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Repairing an HP 1740A Oscilloscope

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cigarsnob

New Member
Hi,

I purchased an HP 1740A scope on ebay a few weeks ago. This is my first time repairing a scope and I have a question(s).

When I first got the item, I noticed the mains setting was not set for US mains. After switched it to US mains, the unit powered on. I took the covers off and probed the power supply board and all the voltages are correct. At this point, I injected a 1khz sine wave to channel A. No signal appeared, so I adjusted the intensity knob, but no luck. I pushed the Beam Find button and then I saw my signal.

This is pretty much where I'm at at the moment. When I get home from work in a couple of hours, I will read through the service/operating manual to see if I can't find any hints in there, but until then, I thought I would post here.

Any feedback would be appreciated...As far as I know, I have the scope configured correctly to display a signal and when I move the volts/div or the times/div, I do see the signal change. It's It's not changing as you would expect it to though. Meaning when you adjust the volts/div with the Beam Find button pressed, the signal looks different from when the Beam Find button is released. I just checked this with an old analog scope at work to confirm this. But again, I can only view the signal when I push the Beam Find button.


Would love to get this little beast back up and running again.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Beam finder defocuses everything.


Scopes can be indauting at times to get a signal displayed.

First use the beam finder and center the resulting trace.
Next look for a trgger light, to see if the scope is triggering. You sould also be able to notice this when using the calibrate waveform and the beam finder. Generally, auto-trigger gives a waveform, but it is sometimes not stab
le.
Next, adjust intensity.
 

cigarsnob

New Member
Thank you Inquisitive for the service/operating manual link. I already have this document and I've been reading it to further help me with this scope.

KeepItSimpleStupid - Yeah I noticed when pressing the beam finder on an analog scope at work that it defocuses everything. That's why I was hoping I was just goofing on setting it up correctly. And you were right. Scopes are a little finicky when trying to display a signal. After reading the operating manual and following there set-up and a little more, I finally got both channel A and B displayed.

I noticed channel A's volts/div when changed from say 1mV to some higher gain, the display would show a signal larger than what it actually was. I noticed this on several different positions, as it would be 1.3x, 2.6x, 2x off. That will be my next thing I will tackle this evening when I get home. Channel B appears to be working just fine though.


Thanks again for your guys' replies.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Knobs typically have a cal position. Usually it's the inner part of the volts/div. Typically there is a light that's labeled cal or uncal and the cal position is full counter-clockwise for calibrated. There is usually a click associated with cal.

Lissajous plots is one reason to use uncalibrated.
 

cigarsnob

New Member
The cal knobs were all locked in the cal position. I recorded the values last night (see pic below) and compared it to my Rigol.


20130709_012901.jpg
 

HiTech

Well-Known Member
I own two HP 1741A which are essentially similar to the 1740A other than they are storage models. The sweep amp caps are known to fail as are numerous switch contacts needing cleaned. First assure all the controls/switches are thoroughly cleaned as most often it's just that at fault. You claim B channel works fine so you have a working section to compare measurements to from Chan A. Those are aging scopes by now and again more often than none, it's oxidized contacts and some of nthe sealed pots at fault. The 1740A and 1741A are amongst the finest o'scopes ever made during that era, surpassing the Tek scopes. The HP CRTs were excellent. The build quality and design is what's made them last so long and it's those two models that forced Tektronix to relook at their line of scopes due to a drop in sales. One of my 1741A scopes was lab calibrated in 2011 which is pretty rare for an eBay scope! I also have a Beaverton-era Tektronix 475A (a quality, workhorse model) and I will take the HP over the Tek any day- it's that superior in numerous ways. You've got a good scope worth repairing.
 

dr pepper

Well-Known Member
Most Helpful Member
'scopes I've owned have all been squiffy with switch contacts, often causing faults, invest in a can of switch cleaner, switch cleaner - not wd40.
Its unlikely you'll get the same fault on both traces, so if both sides dont work then maybe (not allways) the fault is somewhere else.
Having another 'scope is usefull when doing repairs.
Your 'scope probably has a cal connector on the front and is a usefull 600mV squarewave source for self checking.
At least if the beamfind works, the psu, tube and timebase all all probably ok so its worth looking further.
 

HiTech

Well-Known Member
^^^^ What? :confused:

Cigarsnob, try this link: click here
 
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