[SOLVED] Fluke PM6690/Pendulum CNT-90 Auto Trigger not working  - Page 1
Forum section: electronics / repair

author: Cyclotron
author_rank: Unknown
timestamp: July 28, 2025, 03:02:11 pm
content: I purchased the Fluke PM6690 at a reasonable price, as it was reportedly broken. I've upgraded the firmware to the latest version I could find (1.32, thanks to this community and particularly KungFuJosh).The C input doesn't appear to work at all. I've captured thermals of that card, and I will need to determine if the AMPs/dividers are gone.At this point, I've removed the optional card for input C so that it will not impact my testing on the other two channels.Inputs A and B both appear to function correctly when in Manual trigger mode. I have reviewed the settings with the manual to reset everything to its default state, used the Calibration tool, and set everything to its factory default. The manual says "When Auto is active the counter automatically measures the peak-to-peak levels of the input signal and sets the trigger level to 50% of that value. The attenuation is also set automatically."I can see that the system is measuring the peak-to-peak values, but I don't see it trigger at all when set to auto.

author: Cyclotron
author_rank: Unknown
timestamp: July 28, 2025, 04:21:08 pm
content: According to the manual, I can capture the Auto set trigger by setting it to Auto and then switching to Manual. That appears to work, and I get a reasonable trigger voltage.I'm testing against my lab 10 MHz standard reference. I have an Agilent 53132A, a Siglent SDS824x, and a Fluke PM6690 attached to this reference on the same coaxial line. They seem to agree on the peak-to-peak.I have the schematics, so I need to start determining where these measurements are made.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 28, 2025, 04:25:53 pm
content: Do you have a GPIB adapter? You can try and see if the SCPI commands for measurement work, including setting continuous measurement.If you have the GPIB, then try connecting to TestController with the driver I posted in the TC thread. In the Setup section there are buttons to enable and disable continuous measurement, or you can manually send the SCPI commands:INIT:CONT ONand:INIT:CONT OFF. I'm also curious to see if the triggering sent from TC will work despite what might be a flaw in the auto triggering on the device itself.Thanks,Josh

author: Cyclotron
author_rank: Unknown
timestamp: July 28, 2025, 05:03:05 pm
content: Quote from: KungFuJosh on July 28, 2025, 04:25:53 pmI do have some GPIB adapters. I made several based on this design:https://github.com/xyphro/UsbGpibI have one NI GPIB-USB-HS somewhere. I'll try to find that one. We'll see what we can do.

author: Rubo
author_rank: Regular Contributor
timestamp: July 28, 2025, 07:06:13 pm
content: Quote from: Cyclotron on July 28, 2025, 05:03:05 pmThat one does NOT work with TC, I've tried a couple of days ago.You need one that simulates a COM port for it to work with TC (or a network one).

author: Cyclotron
author_rank: Unknown
timestamp: July 28, 2025, 07:24:29 pm
content: Quote from: Rubo on July 28, 2025, 07:06:13 pmWell, that explains why none of the ones I have work. I need to go find a Prologix clone I can build.I tried the AR488, which should work with the NI, but didn't get anywhere.I should go read the thread for TC, but that might be getting me sidetracked. (Squirrel)

author: Cyclotron
author_rank: Unknown
timestamp: July 28, 2025, 07:54:50 pm
content: Quote from: KungFuJosh on July 28, 2025, 04:25:53 pmI issued those commands through the NI VISIA Test Panel and it just says "Write Operation (:INIT:CONT\sON) No Error"

author: Cyclotron
author_rank: Unknown
timestamp: July 30, 2025, 12:18:55 am
content: Today I was able to get the PM6690 opened up and remove the shielding. I have also removed the optional prescaler so it is out of the picture for troubleshooting.I haven't been able to get far due to other work but I was able to get most of the way through the Service Manual's Chapter 2 "Performance Check".All of the sections appear to be validating within reason except for the Input Controls section. I can't validate everything 100% because I don't have the means to generate a 300Mhz signal but nothing in the rest give me reason to believe that would fail. I also do not have a pulse generator *yet*. I have a Standford DG535-4 due tomorrow but it was sold to me for parts or repair so I may need to get that going sooner than I expected.I have briefly looked at the supply voltages and find some issues there but very slightly out of bounds for the most part.I'm starting to thing it may be just simply out of adjustment in the voltage measuring calibration. I can also note that the prescaler card has 2-3 EOL critical components on it. They may be a challenge to source if it has issues.

author: Cyclotron
author_rank: Unknown
timestamp: July 30, 2025, 01:18:11 pm
content: Here are my voltage readings so far. Initially, the PSU 5.1 rail was slightly high, but I've adjusted it.X8, X10, and X62 are concerning. X10 is off by 2 voltsX8 was erratic during measurement, starting at -5.8 and drifting to -6.3.X25 is also off a good bit, given the scale.All of these are on the negative side, so I am going to go back to the PSU and do some more testing on the -15 rail, then I will look at the -12 regulator, as that is the source for X62 and X25.The service Manual is here if anyone wants to check my work:https://assets.fluke.com/manuals/6690____smeng0000.pdfPSU:-15 vdc Reading: -14.937+5.1 vdc Reading: +5.102+15 vdc Reading: +15.346X12: +3.3 VA (from +5 VU) Reading: +3.259X8: -5.2 VA (from -15 VU) Reading: -6.13X10: -12 VA (from -15 VU) Reading: -14.674X62: -5.2 VI (from -12 VA) Reading: -6.327X25: -2.1 V (from -12 VA) Reading: -2.564X16: +12 VA (from +15 VD) Reading: +11.959X17: +12 VU (from +15 VU) Reading: +11.927X14: +3.3 VD (from +5 VU) Reading: +3.288X2: +1.8 V (from +5 VU) Reading: +1.824X15: +15 VD (from +15 VU) Reading: +15.255X11: +5 VD (from +5 VU) Reading: +5.080X13: +5 VA (from +5 VD) Reading: +5.027X66: 1.8V (from +3.3V) Reading: +1.763

author: Cyclotron
author_rank: Unknown
timestamp: July 30, 2025, 03:16:50 pm
content: New wrinkle in things. As I was taking measurements on the bench, the display suddenly dimmed.Now, when I turn the unit on, the display is normal brightness, but then after booting, it dims.I also noticed from the beginning a wave of "flickering" in the display, but I'm unsure if this is "normal" for this device, given that it refreshes the display.

author: Cyclotron
author_rank: Unknown
timestamp: July 30, 2025, 04:15:24 pm
content: I've ordered some LM2991S regulators as I don't have any. I've checked the circuit around the regulator and found no issues.If the new regulator doesn't bring things in line, I'll put in an adjustable resistor and find a value that gets that rail in a closer alignment with the service manual.I have a tendency to be pedantic on supply and regulated voltages. I'm unsure how much these variances in voltages affect the core problem it exhibits.I think of it like a house; if the voltages/foundation aren't level and proper, then nothing on top of it will be.The pulse generator may come in, and I can get that on the bench while I wait for these regulators.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 30, 2025, 08:18:33 pm
content: Did you let the device warm up before testing voltages?You mentioning the pulse generator thing is giving my second thoughts about selling my SML03 with a pulse gen in it. That's the only one I have, but I don't usually neeeeeeeeed it. Crap, I think I could use it as an external pulse gen for my SMIQ. 🤔

author: Cyclotron
author_rank: Unknown
timestamp: July 30, 2025, 08:28:53 pm
content: Quote from: KungFuJosh on July 30, 2025, 08:18:33 pmYes, sir, it sat on my bench running overnight, and I tested the voltages this morning. I got my pulse generator in and I'm taking a quick look at it before I press order on my Mouser cart. I should have the new regulators in by Friday so I can go back to the PM6690 then.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 30, 2025, 08:36:26 pm
content: Sounds like ample warmup time.

author: Smile2016
author_rank: Regular Contributor
timestamp: July 31, 2025, 10:31:59 pm
content: Quote from: Cyclotron on July 30, 2025, 03:16:50 pmIn the main menu, did you try the up/down arrows to adjust the brightness? Flickering has been present on all units I’ve seen so far; particularly those with the Sharp MCU. You can verify MCU type by checking the firmware revision: FWx.xx S or T

author: Cyclotron
author_rank: Unknown
timestamp: August 01, 2025, 03:26:42 am
content: Quote from: Smile2016 on July 31, 2025, 10:31:59 pmWoah, awesome info Smile2016. I could have totally adjusted the brightness on the display by accident while I had my monkey fingers all over it doing voltage validation. I will check this tomorrow. I know for sure that my unit has the Sharp MPU. Thank you for letting me know the flicker is common. That will be something I can address or ignore now that I know it is inherent.I always hate flicker but I don't imaging staring at this intstrument in normal use, I expect I'll use it to characterize things via the GPIB.

author: Cyclotron
author_rank: Unknown
timestamp: August 02, 2025, 06:56:51 pm
content: @Smile2016, you were correct about the display brightness.I've made improvements and have seen "AUTO" work once, but now I get "Clipped" when trying to measure. I replaced the -12V source regulator. (LM2991SX/NOPB)X12: +3.3 VA (from +5 VU) Reading: +3.259 Update +3.25X8: -5.2 VA (from -15 VU) Reading: -6.13 Update: -1.718X10: -12 VA (from -15 VU) Reading: -14.674 Update: -11.917X62: -5.2 VI (from -12 VA) Reading: -6.327 Update: -5.138X25: -2.1 V (from -12 VA) Reading: -2.564 Update: -2.082X16: +12 VA (from +15 VD) Reading: +11.959 Update: +11.966X17: +12 VU (from +15 VU) Reading: +11.927 Update: +11.930X14: +3.3 VD (from +5 VU) Reading: +3.288 Update: +3.288X2: +1.8 V (from +5 VU) Reading: +1.824 Update: +1.823X15: +15 VD (from +15 VU) Reading: +15.255 Update: +15.263X11: +5 VD (from +5 VU) Reading: +5.080 Update: +5.082X13: +5 VA (from +5 VD) Reading: +5.027 Update: +5.032X66: 1.8V (from +3.3V) Reading: +1.763 Update: +1.762X10, X25, and X62 are now all reporting sane voltage levels.X8 is now way off with -1.718 instead of the expected -5.2.I am now going to look at the source for X8. I haven't looked, but I suspect that may be sourced at an OpAMP that was attempting to match output from something impacted by the regulator replacement. That could mean it's not working due to the need for adjustment, or the high voltage on X10 blew something else down the line.Here's hoping it's just an adjustment.

author: Cyclotron
author_rank: Unknown
timestamp: August 02, 2025, 07:49:20 pm
content: Well, it wasn't an adjustment and if my memory had held I would have known that.X8 was wild under measurement previously.I replaced the regulator, another LM2991SX, and now I have -4.812 on that line.The big plus is that the instrument is behaving properly now and works in both Auto and Manual. External Ref is working, and overall, I've cleared the significant issues.This rail is a bit on the low side. I don't see a +/- note about it, but I may adjust the resistors to try to get the voltage closer to spot on while I'm here.So that's two LM2991S regulators in a new unit that were not working correctly. This instrument came in the original box and has the LCD protective coating still on it.Specification from manual 1st test U123 U124X12: +3.3 VA (from +5 VU) +3.259 +3.25 +3.260X8: -5.2 VA (from -15 VU) -6.13 -1.718 -4.812X10: -12 VA (from -15 VU) -14.674 -11.917 -11.914X62: -5.2 VI (from -12 VA) -6.327 -5.138 -5.137X25: -2.1 V (from -12 VA) -2.564 -2.082 -2.083X16: +12 VA (from +15 VD) +11.959 +11.966 +11.966X17: +12 VU (from +15 VU) +11.927 +11.930 +11.930X14: +3.3 VD (from +5 VU) +3.288 +3.288 +3.288X2: +1.8 V (from +5 VU) +1.824 +1.823 +1.823X15: +15 VD (from +15 VU) +15.255 +15.263 +15.266X11: +5 VD (from +5 VU) +5.080 +5.082 +5.084X13: +5 VA (from +5 VD) +5.027 +5.032 +5.03X66: 1.8V (from +3.3V) +1.763 +1.762 +1.766Next up, I'm going to reinstall the divider board for channel 3 and run some tests. If that works, then I'll move to running through a calibration review and adjust as required.I'm super happy about getting this going at the price I paid, and it's basically new.

author: Cyclotron
author_rank: Unknown
timestamp: August 02, 2025, 08:36:22 pm
content: The divider tests good! She counts, and I've been able to calibrate the internal oscillator. I calibrated it 10MHz sourced from GPS through a Leo Bodnar LBE-1420.I've done some measurements of my lab 10MHz rubidium standard, and things look great.I will take a step back and focus on the calibration/adjustments.I'll close this thread as SOLVED since my primary issue appears to be in that state now. Thank you all for the coaching. I blamed Josh for my buying this, but I guess now I have to thank him.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 04, 2025, 02:11:04 am
content: Nice work!