EEVblog #803 - HP1740A Analog Oscilloscope - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: October 03, 2015, 01:48:20 am
content: Dave saved this classic from the dumpster, a HP 1740A 100MHz dual channel analog oscillocope from the mid 1970's.It wasn't supposed to be working so this is a look at the problem, a mini teardown, and some basic PSU troubleshooting.Update: Even after 4 hours it still doesn't fail with the covers off. So likely to be some sort of thermal issue as suspected.

author: edpalmer42
author_rank: Super Contributor
timestamp: October 03, 2015, 02:55:09 am
content: I have an HP 1744A scope (100 MHz analog storage). A few years back, it died. Checked the power supplies and found that one of them had gone. The regulator was one of those round ones in the weird sockets. Wiggled the chip and the problem was gone. Hasn't given me any trouble since. So, rule 1 is check the voltages, rule 2 is reseat all the sockets and connectors.You missed the most obvious reason why all the supplies would drop together. All the supplies are powered and/or referenced from the +15 supply.Ed

author: Barny
author_rank: Frequent Contributor
timestamp: October 03, 2015, 03:39:07 am
content: Nice scope.Yesterday, i found an old 20 Mhz Gould Osziluscope and a analog roughness tester unknown working condition.I think, i give the scope to the neighbor boy.Dave, if you want, I'll send you the analog roughness tester.

author: kallisti5
author_rank: Contributor
timestamp: October 03, 2015, 03:51:13 am
content: I posed the mil-spec (AN/USM-339) on the forum back in 2012.Just took some new pictures. Interesting to compare the Mil-Spec AN/USM-339 to the non Mil-Spec HP1740A (which the Mil-Spec was based on)https://picasaweb.google.com/113129167989960578073/HPANUSM339MilSpecScope

author: Paul Moir
author_rank: Frequent Contributor
timestamp: October 03, 2015, 04:20:36 am
content: Is it just me or does the power LED dim every time it fails?

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 03, 2015, 05:23:26 am
content: As soon as I saw this I was like "yep, that's my baby". Bought one a few years ago from a flea market for $15 as my first scope. It's still my main scope since my tek burned out it's psu randomly.But I've accidentally connected this thing to some old tube stuff with nasty ground faults and let the sparks fly several times and even blew up a homemade mains referenced psu trying to measure ripple.Yet it still works fine and there appears to be no signs of damage, the build quility is amazing. The only problem is that weird backlight it has built into the crt sometimes goes out.

author: steve30
author_rank: Frequent Contributor
timestamp: October 03, 2015, 06:07:40 am
content: Looks like a nice scope. Good luck getting it working.You should use analogue scopes in your videos more often.

author: retiredcaps
author_rank: Super Contributor
timestamp: October 03, 2015, 06:39:52 am
content: I'm surprised during the initial setup of the meters that you didn't use min/max to capture any voltage fluctuations. The audible indicator when a new min or max is made would have come in handy.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 03, 2015, 06:51:10 am
content: Also, try cleaning the input voltage selector switch. It might have dirty or bad contacts causing arcing. This would explain the random jittering on the display and the stink would be the plastic burning from the arc (you'll have to take the cover off the switch to see it).And the power busses are all wired together, possibly to keep them referenced to each other (you can see this in the block diagram). Leave the bottom cover off with the probes connected and rest it right side up on the handle. This would provide a more accurate recreation of ths conditions for the fault.

author: PA4TIM
author_rank: Super Contributor
timestamp: October 03, 2015, 07:33:05 am
content: I like this episode, brings back memories. An other thing, there is a benchbrief about bandwidth problems caused by the delay line. Unwinding the coax and moving it a lot and repeat that over the whole length solves thatI had one and repaired it several times. 4 times a year until the friend I gave it too found the source. Mine was a 1976 model.http://www.pa4tim.nl/?p=564Just things I remember.About the uF, Most times in 60-70 stuff I've seen, they use uF, and some times mF. About nF you are right. On older stuff you see the use of use uuF instead of pF. Look at Tek manuals from that area, they are even nicer. By the way, The 1740 had a separate usermanual.About the scope:Mine showed several problems but one that came back more then once, was floodgun related. These scopes have no normal graticule lights. It uses the floodgun (but it looks very cool) A friend who has it now found the cause of problems. A hard to find cap in the floodgun circuit. It did not fail again since then. I twice had a burned floodgun resistor.The boards are moved every time you push one of those bloody, always problems causing, latching pushbuttons.On mine they gave problems with the solderjoints connecting the boards. But the switches themselves caused problems in my 576, several Philips meters and scopes and other gear. I hate those bloody things. Until I found out that K61 works best for me here.Those big caps are often still better a new replacements but they can fail. I always measure ripple current too. During restaurations I remove and test them on leakage, capacitance and DF.I always reseat parts in sockets. Especially in old Tek scopes from the 7000 series.I never use freeze spray or a heatgun because I'm afraid the fast temp change due to the big temp difference, might kill parts like cracking tantalums or ceramic packages. Do you use it a lot ? I can be wrong here.The dropping of all voltages is weird indeed, but I would check the HV from the crt too. That can cause strange troubles, and in this case the floodgun. In a 60's Philips sample scope the leaking doorknob HV caps caused very hard to find problems.Those 1740's are really nice.

author: Joule Thief
author_rank: Regular Contributor
timestamp: October 03, 2015, 07:58:47 am
content: Quote from: Paul Moir on October 03, 2015, 04:20:36 amNot just you - I noticed the same issue around the 5:00 mark on the video.On closer inspection, the indicator is powered off the 5 volt rail.

author: firewalker
author_rank: Super Contributor
timestamp: October 03, 2015, 08:18:07 am
content: The forces that stresses the boards and could open circuit something, when heated up, is upside down.Alexander.

author: Tothwolf
author_rank: Regular Contributor
timestamp: October 03, 2015, 08:48:27 am
content: I would probably focus on the trigger section. After repairing many analog Tektronix scopes, I found these sorts of issues were common with trigger faults. Trigger faults seem to be second only to PSU faults.In addition to "Thou shalt test voltages", maybe also "Thou shalt inspect and clean electromechanicals"? (switches, interconnects, sockets, and other electrical connectors)Do any of the .156 Molex headers in the scope have round pins (including those used for board-to-board interconnects)? If so, that is also a common problem area. When a round pin header is used, there is only a very small contact surface between the round pin and flat contact surface of the terminal. I replace such headers on sight wholesale with square pin versions just as I do with single wipe DIP sockets. Both Molex and AMP discontinued the round pin versions of their .156 headers long ago due to failures, but tons of them remain in older 1970s and 1980s equipment.

author: Muxr
author_rank: Super Contributor
timestamp: October 03, 2015, 08:49:33 am
content: This is a great scope, although if I had one I would probably 3D print a new bezel around the screen (probably in some hipster color for kicks). As that bromite aging effect looks kind of bad on that plastic.

author: tautech
author_rank: Super Contributor
timestamp: October 03, 2015, 09:03:15 am
content: Quote from: PA4TIM on October 03, 2015, 07:33:05 amYes they are, but as you say if you can keep them going.One I had gave me heaps of grief until I got it sorted, but they are a good unit when right.Got another non-runner under the bench, saved for a rainy few days.

author: hamdi.tn
author_rank: Frequent Contributor
timestamp: October 03, 2015, 09:12:08 am
content: Check this old .. cold war old i think scope , and it's in Russian. i found it in the dump of company i use to work 2 years ago.Sadly it only worked for like one hour before stop displaying anything. i was enjoying it

author: Muttley Snickers
author_rank: Unknown
timestamp: October 03, 2015, 09:32:33 am
content: Thanks for the video, very good indeed....I found it extremely informative and have never used a CRT CRO in my life but have seen more than enough videos of them in use and I've only ever had PC based and LCD DSO's.One thing that I did learn from this video is that the Gossen meters have the amps jack on the right and I looked around at a few images of various models and it appears to be commonplace with the Gossen meters, never noticed it before although granted they do have the input jack shielding mechanism in place.I feel a bit stupid now because a while back in another thread a member was looking at buying a cheap shitty meter that also had the amps jack on the right and I/ we told him to stay away from it for that and other reasons.I cant begin to imagine how many Gossens he has passed up.........Muttley

author: EEVblog
author_rank: Unknown
timestamp: October 03, 2015, 10:26:40 am
content: Quote from: Muxr on October 03, 2015, 08:49:33 amYeah it looks pretty crusty, kinda spoils it.

author: sakujo7
author_rank: Contributor
timestamp: October 03, 2015, 11:06:40 am
content: I thought there might be a problem with the 6th transformer output (unregulated? having trouble reading a scan of the manual...foldouts don't scan well) which could drag the other outputs down, but the drop is too specific.All rails dropped to 79% +- 0.2.It looks like all the regulators depend on the +15V rail. Maybe try adjusting the 15V rail pot and see if the other rails follow it to confirm....Just read further up, +15V is indeed the reference rail. Problem will be somewhere on that. See sections 8-39 and 8-40.

author: tggzzz
author_rank: Super Contributor
timestamp: October 03, 2015, 11:43:25 am
content: Quote from: PA4TIM on October 03, 2015, 07:33:05 amVerytimely and interesting: I have exactly that problem on my 1740, the risetime is ~7ns. Would a simple google uncover the source of your information, or is there a direct link available?Now I'm off to repaint the kitchen door before the rains arrive, but I'll look at the whole thread tonight.

author: longview
author_rank: Contributor
timestamp: October 03, 2015, 12:44:26 pm
content: I just put my 1740A in storage this summer when I upgraded to a DS1074z, it's a really great scope (but stuffed full of HP ASICs as mentioned).Mine has also had issues with the power supplies, but just the backlight fortunately, I suspect it's a bad joint near the bridge or the adjustment components for it. It works 99% of the time so didn't spend more time on it other than reflowing the joints related to it.For testing the PSU there's an interconnect card that disconnects all the PSU outputs that can be pulled out without any further disassembly, a really nice feature IMO.

author: f4eru
author_rank: Super Contributor
timestamp: October 03, 2015, 01:33:05 pm
content: 3 possibilities :1) is there a thermal cutoff on the transformer ? could have gone bad with some resistance2) more probable : a circuit sitting on the +15V drawing too much current there, going to current limit, tying other supplies through the common reference.3) problem on the common voltage reference.

author: elliottveares
author_rank: Contributor
timestamp: October 03, 2015, 01:56:14 pm
content: Quote from: EEVblog on October 03, 2015, 01:48:20 amPlease do a follow up video!

author: EEVblog
author_rank: Unknown
timestamp: October 03, 2015, 02:28:05 pm
content: Quote from: elliottveares on October 03, 2015, 01:56:14 pmAlready partially shot. It decide to co-operate for a bit which allowed some progress. Didn't have long to look at it though.

author: drussell
author_rank: Super Contributor
timestamp: October 03, 2015, 03:14:10 pm
content: Quote from: EEVblog on October 03, 2015, 02:28:05 pmYeah, intermittent problems can sure be annoying to troubleshoot when it won't fail while you're looking at it.I agree with others above to verify the voltage selector switch for proper solid opertation and, of course, the main DC reference. You could also watch a couple of the AC voltages coming out of the transformer. If the AC voltage drops coming in then it's obviously not the reference's fault the DC is dropping and would likely be an intermittent shorted turn in the transformer. Even a single shorted turn will draw a whole bunch of power and drag down all the voltages and will smell that electrical burning varnish smell if it stays shorted for more than a moment.

author: phenol
author_rank: Regular Contributor
timestamp: October 03, 2015, 03:14:21 pm
content: I too had the same bandwidth problem with that scope. Pushing the delay line coil around while feeding 100+MHz to the scope does indeed reveal sweet spots where it meets its specs. At one point i had to stick a pen in there to make it hold its 'good' position, but it only worked for a month or so when it started to deteriorate again.The board to board connectors are prone to failure and fatigue. One of them was actually charred due to poor contact and i resorted to bridging the two boards it connects with wire jumpers.The flood gun on mine only works intermittently.When the scope was 20, a high value resistor in the crt focus section of the HV power supply failed and the beam became defocussed...Quote from: tggzzz on October 03, 2015, 11:43:25 am

author: Alex_arg
author_rank: Contributor
timestamp: October 03, 2015, 04:13:37 pm
content: Hi Dave ! I've got a HP1715 200Mhz the same panel and chassis than 1740, i buyed it 4 years ago , and after cleaning all electromechanical parts it's working OK , but the multi PCBs switches of time base are a pain in the a** , once a year or so, i have to realign the knobs axis with the rotary switches and clean the contacs of the connectors between 3 daughters and mother board. Mine got PCB edge contacts, i've see that yours have a very better plastic connector between boards. TIP: Power supply is almost the same in both scopes and in the service manual says that the +15V regulator serves as reference to al other voltages in the low voltage PSU.I hope this helps.Photos ->https://www.flickr.com/photos/136701500@N04/sets/72157658999271719

author: max666
author_rank: Frequent Contributor
timestamp: October 03, 2015, 05:01:34 pm
content: Dave, don't let your wife catch you with that service manual

author: PA4TIM
author_rank: Super Contributor
timestamp: October 03, 2015, 07:07:38 pm
content: http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-12.pdfNice article about this scope. Including backgrounds about the hybrids/ceramics stuffThe service note about the delayline must be somewhere around 1982. I also have to google for it to find it again. In my case it solved the BW problem. I agree with the rest, about the reference voltage.

author: John Coloccia
author_rank: Super Contributor
timestamp: October 03, 2015, 09:40:15 pm
content: I've got one of these. Really nice scope. Of course, all of the feet are broken off. One of these days, I need to make some new ones in my shop, because it's a LONG scope. The main reason I picked up the Rigol 1054Z is for the size. I still have to whip this thing out sometimes, though, when I'm tracking down noise.

author: Shinobot2
author_rank: Newbie
timestamp: October 03, 2015, 11:57:50 pm
content: I saw the 121V rail droping slowly when the heat gun was pointing to the power supply. Every time Dave pointed somewhere else it started to increase the voltage.

author: Stephan_T
author_rank: Contributor
timestamp: October 03, 2015, 11:59:54 pm
content: Hi Dave,what about replacing four of the five DMMs with a 4-channel storage oscilloscope. Single shot triggered it could wait for hours for the fault to happen, and it could also record in some detail the relation of the different voltages. Like: "which one dumped first".Maybe this could be another argument/example for theOscilloscopes, how many channels do you need?question.

author: boffin
author_rank: Unknown
timestamp: October 04, 2015, 12:27:39 am
content: Quote from: Shinobot2 on October 03, 2015, 11:57:50 pmIt dropped a tiny tiny amount (5th digit), a few tens of mV on a 120V line.

author: AF6LJ
author_rank: Unknown
timestamp: October 04, 2015, 12:44:35 am
content: Quote from: Cyberdragon on October 03, 2015, 06:51:10 amYup.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: October 04, 2015, 02:51:24 am
content: I didn't see the manual posted, so I think this is it.http://cp.literature.agilent.com/litweb/pdf/01740-90909.pdf

author: AF6LJ
author_rank: Unknown
timestamp: October 04, 2015, 03:13:24 am
content: Quote from: HackedFridgeMagnet on October 04, 2015, 02:51:24 amThose are good scopes, it is no wonder they are still working going on forty years later.I have my doubts those LCD all digital scopes we have around us today will have the same longevity.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 04, 2015, 04:39:15 am
content: Quote from: PA4TIM on October 03, 2015, 07:33:05 amMine has the same problem with the graticule light going out sometimes. What parts did you replace?

author: Tim F
author_rank: Regular Contributor
timestamp: October 04, 2015, 05:08:18 am
content: Quote from: sakujo7 on October 03, 2015, 11:06:40 amYeah it's almost definitely this. If there was a problem on the primary side of the xformer you wouldn't expect all the rails to be exactly 79% of nominal because the dropout voltage is going to be different between all the different supply rails.Quote from: Shinobot2 on October 03, 2015, 11:57:50 pmThat's to be expected. It's all discrete components for that rail's regulator so it's easy to heat up one of the feedback resistors or one of the transistors in the differential pair more than the others and cause drift.

author: coppice
author_rank: Super Contributor
timestamp: October 04, 2015, 05:54:43 am
content: In the 70s it was normal to refer to micro nano and pico farads. You have to go back to the 60s to find a time when nano was not used.

author: SeanB
author_rank: Super Contributor
timestamp: October 04, 2015, 07:17:55 am
content: Looking at the socketed IC's in the power supply, and that 10 lead TO99 can with a HP part number. With the age, that is a RCA 723 regulator, just with a HP part number on the can. Most likely the voltage regulator issue is from a socket that is intermittently going high resistance or noisy. Simply reseat all the IC's and sockets in the power supply should fix it. The Moles cannectors on the tin plated transistor leads also give issues and the solution is to simply remove the socket, put a dab of silicone grease ( easiest to use a cotton bud dipped in it to wipe all in the line as you go) on the top and bottom of the leads and put the socket back. The removal insertion scrapes the leads clean of oxide and the grease keeps them from corroding again with time and thermal cycling.

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2015, 07:37:13 am
content: When I had a 1740 years ago I found several x-reference pdf's, Exel lists and pics that were easy at the time to find online. IIRC all HP devices were able to be X referenced.Of those I have there's at least a 1000 Mb in several files, attached is a teaser:Just yell if you want the rest.

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2015, 07:49:47 am
content: Quote from: SeanB on October 04, 2015, 07:17:55 amCorrect, as listed in the BOM Ch6 p20.

author: tggzzz
author_rank: Super Contributor
timestamp: October 04, 2015, 07:51:31 am
content: Quote from: tautech on October 04, 2015, 07:37:13 amIf there's a crossref for the delay line, I'd be interested. Part number 01740-61611.Any other information about it would also be useful, in case it needs to be replaced.

author: tautech
author_rank: Super Contributor
timestamp: October 04, 2015, 08:07:08 am
content: Quote from: tggzzz on October 04, 2015, 07:51:31 amSphere has one @ $19http://www.sphere.bc.ca/test/hp-parts/hpparts6.htmlLast item on the page.Sphere have a HP page with a 20Mb X- ref file that they give instruction NOT to link:http://www.sphere.bc.ca/test/hpparts.html#catalogScroll down, you'll find it.

author: oldway
author_rank: Super Contributor
timestamp: October 04, 2015, 10:18:43 am
content: This is a very great analog oscilloscope. I worked with it for years and I still have one at home.For the delay line problem, see here:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/HP/Publikacje/HP-Bench-Briefs-1983-01-02.pdf

author: AF6LJ
author_rank: Unknown
timestamp: October 04, 2015, 12:24:36 pm
content: Quote from: coppice on October 04, 2015, 05:54:43 amThat's true here they were pushing nano at the college I went to.

author: tggzzz
author_rank: Super Contributor
timestamp: October 04, 2015, 04:02:28 pm
content: Thanks to PA4TIM for alerting me to the delay line's failure mode.Thanks to oldway for locating the specific Bench Brief describing the issue.Thanks to tautech for the pointer to a replacement delay line.Remarkably - to me - squeezing the delay linehasmagically cured the problem. The risetime is back to slightly under 3.5ns, rather than the anaemic 7ns yesterday.Given that the manual states the delay line is ~100ns long, and it looks like it is only ~2.5m long, I wonder what secret sauce they use to get such a long delay. as in Arthur C Clarke's third law

author: tggzzz
author_rank: Super Contributor
timestamp: October 04, 2015, 04:21:18 pm
content: One annoyance I've had with this scope is an intermittent timebase. Eventually I located the problem: inside the multipole rotary switch on the main timebase assembly A8.After disassembling the switch the problem wasn't, as I expected, with the wipers and gold-plated contacts: they are in pristine condition. Instead the problem was that several PCB tracks leading from the contacts to the rest of the board had been abraded and cut. The cut's location were where the edge of the black plastic rotator presses against the PCB, and hence rubs the PCB track every time it is turned, doh.Solution was simple: solder a single piece of 0.2mm wire across the cut. This lifts the black plastic, but apparently not enough to cause problems with the wipers.

author: nfmax
author_rank: Super Contributor
timestamp: October 04, 2015, 07:20:16 pm
content: Quote from: tggzzz on October 04, 2015, 04:21:18 pmWas that the main or the delayed time base? I have a 1740A where the delayed sweep never runs on any setting, in any mode. This video is making me itch to try and fix it!Mine was a 'rescue scope' when the local university electronics department moved to a new building. The only fault was a fried bridge rectifier in the power supply. Because of the sealed case, they tend to run hot, but the absence of outside air flowing through means they stay beautifully clean.

author: tggzzz
author_rank: Super Contributor
timestamp: October 04, 2015, 07:43:09 pm
content: Quote from: nfmax on October 04, 2015, 07:20:16 pmThe main timebase.I suspect there could be similar problems on your delayed timebase because the construction looks similar. But there are two reasons I wouldn't bet on it, from your description:the delayed timebase is rotated far less often than the main timebase, so there would be less wearI saw problems on some, but not all, ranges - and the pattern of faulty ranges gave a strong indication of where to lookPresuming you have a manual, you could start by looking at some of the voltages with a voltmeter. It may even be possible to use the scope to look at its own internal waveforms, but I haven't considered that in detail.

author: nfmax
author_rank: Super Contributor
timestamp: October 04, 2015, 08:16:49 pm
content: QuoteActually I have three additional working oscilloscopes, (as well as two copies of the 1740A manual, and the user guide). It's theprincipleof the thing, you understand...

author: tggzzz
author_rank: Super Contributor
timestamp: October 04, 2015, 09:19:54 pm
content: Quote from: nfmax on October 04, 2015, 08:16:49 pmYes, Idounderstand

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 04, 2015, 11:14:42 pm
content: Hi,I have seen problems, with this family of scopes, where the connector on the secondary side of the line transformer has overheated and become bad.I suggest checking the connector for discoloration.Regards,Jay_Diddy_B

author: helius
author_rank: Super Contributor
timestamp: October 05, 2015, 12:15:01 am
content: Quote from: John Coloccia on October 03, 2015, 09:40:15 pmHere is a site with fantastic pics of test equipment repair, including this HP scope series. You can see the replacement wooden feet:http://www.amplifier.cd/Test_Equipment/Hewlett_Packard/HP_other/1725A.html

author: SilverSolder
author_rank: Super Contributor
timestamp: October 05, 2015, 12:20:36 am
content: Quote from: EEVblog on October 03, 2015, 10:26:40 amApparently, it is possible to remove that yellowing "nicotine look" - I haven't tried it myself yet, but looks plausible:http://www.exisle.net/mb/index.php?/topic/59657-de-yellowing-plastic-the-stain-isnt-permanent-after-all/

author: edpalmer42
author_rank: Super Contributor
timestamp: October 05, 2015, 02:27:31 am
content: Quote from: SilverSolder on October 05, 2015, 12:20:36 amI don't know who did it first, but the web site for this process ishttp://www.retr0bright.com.Ed

author: Tothwolf
author_rank: Regular Contributor
timestamp: October 05, 2015, 07:24:09 am
content: Quote from: edpalmer42 on October 05, 2015, 02:27:31 amIt was a community project involving quite a number of people from the Vintage Computer community. The follow up to the material at retr0bright.com can be found here:Retr0Bright (or RetroBright) treated plastics re-yellowing even with minimal light exposure?

author: han
author_rank: Frequent Contributor
timestamp: October 05, 2015, 07:56:56 am
content: I haven't read whole thread. Is anybody already suggest watt meter at the whole unit.Is the power dip/spike when the error occur?

author: EEVblog
author_rank: Unknown
timestamp: October 05, 2015, 12:44:40 pm
content: REPAIRED!I bet no one can pick it before I upload the next video...

author: krivx
author_rank: Frequent Contributor
timestamp: October 05, 2015, 12:50:44 pm
content: Quote from: AF6LJ on October 04, 2015, 03:13:24 amAny predictions on failure modes? Apart from electrolytic caps I would think they could last a long time, the LCDs are not as fragile or as consumable as a CRT are they?

author: krivx
author_rank: Frequent Contributor
timestamp: October 05, 2015, 12:56:44 pm
content: Quote from: EEVblog on October 05, 2015, 12:44:40 pmPercussive maintenance

author: Fungus
author_rank: Super Contributor
timestamp: October 05, 2015, 12:58:13 pm
content: Quote from: krivx on October 05, 2015, 12:50:44 pmLower power consumption should mean the caps last quite a while.Plus they don't have all that high voltage circuitry, etc.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 05, 2015, 01:02:22 pm
content: Quote from: EEVblog on October 05, 2015, 12:44:40 pmSo either all suggestions given are wrong or you already uploaded the video

author: EEVblog
author_rank: Unknown
timestamp: October 05, 2015, 01:14:22 pm
content: Quote from: PA0PBZ on October 05, 2015, 01:02:22 pmAll the suggestions are wrong.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 05, 2015, 02:00:07 pm
content: There appeared to be a fault with the channel/trace multiplexing logic - my guess would be either faulty logic chip or decoupling cap eventually taking the supply down - either directly operating on the +15V, or overloading the +5V and causing thermal problems in the PSU. All the supplies are referenced to +15V so once it is compromised they all fail.

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 02:03:41 pm
content: Quote from: krivx on October 05, 2015, 12:50:44 pmFrom my mobile radio experience LCD displays degrade in the sunlight. While interior lighting is less destructive florescent light does contain some UV. Plastics age with heat nad time, and this is an issue with all electronics, again drawing from my two way radio experience flex cables will de-laminate over time. Some of the newer flex cable designs are better than the ones made even a decade ago they still are made of plastics and will fail sooner than other connection means.pots can be cleaned and repaired, some rotery encoders like the ones on some of my amateur radio gear are repairable others not so much, and that may not be a big deal in the decades to come, they could actually outlast pots.As far as rotary switches go, HP and others have done a great deal to eliminate the conventional rotary switches which require some degree of skill to clean, just spraying crap on one of those old pheonlic rotary switches will do more damage over the long term, when they get wet they swell up, the rivets holding the contacts loosen up and you have intermittent contact. You can see what HP has done in this scope to make rotery switches more reliable.Over the decades silicon has replaced a lot of discreet components including switches and pots, this is not necessarily a bad thing, but it has its drawbacks. Fifty years ago the mainstay scopes were scopes like the HP-170 and the Tektronix-545. If you have one of those today, you can find replacement parts for just about everything in that scope without robbing parts from another 170 or 545. Each successive generation that fraction of specialized parts has grown The scope mentioned in this thread is on a par with a Tek-465. You can argue til the cows come home how reliable those custome chips are used in both scopes but when one dies, you cannot go to Mouser and buy one, you have to rob it from another 1740 or 465. Sadly this trend toward custom chips is all the rage in test equipment, add to that all that firmware. How do you back up that firmware that runs a modern digital scope? We haven't even touched on the subject of calibration, if the scope in question relies on an external means to access "soft pots" in order to make adjustments the failure of that must also be considered, when was the last time you saw a screwdriver, or a hex head alignment tool fail?We have test equipment with enormous power at our disposal now, the new stuff is truly great in terms of preformance and in some cases makes gear of a generation ago look like junk. Consider the full price that goes with having that gear, how many here have a Rigol service manual for their scope?Sadly our whole civilization is going this way Figuratively speaking all our gear will end up in a landfill (or recycled as raw materials) sooner or later. The trend in consumer electronics is that amount of time before end of life is shorter and shorter, in many cases it is due to fashion trends, in others it is because that TV or cell phone uses a chip that someone once made and no longer does cost of repair being factored out; the trend in how we design gear of all types is toward less repairability while the level o9f reliability remains fairly constant.That ends with a shortened service life for the item in question.

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 02:04:33 pm
content: Quote from: EEVblog on October 05, 2015, 01:14:22 pmI for one am looking forward to seeing what you found.

author: MarkL
author_rank: Unknown
timestamp: October 05, 2015, 02:10:04 pm
content: The voltages on the twitter photo were all down by roughly the same percentage; there wasn't one that was extremely out of line with the rest. I would start looking for something in common to all the rails, like on the primary side.Intermittent candidates: 120/240V selector switch, power switch, connector, bad solder joint.

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 02:12:12 pm
content: Quote from: MarkL on October 05, 2015, 02:10:04 pmThat's the easy part and the part that must be taken care of before anything else.I am of the mind there are two problems, but with so little data it is hard to say.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 05, 2015, 02:13:38 pm
content: I've seen this posted before but I find it very unlikely that all secondary voltages will drop the same percentage if you lower the primary voltage. I fail to see that if 4 different regulators are going out of regulation they will drop the same %.

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 02:33:51 pm
content: Quote from: PA0PBZ on October 05, 2015, 02:13:38 pmI have seen the actual symptom before and repaired it.Yes it can be something as simple as a primary side fault, especially if the voltage sources are referenced to a single source that has a higher drop out voltage than the others. Then they all will track the reference voltage.The bad news is; that may not be all that is wrong with this scope.

author: MarkL
author_rank: Unknown
timestamp: October 05, 2015, 02:38:25 pm
content: Quote from: PA0PBZ on October 05, 2015, 02:13:38 pmThe unregulated inputs to the regulators would track the primary voltage. Once you were below drop-out voltage for the regulator, they would track eachother.I would look at the primary voltage input to the transformer as the next step. Or some of the unregulated rails. But we don't have that data.If not, it could also be something with the +15V supply, since the other voltages are referenced to that voltage. Looking more closely, they're all almost exactly 20% down.

author: han
author_rank: Frequent Contributor
timestamp: October 05, 2015, 02:53:06 pm
content: let me guess, bad main switch contact?

author: AF6LJ
author_rank: Unknown
timestamp: October 05, 2015, 03:15:13 pm
content: Quote from: han on October 05, 2015, 02:53:06 pmMaybe....Something low experience techs would overlook in search of a vastly more complex fault.

author: Len
author_rank: Frequent Contributor
timestamp: October 05, 2015, 04:15:55 pm
content: Spider carcass in the power supply causing an intermittent short.

author: bktemp
author_rank: Super Contributor
timestamp: October 05, 2015, 04:23:14 pm
content: Quote from: han on October 05, 2015, 02:53:06 pmIt sounds obvious because all rails drop at the same time, but I think it is unlikely: The voltage drop and therefore the power dissipation would be high enough to melt or burn all the plastic around the contacts. Dave would have smelled that.My guess is something on the 15V rail draws too much current.

author: nfmax
author_rank: Super Contributor
timestamp: October 05, 2015, 06:03:39 pm
content: Well the obvious thing that might make the 15V rail go wrong would be A16R26 (and or the other resistors in the chain, A16R25 & A16R27). Or it might be bad contacts on the connections to Q4, which is a connector, not soldered.

author: Bickenheuser
author_rank: Newbie
timestamp: October 05, 2015, 08:20:22 pm
content: Dave,Sometime ago I had a HP 3457a that would take off and do very weird stuff, error messages. All of the power supplies dropping about 50%. Sometimes a light smell of electrical burning. Turned out to be the 120/220 AC line switch . The 3457a is in very good nic , must have sat somewhere for most of its life.I have never seen contacts so black from corrosion. If You get back to that scope, Give it a shot. Also that scope is known for bad board to board connections.

author: tautech
author_rank: Super Contributor
timestamp: October 06, 2015, 12:15:09 am
content: Link to the repair:https://www.eevblog.com/forum/blog/eevblog-804-hp1740a-oscilloscope-repair-part-2/

author: drussell
author_rank: Super Contributor
timestamp: October 06, 2015, 02:13:10 am
content: Quote from: MarkL on October 05, 2015, 02:10:04 pmYou're forgetting that in these kinds of equipment the regulators are typically all referenced off of one reference supply. The reason they drop in unison is not because the voltage coming in drops and all of the outputs track it, it is because the voltage they (the regulators) are referenced to drops. Thiscouldbe due to a supply side drop but it would have to be affecting the master regulator before any of the others in order fot them all to drop in unison....Quote from: PA0PBZ on October 05, 2015, 02:13:38 pmYes...

author: tautech
author_rank: Super Contributor
timestamp: October 06, 2015, 02:18:14 am
content: Quote from: drussell on October 06, 2015, 02:13:10 amThat's what many of us suspected, but it wasn't the case.Watch Pt 2

author: MarkL
author_rank: Unknown
timestamp: October 06, 2015, 11:33:58 am
content: Quote from: drussell on October 06, 2015, 02:13:10 amNot necessarily. True that if the master drops they all drop, but another cause could be that they all reached their drop-out voltage at the approximately the same primary voltage. Or, consider that if the other regulators reached their drop-out before the master, they're going to track the input voltage. Then it doesn't matter what the master is doing at that point and it could drop out on its own and then it too would also track the input voltage.And besides, I didn't forget about a reference supply. I said it here:https://www.eevblog.com/forum/blog/eevblog-803-hp1740a-analog-oscilloscope/msg770825/#msg770825

author: EEVblog
author_rank: Unknown
timestamp: October 07, 2015, 03:47:59 am
content: Someone tracked down the guy who tested and signed off on that scope!Quote

author: WN1X
author_rank: Unknown
timestamp: October 07, 2015, 12:37:56 pm
content: Very cool!

author: ale500
author_rank: Frequent Contributor
timestamp: October 10, 2015, 03:48:51 pm
content: I showed my wife the part where Dave shows the manual and repeated Dave's comment on how that was p0rn for electronics people... she looked at me kind of funny

author: AF6LJ
author_rank: Unknown
timestamp: October 10, 2015, 04:14:14 pm
content: Quote from: ale500 on October 10, 2015, 03:48:51 pmSome women don't get it....Then....Some of us Do.