EEVblog #803 - HP1740A Analog Oscilloscope

[phenol]

I too had the same bandwidth problem with that scope. Pushing the delay line coil around while feeding 100+MHz to the scope does indeed reveal sweet spots where it meets its specs. At one point i had to stick a pen in there to make it hold its 'good' position, but it only worked for a month or so when it started to deteriorate again.
The board to board connectors are prone to failure and fatigue. One of them was actually charred due to poor contact and i resorted to bridging the two boards it connects with wire jumpers.
The flood gun on mine only works intermittently.
When the scope was 20, a high value resistor in the crt focus section of the HV power supply failed and the beam became defocussed...

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.

[Alex_arg]

Hi Dave ! I've got a HP1715 200Mhz the same panel and chassis than 1740, i buyed it 4 years ago , and after cleaning all electromechanical parts it's working OK , but the multi PCBs switches of time base are a pain in the a** , once a year or so, i have to realign the knobs axis with the rotary switches and clean the contacs of the connectors between 3 daughters and mother board. Mine got PCB edge contacts, i've see that yours have a very better plastic connector between boards. TIP: Power supply is almost the same in both scopes and in the service manual says that the +15V regulator serves as reference to al other voltages in the low voltage PSU.
I hope this helps.
Photos ->https://www.flickr.com/photos/136701500@N04/sets/72157658999271719

[max666]

Dave, don't let your wife catch you with that service manual 

[PA4TIM]

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1975-12.pdf Nice article about this scope. Including backgrounds about the hybrids/ceramics stuff

The service note about the delayline must be somewhere around 1982. I also have to google for it to find it again. In my case it solved the BW problem.  I agree with the rest, about the reference voltage.

[John Coloccia]

I've got one of these. Really nice scope. Of course, all of the feet are broken off. One of these days, I need to make some new ones in my shop, because it's a LONG scope. The main reason I picked up the Rigol 1054Z is for the size. I still have to whip this thing out sometimes, though, when I'm tracking down noise.

[Shinobot2]

I saw the 121V rail droping slowly when the heat gun was pointing to the power supply. Every time Dave pointed somewhere else it started to increase the voltage.

[Stephan_T]

Hi Dave,

what about replacing four of the five DMMs with a 4-channel storage oscilloscope. Single shot triggered it could wait for hours for the fault to happen, and it could also record in some detail the relation of the different voltages. Like: "which one dumped first".

Maybe this could be another argument/example for the Oscilloscopes, how many channels do you need? question.

[boffin]

I saw the 121V rail droping slowly when the heat gun was pointing to the power supply. Every time Dave pointed somewhere else it started to increase the voltage.

It dropped a tiny tiny amount (5th digit), a few tens of mV on a 120V line.

[AF6LJ]

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.

Yup.

[HackedFridgeMagnet]

I didn't see the manual posted, so I think this is it.

http://cp.literature.agilent.com/litweb/pdf/01740-90909.pdf

[AF6LJ]

I didn't see the manual posted, so I think this is it.

http://cp.literature.agilent.com/litweb/pdf/01740-90909.pdf

Those are good scopes, it is no wonder they are still working going on forty years later.
I have my doubts those LCD all digital scopes we have around us today will have the same longevity.

[Cyberdragon]

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.

Mine has the same problem with the graticule light going out sometimes. What parts did you replace?

[Tim F]

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.

Yeah it's almost definitely this. If there was a problem on the primary side of the xformer you wouldn't expect all the rails to be exactly 79% of nominal because the dropout voltage is going to be different between all the different supply rails.

I saw the 121V rail droping slowly when the heat gun was pointing to the power supply. Every time Dave pointed somewhere else it started to increase the voltage.

That's to be expected. It's all discrete components for that rail's regulator so it's easy to heat up one of the feedback resistors or one of the transistors in the differential pair more than the others and cause drift.

[coppice]

In the 70s it was normal to refer to micro nano and pico farads. You have to go back to the 60s to find a time when nano was not used.

[SeanB]

Looking at the socketed IC's in the power supply, and that 10 lead TO99 can with a HP part number. With the age, that is a RCA 723 regulator, just with a HP part number on the can. Most likely the voltage regulator issue is from a socket that is intermittently going high resistance or noisy. Simply reseat all the IC's and sockets in the power supply should fix it. The Moles cannectors on the tin plated transistor leads also give issues and the solution is to simply remove the socket, put a dab of silicone grease ( easiest to use a cotton bud dipped in it to wipe all in the line as you go) on the top and bottom of the leads and put the socket back. The removal insertion scrapes the leads clean of oxide and the grease keeps them from corroding again with time and thermal cycling.

[tautech]

When I had a 1740 years ago I found several x-reference pdf's, Exel lists and pics that were easy at the time to find online. IIRC all HP devices were able to be X referenced.
Of those I have there's at least a 1000 Mb in several files, attached is a teaser:

Just yell if you want the rest.

[tautech]

Looking at the socketed IC's in the power supply, and that 10 lead TO99 can with a HP part number. With the age, that is a RCA 723 regulator, just with a HP part number on the can.

Correct, as listed in the BOM Ch6 p20.

[tggzzz]

When I had a 1740 years ago I found several x-reference pdf's, Exel lists and pics that were easy at the time to find online. IIRC all HP devices were able to be X referenced.
Of those I have there's at least a 1000 Mb in several files, attached is a teaser:

Just yell if you want the rest.

If there's a crossref for the delay line, I'd be interested. Part number 01740-61611.

Any other information about it would also be useful, in case it needs to be replaced.

[tautech]

When I had a 1740 years ago I found several x-reference pdf's, Exel lists and pics that were easy at the time to find online. IIRC all HP devices were able to be X referenced.
Of those I have there's at least a 1000 Mb in several files, attached is a teaser:

Just yell if you want the rest.

If there's a crossref for the delay line, I'd be interested. Part number 01740-61611.

Any other information about it would also be useful, in case it needs to be replaced.

Sphere has one @ $19
http://www.sphere.bc.ca/test/hp-parts/hpparts6.html
Last item on the page.

Sphere have a HP page with a 20Mb X- ref file that they give instruction NOT to link:
http://www.sphere.bc.ca/test/hpparts.html#catalog
Scroll down, you'll find it.

[oldway]

This is a very great analog oscilloscope. I worked with it for years and I still have one at home.
For the delay line problem, see here:
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/HP/Publikacje/HP-Bench-Briefs-1983-01-02.pdf

[AF6LJ]

In the 70s it was normal to refer to micro nano and pico farads. You have to go back to the 60s to find a time when nano was not used.

That's true here they were pushing nano at the college I went to.

[tggzzz]

Thanks to PA4TIM for alerting me to the delay line's failure mode.
Thanks to oldway for locating the specific Bench Brief describing the issue.
Thanks to tautech for the pointer to a replacement delay line.

Remarkably - to me - squeezing the delay line has magically[1] cured the problem. The risetime is back to slightly under 3.5ns, rather than the anaemic 7ns yesterday.

Given that the manual states the delay line is ~100ns long, and it looks like it is only ~2.5m long, I wonder what secret sauce they use to get such a long delay.

[1] as in Arthur C Clarke's third law

[tggzzz]

One annoyance I've had with this scope is an intermittent timebase. Eventually I located the problem: inside the multipole rotary switch on the main timebase assembly A8.

After disassembling the switch the problem wasn't, as I expected, with the wipers and gold-plated contacts: they are in pristine condition. Instead the problem was that several PCB tracks leading from the contacts to the rest of the board had been abraded and cut. The cut's location were where the edge of the black plastic rotator presses against the PCB, and hence rubs the PCB track every time it is turned, doh.

Solution was simple: solder a single piece of 0.2mm wire across the cut. This lifts the black plastic, but apparently not enough to cause problems with the wipers.

[nfmax]

One annoyance I've had with this scope is an intermittent timebase. Eventually I located the problem: inside the multipole rotary switch on the main timebase assembly A8.

After disassembling the switch the problem wasn't, as I expected, with the wipers and gold-plated contacts: they are in pristine condition. Instead the problem was that several PCB tracks leading from the contacts to the rest of the board had been abraded and cut. The cut's location were where the edge of the black plastic rotator presses against the PCB, and hence rubs the PCB track every time it is turned, doh.

Solution was simple: solder a single piece of 0.2mm wire across the cut. This lifts the black plastic, but apparently not enough to cause problems with the wipers.

Was that the main or the delayed time base? I have a 1740A where the delayed sweep never runs on any setting, in any mode. This video is making me itch to try and fix it!
Mine was a 'rescue scope' when the local university electronics department moved to a new building. The only fault was a fried bridge rectifier in the power supply. Because of the sealed case, they tend to run hot, but the absence of outside air flowing through means they stay beautifully clean.

[tggzzz]

One annoyance I've had with this scope is an intermittent timebase. Eventually I located the problem: inside the multipole rotary switch on the main timebase assembly A8.

After disassembling the switch the problem wasn't, as I expected, with the wipers and gold-plated contacts: they are in pristine condition. Instead the problem was that several PCB tracks leading from the contacts to the rest of the board had been abraded and cut. The cut's location were where the edge of the black plastic rotator presses against the PCB, and hence rubs the PCB track every time it is turned, doh.

Solution was simple: solder a single piece of 0.2mm wire across the cut. This lifts the black plastic, but apparently not enough to cause problems with the wipers.

Was that the main or the delayed time base? I have a 1740A where the delayed sweep never runs on any setting, in any mode. This video is making me itch to try and fix it!
Mine was a 'rescue scope' when the local university electronics department moved to a new building. The only fault was a fried bridge rectifier in the power supply. Because of the sealed case, they tend to run hot, but the absence of outside air flowing through means they stay beautifully clean.

The main timebase.

I suspect there could be similar problems on your delayed timebase because the construction looks similar. But there are two reasons I wouldn't bet on it, from your description:

• the delayed timebase is rotated far less often than the main timebase, so there would be less wear
• I saw problems on some, but not all, ranges - and the pattern of faulty ranges gave a strong indication of where to look

Presuming you have a manual, you could start by looking at some of the voltages with a voltmeter. It may even be possible to use the scope to look at its own internal waveforms, but I haven't considered that in detail.