HP 1740A power supply burn up

[factory]

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

[bd139]

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.

[bitseeker]

Glad it wasn't as bad as it first looked. It'll be great seeing it up and running again.

[bd139]

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?

[Specmaster]

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. 

Surveying 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

The 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?

[BU508A]

I just tested the pulled LM723 and it's short between Vc and Vout. Totally boinked.

A 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 

[bd139]

Thanks for the offer  .  I’ll let you know when I blow up the one I just ordered 

[bd139]

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 wire



New 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...

[bd139]

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 dump
3. 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!

[bd139]

None of the rails are limiting ffs

[bd139]

Something not good happening. If I fire it up under load I can hear arcing. Suspect transformer secondary is duff

Edit: confirmed that's the switch contacts due to the startup current surge 

Current situation: if either the vertical or horizontal boards are attached, the power supply bombs out. Putting it away for the night now

[bitseeker]

Oh, the suspense! The fixes thus far look good, though.

[bd139]

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 low
3. Base voltage is too low.
4. Base current is too low peaking at around 1.4mA max
5. 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.

[bd139]

Oh, the suspense! The fixes thus far look good, though.

Ugh I hope it ends up working or all my efforts are in vain

[bd139]

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 Farnell 

Some 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 

[Specmaster]

That is excellent news then and that flood gun is far better than mine ever was, maybe my regulator was a little weak 

Edit, that 34401A looks lovely on the top shelf.

[bd139]

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 list 

Need to find some feet for this too - it's footless as always

[Specmaster]

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?

[bd139]

Yeah for the 1740A. That'll do the job!

[tautech]

Yeah for the 1740A. That'll do the job!

Beanflying has a good 3DP model for 1740 feet.

[Brumby]

Congratulations on the progress!

And the mess I made fixing it so far 

What mess?

[bd139]

Yeah for the 1740A. That'll do the job!

Beanflying has a good 3DP model for 1740 feet.

Excellent. 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.

What mess?

Actually you can't see the pile of shite because the scope is huge and in the way 

Meant 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.

[tautech]

Yeah for the 1740A. That'll do the job!

Beanflying has a good 3DP model for 1740 feet.

Excellent. 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.

Yep, 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

[Specmaster]

Yeah for the 1740A. That'll do the job!

Beanflying has a good 3DP model for 1740 feet.

Excellent. 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.

What mess?

Actually you can't see the pile of shite because the scope is huge and in the way 

Meant 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.

Yep, 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.

[tautech]

Yeah for the 1740A. That'll do the job!

Beanflying has a good 3DP model for 1740 feet.

Excellent. 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.

What mess?

Actually you can't see the pile of shite because the scope is huge and in the way 

Meant 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.

Yep, 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.

IIRC we found some mechanical drawings of the HP1740 scope and deduced 35mm was long enough.
Then nfmax jumped in the real measurements of surviving feet: 
https://www.eevblog.com/forum/testgear/replacement-knobs-feet-and-fittings-for-test-equipment/msg2375853/#msg2375853