FLUKE PM6680B teardown photos and upcoming repair. - Page 1
Forum section: electronics / testgear

author: H.O
author_rank: Frequent Contributor
timestamp: July 29, 2015, 06:58:03 pm
content: Hi,So I got hold of a FLUKE PM6680B timer/counter for, what I thought, a fair price. I paid $300USD for it untested but "most likely working just fine".It arrived the other day and what I found so far (after about 10 minutes of playing with it) is that channel B seems to be working fine while channel A has some issues. It seems to count fine up to around 130kHz above that it either does what looks like aliasing or just plain stops counting. (No the 100kHz input filter is not enabled).Manuals, service manuals and schematics are available online so as long as it's not an unobtainable part that's failed it should be repairable (it does contain two custom ASICs...).At the moment I don't want to allocate the bench space and time to attempt troubleshooting and repair but since I couldn't find much info about this instrument on the EEVBlog forum I thought I'd take it apart, have a look around, and take some photos.First a general overview of the front and back panels:Taking the instrument apart is really easy, two screws at the back and two at the bottom then the whole thing slides out of the case. The front panel is a fairly thick diecast aluminium piece, the back is piece of sheet metal. Holding the front and back panels together are two pieces T-slotted extrusion to which the PCB is also mounted. A really nice and accessible design.In one of the back corners we have the GPIB board. Made in Sweden, like me :-)In the other corner we have the power supply:The backside auxillary BNC connectors soldered directly to the board:To be continued....

author: H.O
author_rank: Frequent Contributor
timestamp: July 29, 2015, 07:28:22 pm
content: Here's a closeup of the backside of the board. The schematic mentions a delay line, this must be it:Moving on to the input sections hiding under the shielding can. Note the two ferrite rings directly on the PCB:This is where I found the first evidence of damage, a 47ohm 1206 resistor looking quite bad:Initially I thought this was the switchable 50ohm input termination resistor but after looking at the schematic it turns out it's not, the termination is the two thru hole 100 ohm resistors in parallel, pretty obvious once you actually look at it..... Instead it's in series with the input, it's burnt open yet channel A seems to sort of work at low frequencies....Further up on channel A is evidence of either a previous repair (or attempt to repair) or a bodge. There also seems to be more flux residue around the transistors on channel A compared to channel B so I think perhaps someone has been in here before me. The instrument did arrive with intact cal stickers though (for whatever that's worth).After the input section comes the reference oscillator and main chipset, a pair of custom ASICs. This instrument did not come with any options (except the GPIB) but adding a better oscillator should (if needed) be fairly straight forward, the pinheader for it (BU105) is hiding just under the GPIB interface PCB.The digital section with CPU, memory and logic. Ribbon cable to the front panel obviously:I did create a stitched together ~8800x4600 pixel image of the instrument, turned out quite nice, but it's a little too big to upload (26MB).Anyway, once I get to further troubleshooting and (hopefully) reparing the unit I'll report back. In the meantime, if anyone has experience with this instrument please let me know.

author: PTR_1275
author_rank: Frequent Contributor
timestamp: December 06, 2015, 10:25:03 pm
content: How did you go with this repair? Find anything more or have you run into a dead end?

author: H.O
author_rank: Frequent Contributor
timestamp: December 07, 2015, 07:08:52 am
content: I haven't actually got around to it yet :-(Guess it goes to show how much I actually needed a new frequency counter doesn't it?But now that you've reminded me I'll see if I can at least replace that 47ohm resistor and see if that's all it is.

author: HighVoltage
author_rank: Super Contributor
timestamp: December 07, 2015, 08:32:42 am
content: I always wanted one of these but never bought one. And now, that I have my Agilent counters, I will probably never buy one.But I am really looking forward to your repair.

author: H.O
author_rank: Frequent Contributor
timestamp: December 07, 2015, 05:38:06 pm
content: Thank you guys for prompting me to open this puppy up again!I used my WXMT tweezers to remove the damaged 47ohm resistor and the WXMP to solder a new (47.5ohm because that's what I had at hand) one in its place and now look at it:Channel A now counts properly and gives the same readings as channel B. Both slightly low compared to what I dial in on my Rigol DG4162 but I don't know much about its reference so I can't say if it's the Rigol or the FLUKE (or both) that's out. Need a better 10MHz reference.... Tried both 1V and 50mV (RMS) amplitude and various frequencies between 1kHz and 150MHz and it seems to work fine as far as the previously found problem is concerned. That's not to say there can't be other issues with it of course.All in all the fix took less time that it took to write this post.....I need to play around with it and have a read of the manual.

author: Gyro
author_rank: Super Contributor
timestamp: December 07, 2015, 08:51:48 pm
content: QuoteHard to say, Philips/Fluke were/are rather good at oscillators but the board looks configured to use the basic on-board one (non-TCXO, non-oven) which has aging of <5e-7 /month, <5e-6 /year. It would be worth trying to search out and fit one of the higher stability oscillator options.Edit: Quite surprising given the resolution, it's always possible that someone removed the high stability osc before you got your hands on it... the rear panel label will tell you: PM6680B/_2_ is TCXO, /_4_ and /_5_ are OCXOs (/_1_ is the basic on-board one).

author: dom0
author_rank: Super Contributor
timestamp: December 07, 2015, 10:43:29 pm
content: The PM6680 and it's little brother PM6685 (twins) superseded the PM6652 / PM6654 as Philip's laboratory counters (they all have a normal, non-precision, un-ovenized, un-compensated quarz oscillators as the standard time base, ovens where an option), which were most likely connected to a local lab reference anyway.Edit: If memory serves me right there was however a variant of the 6680 with a built-in rubidium standard.

author: Samogon
author_rank: Frequent Contributor
timestamp: April 11, 2019, 04:21:28 am
content: I know that it is really old topic, but short question:I you apply power does it boot or stays in standby mode? There is switch JP16 which is controls power on behavour, but mine can be in two modes always on and always off, no logic controlled boot.Thank you

author: H.O
author_rank: Frequent Contributor
timestamp: April 12, 2019, 06:12:36 am
content: When power is applied to mine it powers up completely, display on etc.

author: Samogon
author_rank: Frequent Contributor
timestamp: April 12, 2019, 06:20:43 am
content: Oh thanks, i though it is some sort of failure.

author: tszaboo
author_rank: Super Contributor
timestamp: October 07, 2025, 07:24:03 pm
content: I'm going to revive this thread. I got a PM6680B recently and I've been doing some basic checks on it.I've connected an (uncalibrated) OCXO on it's input, and got 21-3Hz standard deviation noise when measuring it, and about 30Hz too low frequency. Not sure if this is a good value or not.I've also connected the 1Hz output of a GPS receiver to it, and that shows about 0.999997 Hz. Both the OCXO and the 1Hz seems to indicate that the internal 10MHz is 30 Hz too low. I also see that the 10MHz is not a high quality oscillator. I'm considering fitting it with a custom designed OCXO inside, or even a GPSDO, I didn't see anyone make that modification yet (the GPSDO). I think it would be quite interesting.But for now, do I believe the GPS's output?

author: bingo600
author_rank: Super Contributor
timestamp: October 07, 2025, 07:50:22 pm
content: Quote from: tszaboo on October 07, 2025, 07:24:03 pmhttps://www.circuitvalley.com/2022/05/diy-fluke-philips-pm66xx-ocxo-10mhz-oven-oscillator-option-board-for-pm6680b-pm6681.htmlhttps://github.com/circuitvalley/PM6680_PM6681_OCXO_time_base_optionEven more diyhttps://g5sto.com/?page_id=360

author: tszaboo
author_rank: Super Contributor
timestamp: October 07, 2025, 08:24:26 pm
content: Quote from: bingo600 on October 07, 2025, 07:50:22 pmThanks. I've seen the first one, it's nice because it has the measurements and the pinout of that connector.I've quickly checked and 5V is switched off unfortunately from the internal connector, only 12V stays there in standby. I might opt for no DC-DC converter and just running it directly from that 5V. On the other hand, it would be nice to have a GPSDO always running, without an extra box. And I could take the 10MHz output from the Fluke and distribute that to other instruments.

author: bingo600
author_rank: Super Contributor
timestamp: October 08, 2025, 08:19:03 am
content: Quote from: tszaboo on October 07, 2025, 08:24:26 pmThe DIY $10 China OCXO isn't a gpsdo ...He diciplines the OCXO via a DAC. The DAC is steered via a DS18b20 temp sensor & some "magic code" , i wonder how effective that is.A cheap "Prob good enough" gpsdo for hobby and instrument input are here ... They usually are around $60..80 on Alihttps://www.aliexpress.com/item/1005007363524269.html?https://www.eevblog.com/forum/testgear/10mhz-gpsdo-by-bh3sap/msg5598659/#msg5598659Remember to put "fredo fw" on the BP.You could prob DIY it (BH3SAP) with one of the chinese OCXO modules , a cheap GPS receiver a BluePill w fredo fw , and some PWM filtering on the BP OCXO output steering.But for $60 .... It might be easier to just buy one.Note:You prob have to move the 10MHz wire on the BP module.As you'll get one of the new OCXO's in itYou could buy a - Used ISOTEMP OCXO 143-141 on Ali, and swap the OX256B OCXO (that you'll 90% sure get now) .. If you want the "Older OCXO" in itBut it seems like the "fredo" fw has "tamed" the new OCXO too ... Then it just requires yo to put the 10MHz signal on the correct pin on the BP (move a wire)And i did put a pre-reg switcher in mine + a MIC2940-5 5v reg (quieter)https://www.eevblog.com/forum/testgear/10mhz-gpsdo-by-bh3sap/msg5897836/#msg5897836If you make PCB's for the DIY .... I also have a PM6680B (hint..hint)

author: tszaboo
author_rank: Super Contributor
timestamp: October 10, 2025, 02:53:01 pm
content: Quote from: bingo600 on October 08, 2025, 08:19:03 amYes I understand thatI have some of those cheap CTI OCXOs, so it's going to be that.At the moment, I'm designing a board for it, but decided to poke around in the unit a bit more to understand.Also, service manuals:https://bama.edebris.com/manuals/philips/pm6680The off state power consumption of the unit is 6W. Half of this comes from two resistors:R1256, 15 Ohm @ 5V, which is only switched on when the unit is in standby, andR1323, which is 33 Ohm @5V, which always loads the 5V rail.R1256 gets very warm, over 100C in my unit, see thermal images. The reason to have these two resistors is clearly to load the 5V rail, because that's where the AC/DC PSU is regulating on. So without sufficient load, the other voltages would be out of spec.I think, in case of running an OCXO from 5V, R1256 would be redundant.I think the rest of the power consumption of ~3.5W comes from the efficiency of the AC/DC, it stays warm in standby.5V is on BU113, which is the battery extension, unfortunately it's not fitted on my board, but maybe just wire it from there?There is a nice sine wave 2VPP 10MHz available on the back of the instrument on a BNC, which makes it a good choice to integrate a GPSDO I think. Even if I decide to not use it as a GPSDO, might as well use the same PCB as a OCXO.I can also see that the clock on the OCXO expansion connector goes through two transistors, TS123, TS124, so I don't think I need to square it up myself.Honestly, It doesn't look too difficult to do, seems like I have a schematic for it already.My open questions is:What connectors did they intend on the back side of the instrument for the "input A, input B" Looks like it's BNC, but the hole seems smaller than the regular BNC panel mount connectors.

author: tszaboo
author_rank: Super Contributor
timestamp: October 11, 2025, 11:22:27 am
content: Something important to consider, the power supply has a RIFA capacitor in it.So I'm replacing that first. FN9226-10-06 is a part number they suggested.

author: tszaboo
author_rank: Super Contributor
timestamp: October 11, 2025, 05:19:46 pm
content: So I quickly prototyped a the schematic of the GPSDO I wanted to use. See the attached mess of wires.It's this one, because I like STM32s:https://github.com/AndrewBCN/STM32-GPSDO/tree/mainI had another GPS test board, not a ublox one, so I started that up and forced the code to continue without checking the serial port, which is not connected. It seems to work at least it's software think it works.Code:19:17:13.385 -> Frequency measurements using 64-bit counter:19:17:13.385 -> 64-bit Counter: 553174911119:17:13.385 -> Frequency: 10000000 Hz19:17:13.385 -> 10s Frequency Avg: 10000000.0 Hz19:17:13.385 -> 100s Frequency Avg: 10000000.07 Hz

author: bingo600
author_rank: Super Contributor
timestamp: October 13, 2025, 03:21:43 pm
content: Quote from: tszaboo on October 11, 2025, 05:19:46 pmNEATDid you use a Blackpill , instead of the BP ?Ahh you did the Andrew GPSDO, not the BH3SAP/Fredo

author: tszaboo
author_rank: Super Contributor
timestamp: «Reply #19 on:Todayat 12:53:40 pm »
content: Quote from: bingo600 on October 13, 2025, 03:21:43 pmYes. The code is a bit bloated with other functions but it's possible to remove them.I've made a test fit of the board, seems to be good. Also ordered everything for it, including the PCB. The GPS is ublox MAX M8 or M10.It's a bit ironic that the clock circuit has a faster MCU than the main one.