## What is calibration? Hint: It is not adjustment!

Very often people doing R&D ask me if they need to have their oscilloscope calibrated. And for most of the time, my answer is no unless they need to have the NIST traceability or the calibration sticker to keep the regulatory bodies happy. They often thought it’s adjusting the calibration coefficients (or knobs) to make the unit more accurate. This is COMPLETELY WRONG.

In EEVBlog, they showed a video interview with Agilent Metrologist explaining what calibration actually does: it gives you the sample data points against trusted references about how your test instruments’ references has drifted between calibrations. Actually it’s preferable to not adjust the instruments if it’s already within specs.

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## TDS 500~800 series Monochrome CRT Driver Repair

The most common mode of CRT display failure in the TDS 500~800 series (Monochrome models) is the flyback transformer. The symptom is that after leaving the screen on for a couple of hours, the screen started stretching vertically until it disappears.

It also happens the failure only happens to a batch of CRT boards. The batch I’ve seen looks new (with modern markings that tells you the purpose of the trimpots) and lightly used, so I’m sure it’s infant mortality. Here’s the broken CRT driver board to be repaired:

There’s practically nowhere you can find this obsolete replacement because it’s not a common configuration. I sourced a batch from China that claimed the part number, and I spent whole day cursing the vendor when the flyback transformer arrived. Here’s what I saw:

The one on the right was the broken original flyback transformer, and two on the left were my new orders. Not only that the shapes are completely different, the number of pins doesn’t even match. WTF?!!!

The seller told me that it works. Forget about how to fit that in the board for a moment. How the f*** am I supposed to know which pins goes to which spot? Not to mention there are 11 dots when the original only has 8+1 (actually 7+1, pin#8 is not used). I said dots instead of pins because not all of them are populated with a pin, and the pins that are missing were not even consistent across the transformers in the batch.

I cannot even guess with a multimeter because it’s not a simple, uniform transformer. Even if I know which ones are connected, I could have ruined the whole thing by having the wrong number of coil turns/inductances because I switched a pair or two!

I had to push the seller really hard for him to dig up the actual mapping and draw me the pinouts on the pictures I’ve sent him (I’m sure the whole batch will be trash if I could not communicate with them in Chinese). The ‘product’ must have been designed and the manufacturing line ran by a bunch of village idiots. Nothing is right about it other than the windings inside are electrically usable (can’t even say compatible because I need to hack it really hard to get the correct display). Here’s the pinout:

Here’s another transformer that doesn’t have the ground pin (unnumbered), turns out the transformer works without it:

The space inside the oscilloscope case is pretty tight, and I managed to find one orientation that lines up with the case nicely, but it’s ugly as hell:

I held it down with hot-glue, caulk to stabilize it. A rubber band was put over it so that if the glue fails, the transformer won’t roll inside the compartment causing mayhem (later units I used cable ties since rubber band might deteriorate with heat. You get the idea.):

I have a few of these transformers available for $80/pc. Unless you have the time on your hands to figure out the details, the learning curve is so steep that I wouldn’t recommend DIY. If time is pressing and/or you don’t want to ship the unit back and forth, I have ready-made CRT driver boards for$300 (provided that you send me a bad CRT driver board). Of course, you have to do the CRT screen adjustments yourself since each tube is different.

• HP/Agilent (Older, non-Megazoom) Infiniium series: replacing OS hard drive ($250), cannot calibrate certain channels ($400 for 1 channel + $100 per extra channel). • HP/Agilent Infinium 5483X and above: refer to 54830 series post. • TDS 500, 600, 700 series oscilloscope: SPC fail without other symptoms ($500), Acq/Attenuator fail ($600), Proc board fail ($400), Color screen bubbled ($500), Cal initialization failed ($300), upgrading all possible software options ($200, free with any other service). Contact me at 650-804-5024. All units must be delivered to Irvine, CA for service (and shipped back/picked up) at requester’s cost. 1,156 total views, 1 views today ## Tektronix S1001 DIP switch: Open pin 3 to boot for fast self-test (TDS 744, 744A, 684A, 784A) Normally for most TDS 500 series oscilloscopes, the DIP switch (S1001) typically all closed for normal operation. The only described use in the service manual is opening pin 6 and 7 to reveal the hidden composite test pattern screen for display adjustment. However, I got a TDS 744 scope that boots very slow. Almost 5 mintues! To the extent that it’s unbearable. Initially I thought it’s just the non-A type booting slow. Nonetheless, I got a chance to open up another TDS 744A with boots much faster (in half a minute) with calibration seals on, and I noticed the pin 3 of S1001 is opened. Initially I thought it was out of place so I closed it again. Guess what? A slow 5 minute boot sequence! I suspected opening pin 3 of S1001 puts the oscilloscope in the quick boot mode. I experimented with the pin 3 opened and manually initiating the full blown self-test (utility menu). Turns out I was correct! The full self-test is 5 minutes! Since it was the settings from the calibration house, I looked up the TDS 744A/684A/784A service manual and noticed that the factory default is with pin 3 opened, while the old TDS 544A manual says pin 3 is closed. I suspect that the self-test for the new models 744/744A/684A/784A is unbearably long that the Tek decided to have it disabled for boot-up self-test. In any case, try opening pin 3 to have the scopes boot faster! 1,786 total views ## Agilent Infiniium (Motorola VP22) Motherboard USB Pinouts for DOM SSD The Motorola VP22 motherboard used in a variety of modern Agilent’s Infiniium series oscilloscope (the ones with Megazoom) and logic analyzers has a 5-pin USB port that’s not quite the same as the standard headers on most motherboards. The reason I needed the pinouts of the USB port is because I have a 40-pin DOM SSD to replace the old clunky mechanical hard drive, which needs 5V for power. I don’t want to modify/resolder the original wire harness (for the LS-120 and CD-ROM drive) nor I have the molex connectors to make a power splitter. Luckily, I found somewhere that I can safely tap a 5V for low power devices: the internal USB header. Here’s a picture with the DOM SSD module wired: This post serves as a reminder to myself that the ground is the second pin to the left and 5V is the rightmost pin. The pinout diagram as follow: 2,572 total views ## (Front-end) Attenuator Autopsy of HP 54616B Digital Oscilloscope One day I got a call from a friend saying that he accidentally fed high voltage signal (line voltage) and broke one of the channels. My hunch is that since he didn’t feed kilo-volts into it, the damage is limited to the (front-end) attenuator (an expensive part). I had an extra unit lying around so I just swapped it for him so he can get it up and running. Once I got the time, I opened it up the broken unit to see the damage. My hunch was correct: The 500Mhz attenuator used in 5461XB, 54505B/54506B/54510B/54512B (has to be version ‘B’), 54520/54540 series, and early Infiniiums 54810 series, E1428A (VXI board), 166XCS/166XES (MSO), 167XG-003, etc. are all the same: 1NB7-8303 (or 1NB7-8458). For 5461XB, it’s tucked inside 54616-68401*, which is a big metal case that covers 2 attenuators and an external trigger PCB (see the picture on the right hand side). Everything else is tucked individually inside 54512-63402 metal shield. In this autopsy, the resistive trace of 1NB7-8303 was blown open-circuit (charred), so bad that it spot-welded the metal casing. I took a good attenuator and measured resistance that the blown open ‘trace’ should have been. Turns out it’s a little less than 1Ω so I used 4-lead Kelvin sensing from my Agilent 34401A multimeter. I calibrated that meter using my Data Precision 8200 reference and agreed with my Fluke DMM, so hopefully I wasn’t way off. I could have put a small SMD resistor matching the resistance of the blown trace, but since I have a few back up attenuators lying around, I’d rather not risk the signal integrity by doing so, since I cannot guarantee the temperature specification if I just replaced it with a resistor. The scope was up and running in perfect condition after replacing the attenuator. Now you know what happens when you feed line voltage to a scope. The damage is usually limited, but the front-end (attenuator) is the most expensive RF part that makes a scope truly a scope (see Dave’s rant on EEVblog). The ADC hybrid can get just as expensive, but usually the attenuator takes the hit before your input gets to fry the ADC. * Be very careful about a similar looking block with similar number 54503-63401 or 54503-63402. It has the same metal housing, but inside the attenuator chip 1NB7-8116 or 1NB7-8288, which is NOT compatible with first generation Infiniiums, later 54500 series or 5461XB series. They are used in 54502A, 54503A, 54504A, 54510A. 54501A never showed up in the discussion because 100Mhz scopes does not need the individually shielded metal block. The attenuator was directly soldered to the main assembly. 5,521 total views ## Agilent 54641D (Mixed Signal) 54642A (Digital) Oscilloscope Fan Mod Agilent 54641D has a ADC hybrid (1NB7-8394) converter that runs very hot. The heatsink does a very good job extracting the heat from the chip, but there’s nothing to carry the heat away from the heatsink. The heatsink is actually hand-burning hot when passively cooled. Even with the case, the bottom of the unit gets so hot that it actually warms up the instrument below it. For longetivity, I decided to give it a tiny fan. But given the tight space at the bottom, how can I squeeze a standard 5mm fan? A squirrel cage fan (preferably 5mm side, 15mm thick) might do the trick, but where am I supposed to secure it? Turns out there’s a screw hole for the fan and there’s only one place I can screw it down: I padded the fan with 3 plastic washers to create a surface level with the two ASIC chips the fan is sitting on. Where should I get the power for the fan? It’s not a low-end 100Mhz scope, so I took extra precaution to not have a DC power line flying all over the circuit board to generate noise. I twisted the power wires together just to lower the EMI and follow the same holes Agilent designed to let the power cable go through. It’s a little risky to steal power from the circuit board directly when I don’t have the schematics, but luckily I found a printer power port which nowadays nobody uses which I can steal the power from. I could have tapped the 12V fan power from the power module for the power supply fan, but I noticed the printer power port is even better: it’s around 7~9V: I don’t want the noise from running it at full speed (squirrel cage fan are typically noisier). Finally I made an internal terminal for the printer power so the fan can be easily detached. Weakly wrapped it in a clear heat shrink tube so it won’t get accidentally disconnected yet reminds myself that a connector was built instead of directly soldered on. At last I tie-wrapped the power wire to the other power wires Agilent already secured so they won’t dangle during transportation. Self-calibration expects the temperature to be stable, so the oscilloscope needs to be warmed up before a cal would register. That means before the fan, it’d take a long while for the ADC to heat up to a guaranteed temperature. With the new fan, the calibration needs to be done again because it’s much cooler now. Even better, it takes nearly no time to warm the oscilloscope up for calibration because the steady temperature isn’t high anymore. Of course this mod works for 54641A, 54641D, 54642A, 54642D as well. Technically you can put it in any 5462X oscilloscope, but since they are 100Mhz, you don’t need to cool the ADC down that much. I recently tried it on a 54642A, which has only one stream instead of two, so one chip with glued heatsink and one ASIC (1821-5733) is gone. Looks like I can use the screw hole on the top left (the one with a red condom over it). Unfortunately, since the only way to use that screw hole is to flip the fan over, it became an inferior choice because • The suction part came from the bottom of the case instead of the board. That means I’m blocking ventilation for some components. • The fan’s power line is now close to the board, creating potential signal integrity issues (might not be material, but why mess with EMI issues when there’s no real advantage anyway?) In other words, even if it looks tempting to deviate from my original solution for the non-MSOs, there’s no good reason doing so. Now that I have a seek thermal camera module, here’s the thermal picture after the fan has been installed (accompanied with the picture of the board): You can see from the thermal picture how effective the heatsink is. It took only a very weak breeze to carry the majority of the heat away. The 12V squirrel cage fan running at ~7V is pretty quiet, yet it cools the ADC hybrid down to near room temperature. The real reason why I did this fan mod is because I had some units (like 54641A) bought from the used market that after powering it up for 30 minutes, the signal displayed just went nuts and jumped all over the place. I opened up the ADC and the metal bracket holding it down had a lot of heat stress pattern on it (the golden bracket looked purple-rainbowish). The unit was fixed after swapping the ADC hybrid from a donor unit, but now I know the ADC hybrid really needs to be kept cool to ensure longevity. By the way, if you need a replacement ADC hybrid (1NB7-8394), I have 2 pcs at$660/ea.

First come first served. Call me at 650-804-5024 if you suspect it’s the ADC hybrid that needs to be replaced. The fan mod is complimentary with any repair/service I do for 54640 series oscilloscope that exceeds \$800 after parts and labor.

Since I’m now selling 54640-series oscilloscopes with this as a value-added service, I’d ask my kind readers NOT TO use the technique learned here to offer products and services that might compete with my offerings. Feel free to apply it to units that you’ll keep for yourself. I’ll trust you 🙂

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