HP1740A Oscilloscope – don’t turn it on – Take it Apart - Page 1
Forum section: electronics / testgear

author: balu
author_rank: Regular Contributor
timestamp: March 15, 2017, 09:05:09 pm
content: Hi,I get a new (very old) test gear from ebay just for 10 euro bugs. It was the dirtiest test gear I haveever seen – minimum 10 years lying around in a garage. After cleaning and visual inspection I decideto make some pictures.

author: balu
author_rank: Regular Contributor
timestamp: March 16, 2017, 07:59:47 pm
content: Hello,After power the scope on and checking the inputs I found out, that channel A doesn’t workwith AC input mode. Next step is to check the vertical preamplifier board.

author: balu
author_rank: Regular Contributor
timestamp: March 17, 2017, 06:31:27 pm
content: Hello,I found the problem. C1 the AC decoupling capacitor is damaged. I have no information whythe 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 2ndpicture]After replacing capacitor C1 with a WIMA MKP4 33nF/630VDC capacitor and assembly the scopeeverything works fine.

author: oldway
author_rank: Super Contributor
timestamp: March 17, 2017, 06:42:32 pm
content: 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.

author: apomerleau
author_rank: Newbie
timestamp: March 17, 2017, 06:43:14 pm
content: 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.

author: balu
author_rank: Regular Contributor
timestamp: March 17, 2017, 07:07:17 pm
content: Hello oldway,Thank you for the note about a possible floor gun problem. I use two spot light toilluminate the scope for the photo. One spot shines from the right and the othershines from the left side. The two darker areas on the scope screen are shadowsfrom the spot lights. I have made a photo without light from environment and I canonly see the input signal on the screen.

author: oldway
author_rank: Super Contributor
timestamp: March 17, 2017, 08:09:05 pm
content: And scale illumination ? Does it works ?

author: balu
author_rank: Regular Contributor
timestamp: March 17, 2017, 08:36:29 pm
content: Hello oldway,Good idea, I never checked the scale illumination. Attached to this replay you willfind a photo with maximum level of scale illumination and with very few environmentslight. The scale illumination looks good to me.

author: shteii01
author_rank: Frequent Contributor
timestamp: March 18, 2017, 05:38:35 am
content: 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?

author: amspire
author_rank: Super Contributor
timestamp: March 18, 2017, 05:45:01 am
content: Quote from: apomerleau on March 17, 2017, 06:43:14 pmI had one with failed bridge rectifier - seems to be a common fault. I would probably fit a higher rated rectifier in.

author: tautech
author_rank: Super Contributor
timestamp: March 18, 2017, 07:55:34 am
content: Quote from: amspire on March 18, 2017, 05:45:01 amSo 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/