EEVblog #803 - HP1740A Analog Oscilloscope

[EEVblog]

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.

[edpalmer42]

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

[Barny]

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.

[kallisti5]

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

[Paul Moir]

Is it just me or does the power LED dim every time it fails?

[Cyberdragon]

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.

[steve30]

Looks like a nice scope. Good luck getting it working  .

You should use analogue scopes in your videos more often .

[retiredcaps]

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.

[Cyberdragon]

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.

[PA4TIM]

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 that

I 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=564
Just 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.
 

[Joule Thief]

Is it just me or does the power LED dim every time it fails?

Not 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.

[firewalker]

The forces that stresses the boards and could open circuit something, when heated up, is upside down.

Alexander.

[Tothwolf]

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.

[Muxr]

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.

[tautech]

Those 1740's are really nice.

Yes 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. 

[hamdi.tn]

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 

[Muttley Snickers]

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

[EEVblog]

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.

Yeah it looks pretty crusty, kinda spoils it.

[sakujo7]

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.

[tggzzz]

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 that

Very timely 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.

[longview]

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.

[f4eru]

3 possibilities :

1) is there a thermal cutoff on the transformer ? could have gone bad with some resistance

2) 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.

[elliottveares]

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.

Please do a follow up video! 

[EEVblog]

Please do a follow up video! 

Already partially shot. It decide to co-operate for a bit which allowed some progress. Didn't have long to look at it though.

[drussell]

Already partially shot. It decide to co-operate for a bit which allowed some progress. Didn't have long to look at it though.

Yeah, 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.