## 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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## A Positive Tektronix Customer Support Experience

I often have bad experience with Tektronix product’s design (user convenience, reliability and repairability issues), and the by policy poor support for discontinued products. So far I have yet to get a chance to say good things about Tektronix while Agilent blew them out of water in these 4 areas.

Nonetheless, I have something good to say about Tektronix today. I have a DPO4000 series oscilloscope that the knob and busing popped out during shipping and disappeared, so I had to order them from Tektronix.

The operator on the phone noticed the part numbers and was aware that it’s a common problem that the jog shuttle’s knob and busing (for the Wave Inspector feature) often come loose, and offered to send it to me for free. That’s excellent customer service. The part that I admired the most is that they proactively acknowledged their design weakness and make amends.

Seems like Tektronix takes good care of their customers as long as the models are still supported. Definitely a redeeming quality!

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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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## Keysight Calibration (Performance Validation) for Probes Specifically 1152A

I recently sent a 1152A probe for calibration and was surprised to find out the data on the calibration report tells little about how the tests are done and under what settings. I searched the throughly and called tech support and they confirmed my observation the performance validation procedures are not mentioned anywhere in the published documents.

I called Keysight cal department and was able to reach a super-helpful tech, Markis, who did the calibration for my 1152A probe and he explained to me how the calibration process is done when I called.

HP/Agilent/Keysight probes using AutoProbe interfaces are powered by 1143A (that was intended for 54701A probes) through a N1022A adapter (the one used in 81600 Infiniium DCA) for Keysight’s calibration process, which measures uncompensated probe-only performance. I saw the calibration reports from 3rd party-labs, and probes are are calibrated inside the oscilloscope they are used in, and therefore it’s measuring a compensated system (scope+probe) performance.

There is a 30 minute warm up period.

The procedures resembles to what’s detailed in the old 1144A probe user/service manual, (page 10-14) with the exception that the ‘Gain Accuracy’ done there is ‘AC gain accuracy’ (at 1kHz, 1Vrms) instead of ‘DC Gain Accuracy’ claimed on the report. In fact, given that it’s simply measuring relative error (multimeter reading of the probe BNC output divided by the 5V Fluke Calibrator reference) at one voltage setting, I believe it should be called ‘DC measurement accuracy’. The number on the calibration report was divided by 10 times since 1152A is a 10:1 probe.

The bandwidth test for 1152A is simply looking at attenuation at the advertised bandwidth (2.5Ghz for 1152A) relative to 50Mhz (low frequency reference set at 0dBm).

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## I see dead processor, the first time in my life I've dealt with hundreds if not thousands of PCs since I was a kid

Ever since I got my hands into assembling and troubleshooting PCs when I was a kid, both through my own experience and general consensus in the computer hobbyist community, CPU is almost the last thing to suspect at fault for a non-functioning computer, much less likely if:

• There are no signs physical damages (mechanical or heat stress)
• There were no shorts (burning electronic smells)
• There weren’t any extreme overclocking (at least Vcore was pumped)
• The computer used to boot, but has some random hangs

After 20+ years (and troubleshooted a few hundreds if not a couple of thousand PCs), today I encountered (actually zeroed-in that it’s the culprit) the first bad processor in my life. It was inside a M815G motherboard from a 54854A oscilloscope that I bought that wouldn’t boot into windows without random ‘file corrupted’ errors. Then after a few tries, the board wouldn’t even boot, not even any code on the POST card.

At first I suspected it’s an aging motherboard, since I checked the RAM and passes Memtest86+ on another board. It’d be either the motherboard or CPU, which I never considered it might be the CPU given how unlikely it is both by other people and myself.

I couldn’t be bothered to dig at the moment so I simply replaced the entire motherboard (with CPU and RAM installed) with another unit and confirmed that the 54854A I bought didn’t have any deeper problems. Then I put this ‘bad M815G motherboard’ on my back burner.

Today I was trying to revive a VP22 motherboard (which boots only if I apply pressure on certain areas of the PCB) that didn’t have the Fan+Heatsink+CPU+RAM. I happen to have a spare Pentium 3 and some PC-133 (SDRAM) lying around, but not the heatsink+fan, so I borrowed it from the M815G in the repair-if-I-feel-like-it pile.

The VP22 booted with pressure on the PCB (beeped, checked POST card) but I couldn’t see any display, so I thought of swapping-in the known-‘good’ CPU from the ‘faulty’ M815G to see if I had the wrong revision that the VP22 didn’t support. The VP22 used to get stuck in the boot process, but at least the POST card has a reading, this time after swapping in the CPU from the M815G, it has no POST code at all. No pulse.

I got suspicious and took the the CPU from the VP22 and put it in the ‘faulty’ M815G. Guess what? The M815G in question boots and runs fine!!! WTF! For all that time I thought my M815G has a difficult fault just because I had a marginally failing an then dead CPU, which I didn’t even consider the possibility given how unlikely the CPU is at fault.

And no, it’s not the thermal compound drying up, it’s freshly applied every time I move it to a different motherboard. The CPU was only used in M815G/VP22 which does not even have any means of overclocking. No burns or smells or physical damage, and the computer used to boot. The CPU just died of natural causes.

A black swan day!

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## Aging problem just from storage Working 6632B stored for 10 years has a failed tantalum cap

I fired up one of my 6632B stored for almost 10 years and smelled burned electronics, despite everything is functioning. I tested the unit immediately when I bought them a decade ago and it was working fine, so it’s an example where electronics can deteriorate by storing (even in temperature controlled, dry environment).

Since I see smoke, I turned everything off immediately and investigated. Turns out one of the tantalum capacitors in the processor/controller board gave in:

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## Wobbling rotary encoder in Agilent/HP/Keysight 6630B series 6631B, 6632B, 6633B, 6634B, 6634A, 6635A, 66332A*

6630 series system power supply is sturdy as a rock, but has a rotary encoder sticking out that it’s almost guaranteed to wobble if you buy it used.

I thought they would have known better to secure the rotary encoder with a nut so it won’t wobble (HP usually does a perfect job making their designs reliable. This one is a rare miss), so I opened it up to see what I can do about it.

My initial guess was that the solder joints were weakened as it was used to mechanically support external forces for users of the dial. But I was wrong. Here’s what I’ve found:

The weak metal strip retainers gave in and the whole rotary encoder is about to break loose! The encoder was actually still functioning before I opened the case up. So HP assumed their vendor for the mechanical rotary encoder did a good job withstanding frequent wiggling. Apparently their vendor completely failed them: the metal retainer design was hopelessly flimsy that I wouldn’t even consider using it even in light-usage applications! FAIL!

There’s a huge number of these high quality power supplies on the market because Motorola/Nokia closed down their massive operations, flooding the market with 6632Bs for years to come.

I’ll now strengthen (I came up with a solid technique to make sure the dial will never fall apart again) the 6632Bs I have for sale to businesses that needs a perfect unit (which I sell for $699/ea). If you are a hobbyist, feel free to send me a message and I’ll tell you how to do it, provided that you do not share it with anybody else (I’ll trust you). If you are a business, I can restore 6630B series to a professionally salable state starting at$499.

* Note that I included 66332A despite it’s a mobile communication DC source (66300 series) here because the guts of it is actually 6630 series. Every other 66300 series (3 Amps max) or less has a different form factor (that’s more like a 33120A) and the only odd one out of the series is 6632A (5 Amps max).

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## Option 005 “Vertical Output” port of 54600 series oscilloscopes (54616B, 54616C, etc) A secret backdoor feature that new oscilloscopes lack

Over the last year, I got a couple of requests for 54616B that specifically ask for a “vertical output” port at the back. I have never seen an oscilloscope that came with such a port, including a few hundred of first generation first generation 54600s I acquired from many different sources.

I got curious and looked it up. Turns out it’s a secondary feature of a relatively obscure option (only mentioned in the manuals, but I have never seen one) called Option 005, which lets you analyze (like count lines) and trigger over common TV signals, like PAL/NTSC/SECAM, which is way obsolete today. It also seems that none of the customers asking specifically for the “vertical output” port at the back know that it is a super rare option that is normally not included, so they must be using it for something else other than analog TV signal analysis.

A closer look at the user guide shows that “vertical output” port duplicates the signal source (e.g. channel 1) that the scope is triggering on, limited to what is seen by the oscilloscope, to the said “vertical output” port, a secondary feature to let you chain your signal to instruments like spectrum analyzers for further analysis.

I tried the feature myself by chaining the output to another oscilloscope. Even if the waveform is off-screen for the current vertical volts/div, the vertical output port waveform did not clip. I also played around with input impedance settings 1MΩ and 50Ω for a 50Mhz square wave. Based on what gets the square wave badly distorted, I can confirm that the vertical output signal is the analog signal after attenuator (the amplitude changes only with Volts/div that causes relay clicks) but before ADC, assuming a 50Ω load.

Wait! An oscilloscope that duplicates the input analog signals after being processed by the front end (post-attenuator, pre-ADC) to an external output port?! I don’t have to mess with the original signal path by splitting the signal (passively) or make an amplifier to duplicate the signal? Wow! How come it’s not standard (or at least a purchasable option) in modern oscilloscopes? I’d like to see what’s going on with the analog waveform before the scope processes it! Not only it’s very educational, it allows other instruments to get an accurate insight of what the oscilloscope is seeing. Neat!

Installing the Option 005 is not difficult if you happen to have an unobtainium Option 005 case with labels, and the entire kit with all the necessary interconnect. However, it’s like an unicorn and I’ve never seen one. Drilling professional looking holes for it is a nightmare as we don’t have the dimensions. The hardware is also insanely hard to get as it was made for a specialized crowd for the time and practically nobody cared about analog TV signals nowadays. Even if I can get that, they are most often missing the interconnects. The ribbon cable is missing for nearly all of them, and if you get a standard ribbon cable, you’ll realize the plastic retainer gets into the way of a screw on the main acquisition board so the Option 005 card won’t slide in unless you trim some of the plastic off. PITA!

Nowadays I am already spoiled by high end gears like MSO6054A and 13Ghz Infiniiums (like DSO81304A), but none of them has a convenient analog, post-attenuator output like a first generation 54600 with an Option 005. Given the hardware is scarce, I’ll save it for the top of the line first generation 54600 series, namely 54616B and 54616C.

For those who have this special need (need to tap into the pre-ADC signals up to 500Mhz), I can custom build these Option 005 units for you, depending on parts availability. Call me at 949-682-8145 or reach me at my business website www.humgar.com.

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## Agilent (formerly HP, now Keysight) vs Tektronix

I am much more inclined towards Agilent than Tektronix because

• There’s nothing a Tek scope can do that an Agilent can’t
• Agilent’s user interface is very intuitive that it requires little to no trial-and-error or RTFM.
• Agilent’s people is very generous about helping customers out even if support is discontinued. Tek gets rid of all service information and software after discontinuation by policy.
• Agilent’s gears are very thoughtfully designed and is a pleasure to service, for the ones that I have opened up so far. Tek designed their unit to live barely enough through their support lifecycle, hoping they won’t have to service it.
• Agilent’s old gears lives much longer. Just look at (even better, open up) Agilent 54600 series and the damn TDS 300~800 series and you’ll see what a nightmare Tek is.
• Tek’s autoscale algorithm is a piece of garbage!
Even with TDS6000B/C series that cost tens of thousand of dollars at the time of writing still couldn’t figure out the top Ghz signals and give you a long Time/Div that completely aliases the signal and therefore confuse the heck out of their users. Not to mention Tek’s autoscale is sometimes too dumb to figure out which one channel you are on so that you have to move to (highlight/focus on) the right channel. Never had to deal with this kind of nonsense while using an Agilent scope.
• Agilent’s gears also have much fewer hard/painful to fix aging problem than Tek.
• When Tek scope fails, it’s often followed by a bunch of other unrelated aging problems. The capacitors are not designed to stand the heat for 10 years of usage.

EDIT: It’s not just me bitching about how unresponsive the controls (especially the dials) are in their user interface. Dave Jones did a video review of MDO4000 and a bunch of people share the same frustration in the comments section. I thought they improved after TDS1002B (I stopped following their newer scopes), but I was wrong. Still the same poorly thought-out and laggy UI.

There is Lecroy, but there are much fewer old gears in circulation and I don’t like their user interface much either, but at least the dials won’t take more than half a second to respond like Tek. I once asked Lecroy if they can generously share the schematic for an old unit like Agilent and they sent me one. At least they are not being a d**k about it like Tek.

I have both used Agilent and Tek scopes for sale, but my own bench is all Agilent whenever there’s a choice. Tek is OK if you plan out a difficult measurement setup (for documentation or manufacturing), but miserable if you are poking around to troubleshoot (that’s what I use the gears for). I sell Tek just to cater those who have been brainwashed because Tek got the first-mover advantage back in the days.

Of course my bias is based on their Tek’s gears in the digital age. I heard that they were very good at the analog scope times, so that might be the reason why Tek still have a strong following. HP/Agilent/Keysight pretty much nailed the digital techniques. The part I liked about Agilent is that they are generous about making users of their products happy in general, regardless of whether you recently paid them or not. For deeply discontinued products (like 3+ generations ago), they are happy to pass whatever information they have left to help DIY-ers or non-chartered 3rd parties that are willing to service them (like this one, which people are asking for recovery discs for their 1680 series analyzer and the staff went all the way to dig up their private stash!) so the company can focus on the newer products.

Support culture aside, Tek’s used gears are so problematic (I learned it first-hand, the hard way) that I’m now hesitant about buying them as investments. It looked like an opportunity because Tek stuff often breaks the same way, so I can buy them cheap, fix them, and resell. But the reality is that the labor is simply not worth it. Now I’m just selling whatever Tek leftovers I have strengthened in the past.

You might think Agilent is sabotaging their own market by taking care of users of their old gears. It isn’t. Whoever that has the budget to buy new will do so. Wobblers between buying new/old gears are not worth agonizing over. The ones who are familiar with the older gears will grow fond of the brand and the user interface/environment they are familiar with and will push their employers to buy Agilent when they get a chance to buy it new. I used to have a customer that I convinced them to get a used Agilent one instead of used Tek, and they ended up loving it so much that they bought a new one from Agilent for their second scope. What goes around, comes around.

I realized throughout the years is that whatever hobbyists do with the old gears and can only help the brand image and build a stronger user base. It’s the user base (engineer’s familiarly) that makes or breaks the deal on new gear purchase. I don’t think big companies that pays good money to buy new will switch to all Tek from Agilent all of a sudden when all engineers are comfortable with Agilent’s stuff, and vice versa.

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