HP 1740A on the bench, help please?

[bd139]

It's a trade off really. £30+fuel for parts stock worst case. £30+fuel for working scope with trivial problem best case. Usually it lands somewhere in the middle.

The timebase control looks in relatively good condition however. You can tell a munted one because the first thing to break is the vertical channel knobs. I've seen them damaged on a lot of HP scopes I didn't bother bidding on. I can't see any bent knobs in the picture. In fact it looks like all the knob caps are even present which is a rarity for HP kit.

The tape holding it together looks like someone lost the screws which means it may have had it's innards mucked around with.

It's probably cheapest way to get spares though. It's literally a 50/50.

If that was a Tek 475 I'd probably buy it for the spares.

[tggzzz]

It's a trade off really. £30+fuel for parts stock worst case. £30+fuel for working scope with trivial problem best case. Usually it lands somewhere in the middle.

The timebase control looks in relatively good condition however. You can tell a munted one because the first thing to break is the vertical channel knobs. I've seen them damaged on a lot of HP scopes I didn't bother bidding on. I can't see any bent knobs in the picture. In fact it looks like all the knob caps are even present which is a rarity for HP kit.

The tape holding it together looks like someone lost the screws which means it may have had it's innards mucked around with.

It's probably cheapest way to get spares though. It's literally a 50/50.

If that was a Tek 475 I'd probably buy it for the spares.

All sensible, but be aware that at the end you might still have one almost-working 1740 (plus a working 1740). Only the TEA/GAS-afflicted purchaser can define what they might or night not do thereafter

[bd139]

Well that's obvious, you buy another one! 

[Specmaster]

Yep, I could end up with another working scope, minus the timebase knobs and shafts, because if they are good, they will be transplanted into this one here. Then it would be a case of buy another one to bring the second one into play and so it begins again, the circle of life. 

[Specmaster]

Just what the hell is up with these scopes, it seems almost everyone you come across that has the stupid little allen key grub screws on the knobs, that once you disturb them and take them off, they never ever bite back on the shaft correctly, especially the ones that a series of switches on them like the timebase for instance ARRGHHH 

[bd139]

They're a pain in the arse aren't they.

One thing that relieve the pain I found is one of these: http://uk.rs-online.com/web/p/screwdrivers/8759369/

Yes it's completely the wrong tool for the job but it works much better than the prescribed ones! 

[Specmaster]

They will fit into these allen key headed screws ok and you can make them bite onto the shaft again with that can you?

[bd139]

That’s correct. They bite on better than the Allen keys do as well. If the slot is stripped in the grub then you’ve got a battle on either way. I’ve got a few stripped ones out with these with some difficulty.

[Specmaster]

I reckon that I was spot on about that scope being 120V configured because I see that the seller has changed his wording
to read as
"Tested earlier in the day and it turned on, showing line, however when tested just now, would not turn on. Also slightly taped at side as case has seen better days!
Comes with 2 pin (US) power cable," notice the change from 2 pin Euro to 2 pin (US)?

If that is true, just how much internal damage has been done do you think. would any of the circuit boards and electronics survive being served twice their normal voltage? I suppose thinking about it, the damage might be contained to the voltage regulators and possibly the transformer?

At least he did change the warranty to include a 14 day refund in case I did discover anything that I was not expecting? 

Whatever he has done with it, it can't have damaged the parts that I'm currently primarily interested in   

 

[bd139]

Well two possibilities:

1. He's full of shit, didn't check the line voltage selector before he plugged it in. Probably killed it dead instantly. I doubt it showed a line; that's just fishing for a sale. In this circumstance, I've had a look at the power supply schematic. That probably took out the pass transistors at the very least and popped the fuse. If you're really lucky it didn't toast the transformer before it popped. The deadly bit is the 15v unregulated line. That floats around 20v. I doubt anything on it would take 40v. If the pass transistors went short then everything on the rail is toast. Probably still good as a parts mule however. If you want the mechanical parts and some of the components you're probably good as long as you pull and test them first.

2. There is also the possibility that he has no idea what he's talking about and it's a schuko plug and the fuse just popped because one of the PSU caps was duff. US mains cables don't just appear here in the UK randomly very often.

Best bet is get it home, check the line selector, fuse state, power supply board for physical damage, plug it in and see what happens.

If it's been blown up then raise a claim for £20 back. He's a new seller so won't want to shaft a basic rating.

[Specmaster]

Thats what I thought, assuming it has a fuse fitted and not been bypassed, that best case is its blown the fuse fast enough to prevent anything else occurring and the 15v unregulated supply just feeds the HV oscillator so any damage there will be contained to the HV anyway.

[tggzzz]

Almost forgot...

You'll get your HP1740 so that it is working, but the risetime is too slow on both channels - say 10ns whereas it should be 3.5ns. Do not start fiddling with pots, caps etc.

Instead disconnect it from the mains, wait for things to discharge, stick you hand right into the middle from several directions, and gently massage and fondle the delay line. Seriously.

Remove hand, then look at the risetime

[Specmaster]

Thanks for the tip, but currently rise time looks ok, but sometimes I can see the fly back line faintly. Still have that resistor to replace and calibrate as the vertical is out on some ranges, others there is offset to bring back into line and I think the timebase needs tweaking as well.

[oldway]

Almost forgot...

You'll get your HP1740 so that it is working, but the risetime is too slow on both channels - say 10ns whereas it should be 3.5ns. Do not start fiddling with pots, caps etc.

Instead disconnect it from the mains, wait for things to discharge, stick you hand right into the middle from several directions, and gently massage and fondle the delay line. Seriously.

Remove hand, then look at the risetime

http://www.hparchive.com/Bench_Briefs/HP-Bench-Briefs-1983-01-02.pdf

[bd139]

Interesting failure mode and solution.

[Specmaster]

Interesting failure mode and solution.

Can think of better things to fondle

Sent from my SM-J510FN using Tapatalk

[Specmaster]

Picked up the parts mule and first glance seems it is / was set for 240v so further investigation to see if it's in good shape or not. Might be better then the one I'm working on, soon find out though, I'll put it through the variac and bring up nice and gently.

Visually it is a right turd, all 8 case screws are missing, hence all the tape along the centre rail, all 4 feet are smashed off (seems par for the course with these HP scopes), it does however have the badge left on the fascia but boy, does it need cleaning badly?

It does however have the complete set of control knobs and interestingly, the screen/CRT appears to be a yellow one?

Lets see what happens when I take the covers off and look inside before applying power then 

[Specmaster]

So glad I took the cover off first, the seller claimed that when he switched it on, he got a line on the screen. The vertical output board is missing so is it at possible that he could have got a line with it?

The thing is full of wooly dust all over the voltage regulators on the back panel so its been in a garage or similar, stood on its base and the dirt has gotten in and settled, so tomorrow it goes out in the garden gets blown with an air pump.

Given that its not all and the fact that it was sold for spares or repair, do you reckon I could get a partial refund or would that cheeky?

[bd139]

It's possible he got a line. If the horizontal is working then it will sweep the beam across, but there will be no vertical control in the plates.

I'd try and hit him with a partial refund on the basis that "there is no way that could have ever shown a line" or something so even as spares/repair it was poorly described and dishonest. Go on about rot or something. I'd ask for £15 back myself. Works quite often.

Yellow tube might be long persistence. You might be able to shift that on ebay on it's own as they're quite rare. Check the PN on it (be careful with the HT!)

[Specmaster]

I paid him cash on collection though, I doubt that he would send me the £15 without me driving back to bloody Harpenden for it 

If the tube is rare is it worth swapping it over (assuming it works) or is it not worth the hassle?

[bd139]

Ah I'd skip it then. At least you got a healthy stock of parts out of it. I've paid a lot more for worse.

I wouldn't swap it TBH - requires a full recal if you change the tube. Someone might be interested however. I sold an HP tube a few years ago for £75 for a 180C scope which is very similar to the tubes in the 1740A (I think it was the only thing they kept between the models). Not sure you'll get that now but it might cover your outlay.

[Specmaster]

Yeh, that makes sense, well if I do sell it on Ebay, I'll leave it in the case then and then they can remove, not only that but it would the best form of protection in transit for it surely? I'll extract all the boards and other bits and bobs first though. I could also extract that input resistor I need complete with its shield and transplant it.

Already extracted the timebase knobs with their shafts and just about to perform an organ transplant with them into my working one after giving them a though clean up without separating of course to retain the integrity of the mechanism between main and delay controls   

[tggzzz]

Simple question: did you get what you needed for a fair price? If so, don't worry; move on.

[Specmaster]

Simple question: did you get what you needed for a fair price? If so, don't worry; move on.

I did, its the antique dealer in me looking for the "Real Deal" 

[tggzzz]

Simple question: did you get what you needed for a fair price? If so, don't worry; move on.

I did, its the antique dealer in me looking for the "Real Deal" 

When I was backpacking around India, many decades ago, I went to a market to get some food. Just outside the market I found a girl on the pavement with a few satsumas in front of her. I bargained with her to get 5 pieces for, IIRC, 2Rps (~2p).

About 5 mins late, while walking back to the hotel, I realised what I had done, and went back and gave her 5Rps. She was rather surprised and happy.

Lesson learned.