Fluke 21 Series II repair - Page 1
Forum section: electronics / repair

author: oilburner
author_rank: Contributor
timestamp: January 30, 2018, 08:40:21 pm
content: I didn't get very far...With a new/good battery the display flickers on for a second and goes blank. Turn off the meter and it flickers on for a second and goes blank.Got the service manual and TP1 = 5.4VdcTP3 = 0U1 pin 48 = -4VDoes this mean U1 is not good?Thanks!Greg

author: ModemHead
author_rank: Frequent Contributor
timestamp: January 31, 2018, 12:19:20 am
content: The display flicker during power on and power off suggests that the clock is not running. That and the TP1 voltage being off suggests that U1 is bad. Maybe check the crystal anyway.There have been reports that some of the 70 series suffered from bad soldering for U1, so maybe reflow the leads on U1 just in case. I found such a case once, using a plastic stick to press on things until I found the bad connection.

author: mzacharias
author_rank: Frequent Contributor
timestamp: January 31, 2018, 11:58:52 am
content: Assuming the battery is good. I know but it had to be said.

author: oilburner
author_rank: Contributor
timestamp: February 02, 2018, 06:24:15 pm
content: Thanks ModemHead and mzachariasThe crystal is singing at 32768 Hz but its at about negative 2 V, is this normal?U1 solder looks ok and poking it does not make the display come on, just cycling power shows a second of "life"...The battery is at 9.38V : )Greg

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 02, 2018, 10:21:52 pm
content: Quote from: oilburner on February 02, 2018, 06:24:15 pmI checked this on a 77-II (same chipset). The right side of the crystal is a nice 2Vp-p sine, and swinging from about +0.5 to +2.5V wrt ground (the COM jack.) I see the manual says 600mVp-p, but this is what I get.The reason you're seeing something different is probably because your TP1 voltage is wrong. U1 has internal regulators to create the digital ground 3.1V below the positive battery terminal, the remainder of the battery voltage makes up the negative supply. Your TP1 is 5.4V, so something is definitely wrong with this function of U1.TP3 is one of the LCD drive lines, which all should be swinging from 0V to 3V with a nice slow 50% square wave. This would normally read 1.5 VDC. If yours is 0V, the digital processor U2 must not be running. I figured this was due to lack of a clock, but it may be power supply related instead.Quote from: oilburner on February 02, 2018, 06:24:15 pmAt least that seems right.TP1(5.4V) - U1P48(-4.0V) should equal the battery voltage. Kirchoff must have been onto something.Not sure where to tell you to go from here, seems everything is pointing at a power supply problem related to U1. Maybe check the battery current draw. Should be about 280uA in VDC mode. At least it's another data point to consider.

author: oilburner
author_rank: Contributor
timestamp: February 03, 2018, 10:08:52 pm
content: Hey Thanks ModemHead!! I have a few flukes and another one was acting the same way, but would blink on when I pushed some pins on U1. I reflowed as you suggested, and it works!(off to buy some probes...) : )Greg

author: oilburner
author_rank: Contributor
timestamp: February 11, 2018, 06:43:54 pm
content: Reflow also fixed the first one, thanks all-Greg

author: ModemHead
author_rank: Frequent Contributor
timestamp: February 11, 2018, 09:32:32 pm
content: Quote from: oilburner on February 11, 2018, 06:43:54 pmThat's great! Thanks for reporting the results.