54855A/54854A Oscilloscope won’t power on

Recently I got a 54855A oscilloscope sent back to me for service under the 1 year warranty I underwrote for most of the unit I’ve sold direct. The unit would not turn in at all after sitting for a long time.

I looked up the forum and it turns out other people had this problem with a certain generation of Infiniiums and sometimes changing power supply or motherboard would disturb the setup a little bit and the unit might power on again. When I tried to do that, the unit does boot a quite few times but the problem randomly came back again. This is frustrating as it’s a heisenbug. I almost thought I was done when the unit worked consistently for a week then it comes back. It just looked like something component was acting borderline and disturbing the setup and got it to click.

In the past I had 54830s that’s ‘fixed’ by changing either one of the motherboard or power supply, but turns out that those were flukes, but I didn’t get lucky with 54850s this time.

The customer gave me time to troubleshoot deeper instead of just getting something to work as a fluke by just blindly changing modules which might break down again at a random time if it’s a gravitating aging problem (i.e. if you figured out a problem, it tends to be a wave of manufactured gears that’ll trip on the the same issue as they age). So I spent a whole month troubleshooting through reverse engineering the circuit and nailed down the cause and the fix.

There’s a lot of mixed info going on in the forum with different modes of failure, but those might not be the real cause, as replacing components disturbs a set up that wasn’t supposed to be in that state in the first place and the unit is prone to get trapped into a bad state when the unit ages. The real fix is to plug the path to the bad state in the first place instead of rocking the boat hoping the new combination doesn’t trigger the bad state.

The customer made the right investment not trying to save a few grands buying from whatever the cheapest source with a 30 days warranty. This is the kind of agony I’d charge $10k+ for the first time research to get this deep. Had he bought this from others without the warranty, he might have to write off the unit and raise a new purchase request/funding, and might run into the same issue later as the 2nd unit also aged for the same reason. It’d just be a endless cycle of frustration that typically makes people give up and buy new instead.

Agilent/Keysight Infiniium oscilloscopes are relatively reliable. The computer section, which was not made by Agilent in house, is their weak spot as PCs made at the time has a lot of common failures at this stage so the problems are not exclusive to Agilent, but all computer-based scopes and most people are scared by the frustrations as it’s nothing like the old-timers’ folklore in analog section repairs. I got past the steep learning curve, so if you buy from me or have me repair the unit, you’ll often have it fixed for good and I often underwrite the units/repairs with a warranty.

Tektronix on the other hand, is an impending multiple organ failure when the first problem show up. It’s like my Maxima 2000 I bought for $3k back in the days: fixing the problem last you a few months before another shows up, which is a money pit. Old Tek scopes are like that. If you want to buy use scopes to save money, buy HP/Agilent/Keysight, not Tektronix.

Actually if you buy new, I’d still say Keysight is the safe bet as they DESIGNED their products and their support practices to keep a good long term relation with the customers. HP/Agilent had the same realization as I do: engineers would not try to pick the cheapest instrument just to save his boss a few bucks now so they’d risk doing uncompensated work later (like raising a new purchase request, rewriting code/procedures because the out of support model that nobody else could repair). There are so much goodwill and intangibles built in the brand that disposable marketers and sales force cannot replace as the customers of the industry is highly technical and you can’t fool them consistently. The peace of mind and knowing somebody will back you up or dig you out of troubles is worth a lot more than the price difference while shopping.

Not to mention the user interface in HP/Agilent/Keysight instruments are way more intuitive to use and have little learning curve (RTFM is rarely necessary). Agilent’s instruments are designed to be easy to service and they don’t do obviously stupid things like Tektronix having wrapping chassis like a lettuce so you need to disassemble a dozen of irrelevant parts to get to something simple. Tektronix does a lot of inexcusably stupid things in their design, like putting electrolytic capacitors at touching distances from large transistor/IC heatsinks and wonder why even genuine Nichicon capacitors leaked like hell. You can tell some somebody in Tektronix’s engineering genuinely doesn’t care when you see a SATA->PATA->SATA converter chain for removable hard drive.


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