[HELP] Tektronix 2400 series U400 channel switch hybrid replacement - Page 1
Forum section: electronics / projects

author: mrnuke
author_rank: Regular Contributor
timestamp: August 04, 2016, 10:41:36 pm
content: I have a 2465B scope with a failed U400 channel switch. Since these hybrids seem to be either impossible to find, or unaffordable, I needed a more "modern" approach.THEORY OF OPERATION:U400 should switch the signals from the analog frontend amps to the display amplifier. Simple! The service manual has tons of information on the operation of U400 (except impedances, of course, as we'll see later).The input signals are passed through analog switches with suitable bandwidth. The Vishay DG4053 has a-3dB bandwidth of 730 MHz, so more than suitable. The assumption is that the signal sources are perfect current sources, so that both the following apply:Resistance of switches does not affect signalSignals can be summed up by simply closing two or more of the switchesAfter getting the logic table back-to-front on the first revision, I finally have a module that can display something on the CRT.

author: tggzzz
author_rank: Super Contributor
timestamp: August 04, 2016, 10:49:07 pm
content: Quote from: mrnuke on August 04, 2016, 10:41:36 pmDon't be a tease: show us a picture of the screen with a 1ns step input

author: mrnuke
author_rank: Regular Contributor
timestamp: August 04, 2016, 10:56:45 pm
content: Of course, the problem here is that the signal reaching the CRT is significantly attenuated. At this point, I haven't gotten too far info figuring out what the root cause is. I'm not sure if it's the resistance of the switches, or if I need a fully-differential summing amplifier. As you can imagine, the service manuals are dead-silent on the issues of impedance.The only hint that we get is the single-ended to differential amplifier for the readout (CH5):I've only quickly glanced over it, so I might be reading this incorrectly, but it seems to have 300 ohms series resistance between the emitter-followers and the switch. If I try to measure the DC resistance at the output pins (that is, looking into the output side of U400), I get around 127 ohms, though hooking a multimeter in ohms mode is hardly a conclusive test."Original" denotes display with known-working U400 hybrid. "Prototype" denotes display of same waveforms when the prototype is installed instead of the ceramic:That's a ratio of 2.4, which is similar, but not the same) to the ratio I'm seeing the attenuation of the traces on the display, which I measured around 2.8.And there are a bunch of other issue, as you can see from the attached comparison shots. What do you think I should do next to get this up and running on par with the original module?

author: mrnuke
author_rank: Regular Contributor
timestamp: August 04, 2016, 11:04:31 pm
content: Quote from: tggzzz on August 04, 2016, 10:49:07 pmHehe, you reply faster than I can type my posts.

author: David Hess
author_rank: Super Contributor
timestamp: August 05, 2016, 01:14:16 am
content: I applaud you for taking this on but I suspect you could get an economical replacement for the original hybrid from a broken parts donor.The original channel switch internally terminates each input signal pair and amplifies it with a differential transconductance amplifier which might be configured in an Ft doubler configuration. The outputs are current switched and probably drive a cascode which may be the current switch itself. Older channel switches made by Tektronix have documentation showing the general configuration. There may be a special output circuit for driving the delay line.So the problem with your configuration besides likely bandwidth and transient response issues is lack of gain.

author: george.b
author_rank: Frequent Contributor
timestamp: August 05, 2016, 02:06:59 am
content: That's way cool! Congrats on what you have so far, and I hope you get it working properly!

author: mrnuke
author_rank: Regular Contributor
timestamp: August 08, 2016, 03:54:49 am
content: Quote from: David Hess on August 05, 2016, 01:14:16 amThanks fir all the info. I was afraid Tektronix might have made something a little more complex than a "switch". I've put an ohm-meter to a known-good module, and it seems only channels 3 and 4 are terminated. I've also tried straight connecting one of the channels to the output, and the signal is still attenuated, as suspected.What I'll do is make a breakout board and bring all the signals to a breadboard through a flat-flex. That way I can at least quickly prototype to get the correct configuration before I start worrying about bandwidth and high-frequency response.Quote from: David Hess on August 05, 2016, 01:14:16 amDo you know off-hand a scope which uses a documented channel switch module? I wasn't yet able to find a specific model whose service manual shows the switch configuration. Closest I got was the service manual for the 2236, which implements the channel switch with discrete ICs.

author: mrnuke
author_rank: Regular Contributor
timestamp: August 10, 2016, 03:14:45 am
content: Next step is to have a better way to probe the signals and test different circuit topologies. I think this will help nicely:

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2016, 06:16:58 am
content: Well, I posted a long response a couple days ago answering your questions but the forum inexplicably deleted it. I am only going to summarize what I posted.The 2236 schematic shows everything you need but check out the datasheet for the Tektronix 155-0022-00 channel switch which shows transistor level details.In the 2465, the channel switch is also the delay line driver and I bet the delay line is shunt terminated at both ends explaining why the 2465 needs to have gain.

author: jonpaul
author_rank: Super Contributor
timestamp: June 29, 2021, 05:03:15 am
content: Bonjour, we also struggled with a defective U400, had dynamic compression on CH 1,2 in lower part of CRT display. Fixed with new U400, perfect now. Cost was $100Did you try to remove and reseat the original U400 hybrid?No CMOS switch will have the low shunt C, controlled ON R, and HF response of U400.The input pins of U400 are biased during the CAL procedure. Any replacement will not CAL.Look for U400 replacements, eg Qservice in Greece.Beware the newer Maxim parts, you need the original TEK.PS: DO NOT buy from China!After you finish do a complete CAL with correct CAL GENs, then check BW, rise/fall time, transient aberration etc.Bon chance,Jon

author: woodchips
author_rank: Frequent Contributor
timestamp: June 29, 2021, 11:30:03 am
content: Well done, good work.