Category Archives: Electronics

Tektronix TDS 500~800 series Color CRT adjustments

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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.

 

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Switching oscilloscope attenuator relays with H-bridge driver IC

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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.

 

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EMC PCB Layout Notes

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  • Implicit RLC (potentially filters) and antennas formed by traces
  • Large ground/voltage planes serves as EMI shield, low impedance path current sink
  • True differential signals can be generated by current sources
  • Decouple with ferrite beads if radiation inevitable by geometry/placement
  • Avoid / minimize large current swings on analog plane (e.g. buffer digital signals)
  • Star ground when splitting sections: don’t let heavy digital current sink through analog ground by cascading the grounds.
  • Don’t really need to split planes as long as large digital current’s preferred return paths are localized and far away from the analog section.
  • AGND/DGND refers to the grounds responsible for different sections of a mixed-signal IC. Has nothing to do with which actual ground to tie to. (e.g. DGND pin in ADC chips still goes to analog ground plane as it has low switching current)

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Lepy LP-2024A+ Class T Amplifier Mod

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My traditional Hi-Fi amplifier drains a lot of power and heats up my room when I’m not using it. The summer heat prompted me to look into Class-D amplifiers as they’re highly energy efficient.

I bought a Lepy LP-2024A+, a Class-T amplifier (It’s Tripath’s improvement over Class-D amplfiers) for $22 shipped. It sounded good over a narrow range of volume, as I can hear the background sound details on my ADS 200 speakers.

Unfortunately, strong bass components in certain music gets distorted, a sign that the amplifier cannot deliver fast energy impulses. To put it simply, I enjoyed the treble but not the bass with this amplifier.

I saw some mods reports on the older TA-2020A+ based units (like LP-2020A+), but as of now, only one Japanese blog site talk about switching out input stage op-amps without other changes. So I decided to do my own mod and post the results here.

First, the unit came with a dinky 2200uF capacitor for power smoothing. I upgraded it to 6800uF. I happen to have a 12uH inductor with thick wires, so I replaced the toroid inductor (as in the LC power smoothing pair) with it while I’m at it.

Then I replaced all the remaining white-label capacitors with decent brands (Wruth, Nichicon, Panasonic, CDE), all rated at 105 degC, sometimes with higher voltage ratings depends on which brand-name parts Newark has on sale when I order it.

Then I upgraded the 4 output stage inductors with Wruth 7447452100 rated 10uH 4.5A. Tripath’s datasheet says 2A, so I supposed the one populated on board would be less than that.

I also replaced the SMD (1206) ceramic capacitors at the output stage (very close to the speaker wire terminals) with Polyester Film (for the taller 0.47uF ceramic chips) and NP0/C0G (the thinner 0.1uF ceramic) to improve linearity. I suspect this change helped to reduce the listening fatigue for treble components of the music I’m listening to. Now I don’t have to tone down the treble gain (knob) that aggressively.

As prompted by the Japanese blog site, I ordered some LT1364, but the improvement isn’t that big since the NE5532 wasn’t bad in the first place:

The bass and drums are much more enjoyable after the mod, since the improved power handling reduced the bass distortion on bass impulses. The amplifier is still 15W (7.5W+7.5W) dictated by TA-2024A+, but I rarely want to crank the music up louder than that anyway.

As a bonus, I took a thermal image with my Seek Thermal Imager:

The input stage ICs LT1634 are 118 deg F:

Update [02/06/2018] I did some experiments with external capacitors and realized that the real problem is the crappy 13.5V@3A power supply that came with it. Yes, I tested it with a DC load and it can really do 3A, but it has a weird behavior: when there’s a huge power draw (like from a bass drum) that drops the voltage level below 11.5V, the power supply starts oscillating from 10V to 11.5V (never gets above that even when I stopped the music) with a regular hissing noise of around 1Hz physically from the power supply itself. I had to turn the unit off for it to ramp back to 13.5V.

Then I used my HP 6033A systems power supply (can do 30A, relatively clean power with fast transient recovery) and observed the rail voltage and compared it to when I power the LP-2024A+ with a big capacitor + original power supply. It’s clear that no capacitor is big enough to cover the flaw of that crappy power supply that came with the amp.

After the power supply issue is resolved, even loud music sounds smooth, expressing the 3D acoustic image crisply through my ADS 200 speakers: I felt like a person is talking/singing right in front of me than some loudspeaker generated sound. It’s so crisp that I can hear each individual string pluck. Bass is deep too after I added a ADS sub 6 subwoofer. I’ve been listening to “the Phantom Of the Opera” CDs ever since high school and I’m still rediscovering new musical details with this amplifier + ADS speakers! 

Before the mod, I would hesitate to make it my main amplifier and might want to go back to my Denon AVR-988 for serious listening. It took me quite a while to tune my Denon to sound good, but it works right out of the box on my modified Lepy LP-2024A+!

Update [08/21/2017]: Based on people’s comment (in audiokarma) on TA-2020A+ vs TPA3116, I decided to give it a shot and ordered Nobsound’s 50W model.

Actually before this LP-2024A+, I modded a Pyle TA-2020A and was disappointed at the clarity is totally lacking compared to the lower powered LP-2024A+. I thought it could be Pyle’s terrible implementation of TA-2020A+, but after hearing TPA3316, I observed these:

  • TPA3116 has less distortion than TA-2020A+ and it sounded slightly tighter at all frequencies, especially the low ones
  • Both TA-2020A+ and TPA3116 lack the clarity at vocal frequencies and above. TA-2024A+ beat them both hands down.
  • TA-2024A+ draws 0.15A when no music is played while TPA3116 draws 0.03A. The reading is from my HP 6032A (60V, 50A) power supply.

TPA3316 might have a tighter bass than TA-2024A+, but I wouldn’t trade the vocal and up for that. TA-2024A+ might be marginally OK for N.W.A., but certainly not for parties. It’s an amp for music, not for acting cool.

 

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