16GHz Tek DSA71604C. What were they thinking? - Page 1
Forum section: electronics / testgear

author: Jwalling
author_rank: Unknown
timestamp: September 25, 2019, 02:45:14 pm
content: This is the acquisition board in a Tek 16GHz DSA71604C. There are two conductive foam assemblies and each surround 4 chips. They are crumbling into dust (Conductive dust, mind you!) and there's black dust all over the board. They have the consistency of burnt toast. I can literally crumble them with my fingers! This can't be good.Shown is the bottom sheet metal with arrows showing where they are located.WTF is this supposed to accomplish? Does it perhaps help to channel air into the heatsinks?Then there's this rubber bumper that presses down on the reference TCO. Well, at one time it was a rubber bumper. Now it has the consistency of a melted marshmallow - sticky and quite gooey. I can't imagine what this was supposed to accomplish...

author: capt bullshot
author_rank: Super Contributor
timestamp: September 25, 2019, 04:38:06 pm
content: Conductive foam might be used to dampen RF leaking from the traces.

author: DaJMasta
author_rank: Super Contributor
timestamp: September 25, 2019, 05:00:54 pm
content: Maybe they're trying to use the foam as some kind of gasket to force air through those central heatsinks?Whatever the reason.... it seems like the wrong choice, though

author: Mechatrommer
author_rank: Super Contributor
timestamp: September 25, 2019, 05:17:46 pm
content: Quote from: DaJMasta on September 25, 2019, 05:00:54 pmLecroy older WaveMaster / WavePro DSO also have this foamy thing and mine also start to disintegrate upon touching, so i dont touch them too much. i didnt check whether they are conductive or not. they are directly under the ventilation hole from PC area where user can open and got exposed. so i guess they are there to avoid dust, debris or worst metalic items such as screws from falling and shorting whats under there or changing rf characteristic thereof. under is just some relays and coax cable

author: edpalmer42
author_rank: Super Contributor
timestamp: September 25, 2019, 08:44:16 pm
content: The only time I've seen rubber bumpers used like that was to ensure that the circuit board couldn't flex enough to cause a short to whatever was beside it (e.g. the case or another board).Ed

author: technogeeky
author_rank: Frequent Contributor
timestamp: September 26, 2019, 01:55:26 am
content: Quote from: DaJMasta on September 25, 2019, 05:00:54 pmI think we can logically deduce that since this foam is conductive (intentionally), then it's not for airflow reasons. We might be able to further deduce that it's there for RF reasons, but I'm not sure.Could the goop on the TCXO be both calibration blocking and (thermal) mass loading? Maybe if you take a TCXO and make it much more dense and massive, its thermal fluctuations will be dampened and therefore its properties will be improved?I'd love to hear expert opinion on this.I feel like it's still in the realm of possibility that everything (except perhaps conducive dusting) is in the realm of normal behavior?

author: tautech
author_rank: Super Contributor
timestamp: September 26, 2019, 02:09:04 am
content: Any of the ESD safe foam sheets should work for this and what you have left over you can store your IC's in.https://nz.rs-online.com/web/c/safety-security-esd-control-clean-room/esd-control-clean-room/esd-safe-foam-film-wrap/?searchTerm=conductive%20foam

author: Someone
author_rank: Super Contributor
timestamp: September 26, 2019, 03:19:21 am
content: Looks and sounds a lot like dissipative foam RF absorber material. Its nasty stuff so they must have included it for a good reason.

author: KE5FX
author_rank: Super Contributor
timestamp: September 26, 2019, 06:14:30 am
content: Quote from: Someone on September 26, 2019, 03:19:21 amCrosstalk between those huge differential pair traces would be my guess. If those aren't routed with care, they won't be quite as "differential" as the designer hoped.It would have been a bad day in Beaverton when whatever prompted that foam to be added was discovered.Thatmuch, you can bet on.The goop on the TCXO might have been applied to keep air currents away from it, including convection currents from the surrounding PCB itself. Pretty standard stuff there. I guess it doesn't cover the whole thing, so maybe just covering the trimmer would have been enough.

author: JimS
author_rank: Contributor
timestamp: September 26, 2019, 09:12:49 am
content: Looks like the foam would direct air from the fans to flow under the foam then up through the seat sink fins.I saw something similar to the dissolving bumper on a Fluke 5725A.That bumper was to keep a metal part from contacting a PCB.

author: Jwalling
author_rank: Unknown
timestamp: October 01, 2019, 05:35:46 pm
content: The reason I'm messing around with this. One of my customers sent this to me for repair.Symptoms: Unit would power on for a few seconds, then shut down.There's a lot of reasons why this can happen. The power interconnect that contains a bunch of DC/DC converters has an Atmel 8 bit RISC processor that monitors all voltages including the ones on the ACQ board. It also monitors that all the fans are running.All the DC/DC converters in the scope are powered by a 48VDC 25A 1200W Emerson power supply.After mucking around with this scope for a few days, I finally found the culprit. There is a short on -5VDC rail somewhere on the acquisition board. A 100uF ceramic cap that is soldered on top of another cap. I removed the one next to it to get the value using my Fluke 289 DMM.For now I've removed the cap and the scope is running. It throws errors, but this is likely due to the RF coaxial relays/attenuators being removed.Some more pics of the solder side this time, along with the shorted cap and more gooey stuff. There's also a picture of what's left of the conductive foam. Not much... I'm not sure whether I should replace it...Each channel of the scope is powered by its own 1.8V 40A buck power supply. Pretty impressive. Not sure what -5V powers... A/D converters, possibly.I don't think there's anything too critical about a replacement cap, but thoughts are welcome!

author: nctnico
author_rank: Super Contributor
timestamp: October 01, 2019, 05:38:19 pm
content: I'd take the cap next to it from the board and measure that one.

author: DaJMasta
author_rank: Super Contributor
timestamp: October 01, 2019, 05:39:18 pm
content: That goop looks quite unappetizing, but a board that size full of ADCs is a sight to behold.

author: Jwalling
author_rank: Unknown
timestamp: October 01, 2019, 05:42:13 pm
content: Quote from: nctnico on October 01, 2019, 05:38:19 pmErr... That's what I said above.

author: Kilrah
author_rank: Unknown
timestamp: October 01, 2019, 05:42:46 pm
content: That board is insane... and a 1200W PSU for a scope, and 70W/channel?

author: Cubdriver
author_rank: Unknown
timestamp: October 01, 2019, 05:49:48 pm
content: As far as the foam goes, I'd be inclined to replace it. I don't think manufacturers like Tek are inclined to put things like that in unless there's a very good reason for doing so.-Pat

author: Jwalling
author_rank: Unknown
timestamp: October 01, 2019, 05:51:13 pm
content: I'm leaning toward this one.https://www.digikey.com/product-detail/en/kemet/C1210C107M8PACTU/399-11631-1-ND/491888710V X5R which is twice the rail voltage.Still guessing about the foam. To foam or not to foam...It sits at least 2cm off the board so could this still be a case for RF shielding? And if I don't use conductive foam, should I use something else?The 6 exhaust fans blow outward, pulling cool air through the bottom cut-outs shown in my first post. Kind of makes my think that the foam is used to force air through the fins on the chips...EDIT: Didn't know this stuff is so expensive.https://www.digikey.com/products/en/rf-if-and-rfid/rfi-and-emi-shielding-and-absorbing-materials/869?k=conductive+foam&k=&pkeyword=conductive+foam&sv=0&sf=0&FV=ffe00365&quantity=&ColumnSort=0&page=1&stock=1&pageSize=25Granted it's 2' x 2'.

author: Someone
author_rank: Super Contributor
timestamp: October 01, 2019, 10:45:19 pm
content: Quote from: Jwalling on October 01, 2019, 05:51:13 pmIts an absorber, usually for suppressing crosstalk in RF systems.

author: thm_w
author_rank: Super Contributor
timestamp: October 02, 2019, 01:06:49 am
content: Quote from: Jwalling on October 01, 2019, 05:51:13 pmI'm sure that would be fine.Although its 66% down at 5V, so for future reference, this one for example would be equivalent (~33uF):https://www.digikey.com/product-detail/en/taiyo-yuden/EMK325BJ476MM-P/587-5436-1-ND/7067112

author: EEVblog
author_rank: Unknown
timestamp: October 02, 2019, 01:28:38 am
content: What age are we talking about here?

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: October 02, 2019, 02:49:17 am
content: Quote from: technogeeky on September 26, 2019, 01:55:26 amYes, adding thermal mass on a TCXO is a good method to improve its stability. However, in this case, it looks like some kind of tape which will not add much thermal mass. However, it can prevent gusts of air from cooling the TCXO rapidly. This it can help reduce the temp drift.

author: EEEnthusiast
author_rank: Frequent Contributor
timestamp: October 02, 2019, 02:51:53 am
content: Quote from: Kilrah on October 01, 2019, 05:42:46 pmThe high speed ADCs are suckers for power. They draw in couple of Amps when running at 3Gsps. So this one would have at least 6 of them per channel. Now multiply that with the number of channels and you have some black hole for current.

author: TiN
author_rank: Super Contributor
timestamp: October 02, 2019, 05:14:36 am
content: Quote from: EEVblog on October 02, 2019, 01:28:38 amCirca 2010.

author: Jwalling
author_rank: Unknown
timestamp: October 02, 2019, 09:05:14 am
content: Quote from: TiN on October 02, 2019, 05:14:36 amI'd say you're dead-on if that's a date code. 31st week of 2010.

author: Jwalling
author_rank: Unknown
timestamp: October 02, 2019, 09:15:01 am
content: Some more pics, now with the attenuators in place. (even they have heat sinks!)The rectangular foam still in place is flexible, they must have used a different type of material for them.Scope is pretty well optioned.CH4 seems a bit noisy, but I'm running it without two of the 100uF caps which I will order today along with the arm and a leg foam.SPC was last run in May of 2019 where it was passing.

author: Jwalling
author_rank: Unknown
timestamp: October 02, 2019, 09:22:18 am
content: After running SPC, the excess noise seems to have disappeared! Interesting, given that it was last run in May and passing, so it should have been pretty close.I'm running it on an external monitor as the backlight is not working for some reason. It's getting +12VDC, so I'll have to find a datasheet for it to see if the enable signal is asserted.

author: Jwalling
author_rank: Unknown
timestamp: October 02, 2019, 09:25:20 am
content: Quote from: thm_w on October 02, 2019, 01:06:49 amThanks.A 16V part can be found in a 1210 package. Perhaps that would be a better part?

author: RoGeorge
author_rank: Super Contributor
timestamp: October 02, 2019, 09:35:08 am
content: I guess that's an RF absorbent foam (antireflective material) in the OP pics.Without the dampening RF materials, reflections from the enclosure will create unwanted/uncontrolled resonances, thus ruining the linearity of the circuit/instrument, which translates in a distorted waveform on screen or fake spectral indications.

author: Jwalling
author_rank: Unknown
timestamp: October 02, 2019, 10:19:30 am
content: So the fuse is blown on the inverter. Probably a pre-existing condition that the customer did not disclose.NEC 121PW181https://www.newark.com/nec/121pw181/inverter-5ma-lcd-module/dp/01P4251?CMP=AFC-SF-OEMNo real stock from reputable vendors.I'll order a fuse and see if it opens up again. It's 3.5mm in size. Datasheet says: rating 1A, fusing current 2A.Part # CCP2E20HTTE. Annoying datasheet does not allow copy-and-paste.There's some inverters available on Ebay. The customer is in a rush (I think they are military) so I've been buying parts with over-night shipping.Might just buy an inverter as well, JIC.

author: Jwalling
author_rank: Unknown
timestamp: October 04, 2019, 11:41:15 am
content: Using a plastic bag for a template, a sharpie, and an X-acto blade I made the new foam inserts.Not perfect, but it should do the trick.

author: Jwalling
author_rank: Unknown
timestamp: October 04, 2019, 01:02:50 pm
content: Lastly,Replaced the fuse on the inverter and it did not blow again.1. Shown is a 6GHz 300mV sine wave from my SMT06.2. The obligatory rise-time of 55.38pS on Leo Bodnars excellent BNC pulse generator. Scope is set to 5TS/s ET.3. Passing SPC and full self tests.4. Power consumption on my AC watt meter. This scope would probably keep my small lab fairly warm in the winter.5. My dog thinking that he should get a cut of the repair job and it should be paid with a Filet Mignon. Never-mind this lousy fake bone...

author: Someone
author_rank: Super Contributor
timestamp: October 04, 2019, 10:03:20 pm
content: Love the crack across the casing. Imagine returning a rental/company car with the bumper snapped in two, but hey its just some plastic box that looks like a computer no need to take care of it.

author: Jwalling
author_rank: Unknown
timestamp: October 05, 2019, 09:54:20 am
content: Quote from: Someone on October 04, 2019, 10:03:20 pmThose front bezels are really flimsy, and these scopes are HEAVY! Pretty sure this happened during shipping.

author: nctnico
author_rank: Super Contributor
timestamp: October 05, 2019, 11:24:36 am
content: Quote from: Jwalling on October 02, 2019, 10:19:30 amThe cause could be worn CCFL tubes in the display. So I'd replace them to make sure. Otherwise chances are high you'll see the oscilloscope back on your doorstep for a free repair.

author: Jwalling
author_rank: Unknown
timestamp: October 05, 2019, 02:12:05 pm
content: Quote from: nctnico on October 05, 2019, 11:24:36 amIME, they usually get dimmer. This display is pretty bright, but I did pull them out to have a look at them. They were nice and white with no signs of browning.The customer said that it was working fine before the main failure.

author: nctnico
author_rank: Super Contributor
timestamp: October 05, 2019, 03:09:27 pm
content: Then it is weird the fuse has blown on the inverter.

author: KE5FX
author_rank: Super Contributor
timestamp: October 06, 2019, 12:13:55 am
content: Agreed that this is an odd combination of failures, but why would worn CCFL tubes cause inverter fuses to blow?This seems like a really nice scope, probably deserves an LED backlight upgrade. (And an HD image backup, while you have it on your bench!)