Tektronix 2901 Time Mark Generator Tear Down & Inspection - Page 1
Forum section: electronics / general

author: BravoV
author_rank: Super Contributor
timestamp: November 15, 2011, 06:56:21 am
content: Sharing few shots of this Tek vintage scope calibration tool that just received recently. Did a brief external inspection, it looks fine and well preserved. Powered up fine too, its just I haven't hook it to the scope to see it's output.Planning to check it's signal and an internal inspection tear down to see what and how it look inside now, wishing no bad surprise, and hopefully I can manage to do it at this weekend, will post the innards photo shoots once its available.This little thing is quite heavy for it's size.Top front panel view, serial B031078 with few stickers overlapped with years printed on them, calibration date maybe ?Front panel, with a date 1981 written at yellow sticker, probably that is the last traceable calibration date conducted. The main crystal oven led at the right side was on once its plugged at the mains, even the unit wasn't turned on. It blinked on and off periodically, looks like its maintaining the oven temperature, measured with kill-a-watt meter, while the unit was off, when that led was on, the oven it self consumes approx. 14 watt.Rear panel, with switch and bnc for external reference clock.Top and bottom view.

author: tekfan
author_rank: Frequent Contributor
timestamp: November 15, 2011, 08:36:11 pm
content: Nice one! Looks like it's in good cosmetic condition. These things are really handy for calibration of oscilloscope time bases. I'm surprised there is actually a proper ovenized oscillator in there.

author: Unknown
author_rank: Unknown
timestamp: November 15, 2011, 09:55:26 pm
content: I believe this model was between the 184 (tubes) and TG501 (compact TM500 module). Appears to share some mechanical parts with the TM500 series. It will be interesting to see the insides. Too bad time mark generators are kind of a one trick pony, not much else you can use them for apart from calibrating analog scopes.

author: BravoV
author_rank: Super Contributor
timestamp: November 17, 2011, 05:04:08 am
content: Quote from: tekfan on November 15, 2011, 08:36:11 pmThanks, yeah, external condition is still very well preserved as you can see, thats one of the reason I grabbed it, quite a gamble though without knowing how it looks like inside.I was also quite surprised its still working very well, did a brief 15 minutes test with my frequency counter showed the decades old ovenized 10 Mhz oscillator inside is still at pretty good accuracy, will post the result once I opened it up.Quote from: alm on November 15, 2011, 09:55:26 pmActually it was kinda impulse buy even I knew it does only that.Stay tuned, definitely will post it's internal parts shots here.

author: elementip
author_rank: Newbie
timestamp: November 17, 2011, 07:35:36 pm
content: I have one of these 2901's - neat units. Ideal for scope calibration. One of the cool features is you can press in just about as many of the marker selector switches as you want at the same time - it will output markerss for all of them.How long did you let it warm up? I think Tektronix recommended at least an hour and I seem to recall they also recommend leaving it on all the time if possible.

author: BravoV
author_rank: Super Contributor
timestamp: November 19, 2011, 01:20:14 pm
content: Quote from: elementip on November 17, 2011, 07:35:36 pmThanks, I didn't know that !Btw, do you have the manual ? This one is bought from a surplus tech shop which has nothing else with it, just the unit.Quote from: elementip on November 17, 2011, 07:35:36 pmNot sure, I guess its an hour like other classic ovenized crystal does at measurement instruments.Btw, just curious, does your oven led blink ? I mean like turned on maybe for 10 or 15 seconds, then off for another 10 or 15 seconds or so and so on.

author: BravoV
author_rank: Super Contributor
timestamp: November 21, 2011, 02:57:09 am
content: The internal photo shots.It has two compartments top & bottom which has it's own pcb, and isolated by aluminium plate.I leave all pictures at their thumbnail state to save bandwidth when viewing this thread, the file name of the picture should be self explanatory.Interesting spot :"10. Dual Coils & Old School Transistors.JPG" shows there are two coils windings but using different materials, wonder why used two different materials for the coils ?

author: BravoV
author_rank: Super Contributor
timestamp: November 21, 2011, 03:20:09 am
content: ..continued..These are more details close up shots at some parts, few interesting spots which are :- "16. Bulova Ovenised Crystal.JPG" shows the toaster of this unit, actually the whole unit feels very warm especially at the rear after the unit was turned on just for few minutes. Time to find for an external 10 Mhz maybe ?- "17. Shiny Coils.JPG" , the silvery wires still so shiny after all this years, are they silver ? or other metal ?- "18. Solder flux residue.JPG", the Semtech rectifier is the only semiconductor that is soldered while others like transistors and ics are socketed. Also at the inset, the solder points have the flux residue while others are clean, just curious if its manually replaced ?- "19. Trim Cap Winged Bug.JPG" , funny looking trim cap, looks like a bug with folded wings.

author: BravoV
author_rank: Super Contributor
timestamp: November 21, 2011, 03:24:18 am
content: Interesting spot :- "20. Very Clean Untarnished PCB.JPG" shows the pcb is still very shiny and almost untarnished at all.Last, wonder what happened to Ray and where is he now ?

author: codeboy2k
author_rank: Super Contributor
timestamp: November 21, 2011, 05:21:17 am
content: Great pictures, thanks!The socketed transistors blew me away.The coils look almost like nickel plating. Silver is a little duller even when it's first applied, and then tarnishes very quickly.I used to build much larger silver and nickel plated coils for high power RF equipment, so I've seen them both after a few years.

author: david77
author_rank: Frequent Contributor
timestamp: November 21, 2011, 02:32:46 pm
content: What a brilliant piece of kit. I love that PCB, you don't get stuff like that anymore.Hard to believe that people actually bothered to but simple transistors in sockets.

author: tekfan
author_rank: Frequent Contributor
timestamp: November 21, 2011, 03:03:22 pm
content: Very nice! The OCXO is made by Bulova the famous company that made the wristwatch with the tuning fork oscillator. Nice old HF design for the 500MHz multiplier. Looks like Tektronix had a habit of putting as many components in sockets so they can easily be replaced if they should ever fail.If you're interested you might want to see how Tektronix made the circuit boards back then:http://www.classictek.org/index.php?option=com_content&view=article&id=185:tektronix-circuit-boards&catid=88:tektronix-videos&Itemid=80

author: Unknown
author_rank: Unknown
timestamp: November 21, 2011, 08:55:50 pm
content: Great pictures. I wonder why the sockets for small signal transistors. Was this a holdover from the tube days which would have to replaced regularly? I remember that some manuals mention that it's not recommended to periodically remove and check semiconductors, suggesting that it is recommended for some equipment. It may also be because they considered semiconductors expensive, just like how people often used sockets for expensive DIP parts. Not much choice now most expensive parts are BGA, however.I think the sockets cause more issues due to bad connections than they solve by making part replacement easier.I believe the OCXO was not very accurate. A decent modern day XO will have a similar performance, but you don't exactly need 0.01 ppm performance for calibrating an analog scope.

author: BravoV
author_rank: Super Contributor
timestamp: November 23, 2011, 02:25:36 am
content: Quote from: codeboy2k on November 21, 2011, 05:21:17 amWhy used two different wires type instead of just using the copper enameled for both ?Quote from: tekfan on November 21, 2011, 03:03:22 pmThanks, that is a great website, bookmarked.Its so amazing how complex they were made back there.Quote from: alm on November 21, 2011, 08:55:50 pmAgreed, beside personally I don't need that kind of accuracy either. Anyway, looking at the results, what do you think of that decades old ocxo's performance ?

author: Unknown
author_rank: Unknown
timestamp: November 23, 2011, 04:49:44 pm
content: I don't see any reason to doubt the accuracy based on your results, but an analog scope isn't exactly a hard test for an OCXO. My comment was based on the 20ppm accuracy spec after warm-up, which is close to the performance of a normal XO.

author: JoeyP
author_rank: Frequent Contributor
timestamp: November 24, 2011, 01:14:53 am
content: I have one of these too! Thought I was the only one still hanging on to one. They're not needed for digital scope calibration, but they do still have an interesting use. The time marks are essentially impulses, and so are very rich with harmonics. You can use them as a comb generator for RF work.

author: BravoV
author_rank: Super Contributor
timestamp: November 24, 2011, 04:15:36 am
content: Quote from: alm on November 23, 2011, 04:49:44 pmYou're right actually, brought it to my friend's counter with gps reference, this one its still within 15ppm after about 30 minutes of warming up.Quote from: JoeyP on November 24, 2011, 01:14:53 amJoey, any chance you could share the manual ?

author: JoeyP
author_rank: Frequent Contributor
timestamp: November 24, 2011, 07:07:01 pm
content: I don't have the manual in electronic form. I don't have a scanner, but if there was something specific you needed out of it, I could take a hi-res photo of the page. If you want an entire manual, check this out:http://www.ebay.com/itm/TEK-2901-TIME-MARK-GENERATOR-SERVICE-AND-OPS-MANUAL-/370221905595?pt=BI_Books_Manuals&hash=item5632f1f6bbI have some of their CD manuals for other instruments, and they do a really good job.

author: Unknown
author_rank: Unknown
timestamp: November 24, 2011, 10:20:29 pm
content: Quote from: JoeyP on November 24, 2011, 01:14:53 amThe OCXO reference may actually be important for this application.You can probably get the manual cheaper if you buy it directly fromhis website. Especially if you opt to download it instead of having a CD shipped. The 2901A not listed on his website for some reason, but you'll probably get a quick response if you send an email. He's a very reliable source and has a good reputation within the Tek/HP repair communities.

author: BravoV
author_rank: Super Contributor
timestamp: November 25, 2011, 03:40:18 am
content: Quote from: JoeyP on November 24, 2011, 07:07:01 pmThanks for pointing that out, actually I purchased the physical manual just last week, its just it hasn't arrive yet and prolly will take few weeks.Bought it from ebay seller that looks like he has lots of this 2901 manuals, and still offering them like ebay deal no 130601052761.Just curious about it's full technical specification, if its not troubling you too much, appreciate if you can take the shoot on that particular tech spec page.Quote from: alm on November 24, 2011, 10:20:29 pmThanks, yes, I'm aware of that Dave's offering.Quote from: alm on November 24, 2011, 10:20:29 pmGents, mind explain what is this all about ?

author: JoeyP
author_rank: Frequent Contributor
timestamp: November 29, 2011, 10:05:03 pm
content: Quote from: BravoV on November 25, 2011, 03:40:18 amBecause the time marks are narrow, with a relatively fast rise time, they are rich in harmonics similar to a square wave. The difference is that an impulse has a much lower level for its fundamental and near neighbors. This means the harmonics are relatively much flatter in amplitude compared to a square wave, and that allows them to be useful over a wider range for a given full-scale amplitude. Hence the term "comb generator".One common use for a comb generator is to test the frequency span accuracy for RF spectrum analyzers (analog hardware-based analyzers, not talking digital FFT analyzers here). For example, if you set your time mark generator to 1us, it will produce many 1MHz harmonics. So feed that into your spectrum analyzer and set it to 1MHz/div, and you should see one harmonic per division. You can instantly view the linearity and overall accuracy of the analyzer's sweep.

author: BravoV
author_rank: Super Contributor
timestamp: November 30, 2011, 01:57:28 pm
content: Joey, thanks for the info, it might be useful someday.

author: ciccio
author_rank: Frequent Contributor
timestamp: November 30, 2011, 10:04:04 pm
content: Quote from: tekfan on November 21, 2011, 03:03:22 pmGreat memories.I was very good in designing and drafting PCBs with the adhesive pads and tape.I used a slightly different method: a sheet for pads only, two sheets (component side and solder side) for tracks only, then with the reprocamera we made three negatives (pads + component side tracks, pads + solder side tracks and pads only for the solder resist).I worked at a 2 to 1 size, not 4 to 1 as Tek did: film, pads and tape was too expensive..I still have some Mentor Graphics and Mecanorma supplies and tools in a closet: if my computer fails, I can still design boards!Please note that nobody, in the board fabrication plants, wears gloves or eye protections or masks: workplace safety was less important, in those days.

author: BravoV
author_rank: Super Contributor
timestamp: December 01, 2011, 04:36:21 am
content: Quote from: ciccio on November 30, 2011, 10:04:04 pmYeah, just watch that pretty lady at the solder reflow station, her face was so close to that molten solder when she adjusted the components placement at the pcb, a single splash of that hot molten metal by accident will ruin her pretty face and eyes, quite scary.

author: SoundTech-LG
author_rank: Frequent Contributor
timestamp: April 17, 2014, 09:51:04 pm
content: Quote from: BravoV on November 17, 2011, 05:04:08 amActually there is at least ONE other trick that these ponies can accomplish, and that is audio loudspeaker impulse testing."LoudspeakersAn application that demonstrates this idea was the development of impulse response loudspeaker testing in the 1970s. Loudspeakers suffer from phase inaccuracy, a defect unlike other measured properties such as frequency response. Phase inaccuracy is caused by (slightly) delayed frequencies/octaves that are mainly the result of passive cross overs (especially higher order filters) but are also caused by resonance, energy storage in the cone, the internal volume, or the enclosure panels vibrating.[citation needed] Measuring the impulse response, which is a direct plot of this "time-smearing," provided a tool for use in reducing resonances by the use of improved materials for cones and enclosures, as well as changes to the speaker crossover. The need to limit input amplitude to maintain the linearity of the system led to the use of inputs such as pseudo-random maximum length sequences, and to the use of computer processing to derive the impulse response."http://en.wikipedia.org/wiki/Impulse_response#LoudspeakersAnother benefit of the Tektronix 180A, is a space heater, with 54 tube heaters glowing in the fan cooled cabinet, it can provide some comfort on a chilly evening. Almost every time constant cap had changed value, and were changed out. The electrolytics are still good after 54 years. They don't make them like this anymore!

author: BravoV
author_rank: Super Contributor
timestamp: April 18, 2014, 02:11:44 am
content: Thanks for the bump, this thread is quite old that I almost forgot that I created it.Btw, this gadget since it was bought, its frequently borrowed among friends used it as comb generator to check their analog spectrum analyzers.

author: marcuswilson
author_rank: Newbie
timestamp: October 31, 2014, 01:17:17 am
content: Why used two different wires type instead of just using the copper enameled for both ?Because the silver or nickel one solders in instantly and the enamelled one needs to have it's ends cleaned, so it's only used where insulation is required.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: October 31, 2014, 12:34:55 pm
content: Just be careful with that diode in the clip in the 500MHz section. I lost mine and it's impossible to find a substitute.

author: TerraHertz
author_rank: Super Contributor
timestamp: November 07, 2014, 07:09:29 am
content: Quote from: Alex Eisenhut on October 31, 2014, 12:34:55 pmI was wondering about that. While admiring the photos I noticed that tiny little thing positively glowing with essence of gold-plated unobtainium.What is it, a tunnel diode as the active element in that stripline oscillator?Now I'm all envious and want to buy one of these things. Too bad the current ebay crop is quite high priced.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 07, 2014, 03:40:38 pm
content: Quote from: TerraHertz on November 07, 2014, 07:09:29 amIt's a varactor used in the frequency multiplier. I don't know how it works so I don't know what modern part you could shove in there.The OCXO in my 2901 is not great, you can see the frequency shift up and down as the oven turns on and off. It's quite crude.

author: Chipguy
author_rank: Unknown
timestamp: November 07, 2014, 06:54:03 pm
content: Awesome!IC's with datecodes starting with 69You don't see that very often

author: TerraHertz
author_rank: Super Contributor
timestamp: November 08, 2014, 12:27:36 pm
content: Do the videos herehttp://www.vintagetek.org/video-gallery/work for you?The page source has them as Flash things, and none are working for me. Not sure if due to my browser, or their server.Alternatively, does anyone know of copies hosted anywhere else in a normal video format, that can be downloaded and saved locally?Edit: Nevermind the 2nd, I found:https://www.eevblog.com/forum/testgear/historic-tektronix-videos-back-online-neat-stuff-for-test-equipment-geeks!/ft p://www.helpedia.com/pub/mirrors/www.vintagetek.org/video-gallery/ (remove space)

author: David Hess
author_rank: Super Contributor
timestamp: November 08, 2014, 05:25:07 pm
content: Quote from: alm on November 21, 2011, 08:55:50 pmIn the past transistors were less reliable and relatively more expensive so the extra cost of using sockets was small. Eventually reliability become high enough that the sockets themselves caused more failures than bad transistors so they stopped using them.This also happened with ICs and IC sockets although in this case it may have been helped along by those cheap edge wipe sockets Texas Instruments made which were unusually unreliable.Quote from: Alex Eisenhut on November 07, 2014, 03:40:38 pmVaractor multipliers rely on non-linear change in reactance whereas normal diode multipliers rely on non-linear change in resistance. Varactor multipliers are higher efficiency but have much narrower bandwidths and are more difficult to tune. In the schematic you can see where they used a pair of tuned circuits (idlers) to short out two of the harmonics; probably the 2nd and 3rd. (I just now looked in the theory section and it was indeed the 2nd and 3rd.)The specifications for the diode Tektronix used are available and it would not be difficult to find a suitable surface mount varactor diode to replace it. They are not expensive so a bunch of different diodes could be evaluated to find the best one.QuoteThe earlier type 184 time mark generator uses the same OCXO with the on-off oven. I never understood what application Tektronix had in mind for these which would require an OCXO. Analog sweep calibration does not require this kind of accuracy.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 09, 2014, 08:52:31 am
content: Yay! Got one! Ebay 151502733003. Seems in nice condition, and cheap enough.Will be a while before it gets here, and fingers crossed it works. From seller's description that's not guaranteed.Just so long as that tiny gold plated unobtainium varactor is OK.Quote from: David Hess on November 08, 2014, 05:25:07 pmArrgh! Those damned TI sockets... how I hated them. How did TI manage to make anything so flakey? I never could decide if it was mainly due to the very weak and stretchy spring contacts, or the plating which seemed to become non-conductive.Before learning about those, I once made the mistake of constructing a fairly complex prototype board with them, and point to point soldered kynar wire. Urrrgh...After that I always used Augat machined pin sockets. Expensive, but they never failed.

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2014, 01:42:00 pm
content: Quote from: TerraHertz on December 09, 2014, 08:52:31 amWith some diagnostic work, the varactor can be replaced with a modern surface mount one.QuoteTI has a history of making mistakes like this and then ignoring or covering them up. I try to avoid their products.QuoteI like the machined pin sockets for repair work because if I am expending that much effort, then I only want to do it once. I like cutting the pins out of the socket and mounting them individually as shown in the attached photo.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 14, 2014, 09:40:29 am
content: Yes, looks nice and you can see the tracks. I did the same thing a few times, I recall.Needs something plugged into them to hold them in line for soldering.Those other sockets are the offending items, aren't they? Are you replacing them all?Ha ha, and the same people that chose those sockets for that board thought it was OK to alternate the orientation of ICs.I spy tantalums. Together with *those* sockets I bet that board was soooo reliable.Edit: PS I should have the 2901 in about 3 days.

author: BravoV
author_rank: Super Contributor
timestamp: December 14, 2014, 09:55:20 am
content: Quote from: TerraHertz on December 14, 2014, 09:40:29 amPlease, it its not too troublesome, do a detail macro shots as well, especially if you spotted there are differences vs mine, thanks in advance.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 14, 2014, 10:06:43 am
content: Quote from: BravoV on December 14, 2014, 09:55:20 amSure, I was intending to. Though, just yesterday my camera lens (Canon) developed a problem. So now I have two Canon bodies (EOS 20D & EOS 40D) but no fully functional lens. Not sure how long that will take to fix.Thehttp://w140.com/tekwiki/wiki/2901page could do with a lot better info.

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2014, 12:47:04 am
content: Quote from: TerraHertz on December 14, 2014, 09:40:29 amNo, I just friction fit them and solder from the top if necessary. You have to be very careful about plugging something in to hold them because if it is tin plated, the tin will melt during soldering and fuse it into the socket.QuoteThey are but the ones I replaced were used for ceramic packaged 723 regulators which ran hot and the heat caused the sockets to become loose. There was not an actual failure but I did not like it.QuoteThat does not bother me. I am sure they did that to make the layout easier.QuoteIt is a Tektronix DC505 universal counter. It had some bad transistors and a design flaw but none of the solid tantalum capacitors failed.

author: TerraHertz
author_rank: Super Contributor
timestamp: December 20, 2014, 03:16:39 pm
content: Here's the teardown and repair of my Tek 2901.http://everist.org/NobLog/20141220_Tek_2901_teardown.htm

author: robrenz
author_rank: Super Contributor
timestamp: December 20, 2014, 03:45:33 pm
content: Quote from: TerraHertz on December 20, 2014, 03:16:39 pmVery nice

author: extide
author_rank: Regular Contributor
timestamp: December 23, 2014, 06:55:34 pm
content: Can someone point out what pic this rare diode is in and where it is located? I am trying to find what you guys are talking about...

author: SeanB
author_rank: Super Contributor
timestamp: December 23, 2014, 07:22:22 pm
content: http://everist.org/NobLog/pics/20141220/05_9163_top_close.jpgSee the little clip between the 3 open core inductors at the bottom. That is the clip and the tiny gold button under it is the varactor in question.At least is is possible to replace that, try getting a tunnel diode or even worse a backwards diode. Noone dopes a zener diode that heavily any more, you would have to use pieces of zone refining ends to get silicon that heavily doped.

author: extide
author_rank: Regular Contributor
timestamp: December 23, 2014, 08:15:46 pm
content: Ah, ok, I had seen that but didn't think it was it, heh. Thanks!

author: BravoV
author_rank: Super Contributor
timestamp: December 24, 2014, 03:56:24 am
content: Quote from: robrenz on December 20, 2014, 03:45:33 pm+1, thanks, looking forward to see you fix the other problems as well.

author: BravoV
author_rank: Super Contributor
timestamp: December 24, 2014, 03:57:56 am
content: Quote from: extide on December 23, 2014, 08:15:46 pmThis one has better view as in one of my photoshoot.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: December 24, 2014, 04:32:09 am
content: It's so small you'd hardly miss it if you put it in an envelope and mail it to me.

author: SeanB
author_rank: Super Contributor
timestamp: December 24, 2014, 05:27:27 am
content: It would probably fall out of the envelope though, unless you tape it to a card.

author: Docholiday
author_rank: Regular Contributor
timestamp: February 26, 2015, 09:20:35 pm
content: I was fortunate to come across one today. But there are two issues, one is the power switch and then the power indicator which I guess is incandesent? Anyway is it possible to replace both by just removing bottom cover?Thanks,Nicholas

author: SAUL BRITTO
author_rank: Frequent Contributor
timestamp: March 08, 2016, 06:04:18 am
content: I am a new owner of a 2901, hope it correspond to specs.There are some people are having problems withA heater and a crystal inside it,do you see any problem in your unit around this?

author: Docholiday
author_rank: Regular Contributor
timestamp: March 08, 2016, 11:19:48 am
content: I have had no issues in thst area. The only thing I had to do was reseat all most all of the components. Other than that replace the power indicator as it was broken and the power switch both easy replacements.

author: BravoV
author_rank: Super Contributor
timestamp: March 09, 2016, 07:10:36 am
content: Mine is still working, no complaint what so ever.Heck, even the electrolyte caps are still fine.

author: Docholiday
author_rank: Regular Contributor
timestamp: March 09, 2016, 08:11:57 am
content: Yep, wonder if Saul got his working or not? Sounded that he was not sure that it was working or not.

author: 1xrtt
author_rank: Regular Contributor
timestamp: March 10, 2016, 11:57:28 pm
content: Saul,I have a 2901 also, found it at begining of this year in one of the many electronic junk stores in Sao Paulo. It was in a sorry state, completely covered in dirty. I don't know how it is yours, but a good starting point is to get a good contact cleaner and isopropyl alcoohol and clean all the switches. Also, all transistors are socketed, and they may have bad contact, so cleaning them is also a good idea.Make sure you let it dry for a day before turning it on.Terrahertz posted the link for the manual on this thread, so I won't be redundant. He and Bravov both made an amazing job with the teardown of their units, make sure you read this whole thread, it is unbeliavable how much one can learn from these guys.Have fun with it! I understand it is an equipment with a very specific use (to calibrate scope time base) and may not be of much use outside of that , but it sure is a nice example of engineering.