HP1740A Oscilloscope – don’t turn it on – Take it Apart

[balu]

Hi,
I get a new (very old) test gear from ebay just for 10 euro bugs. It was the dirtiest test gear I have
ever seen – minimum 10 years lying around in a garage. After cleaning and visual inspection I decide
to make some pictures.

       

[balu]

Hello,
After power the scope on and checking the inputs I found out, that channel A doesn’t work
with AC input mode. Next step is to check the vertical preamplifier board.   

[balu]

Hello,
I found the problem. C1 the AC decoupling capacitor is damaged. I have no information why
the capacitor was broken and lost a connector. The capacitor will be shorted with switch A
(closed) for DC signal input mode and in AC mode the switch A will be open. [see 1st and 2nd
picture]

After replacing capacitor C1 with a WIMA MKP4 33nF/630VDC capacitor and assembly the scope
everything works fine.   

[oldway]

Great oscilloscope, I have one too....It seems that you have still a problem with flood gun of scale illumination. It must cover all the graticule area.
Also check the bandwith.
This scope is known to have an oxydation problem of the delay line, reducing the bandwith...Easy to fix.

[apomerleau]

What a great find.  And a nice repair job.  Awesome.

I have a 1740A found in a barn for $45.  It does not yet power on as the power supply board has a couple of failed bridge rectifiers.  I haven't had the time yet to complete initial repairs, but I do have it opened and disassembled.  I hope I can get it working.

[balu]

Hello oldway,

Thank you for the note about a possible floor gun problem. I use two spot light to
illuminate the scope for the photo. One spot shines from the right and the other
shines from the left side. The two darker areas on the scope screen are shadows
from the spot lights. I have made a photo without light from environment and I can
only see the input signal on the screen.

[oldway]

And scale illumination ? Does it works ?

[balu]

Hello oldway,

Good idea, I never checked the scale illumination. Attached to this replay you will
find a photo with maximum level of scale illumination and with very few environments
light. The scale illumination looks good to me. 

[shteii01]

I also have one that I picked up at an estate sale a year or so ago.  Works fine.

One question.  My zoom feature does not work.  Anyone run into this problem?  Any guides on fixing it?

[amspire]

What a great find.  And a nice repair job.  Awesome.

I have a 1740A found in a barn for $45.  It does not yet power on as the power supply board has a couple of failed bridge rectifiers.  I haven't had the time yet to complete initial repairs, but I do have it opened and disassembled.  I hope I can get it working.

I had one with failed bridge rectifier - seems to be a common fault. I would probably fit a higher rated rectifier in.

[tautech]

What a great find.  And a nice repair job.  Awesome.

I have a 1740A found in a barn for $45.  It does not yet power on as the power supply board has a couple of failed bridge rectifiers.  I haven't had the time yet to complete initial repairs, but I do have it opened and disassembled.  I hope I can get it working.

I had one with failed bridge rectifier - seems to be a common fault. I would probably fit a higher rated rectifier in.

So did Dave:
https://www.eevblog.com/2015/10/03/eevblog-803-hp1740a-analog-oscilloscope/
https://www.eevblog.com/2015/10/06/eevblog-804-hp1740a-oscilloscope-repair-part-2/