Fluke 89 IV reads AC voltage incorrectly

[vladc77]

I am hoping someone can help to clarify if the Fluke 89 IV multimeter is in faulty state.

I recently bought this old multimeter and do some testing now. I tested it in a few modes including resistance, DC voltage, Temperature, and checking the fuses. Everything is correct and identical to the new Fluke 77 IV multimeter readings. The only issue I found is with AC voltage readings out of power outlet. While Fluke 77 IV reads 119-120 AC V, Fluke 89 IV reads a nonsense number of 0.2169 AC V. The Hz readings are correct and identical to Fluke 77 IV which is 60Hz.

I do not have much experience using Fluke 89 IV. As a result, I may be doing something wrong or it is a faulty. I am wondering if someone can help me to understand what can be the issue. I also would like to know if it is possible that most of the readings are correct while some other are not. I thought that if something is out of specs than it will affect all modes.

Any ideas are highly appreciated! Thank you in advance!

[helius]

If the TRMS converter chip is faulty it would affect both AC Volts and AC Amps modes, and nothing else.

[vladc77]

Thank you for the tip. How to make sure if the chip is faulty? How to identify the chip on the board? Thank you again.

[vladc77]

I am still trying to figure out what is the issue and find a possible fix for it. I am not totally sure if it is TRMS chip faulty. Maybe, it can be firmware issue or something else. I'd like to get a second opinion if possible. Thank you again.

[Wytnucls]

Post a picture of the PCB to see if the true RMS chip can be identified. Usually AD636 or similar.
Try measuring AC amps to see if you get a proper reading on that setting.
Try also AC+DC setting on volt and amp.

[vladc77]

Great! I'll post it in a few hours.

I tested it in AC+DC mode and result was close to what was under AC, but not the same.  if I remember correctly, the reading was about 0,018 AC but not near close to 120 V. Does it say anything? Thank you!

[vladc77]

Here are the images. Please let me know if you can see RMS converter chip. Thank you.

[lowimpedance]

Cannot see any separate RMS converter on the pcb either side !. Maybe its done inside the chip with the white label stuck on top. Hard to say without tracing switch position traces (not a fun prospect).
 Is it me or the photo, but are the legs on the main IC's have some corrosion on them??. They just don't look clean.

[vladc77]

Which picture are you talking about? I think that main chip probably was replaced and the soldering is not as clean as it would be from production line. I also noticed that 3 legs on one of the chips are soldered together. Maybe, it is supposed to be this way.

Does it still make sense that all other modes I described above are working correctly, in case if RMS converter is inside the chip? What can be other issues that may affect incorrect AC voltage readings? It seems like if it is RMS converter, then I will not be able to repair this multimeter.

[lowimpedance]

Which picture are you talking about? I think that main chip probably was replaced and the soldering is not as clean as it would be from production line. I also noticed that 3 legs on one of the chips are soldered together. Maybe, it is supposed to be this way.

Does it still make sense that all other modes I described above are working correctly, in case if RMS converter is inside the chip? What can be other issues that may affect incorrect AC voltage readings? It seems like if it is RMS converter, then I will not be able to repair this multimeter.

Okay I was referring to the three large IC's, two on the bottom and the one with the white label on top. Maybe a good scrub with Isopropyl Alcohol around those pins then let dry and retest.
Its a shame that there appears to be no circuit diagrams (that I could find anyway) so that means tracing the range switch from the AC V position to see where the signal ends up.
 In the 83_85_87 service manual that do have circuits the 83 and 85 have AC converter circuits internal to the main IC, (passives external), but the 87 has the extra external RMS IC at the same area of the circuit.

[vladc77]

Thank you for trying to help!

I just want to clarify that the picture with two large IC's is taken from front side of the board. These ICs sit under LCD display which I removed for this shot. The third IC with white label sits on opposite side of the board. It was covered by metal shield that I also removed. The LCD display and metal shield can be seen in the first two images. It less likely will help much but I just wanted to avoid any misunderstanding of the board layout.

I did a little more testing on the multimeter in other modes and also checked if setup features (like setting logging) are accessible. It looks like everything is working just fine. It is just sort of not logical that only one feature is in faulty but the rest is fine.

I will try to clean the areas around IC, who knows maybe it will help.  I am still wondering if there can be some other causes for the incorrect AC Voltage readings? So far, my list of possible causes is short:

• Faulty RMS converter
• Dirty lead connections
• Dirty surface around ICs and between legs

Is anything else that can be added to the list? Thank you again.

[Wytnucls]

The true RMS converter is probably internal. Looks like someone attempted a repair on the labeled iC already.
Try to find a picture of another meter, showing the affected pins, to see how they were connected originally.
Try a clean up first. Then I would attempt a reflow of the pins solder with a good dollop of flux. iC pins shouldn't be connected together.

[vladc77]

Thank you for the tips. I will clean it more.

I found another thread with the photos out of Fluke 187. https://www.eevblog.com/forum/testgear/fluke-187/
Photo# 187_14.JPG with ICs is from identical PSB. The same 3 pins are connected there. I believe I may conclude that this is OK then.

Is faulty of RMS converter common issue for fluke multimeters?

Update:
I thoroughly cleaned all ICs pins but it did not fix the issue. It seems like I will for other solutions.

I called to Fluke and they stated that this device can be repaired under warranty if none damage from neglect, misuse, contamination, alteration, accident or abnormal conditions of operation or handling, including overvoltage failures caused by use outside the DMMs specified rating, or normal wear and tear of mechanical components. They suggested to send multimeter to them and they will let me know if they can fix it for free.

I am wondering if someone had experience dealing with Fluke customer service and what are the chances the multimeter will be repaired for free. Thank you!