not sure what to make out of this, because any difference could also be due to difference in calibration, right?

/edit: I just noticed if i use "AC Volts" mode and e.g. 200 mV range, there is a voltage at the "input" present corresponding to the value shown by the meter. In addition, if i select "AC mA" mode and e.g. 200 µA range, i see 22 +/- mV between "V/Ohm in and LO" which is also shown in the display of the instrument ... as "mA"

So somehow a tiny voltage creeps all the way to the "input" jacks.

/edit2: With the AC Board removed, the "phantom voltage" is gone from all input jacks. Even if i remove the AC V IN Connection (a single wire connected to S2A (mA) Switch) these "ghostly low voltages" are present.

[Harry_22]

Hi!

Your tests have confirmed that the AC Converter is working properly.
Although there are still two points left: zeroing and calibration.

/edit: I just noticed if i use "AC Volts" mode and e.g. 200 mV range, there is a voltage at the "input" present corresponding to the value shown by the meter. In addition, if i select "AC mA" mode and e.g. 200 µA range, i see 22 +/- mV between "V/Ohm in and LO" which is also shown in the display of the instrument ... as "mA"

So somehow a tiny voltage creeps all the way to the "input" jacks.

/edit2: With the AC Board removed, the "phantom voltage" is gone from all input jacks. Even if i remove the AC V IN Connection (a single wire connected to S2A (mA) Switch) these "ghostly low voltages" are present.

You have made a very interesting discovery: phantom voltage. It can be interference, since the input of the "V/Ohm" is open at AC(mA) mode. But /edit2 I cannot explain.

Try checking the pulsation on +5V and +/-15V power rails.

[kblue]

Hi!

[...]

Try checking the pulsation on +5V and +/-15V power rails.

Pulsation? Do you mean ripple? +15 and -15 are clean so far. The 5V rail seems a little "odd" (Osci measuring AC).

Just checked again, there is even AC voltages present at the input-jacks which seems kinda odd. My next step would be to remove the Power Board (because in the past, this was a battery powered unit) and feed the unit directly with clean -15, +15 and +5 Volts.

/edit: Feeding +15, +5 and -15 directly from a lab power supply didn't changed much (except all rails are now clean). But the "offset" or whatever this value represents is now smaller. 0,45 which will be shown in AC Volts (200 mV range) while inputs are shorted and its the same "number" shown in all AC mA ranges, just with a different comma position. Mayhaps i need to soak the AC board in some IPA for a good cleaning.

[floobydust]

Quick check of the 8600a schematic, the AC converter has trimpot R72 for DC Offset Adjust on the JFET diff amp.
Have you set that? 4-52 thru 4-54 check DC offset there. pdf pg. 36/108
AC converter output is pin 9 of edge card connector, and it's compared to pin 10. 4-54 says the voltage difference should be in range of 20-30mV. I'm not clear on what to set it for though.
I would also expect the bigger capacitors to have died of old age. Tantalums C12,13,15,16 etc.

Fluke 8600A manual 1974 when they included the schematic and BoM. Pdf pg. 101/108

[kblue]

Quick check of the 8600a schematic, the AC converter has trimpot R72 for DC Offset Adjust on the JFET diff amp.
Have you set that? 4-52 thru 4-54 check DC offset there. pdf pg. 36/108
ACMA+DCV+1200 buttons set, it reports -25,0 (so its in the ballpark of -20 to -30) and seems stable.

AC converter output is pin 9 of edge card connector, and it's compared to pin 10. 4-54 says the voltage difference should be in range of 20-30mV. I'm not clear on what to set it for though.
Pin 10 is rather stable 24,53 - 24,54 mV with shorted input (ACV + 200 mV range set)
Pin 9 is a bit erratic and unstable between 24,95 - 25,12 mV; so the difference does mess up the reported value

I would also expect the bigger capacitors to have died of old age. Tantalums C12,13,15,16 etc.
Already changed C12 and C13 (blue tantalums) but other Caps are still untouched blue tantalums ... will change these too)

Fluke 8600A manual 1974 when they included the schematic and BoM. Pdf pg. 101/108

I already have the schematics and PDF but thanks anyway! I'm tempted to get another (hopefully working) Fluke 45 just to see which "board" seems to the problem.

Best wishes!

[floobydust]

I notice the Fluke 8810A AC Converter is quite similar. But again, no instructions on how to calibrate the converter DC offset trimpot R30. pdf pg. 112/115.
https://bama.edebris.com/manuals/fluke/8810a
I would keep looking at other models with a similar AC converter, see if there is any better info on that, if this is a calibration issue.

[Jeff eelcr]

In the 8800A manual page 4-7  4-47 bias adj is for R30.
I do not have my 8810A manual here.
Jeff

[elecdonia]

The unit is NOT powered from 230V, rather from externally 5V Supply

Is this a unit which was originally powered by 4 NiCd cells?

The Fluke 8600A is a favorite of mine. Over the years I've owned and restored several, both "AC powered" and "battery powered" units.

If your 8600A is the battery-powered version then please check these items:

1) All traces of battery leakage must be cleaned from the PC boards. Switches and DIP IC sockets may need to be unsoldered and removed to clean them and the PC board underneath them. Sometimes I replace the IC sockets. I've seen some horribly corroded PC boards in battery-powered 8600A units. I succeeded at cleaning all but one which had numerous PC board foils corroded into open circuits.

2) The original battery-powered circuit utilized the 4 NiCd cells as the "bulk filter" for the 5V supply rail. The only additional filter/bypass capacitor in a battery-powered 8600A is C29 (10uF).   It's recommended to add a very large filter capacitor in parallel with C29 when powering the 8600A from an external 5V source. The added capacitance can be from 4,700uF up 10,000uF. I also suggest adding several low impedance 100nF ceramic bypass  capacitors between 5V rail and ground distributed around the main PC board. The 5V rail needs to be as clean, stable, and noise-free as possible.

3) The +/-15V rails are derived from a very simple DC-DC converter located on the "battery power supply" PC board. Check ESR and uF of the capacitors on this board (C202, C203, C204). Also confirm the 2 voltage regulator chips (U202, U203) are functioning properly. Check for high-frequency ripple on +/-15V rails with oscilloscope. There should be very little ripple or noise (only a few millivolts). As with the 5V rail, getting these +/-15V rails to be as stable and noise-free as possible will improve the functioning of the 8600A.

The following items are common to both AC-powered and battery-powered 8600A units:

1) Examine all connector pins and sockets for the small plug-in PC boards very carefully. Look for corrosion, looseness, and broken solder joints. The larger connector pins are secured by eyelets through the main PC board. These may fail to make good electrical connections. Solder in short jumper wires to bridge across these eyelets if necessary.

2) The "mA" input has 4 protection diodes (CR1, CR2, CR6, CR8). Check all 4 for shorts, opens,and leakage. 
   

[kblue]

Huge thanks for all the input (especially elecdonia!), i'll report back once i can try these things and yes currently i just "replaced" the batteries and power it with 5V.
Having such a large CAP parallel to C29 wasn't something i tried yet - will be interesting to see what difference it will made with all the other additions.