EEVblog #803 - HP1740A Analog Oscilloscope

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.
Original HP Journal article on the HP1740A:

Acknowledgments
The 1 740A design group was led by Stan Lang until the start of pilot production when he transferred to another project. The design team included Jim Garner, mechanical design including the vertical
attenuator switch, Eldon Cornish, who designed the horizontal section, and Van Harrison who designed the CRT circuits, power supplies, and gate amplifier.

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[youbecha]

That is the most realistic type of troubleshooting...the kind where it won't fail while you watch it.

[ebayscopeman]

HI Dave,

I have a mint one of these that my best friend and TE aficionado Ken Eastep left me in his will. I was with him when he ordered and took delivery of it from HP in the late 70's. I always wondered if his wife knew what he paid for it (you could buy most of a new car for the price of a 1740A). The 1740A was designed to compete with the Tektronix 465. Somewhere I even have the HP shipping box. A great scope. Now that you have it working check the risetime of the scope using a fast rise source. Chances are by now it is marginal. Fix this by massaging the coaxial delay line. The plastisizer in the jacket of the coax reacts with the silver and causes issues. Moving it around will cure this ill. On your bridge rectifier failure I had one of them fail in the +8V supply of a Tek 475 in exactly the same fashion, so though rare now I know of two!! Great job on a classic analog scope! How about that flood gun scale illumination! Worked great for Polaroid pictures.

P.S. Kenny would have loved your teardowns, repairs and videos and also would be right up to speed with current electronics development. He was born 30 years too soon!

[FractalD]

At 33:00 when you started to apply the heat gun to the electronics, I noticed that when you were focusing on the big long caps the black multi-meter started to dip constantly. When you pointed the heat gun away from them the black multi-meter began to rise again. I would suggest applying more heat to that area and see how low it goes and to hopefully (or not) break it again.

[KennethScharf]

I have two TEK's, a 454, and a 465. The 465 I got off Ebay from a seller that professionally reconditions them. IIRC I paid under $400 for that one. I've had my 454 for 30 years now, I bought it from a former employer who was auctioning off outdated test equipment to the employees. I paid under $250 for it at time, and they threw in a viewing hood, several probes, and a scope cart!

[brainndamage]

Well, I don't know if you can see it, but it seems that all the rails are deriving their reference fron the +15V rail, so anything wrong with the 15V rail would logically effect all of them. The +15V is the only one deriving its own reference (U1 pins 3 and 4)

[michaelhawthorne]

I can't believe your timing with uploading this video Dave.

Last week I dragged out of mothballs from the back of the factory where I work the very same CRO. It had been sitting there for 15 years or so. It was put there long before I was employed there.
It wasn't wrapped up or protected in any way so everything looked crusty, rust on the BNC's dirty to look at. It past the PAT so I turned it on.
Boy was it ill, Problems with the main Time Base, Vertical problems. Not quite like what you were getting Dave but felt it would need a lot of work to put it right.
The traces would initially jump around even if you breathed on the control knobs. After awhile things started to settle a bit but never really stabilised.
I attached a spectrum analyser to it, an SA450B which was also dragged out of mothballs and it produced a reasonable trace.
It was rather unstable though and was inconsistent if you changed a Time Base setting.
The general condition and the time needed to sort it out were prohibitive,

It unfortunately had to make an appointment to see it's maker.
Judging by how old it was, it will probably meet them too. lol

Nice video though, thanks for that.

[UniqueDesignsInc]

Another common failure with the 1700 series is the gold interconnect headers that connect the various boards. They commonly develop hairline cracks and what looks like oxidation around the gold plaited pins, where they contact the solder. I have had positive results when the old solder is removed and the pins are cleaned, then re-soldered with new solder. The horizontal time-base pcb's flex a little when changing the time-base setting and this may be why it's more common on them.

Most of the IC's are standard TTL parts with the exception of the (2) Horizontal ASIC's (Gold Plated), (2) White Ceramic ASIC's, one near the CRT and one on the bottom PCB under the vertical calibration shield.

One other common chip prone to fail is the 14 pin DIP chip on the Focus/Intensity/Beam Finder PCB, (HP 1821-0002) which is a CA3045 Transistor Array.

[GoldSrc_]

What's not to love about analog oscilloscopes, that phosphor glow is just beautiful.
You have to fix this one Dave.
Also, I laughed when you said "reboot it" lol.

[AndyHullMcPenguin]

1st golden rule, measure voltages, 2nd golden rule, clean the switches. 3rd golden rule suspect the capacitors.

[dhpbear]

28:30 - David, you continue to amaze. I love the 5-DVM setup!

[gjsmo]

I HAVE ONE OF THESE!! This is my first scope and as a matter of fact the only working one I have. I love this thing! Wish I could afford a digital scope but I have to settle with my school's equipment if I need that capability. Still a great piece of hardware for analyzing the signals I work with. So great to see a piece of equipment I own on the channel.

[PeterWalkerHPc]

5:41 ... that folks is why HP went from three guys in a garage to a loved and trusted, massive multinational instrument company.

[stonent]

I noticed the reset light blanked out when the scope glitched and then started flickering when you had the crap jumping all over the place.

[IN]

The manual!! It is like an old style Aircraft Maintenance Manual!(believe it or not we still have those as a backup for old aircraft.)

[TheEPROM]

Looks like one you will have to come back to. Love vintage test gear.

[suzesiviter]

The scope has been in Australia so long the electrons are running backwards.

[TKomoski]

Looks like a vertical deflection problem, when you adjusted the trace position the vertical collapsed. You might want to look at the vertical output stage with the two wires going to the deflection plates. Your right with the case off it can cool, and operate normally. You might want to tack on a thermal probe and put the case back on and watch the temp. From what they say on the net, the
vertical seems to be a common fault. Maybe apply some new thermal paste and that might do the trick. Worth a shot Dave ...

[Oj]

That scope is honeetly the most beautiful piece of engineering I have ever laid eyes upon.

[khurai]

That's pretty quality material for a 70's device. Metal Film resistors!!

[dsmbaby]

I absolutely share your enthusiasm for that exemplary operating manual! I wish I had something that thorough for the Lecroy scope I've been playing with lately.