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 in connector just fell off the board with the pad:
That means the RF in 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!
For TDS 500~800 series, a batch of CRT driver boards, color and mono, regardless of how heavily they are used, have bad flyback transformers. After turning the unit on continuously for half a day, the screen might stretch and disappear.
If you have a matching CRT driver board with a CRT tube, I recommend instead of swapping the CRT driver (seemed more straightforward), extract the flyback transformer from the donor board instead. The reason is that the adjustments needed from replacing the flyback transformer is far less than re-tuning a different CRT driver board to match the tube.
It’s impossible to tune the CRT driver board while it is in the case, since the processor board covers it during operation (unless you have special cables for the Acq/Proc interface to replace the interconnect PCB card), it’s done ex-vivo like this:
I bought a ribbon cable extender and built a 2-pin jumper extender by salvaging them from CRT driver boards with toasted flyback transformers:
The first thing to check for is the +21V which is used to generate many voltages across the board (pun intended here): it affects brightness, scale, offset and linearity everywhere. If there’s any adjustments to be made, this need to be done first.
This voltage can be tapped by hooking the positive (red) lead to the center (output) pin of LM317 (3-pin linear regulator) at U90. If you have an alligator clip instead of a grabber, you can also hook it up to ‘pin 4’, which is the body of the regulator.
You can pick many spots for the ground pin. Since I’m using a grabber, I’d pick another big 3-pin IC sitting on a heatsink for the ground lead. In this case, it’s Q10, the transistor that drives the flyback transformer. It’s the pin nearest to the short edge of the board (behind the red lead, sorry):
Here’s a picture of blank board showing how many trimpots are there:
Only the brightness and contrast dials are documented in the service manual. The rest, I had to locate them in the schematic one by one. Before that, I kind of figured out most of them by trial-and-error but had a few of them wrong, especially the voltages (there are three: +21V, screen and HV adj.): they all have the same effect. There are also some more obscure trims like center focus and horizontal focus (variable inductor). Now I know exactly what each dial does.
It’s hell of a lot of work to figure this out. I have some new old stock CRT straight from Tektronix at Beaverton, and it’s the reserve to support customers who bought color TDS 500~800 units from me. Almost all used units out there have problems (or going to have problems soon), and so far I’m the only one selling units with 1 year warranty (extendable to 3 years for extra).
If your unit is not under warranty included when you bought from me, and want one of these new color CRT tubes with shutter, I’ll almost require you to send your unit to me for installation unless you can guarantee that you can figure it out without my help. It’s $500 full-service with the tube included. Call me at 949-682-8145.
One day I was working on an old first generation Agilent Infiniium oscilloscope (namely 54810A) while I was too tired and rushed, I accidentally short-circuited the main acquisition board because I forgot to reconnect the probe-compensation port pin back to the acquisition board after taking out the front-panel and putting it back: the jumper with exposed metal (the heat-shrink over it was a little short the way Agilent manufactured them) swiped over something and I heard a loud bang, the scope shuts off, and I smelled the magic smoke.
My heart sank. Just saving a few extra minutes being careful checking everything (despite I opened and closed those front panel ten dozen times) I thought I lost an expensive PCB that’s almost the cost of the whole oscilloscope, and even if I can fix it, it might drain me at least a week.
Given that I know a short fried something (there’s a bad smell). I didn’t even bother to turn the unit on again until I’ve located what has fried. Turning something that you know it’s fried on again just risks further damage as it can load other parts of circuits.
Following the smell, I found a burnt IC on the other side of the board (needs to be painfully disassembled as the front panel /w BNC nuts needs to be taken out all over again), and I looked up the part number: ST L6201. It’s sitting on Channel 1’s front end section between the attenuator relay block and the ADC hybrid and there are 2 of them per channel.
Given the location of the component, it’s clearly the L6201 populated at Ch3’s slot is not used since it’s a 2 channel oscilloscope. So I transferred the chips to replace the broken ones at Ch1:
What is an H-bridge driver (L6201), that’s supposed to control motors, doing in an oscilloscope, especially the front-end section? I googled “H-bridge driver in oscilloscope” and nothing relevant turned up. Then I went back and read a little more on how an H-bridge driver is really used. L6201 is a DMOS Full Bridge Driver with four power MOSFETs (switches) that basically sends current through an inductive load (typically motor) that might have stored energy (momentum) that might need to be drained (brake) to stop faster.
Turns out it’s a slick way to drive mechanical relays in the input attenuator given the amount of due care needed to accurately manage the current demand and switch transitions. There are 3 relays in the attenuator module. I suspect up to two relays can be switched with each L6201 by placing a diode in serial with with a relay coil, and repeat it (in parallel) with the diode reversed in the other branch. Is it an overkill? Let me know in the comments section. Please see the comments section
I also noticed, while dealing with the main acquisition board for 54615B/54616B, there is a L293D chip under the attenuator block shield for each channel. It’s a 4 channel driver with half-bridge, also intended to drive inductive loads. This one is more explicit about being used to drive relay solenoids as well as motors. So this is nothing new; It’s just not too many people talked about using it on oscilloscope architectures.
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.
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.):
If you are not a hobbyist and don’t want the hassle of disassembling whole bunch of stuff just to take out the CRT driver, rebuild it with the said flyback transformer and re-tuning the CRT driver (not only it’s a huge pain, the working room is very tight if you don’t have the extension cables), I recommend sending the unit to me for a full CRT surgery (you pay for shipping costs both ways). I also charge a lot less if you combine it with other services such as re-capping (strengthening), NVRAM replacement, etc, in one trip.
Update (2023/07/02): the prices 6 years ago is no longer practical. Luckily nobody asked. This surgery is just way too much hassle to charge this little. If you have a big customer who really need to keep exactly the same model to avoid changing their process/certification/software, ask me for a spot quote. I usually give very generous combined discounts if there’s more than one thing to work on.
I don’t think anybody else have new compatible flyback transformers for these displays that has the same fate anymore. I’ll update this post when the ones I saved are used up.
