Tektronix 7904:  Might have had two S-52 Pulse Generators Fail? - Page 1
Forum section: electronics / testgear

author: Electro Fan
author_rank: Super Contributor
timestamp: May 05, 2020, 06:42:50 am
content: Every once in awhile I dust off the Tek 7904 and keep my fingers crossed it's all still working.Maybe not so much luck today. Normally the S-52 plays nice with a S-4 or S-6 in the 7S12 but after hours of chasing my tail I think maybe the S-52 (two of them?) might have given up their usual good performance.Attached is a photo I found that I posted way back when I first was trying to get the scope system (mainframe plus plug-ins) running and eventually did. This photo at the very end - with the red markings on the blue trace - shows what I think might be a healthy pulse waveform.The other photos without any red markings are what I'm seeing now. Somehow it seems that I have lost two S-52's. (In one photo you can see S-52 #2 shows more gain than S-52#1 but the larger issue is no pulse evident with either.) I'm pretty sure the S-6 and the S-4 are good, but there is some possibility the 7S12 has an issue. I've tried swapping plug-ins in various slots and I think the 7904 is working but there is some medium to small possibility the issue is the mainframe.What do you think? The new pulses don't look like the old pulse with the red markings.... yes/no/maybe? Thanks

author: Electro Fan
author_rank: Super Contributor
timestamp: May 06, 2020, 12:37:06 am
content: Below is a signal generated by a Tektronix demo board that supposedly can produce a 200 ps rise time. It is being fed into a 7A29 on the 7904. It looks about as expected (the 7A29 has bandwidth of 1 GHz but the 7904 is spec'd to 500 MHz so the rise time should be about 700 ps, which is roughly what the trace is showing or maybe a tad better) so I think the mainframe is maybe/probably ok.Any reason why this same signal fed into an S-4 in the 7S12 shouldn't display properly? Maybe the S-4 or the 7S12 are faulty? I can't get the S-4 in the 7S12 to display this same signal using either the 7S12 timebase controls (in the A position) or a known good 7B92A (in the B position). Thoughts? Thanks(not sure why the thumbnail is rotated but when you click to enlarge it renders properly)

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2020, 06:36:50 pm
content: As I replied in my private message to you, there is no reason that signal should not work with an S-4 or S-6 and 7S12.

author: texaspyro
author_rank: Super Contributor
timestamp: May 08, 2020, 02:02:03 am
content: Open up the unit. See all those 1/8 watt carbon composition resistors? Pretty much all of them will have drifted way off of their proper value.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 09, 2020, 08:56:03 am
content: Quote from: texaspyro on May 08, 2020, 02:02:03 amThanks texaspyro. Just to clarify, when you say open up the unit to examine the resistors are you referring to opening up the sampling heads, the pulse generators, the 7S12, or the 7904 mainframe?I think it could be the sampling heads - but it seems strange that I would have had four of those fail (two S-4s, one S-6, and one S-1). On the other hand it's also somewhat strange to think that two S-52 pulse generators have failed.I'm sort of leaning toward the problem is with the two S-52s because when I send the signal from the S-52 (either one) through the S-6 passthrough to either a Rigol scope or another Tektronix (2647B) scope, the tall pulses (previously marked with red in the image below when things were working properly) are missing. Also, if I send the signal from the S-52 generator (either one) directly to the other scopes (without going through the S-6) the tall pulses are missing. So it seems more likely that the pulses are not being generated in the first place (by either of the two S-52 generators). Or maybe there is something in the 7S12 that is keeping the S-52s from generating the tall pulses. I'm kind of leaning toward the S-52s as both having failed but I can't quite say for sure, much less why.Seems like the key question is: if the tall pulses are missing is that more likely to be caused by a generator or a sampling head, or something else? Thanks

author: EV
author_rank: Frequent Contributor
timestamp: May 09, 2020, 01:07:14 pm
content: Here is picture (single shot) of S-52 waveform and Type 285 power supply where the S-52 is installed. S-52 is connected with BNC cable through 50feed trough connector to 1 Mconnector of Rigol DS2202 (not A).

author: EV
author_rank: Frequent Contributor
timestamp: May 09, 2020, 03:20:01 pm
content: Quote from: Electro Fan on May 09, 2020, 08:56:03 amThe problem is in S-52 or its connections to scope or 7S12. S-52 needs only correct voltages from 7S12 not else to work correctly.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 09, 2020, 06:49:14 pm
content: More testing....I powered and tested the S-52s (both of them) in various ways and for now I'm going to focus on one of them that I think behaves better.With the better S-52 I powered it from a 7S11 in both the right vertical and A horizontal slots to confirm it is getting power - to make sure the slots are providing power.In the first two images below you can see that the S-52 has power and is sending a signal directly to the Rigol above. In these photos you can see I am now getting some pulses, as opposed to the third photo which shows Pulses Missing. So this is an improvement (maybe there is an intermittent problem?), but I am not clear on whether the tall pulses shown in the first two photos are proper (sufficient.)In the photo from dabbledoo you can see a trace that looks similar and below is an explanation regarding the pulse and the pretrig output. Maybe I am missing the pretrig output?Also is a photo (called Good Pulse) I've posted before showing some red markings on a blue trace - this was from when things were working properly. It shows both a relatively a narrow pulse and a wide pulse.So the question is, why don't both the narrow and the wider pulse show up on the new photos of the signal going directly from the S-52 to the Rigol?Please feel free to straighten me out - I'm pretty sure I am on the edge of confusing any number of things. Thx!Edit: for some reason the

author: Electro Fan
author_rank: Super Contributor
timestamp: May 09, 2020, 07:07:57 pm
content: Here is a repost of EV's waveform which seems to show the trigger occurring on on the narrow pretrigger output circled in red, and two of the pulses used to measure risetime. So, the question is am I missing the pretrigger output? (Sure looks like it.) If so is it the fault of the S-52, or something else? Thx

author: EV
author_rank: Frequent Contributor
timestamp: May 09, 2020, 10:03:19 pm
content: Quote from: Electro Fan on May 09, 2020, 06:49:14 pmThe first picture looks good. I marked possible trigger points to your picture. If you use single shot triggering scope can trigger to either of them. It you use auto triggering scope triggers both of them and you get mixed picture. It can be avoided by using delayed triggering.In your picture the narrower pulse is so narrow that it can not be seen in the picture. It becomes wider if you use faster time scale.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 10, 2020, 01:07:44 am
content: Thanks for the patient and persistent help with this.I have a bunch of questions but before going back to debugging the scope, plug-ins, generators, and sampling heads here is an attempt to get me regrouped back at square one.In Illustration 1, there are three pulses marked A, B, and C. Just to be descriptive I'm saying A is a wide pulse, B is a medium pulse, and C is a narrow pulse. If I was guessing (and I'm sure I'd be wrong), I would have guessed that one of the three wassome part ofthe fast rise time normally measured as about 40 ps, and another one of the three might have been a pretrigger. I don't know what would be the significance or purpose of the third pulse.To be still more candid, I'm not sure why a pretrigger is needed if the scope can trigger on the fast rise time itself (otherwise, how do we ever display a rise time such as what is shown in Illustration 2?). Unless maybe the fast rise time is found/captured based on a calculated distance from the pretrigger? All just guesses, obviously.In any event, what are A, B, and C? (Why are there three types of pulses shown - and what are the reasons for the three different pulse shapes and widths shown in Illustration 1?)With all that as a baseline, in Illustration 2 we see the signal of ultimate interest which is the 40 ps rise time. I am calling this "D".Where should D be located relative to A, B, and C on Illustration 1?Thanks!

author: texaspyro
author_rank: Super Contributor
timestamp: May 10, 2020, 01:45:40 am
content: Quote from: Electro Fan on May 09, 2020, 08:56:03 amThe sampling heads... I've never seen one without resistor issues. That said, I'd check every small carbon comp resistor in Tek equipment from this time frame. Larger sized resistors may also have issues, but those 1/8 watt devils are (multiple expletives deleaed).

author: EV
author_rank: Frequent Contributor
timestamp: May 10, 2020, 06:12:14 am
content: Quote from: Electro Fan on May 10, 2020, 01:07:44 amPulse D is same as pulse B. You can use pretrigger pulse C to get steady picture at auto trigger mode. It is pulsing at same frequency as pulses A.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 10, 2020, 09:51:45 pm
content: Hi EV, Thank you. If D is the same as B, let's call it B going forward. So, next question: are B1 and B2 as shown below? Thanks again.

author: EV
author_rank: Frequent Contributor
timestamp: May 11, 2020, 08:15:47 am
content: Quote from: Electro Fan on May 10, 2020, 09:51:45 pmYes, and B3 is also as I added to your pictures.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 11, 2020, 08:09:36 pm
content: Quote from: EV on May 11, 2020, 08:15:47 amThanks.B2 should be about 200 ns? Thx

author: EV
author_rank: Frequent Contributor
timestamp: May 12, 2020, 05:44:43 am
content: Quote from: Electro Fan on May 11, 2020, 08:09:36 pmYes, that pulse is about 200 ps wide and about 250 mV high.

author: EV
author_rank: Frequent Contributor
timestamp: May 12, 2020, 08:57:15 am
content: I connected Leo Bodnar's pulse generator to my 7904 with S-4, 7S11 and 7T11. It was very easy to get picture of the rising edge of 10 MHz repeating signal to the screen. I used external trigger and sequential mode. Here are some pictures attached.

author: EV
author_rank: Frequent Contributor
timestamp: May 12, 2020, 10:59:24 am
content: I connected also S-52 with its power supply to my 7904 using S-4, 7S11 and 7T11. It was not so easy to get picture of the rising edge of its 65 kHz repeating signal on the screen. I used external trigger and sequential mode. It was not possible to see rising edge with faster time bases than 100 ps because of long (85 ns) delay time of the S-52. Also the rising edge does not look very good maybe because of the long cables. Here are some pictures attached.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 21, 2020, 03:15:04 am
content: Thanks for all the help. I appreciate it.Back on line.... might be about 34 ps or maybe a tad better.

author: EV
author_rank: Frequent Contributor
timestamp: May 21, 2020, 02:37:41 pm
content: Quote from: Electro Fan on May 21, 2020, 03:15:04 amI think it looks to be 28 ps.

author: Electro Fan
author_rank: Super Contributor
timestamp: May 22, 2020, 05:59:08 am
content: Quote from: EV on May 21, 2020, 02:37:41 pmOh, wow, guess I missed that. I need to work on better minor division mark reading. Thanks for pointing this out. Not just any 6 ps that went missing, it was 6 that contribute to a sub-30 ps. Thanks EV!