HP 1740A power supply burn up - Page 1
Forum section: electronics / testgear

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 02:11:54 pm
content: Just nabbed myself a fairly clean looking HP 1740A off ebay. Unfortunately it was dead. Quick poke and prod session showed that the -15V and 8V rails were off.Pulled off pass transistor for that rail and fired it up and the thing sprang into action. Seems ok, at least after the power supply!Investigating the power supply lead to an interesting discovery. Transformer secondary connector appears to have melted and taken half the board with it.This is what I discovered..Decided to pull the connector out and it fell to bits in my hand unfortunately.After desoldering the burned connector and rectifiers I am left with this:I've just tested the bridge rectifiers and caps and they are all fine. This suggests that it was the connector or the soldering on the board that caused this.Anyone have any repair tips for this one? I did professional rework years ago and this was "nah chuck it" territory.I am considering:1. Clean and grind out any charring.2. New connectors on both ends.3. Lift any burned up traces and jump with thick copper wire.4. Apply copious amount of Kapton tape.

author: CDaniel
author_rank: Frequent Contributor
timestamp: August 23, 2019, 02:32:57 pm
content: The cause of this failures is usually the connector itself , not good enough to sustain that current for the life of the equipment . It develops contact resistance that produce heat , then more contact resistance until is cooked and burned ... Or insulation failure between pins . Maybe the rest is good , a short should have blown a fuse , not the connector .

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 02:34:55 pm
content: Yeah transformer is good and secondary comes up. Agree it's the connector that burned up. Looks like a Molex KK 394. Likely to replace it with something else.Sooooo off to find some 3.94mm pitch connectors that aren't shiteEdit: have checked for conduction between traces and we're good there. Looks like it is only the surface that is carbonised and the plated holes gone. Shouldn't be too painful to repair actually even though it looks like crap now.

author: BU508A
author_rank: Super Contributor
timestamp: August 23, 2019, 02:51:12 pm
content: I don't like these Molex connectors.What I would do:- cutting a drill board (Vero board) so it has enough space for this:A Metz AKL 310-12 terminal spacinghttps://www.reichelt.de/12-pin-connection-terminal-5-0-mm-spacing-akl-310-12-p153958.html?and two of these spacers:https://www.reichelt.de/spacers-metal-6-edge-m3-12-mm-vt-da-12mm-p231459.html?- solder the terminal on the veroboard.- drill two 3mm holes in the PCB for the spacers- solder wires from the PCB to the veroboard- fix the veroboard to the PCB- attach the wires to the connector of the connection terminal- have fun

author: Specmaster
author_rank: Super Contributor
timestamp: August 23, 2019, 02:58:02 pm
content: Quote from: BU508A on August 23, 2019, 02:51:12 pmYep that sounds like a good plan.Sent from my POT-LX1 using Tapatalk

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 02:58:20 pm
content: I don't like Molex connectors now either. Bloody things. The usual suspects used to burn up in PCs as well years ago.Good idea but alas I've opted to buy 5 pairs of 6-way Molex ones from a UK supplier who does packs of the crimps, the headers and the shells for now as it appears you can only get alternative connectors in packs of 500 million parts etc or delivery nearly as much as I paid for the scope

author: Specmaster
author_rank: Super Contributor
timestamp: August 23, 2019, 03:02:32 pm
content: Quote from: bd139 on August 23, 2019, 02:11:54 pmTotally disagree, it's not "nah chuck it territory", you could always solder the secondary direct to the power board if you can't find suitable connectors. You rework guys, I don't know Sent from my POT-LX1 using Tapatalk

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 03:05:08 pm
content: If someone else was paying the bill it would be chuck it. This one is for personal interest though as I'm sure you know

author: ogden
author_rank: Super Contributor
timestamp: August 23, 2019, 03:10:21 pm
content: Quote from: BU508A on August 23, 2019, 02:51:12 pmIMHO way too complex. I would simply introduce short pigtail for cable-cable connector (2x6).

author: james_s
author_rank: Super Contributor
timestamp: August 23, 2019, 03:15:03 pm
content: That looks like a Molex KK, the same problem is very common in pinball machines. There are high current versions available that hold up much better, any number of sites that sell pinball parts will list recommended types.

author: BU508A
author_rank: Super Contributor
timestamp: August 23, 2019, 03:19:12 pm
content: Quote from: ogden on August 23, 2019, 03:10:21 pmHmm, not sure what you mean.Can you show me an example? Just curious...

author: ogden
author_rank: Super Contributor
timestamp: August 23, 2019, 03:28:06 pm
content: Quote from: BU508A on August 23, 2019, 03:19:12 pmHopefully pigtail does not need illustration. Connector could be automotive or similarhttps://www.te.com/usa-en/product-794198-1.htmlhttps://www.te.com/usa-en/product-794199-1.html

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 03:41:54 pm
content: Ok looks like this isn't the connectors' fault. Looks like cascade failure. Going to bring the entire supply up on a pile of bench supplies and see where I get to.

author: Specmaster
author_rank: Super Contributor
timestamp: August 23, 2019, 06:00:42 pm
content: Quote from: bd139 on August 23, 2019, 03:05:08 pmYep I know OK, but I'd still repair it if it was a repair for someone else, the repair it self need not that expensive. If push came to shove, I'd get some single header pins, solder to power board and then sold the secondary wires to the headers directly. Sent from my POT-LX1 using Tapatalk

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 06:13:29 pm
content: Makes sense.Ok further debugging. Got the board up and 15V rail up on bench supply. Now there is zero regulation on the 5V rail which suggests that is knackered or something is cocking it. Further debugging continuing.Found a source of TO100 LM723s which doesn’t hurt too bad as well. I have some DIP ones but that’s fugly hack territory

author: Specmaster
author_rank: Super Contributor
timestamp: August 23, 2019, 07:01:28 pm
content: I would say that the scope has been left switched on for extended time, possibly been in an environment where the owner switched everything on in the morning and off at the end of the day, regardless of if the kit was going to be used or notTotal waste of power and not to mention premature failure of the kit self.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 07:06:07 pm
content: Yeah could be right there!Anyway debug stuff...Ok managed to step test the power supply. With the board out, I rammed MJE3055T transistors in the sockets as that's all I have around with the same pinout (no heatsink required here as no load!). 15V supply brought up - no problems at all. 5V came up, hmm not good - rammed at the max voltage on the rail. transistor in saturation. Measured voltages around the LM723 on that rail and it's FUBAR. Desoldered it carefully and chucked another supply on the 5V as a reference and brought the (floating) -15V input supply up. So the source of the problem appears to be the LM723 on the 5V rail was toast. Checked pass transistor in the chassis and that is good so it's possible that something murdered the LM723.HP power supplies debugging HP power supply...Found some terrible dry joins on the connectors so have cleaned and resoldered these.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 23, 2019, 07:17:34 pm
content: Quote from: bd139 on August 23, 2019, 07:06:07 pmThat just looks terrible! Also, the caps look like they lost their underwear but that could be a trick in the picture?

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 07:51:37 pm
content: Caps are fine up close - bad angle makes them look nasty.

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2019, 08:04:54 pm
content: Quote from: PA0PBZ on August 23, 2019, 07:17:34 pmCommon in HP1740's.The connector series used, their long pins, tension on the interconnects, thermal cycling etc cracks many of the pins.Good idea to reflow all of them in the scope.Sometimes you find the original solder fillets a bit on the skinny side and they need to be bulked up for physical strength.These post cracks are generally most prevalent around the interconnect PCB.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 08:09:29 pm
content: Yep have reflowed them all now. Disappointing for sure. Project shelved for a few days now as it's a bank holiday weekend and stuff isn't going to make it here until next week. Awaiting:10x bridge rectifiers.1x LM723H.5x 6-pin Molex connectors (shells, headers and pins)

author: tautech
author_rank: Super Contributor
timestamp: August 23, 2019, 08:23:38 pm
content: If you think some gorilla has been in this scope before you take note of how the timebase boards, timebase shaft and shaft lock rings have been installed as it affects the timebase operation and the tensions placed on the selectors which can impact on the wear and life of the contacts.It's not hard, just keep your eyes open for anything that seems misaligned.

author: BU508A
author_rank: Super Contributor
timestamp: August 23, 2019, 08:33:40 pm
content: Quote from: bd139 on August 23, 2019, 08:09:29 pmOnly one LM723H? What, if it explod^W let out the magic smoke again for some reason?

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 08:59:54 pm
content: I'll just have to wait another 3 daysI'm a bit annoyed because I had a box of the damn things a few years back. Said to myself: "hey I've never used these, I'll chuck them on ebay". Went for a small fortune which I wasted I'm sure. Should have nicked a couple out of the box first.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 09:13:13 pm
content: Quote from: tautech on August 23, 2019, 08:23:38 pmSurveying the rest of the instrument suggests it hasn't been used that much and is in perfect condition. There has been another repair on the power supply by the looks evidenced by a lot of flux that shouldn't be there. This might explain why the flood gun illumination isn't working. Will debug that one once I've got the smoke to stay in this part of the power supply

author: factory
author_rank: Super Contributor
timestamp: August 23, 2019, 09:23:48 pm
content: I hate that style of connector too, one product at work used lots of them with the double wammy of using IDC plugs, the solder joints got dry or the wires loose from thermal cycling/vehicle vibration over 15 to 20 years of use.It was always the connections/wires that passed the most current that got burnt up, the boards were unobtainable so we repaired them, by cleaning all traces of charred PCB, fitting new connectors, used slow set araldite and grafted in new PCB material where needed. The PCB traces were replaced by using appropriate size tinned copper wire.The connectors were ITW Pancon Mas-con 3.96 pitch.David

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 09:37:35 pm
content: Oh nice. I hate IDC connectors even more. I noticed a lot of them on the market when looking for replacements here.Plan here is to do pretty much the same. I've ground out all the charring now and there's no sign of conduction at all. About 50% of the pads are salvageable so I'll use that to anchor the connectors then epoxy, then mask any existing traces with kapton, then copper wire jump what's left.I just tested the pulled LM723 and it's short between Vc and Vout. Totally boinked.

author: bitseeker
author_rank: Super Contributor
timestamp: August 23, 2019, 09:47:53 pm
content: Glad it wasn't as bad as it first looked. It'll be great seeing it up and running again.

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 09:54:42 pm
content: Yeah fingers crossed for this. It "sort of" worked to start with, minus a -15V rail, jumped here with my 6284A...Only saving this because the rest of it looks like it works and it's immaculate otherwise. Starting to wish I hadn't walked past that one last year for a fiver.Edit: just found another problem. The return spring on the power switch is gone. I'm going to shake it out as if that's loose it can cause damage. Wonder if it started this carnage?

author: Specmaster
author_rank: Super Contributor
timestamp: August 23, 2019, 10:30:32 pm
content: Quote from: bd139 on August 23, 2019, 09:13:13 pmThe flood gun illumination works best after its warmed up, I have have never seen one that worked brilliantly right from the moment you power up, as long as it works after the scope reached operating temp then I'd not worry about it. AS to the poor solder joints on the interconnection pins, this is what I was saying in the normal TEA thread is common with these scopes because there is always going to be a certain amount of flexing on the boards as the unit is used due to thermal movement and also on the boards with rotary switches in particular there is lot of flexing during rotation of the switches themselves.As to what caused the regulator to be toasted, it could be possibly transients set up from the arcing that has evidently been going on to have burnt the power connector in such bad fashion?

author: BU508A
author_rank: Super Contributor
timestamp: August 23, 2019, 10:54:59 pm
content: Quote from: bd139 on August 23, 2019, 09:37:35 pmA few weeks ago I have bought ca. 4kg parts which are declared NOS coming from R&S.There was also this one:If you want/need it, let me know.And I can give you some UA709AHM as well, Have here enough for a lifetime

author: bd139
author_rank: Super Contributor
timestamp: August 23, 2019, 11:10:32 pm
content: Thanks for the offer. I’ll let you know when I blow up the one I just ordered

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 05:30:03 pm
content: Ok finally got all the parts for this and got an hour on it after work carnage. The LM723 was replaced, power supply connector replaced (both ends), power switch replaced, reworked the board, replaced three of the bridge rectifiers as a precaution.So first the rework... epoxy and copper bus wireNew Molex connectors and bridge rectifiers:Nwe Molex connectors on the transformer:After this the thing was powered up and the rails were all good.So I plugged the interface board back on and the rails bombed pretty quickly which is not good. Visual inspection performed next and I found this bastard hiding which looks suspiciously like a possible culprit for this whole situation...

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 06:11:42 pm
content: Further diagnostics now I've eaten:1. Wasn't that skanky cap. Despite the appearance that's fine and is part of the timebase.2. Rails fine and HT comes up until I plug the interface board on then they dump3. No obvious shorts on either board.4. Connecting the interface board only to the horizontal assembly, issue occurs.5. Connecting the interface board only to the vertical assembly, issue occurs.Suspecting the power supply current limiting on the 15V rail is wonky. Off to experiment!

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 06:35:39 pm
content: None of the rails are limiting ffs

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 06:54:54 pm
content: Something not good happening. If I fire it up under load I can hear arcing. Suspect transformer secondary is duffEdit: confirmed that's the switch contacts due to the startup current surgeCurrent situation: if either the vertical or horizontal boards are attached, the power supply bombs out. Putting it away for the night now

author: bitseeker
author_rank: Super Contributor
timestamp: September 04, 2019, 08:52:37 pm
content: Oh, the suspense! The fixes thus far look good, though.

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 08:56:54 pm
content: Ok some further diagnostics:1. collector voltage on pass transistor on 15V rail is fine. Tested by pulling and checking beta and substituting with a BD139 temporarily.2. Emitter voltage is too low3. Base voltage is too low.4. Base current is too low peaking at around 1.4mA max5. Reference voltage divider is correct.6. Reference output is correct on LM723.This feels like the LM723H has a weak output transistor or is wonky and can't kick out enough current. Ordered another one and will replace and see what happens. This will take a week to arrive...Edit: going to confirm rest of power supply functional by pulling the pass transistor off and jumping the 15V supply with a bench supply.

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 08:57:43 pm
content: Quote from: bitseeker on September 04, 2019, 08:52:37 pmUgh I hope it ends up working or all my efforts are in vain

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 09:12:18 pm
content: Wuhahahah progress. Jumped the 15V with a power supply and it's coming up fine. This is almost certainly confirmed as a weak LM723H. There's literally nothing else to go wrong in that circuit. Ordered another one ... only £3.09 fortunately rather than the £12 each on FarnellSome pictures. 50MHz sine with flood gun going too. I'd forgotten how nice the displays on these are!And the mess I made fixing it so far

author: Specmaster
author_rank: Super Contributor
timestamp: September 04, 2019, 09:15:58 pm
content: That is excellent news then and that flood gun is far better than mine ever was, maybe my regulator was a little weakEdit, that 34401A looks lovely on the top shelf.

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 09:21:47 pm
content: I had to adjust this one until the flood gun worked properly. Also as you suggested elsewhere it takes a while to warm up.Extremely happy with the 34401A. Definitely deserving of the reputation they have.Turning into an HP fan boy. Got some more of their stuff on the watch listNeed to find some feet for this too - it's footless as always

author: Specmaster
author_rank: Super Contributor
timestamp: September 04, 2019, 09:33:39 pm
content: Feet for what the 1740A or the 34401A? If its the former, you can use an extra deep rubber door stop x 4 and screw then onto the metal back plate in the corners, there is just enough space there for them, something a bit like this but you may need a small packing piece beneath them, if I recall correctly.https://www.ebay.co.uk/itm/White-Rubber-Door-Stop-Stops-Stopper-Jam-Wedge-33mm-1-1-4-PACK-OF-4/161255611194?

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 09:40:57 pm
content: Yeah for the 1740A. That'll do the job!

author: tautech
author_rank: Super Contributor
timestamp: September 04, 2019, 09:52:52 pm
content: Quote from: bd139 on September 04, 2019, 09:40:57 pmBeanflying has a good 3DP model for 1740 feet.

author: Brumby
author_rank: Unknown
timestamp: September 04, 2019, 10:12:04 pm
content: Congratulations on the progress!Quote from: bd139 on September 04, 2019, 09:12:18 pmWhat mess?

author: bd139
author_rank: Super Contributor
timestamp: September 04, 2019, 10:15:17 pm
content: Quote from: tautech on September 04, 2019, 09:52:52 pmExcellent. I went digging based on that comment. Source:https://www.eevblog.com/forum/testgear/replacement-knobs-feet-and-fittings-for-test-equipment/msg2267919/#msg2267919... will get some local quotes once I've fixed the power supply.Quote from: Brumby on September 04, 2019, 10:12:04 pmActually you can't see the pile of shite because the scope is huge and in the wayMeant to say earlier, the return spring for the old power switch fell out of the chassis today. Get the feeling I may have been right about that causing this issue.

author: tautech
author_rank: Super Contributor
timestamp: September 04, 2019, 11:09:38 pm
content: Quote from: bd139 on September 04, 2019, 10:15:17 pmYep, that's them.Bean sent me some and they look shit loads tougher than originals.Bean did a vid on destruction tests:https://www.eevblog.com/forum/testgear/replacement-knobs-feet-and-fittings-for-test-equipment/msg2269716/#msg2269716

author: Specmaster
author_rank: Super Contributor
timestamp: September 05, 2019, 12:50:52 am
content: Quote from: bd139 on September 04, 2019, 10:15:17 pmYep, the spring was more than likely the culprit, little bugger. I'd be inclined to double check the dimensions of the feet, especially the length of them as some of the 1740A scopes I think slightly different CRT cover caps fitted and you don't to have it stood on the cover do you.

author: tautech
author_rank: Super Contributor
timestamp: September 05, 2019, 02:12:35 am
content: Quote from: Specmaster on September 05, 2019, 12:50:52 amIIRC we found some mechanical drawings of the HP1740 scope and deduced 35mm was long enough.Then nfmax jumped in the real measurements ofsurvivingfeet:https://www.eevblog.com/forum/testgear/replacement-knobs-feet-and-fittings-for-test-equipment/msg2375853/#msg2375853

author: jklasdf
author_rank: Regular Contributor
timestamp: September 09, 2019, 04:57:34 am
content: Quote from: bd139 on September 04, 2019, 09:12:18 pmThe displays on HP's old analog scopes really are quite nice. It's apparently because the CRTs don't use a scan expansion mesh, like a lot of Tek scopes of the same/slightly later era.

author: bd139
author_rank: Super Contributor
timestamp: September 09, 2019, 06:32:11 am
content: Agreed. I’ve had a fair few teks and the HP CRTs are much better for sure.

author: bd139
author_rank: Super Contributor
timestamp: September 12, 2019, 11:46:06 am
content: Looking good now. Last new LM723 arrived so I chucked it in. Fortunately this one is socketed so it didn't mean removing the power supply again and was really easy.So firstly the new LM723 regulators installed. Fortunately one of them was still good:The repaired and re-connectored power supply. 6-way connectors were cheaper so I just got them. Also made for easier debugging which was handy:Fired it up and voltages were slightly too high so I trimmed them down again to 15.000V and all the rails are now spot on.Anyway first time this is running under its own power supply:Seems to work ok. The timebase needs a bit of a squirt of contact cleaner however. Sort of expected with the ganky switches in these scopes.Next steps are:1. I'm going to add a small heatsink made out of a bit of bent alu sheet to the 15V rectifier as it runs very hot. This should stop it taking the rest of the damn thing out.2. Clean timebase out and calibrate it.3. Feeeet. beanflying's 1740A feet will be printed by a friend of mine at some point in the next couple of weeks. He's got a relatively decent printer and is doing it for free if I set up his WiFi extender

author: bd139
author_rank: Super Contributor
timestamp: September 12, 2019, 12:31:38 pm
content: Heatsink done. As mentioned elsewhere I have virtually no metalwork tools so this is not prettyBingo - looking alive again:

author: bitseeker
author_rank: Super Contributor
timestamp: September 13, 2019, 01:56:57 am
content: Woot! Looks great, bd. Congrats.

author: jklasdf
author_rank: Regular Contributor
timestamp: September 13, 2019, 04:39:39 am
content: Looking good! The timebase switch is probably the worst part about these scopes, as great as the display is.

author: bd139
author_rank: Super Contributor
timestamp: September 13, 2019, 06:47:09 am
content: Yeah the timebase switch on this isn’t totally healthy yet. Need to do some work on it. The delay sweep doesn’t turn off properly and the rotation is gappy.

author: tggzzz
author_rank: Super Contributor
timestamp: September 13, 2019, 06:54:11 am
content: On my 1740A the PCB tracks under the timebase switch were abraded by the plastic part of the switch. The parts contacted by the switch fingers were fine.I soldered a thin wire across the gap, ensuring no solder flowed onto the part contacted by the switch's fingers.

author: bd139
author_rank: Super Contributor
timestamp: September 13, 2019, 07:05:07 am
content: I shall bear that in mind. I suspect something similar is afoot. Thanks for the heads up

author: bd139
author_rank: Super Contributor
timestamp: September 14, 2019, 12:03:08 pm
content: Ok so I managed to get inside the failed LM723 and found that one of the bond wires had melted. This happens to be the internal pass transistor's collector so it looks like it went over-current at some point, probably when the connectors melted and the bridge went short.Photo inside LM723 showing the melted bond wire. You can see the blob on it.Blown up bit on datasheet:On a positive note, the reference part of it still works so I'm going to attempt to put its hat back on and play around with it and my 34401AAlso a hack. This could be repaired by changing the pass transistor to outboard. Snip off the collector and emitter pins and use a 2n2222 or something with base connected to the frequency compensation pin.