Crack open wall wart (power brick): hot air method (READ or risk more damages!)

The power brick that came with my U1620A order has a rattling noise when I shook it. I suspected a piece of soldered cracked and was wandering inside so I chose not to power it on as it might short something. So I had to crack open the wall wart, which turns out many people have little luck open it up non-destructively.

Just prying it open by force with a screwdriver will ruin the plastics. I needed to somehow soften the glue first and work into it (like using a cutter) to dislodge/cut the glue. Rubbing alcohol doesn’t work that well. The glue is quite stuff so it’s not the easily dissolvable kind.

After some experimentation, I used my hot air station and set it to a temperature right below it melts the hard ABS plastic outside (143 deg C, max air flow), and carefully pried it after taking the heat away.

WARNING: Do not pry while the hot air is blowing as the ABS plastic might be slightly softened! The hot air might overshoot in its temperature feedback) so you might accidentally remold it. When working at the borderline temperature that barely soften the ABS plastic, the second you take the hot air away the plastic cools back (hardens) enough for you to pry.

Out of practicality, I heat up one edge at a time, then focus on 1/3 of the area at a time get an entry point prying one hook open at a time. It takes a lot of patience.

After I got all the 4 sides pried open for some reason I couldn’t release the case and it felt something was stuck in the middle. Turns out the wall wart I have happened to have a center screw hidden from plain sight under the label. The label quality was so good that just pressing it around with a pair of tweezers won’t find the screw hole (because of the tension). I used hot air (at the same 143 deg C) to probe with tweezers to locate the screw hole. I cut a hole in the label for the screw hole because it’d be a pain to use hot air to get the label off in one without ruining it and be able to put it back like new.


Bonus discovery: molded plastics have some memory!

I dented the ABS plastic case during prying (before I thought of using hot air), but I discovered this magical temperature (143 deg C) actually heals the plastic when I blow it in a direction that undoes the dent! The small (like 2mm) dents magically smooth itself out and went back to rough where it was before I screwed it up (pun intended)!

Yes, for molded plastics, small dents caused by prying and bumping, heating it up to the temperature right before it softens will return it to its near original shape without external forces!

Of course, do not go past the softening temperature (143 deg C). If you heat it to a melting temperature, the plastic will lose its memory so you need to apply external force to beat it to the shape you wanted (which is a pain in the butt and it requires a lot of post-processing to get it to look like original).

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Agilent DSO6000 series was internally called 54670/54680?

Architecturally, the DSO6000A series shares common designs with 54830 series oscilloscope and 54640 series oscilloscopes. I noticed the Acq board numbers for my DSO6052A start with 54672 and the my M/DSO6104A starts with 54684. Looks like the 5467X means 500Mhz and 5468X means 1Ghz while the X is the number of analog channels.

This is ‘confirmed’ by a slip up in the documentation (user guide) which they forgot to update the model number in their vector graphics:

I think I’m getting a hang of Agilent’s oscilloscope’s hardware to do deep level board repairs as I have various model to compare. I used If you need yours repaired, please reach me at 949-682-8145.

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Stanford SRS PS350 High Voltage Power Supply Repair

I received two PS350 power supply units that one of it has sparks when output is enabled and the other just won’t output anything at all.

The only repair info I found is from one of my favorite youtube channel the Signal Path. However, his unit has a much easier problem: the solder joints cracked because PS350 uses the metal case as a shield that are subjected to mechanical stress.

However, after difficult troubleshooting, I realized one unit has a fried resistor in the HV section, and a few core MOSFETs shorted.

The other unit is much more difficult: not only the HV capacitor is blown, resistor is blown, diodes shorted (won’t be able to detect it by probing in-circuit because of the capacitor ladder), PCB trace to the feedback path vaporized (without that the voltage will rise uncontrollably until something’s fried), and a bunch of MOSFETs, transistors and regulators ICs needs to be replaced.

Likely both units are broken because the users switched polarity without turning the HV section off (and let the voltage bleed out). This is very important and the markings on the case already warned the user NOT to do so.

You absolutely must NOT change the polarity while the output is live because the components in the HV section are marked for 4~6kV, so there is little room for a voltage spike past the operating voltages. The act of switching out the polarity (by mechanically swapping the pins through the dial switch at the back) doubles the voltage stored in the capacitors in a voltage multiplier ladder, so you are almost sure to crack the HV capacitors and likely the HV diodes.

Since I’ve developed experience for repairing SRS PS350 by reverse engineering some of the circuit sections, I welcome request for repair evaluation (no fix, no fee). Please call me at 949-682-8145, or meet me at www.humgar.com.

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Data Precision 8200 Repair Service

I just bought a big lot of Data Precision 8200 and some Analogic AN3200 DC Voltage/Current Calibrators with a bunch of hard to find (unobtainium) genuine parts (relays, switches, hardware, regulator and amp ICs, whole modules, transformers) that that I believe it’s the leftovers of a closed down repair shop.

That means I’ll have all the materials needed to service and upgrade Analogic / Data Precision 8200 that you are unlikely to be able to find elsewhere.

Data Precision 8200 is the official unit to field adjust TDS 500~800 series oscilloscopes as the automation software (GPIB) was hard-coded to this model. Nonetheless, I find it a reliable reference for verifying oscilloscope performance and adjusting my multimeters as well.

Call me at 949-682-8145 for a repair quote or if you are interested in buying a unit. GPIB and 1kV option can be ordered for extra, either with the unit or service upgrade.

 

 

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Tek Evil: Repairing MDO4000C Series MDO4014C stuck at splash screen on start

I recently got a repair evaluation order that took me two intense weeks to nail the problem. It’s a MDO4104C that does not get past splash screen.

Worst of all, the unit failed right within 3 months right after the warranty expired. The unit didn’t show signs of heavy use. In fact, I nailed the problem so I can tell for sure the unit DIED OF NATURAL CAUSES. It’s not the first time I (and other people on forums like EEVblog) bitch about Tek designed their unit to last beyond the support/warranty period. This one takes the cake.


Basically everything in MDO4000C series happens in the main acq board. Anything that goes wrong there you might as well just buy another unit.

It runs embedded linux (uboot) and it’s slow to boot as I always expected from Tek (Keysight/Agilent use VxWorks for their modern embedded scopes and they boot fast).

There are no UART debug consoles anywhere and the only two test points with digital signals that idles high at 3.3V. The data pulses are quite long for the frame, and one of them looked more like clock bus in either SPI or I2C. So no UART. The firmware file mentioned BDI3000, which is a JTAG, which I suspect there’s a 10 pin IDC (ribbon) port on the right hand side of the main acq board, but this is as much as what I can get getting debug info.

There’s a RS-232 driver chip on the peripheral board, but it goes to the ID pin of the VGA port. I didn’t see any data traffic on it on boot.


Getting stuck at the splash screen practically tells you nothing about what’s wrong. The only thing you can infer from it is that the unit powers on and the display/keyboard is good. This is pure evil. HP/Agilent/Keysight designed their products to make it easy to service, and Tek has always been a pain to service, and the new ones are no exception.

They don’t even mark the grounds in test hook points. It’s just a bunch of TPXXXX numbers. Apparently they are there for testing the design, not repairing it.


It’s obvious from the way they designed their product to the service manual, Tektronix definitely don’t want any people to do board level repairs: ultimately they want you to blindly send it in for factory repair within the first 5 yr of production, and make you buy a new one if the model is discontinued.

Despite I hated it, Tek has a valid strategy. If a product is cheaply made, even at the cost that it only last long enough through their warranty period, the manufacturer doesn’t even need to bother with board level repairs because it’s cheap for them to just give you a new board/unit instead of figuring out what’s wrong with it.

Putting the frustrating to use UI designs aside, Tek works if you are a company buying the scope for a short 3 yr project. I wouldn’t recommend Tek at all if you plan to buy it for a long haul (because they are not designed to last given ones I’ve serviced), or plan to use it as a daily troubleshooting scope (because their user interface is slow and clumsy).

Tek has a huge following from the analog days when they did things right. Ever since the digital age, the learning curve for Tek’s UI was so steep that inevitably it turned into a customer lock-in where Stockholm syndrome kicks in.

Nonetheless, Tek was a little ahead of Agilent in terms of the MDO concept that correlates time domain measurement with the built-in spectrum analyzer. If that’s a feature that is important to you to the extent that you are willing to live with Tek’s clumsy UI and it might break down right after the warranty expires and it’s nearly impossible repair it yourself, it’s reasonable to go with Tektronix for this one given the lack of functional alternatives. Do NOT get MDO3000 series for this reason though, since the spectrum analyzer and oscilloscope are not time-correlated there’s no material innovation out there. MDO4000 series and up are time-correlated with the spectrum analyzer.

Again, I’m not saying Tek is bad. Tek is just mean towards their customers from a product design’s point of view, when you contrast them with how considerate (nicely thought-out) HP/Agilent/Keysight products are designed. Tek is still much better than the RCCC (Random Cheap Chinese Crap) in every way.


EDIT (2017-07-02): I just got a call this morning from somebody who bought a TDS 220 from me years ago. The BNC connectors broke off and he has found somebody selling a kit specifically to fix the problems in TDS 200 series. That’s pathetic. I knew the design was obviously flimsy back in the days, but I trusted the Tek engineers knew what they are doing given their brand reputation. Turns out common sense is right. A bad design cannot last even if they had solid parts/manufacturing.

From my experience opening up many Tek and  HP/Agilent/Keysight units so I can compare their mindfulness, I can see that Tek is one of those high pressure companies that cut corners to get stuff out fast with low manufacturing costs. It’s not necessarily a bad thing from a business standpoint, it’s just Tek and HP/Agilent/Keysight operate with very different set trade-offs (or say, philosophy). Apparently Tek’s kind of trade-offs makes their used equipment a terrible choice unless it’s covered by their extra warranty.

Since I deal with old gears, I’m not impressed with the outcomes of used Tek products that I have/had. If I were to buy from Tek, I’d rather lease it since I don’t expect their products to last beyond their warranty period without multiple breakdowns that are costly to fix. My experience with used HP/Agilent/Keysight gears is that the problems are more predictable, limited to a small area and easy to reach and fix. For example, the caps in power section and CRT driver of TDS 500~700 series starts to fail, and the flyback transformer dies year after recapping the SMDs, and the units were just sitting there, not actively used. On the other hand, early 54600 series are almost always problem free other than an occasional cheap capacitor in the CRT driver drying up. This gave me a very bad impression about how Tek’s made their stuff.

Even if I could lease Tek products on other people’s budget, I still won’t consider until they radically changed their clumsy user interface and dumb autoscale mechanism that doesn’t even detect all channels and show you all relevant signals.

So far I haven’t see the kind of extra attention to detail in Tek products when I put it side by side with HP/Agilent/Keysight. This is how corporate culture reflects in their products: if you treat people well and trust them, they’ll get to all the nooks that managers and processes can’t reach and do it right. Tek just did enough that it will work as marketed, but whatever that’s cannot be objectively claimed in the advertisements, you are on your own.


I have spare parts to repair MDO4104C, MDO4054C, MDO4034C, MDO4024C

If you have an oscilloscope that you’d like to send it to me for repair evaluation (no fix, no fee), please call me at 949-682-8415 or email owner@humgar.com.

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Agilent N9340B Handheld Spectrum Analyzer Repair and Teardown

I received an Agilent N9340B 3Ghz Handheld Spectrum Analyzer with a note that it passes all self-tests but does not respond to input signals. I took the gamble that it’s the RF input connector got disconnected somehow.

I opened up the case and noticed that the 40Mhz cable was unplugged, so I was half-correct. I connected it and got a signal at the precise frequency, but the amplitude doesn’t look quite right. It’s around -20dB off. When I scanned it across the full 3GHz band, I noticed the amplitude roll-off when I scan below 800Mhz, and I got very little signal left when I get to somewhere near 10Mhz.

I tried running a user calibration with a 50Mhz CW source but it failed amplitude calibration. Apparently the unit is not fully working. No self-test errors though.

So I opened up the unit and the RF section. The front side of the board doesn’t have any visible signs or unusual smells, so I suspected the improper gains is caused by the input attenuator HMC307:

I was about to order the chip, but because of the lead time, I decided to just take a picture of everything and analyze it off-line:

After removing the screws holding the N-type terminal so I can get to the back side of the board for taking pictures, I noticed the RF out connector just fell off the board with the pad:

That means the RF out is not touching the board! I resoldered the connection after I put the board and the connectors back to the RF module slab. Once I put unit back together, I turned it on again and everything works perfectly! The power level is flat and within 1dB of what my 8648C pumps out. I did the user amplitude calibration again, it passed, and everything was spot on!

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