[SOLVED] Prestine Fluke 75: is it worth searching for a new U1 chip? - Page 1
Forum section: electronics / repair

author: wwjd1924
author_rank: Contributor
timestamp: January 22, 2022, 04:10:54 pm
content: I bought a Fluke 75 in very, very good condition. only 1 tiny little scratch on the display plastic.But I knew it was not working and I hoped to revive it but I failed.I cannot get a value displayed at VDC or VAC setting.- I did resolder the U1 contacts- cleaned S1- R1 is not open: ~1K- RV1 and RV2 not shorted- checked over and over again the pcb with a microscope- followed the troubleshooting procedure:-- The VDC signal path check procedure 3-21 of the service manualfailed: Instead of 200mV I measured 0.2mVAlso when I apply a voltage on the input of the MM, the current limiting kicks in.I did not set the current on the PS higher than 100mA but is should not short when measuring.In the Ohms setting I get 2.7xx MOhms when shortedI still like the Fluke! I could not afford it 40 years ago. I would love to get it working again!Any suggestions??Thanks!

author: abdulbadii
author_rank: Frequent Contributor
timestamp: January 23, 2022, 01:14:11 am
content: Dutch not importing cheap Taiwan/China/Korea products?we'll get 2-3 times better quality for the same cost

author: rsjsouza
author_rank: Super Contributor
timestamp: January 23, 2022, 01:26:51 am
content: If you are suspecting the main ICs are busted, perhaps consider buying another meter as a parts donor?Quote from: abdulbadii on January 23, 2022, 01:14:11 amUnless the OP paid hundreds of dollars for it, there's no comparison in quality with China-made DMMs. Taiwan-based Brymen is fine, as well as Malaysia-made Keysight. UEi is the only one I know from Korea, and they are not good.

author: tooki
author_rank: Super Contributor
timestamp: January 23, 2022, 02:20:09 am
content: Quote from: abdulbadii on January 23, 2022, 01:14:11 amNo. You get 2-3 times as manyfeatures, but with lower quality, at the same cost. Do not confuse features with quality.

author: lowimpedance
author_rank: Super Contributor
timestamp: January 23, 2022, 02:43:25 am
content: As the 'OP' has made measurements on this meter the assumption can be made that he has other meters, so this post is more about resurrection of a classic FLUKE rather than having a need for any specific function.@OP, when you say section 3.21 failed do you mean you measured 0.2 mV at Z1 pin 3 or further along at the filter ?.One other quick check to make sure the crystal y1 is oscillating.However if your sure U1 is dead then as suggested your only bet would be a junker unit , keep an eye out on your favourite 'auction site' or perhaps put a WTB request in the BUY/SELL/WANTED board and hopefully one will turn up.

author: wwjd1924
author_rank: Contributor
timestamp: January 23, 2022, 02:01:21 pm
content: Thanks to all who replied!Yes I have another DVM: a Fluke 179... and yes I know this is an overkill for a hobbyist...But I love well made products...This Fluke 75 meter, as I said, was a unreachable dream decades ago...Even if I cannot revive it, I still am glad to own it now... Silly maybe?!I can understand that putting time into this for members of this great forum is a waste of time but I do appreciate any help!To answer the question of @lowimpedance:Quote from: lowimpedance on January 23, 2022, 02:43:25 am- I will certainly look for another unit for spare parts... I saw a Fluke 23 in working order for 57 euro's... But a broken unit will be cheaper hopefully- the DVM has problems coming out of the display selftest now, after 3 minutes it passes that 'test'...- The VAC and VDC modes give a overrange 'OL' continually- The oscillator is working! I measured it: 32.76Khz but the voltage pp was 2.2V instead of the 600mV stated?- pin 1 of the input divider Z1 is 2V DC as it should be so no shorted MOV or open R1- pin 3 of Z1 should be 200Mv but is basically zero! Yesterday I said I measured 0.2mV but in the Fluke 179 mV range that is just noiseI will investigate the divider, solder it out and check it.after that I think removing the S1 rotary switch and checking it will be useful.again, thanks to everyone willing to think with me to restore this beautiful DVM.

author: wwjd1924
author_rank: Contributor
timestamp: January 23, 2022, 08:03:19 pm
content: Update:Today I desoldered all capacitors (except the ones around the U2 chip), the S1 switch and the voltage divider. Checked all of them: The S1 switch was OK as was the Z1 voltage divider.The capacitors are as good as new.Measured all resistors and checked the diode and zener: all ok...Measured all pcb traces going from the U1 chip, one by one: no problems there also.So I must conclude that the U1 chip is in need of replacement.Could the cause of the blow U1 chip be that the rotary switch S1 can be rotated 360 degrees? I don't know if there should be end stops?

author: Shock
author_rank: Super Contributor
timestamp: January 24, 2022, 05:13:12 am
content: They are nice meters, I have a few of the original series. The Fluke 27/FM have more features though.The cover/function knob prevents the switch from rotating fully. Not sure if it damages anything but it's not a good idea to rotate it past that point if broken. Here is a repair Mr. ModemHead did by using a small screw with the top filed. I'd probably use part of a small motor shaft or pinch roller pin from an old cassette deck.

author: wwjd1924
author_rank: Contributor
timestamp: January 24, 2022, 01:11:46 pm
content: Thank for the tip, I will certainly fix that knob...!

author: wwjd1924
author_rank: Contributor
timestamp: February 18, 2022, 06:33:32 pm
content: A-MA-ZING recovery...The meter is as new again... Fully unexpected it started working again...I removed the U1 A/D converter chip and replaced it with another U1 chip from a donor Fluke 75 but that didn't work at all.Then, to at least have some reading I resoldered the original U1 and to my amazement the meter started to work again!At first the reading were a bit off and when measuring a voltage, the reading was not stable but lowering slowly which is strange of course, as you were measuring a discharging capacitor.Then I reheated/resoldered all pins of the U1 chip again and after that the readings were stable and correct.After calibrating it with R8, the Fluke 75 is now *very* accurate and as new...I suspect the meter never has worked because the case and acrylic display cover are without any scratchesMaybe the heat from removing the U1 chip with a hot air rework station and resoldering has revived it?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: February 18, 2022, 07:33:56 pm
content: Good Morning Wwjd1924, Congratulations on your successful repair, (great feeling, isn't it?). And thank you for coming back to post what you did, as it surely will help others with similar problems.

author: retiredcaps
author_rank: Super Contributor
timestamp: February 18, 2022, 08:26:13 pm
content: Quote from: wwjd1924 on February 18, 2022, 06:33:32 pmMember Excavatoree has mentioned that sometimes resoldering U1 makes the unit work again.

author: wwjd1924
author_rank: Contributor
timestamp: February 19, 2022, 04:59:39 pm
content: Quote from: BILLPOD on February 18, 2022, 07:33:56 pmThat was one of the first things I did, but that didn't work in my case...The hot air rework has heated the chip much more so just maybe that's the explanation in my case??