HP 1746A - new acquisition for repair - Page 1
Forum section: electronics / testgear

author: qu1j0t3
author_rank: Regular Contributor
timestamp: September 26, 2017, 04:50:13 am
content: Just obtained this pretty thing:DOA - as in no life, no Line lamp. Will be a repair project (which means a learning project, as I'm not experienced in scope repair

author: WastelandTek
author_rank: Frequent Contributor
timestamp: September 26, 2017, 05:47:15 am
content: Looks good cosmetically, and frequently no life at all is better than some malfunctioning operation as it may well point to a simple power supply fault.Best of luck!

author: edpalmer42
author_rank: Super Contributor
timestamp: September 26, 2017, 06:13:24 am
content: Damn, that thing looks brand new!I have a 1744A scope. Many of the transistors and ICs are in sockets, but you have to look really close to realize that there is a socket there! There's no plastic surround, just single-pin sockets that are soldered into the PC board. A few years ago, my scope developed a power supply fault. Wiggling an IC is all that was required to fix it. Be sure to check for that.Ed

author: qu1j0t3
author_rank: Regular Contributor
timestamp: September 26, 2017, 05:14:15 pm
content: @edpalmer42 Thanks for the tip. I haven't even cleaned it up yet - that is as received! and no broken knobs, which my other HP's suffer from. I'm still looking for replacements; the hardest may be the Volts/Div knob for 1703A.Stranger yet, it was ex-University of Washington; I might have expected more wear and tear on a lab scope.@WastelandTek Yep, that's what I'm hopingI started reviewing the schematics yesterday. I noticed the usual big smoothing caps in the LVPS. Is it typical to replace these in a 40 year old instrument?

author: edpalmer42
author_rank: Super Contributor
timestamp: September 26, 2017, 06:25:57 pm
content: Quote from: qu1j0t3 on September 26, 2017, 05:14:15 pmI know a lot of people would automatically replace them, but if the DC and AC ripple voltages across them are acceptable, I don't see the point. At the very least, I'd put it low on my list of priorities.Ed

author: tautech
author_rank: Super Contributor
timestamp: September 26, 2017, 11:04:00 pm
content: The line filter may have gone poof and blown the fuse.Keep your eyes wide open for someone else's handywork when you open her up. Take your time and expect any and everything. Look for stupid simple stuff first and confirm line filter/switch/fuse and general wiring connectivity with continuity tests.Dim bulb tester inline with the mains is good for powering devices safely of unknown condition. You'll need ~100W incandescent bulb for this unit.

author: qu1j0t3
author_rank: Regular Contributor
timestamp: September 26, 2017, 11:36:26 pm
content: Fuse is OK.Should I use an isolation transformer for all PSU repairs like this?

author: edpalmer42
author_rank: Super Contributor
timestamp: September 27, 2017, 12:36:51 am
content: Quote from: qu1j0t3 on September 26, 2017, 11:36:26 pmNo, you don't need an isolation transformer. This scope uses a linear supply. The transformer is built in. Any testing you need to do on the primary side of the transformer can be done with an ohmmeter while the scope is unplugged.Tautech is right when he suggests looking for stupid stuff. I recently bought an HP power supply that was dead. Okay, start checking voltages. The test points are on the bottom. Pull off the bottom cover. Oh..... Maybe this cable should be plugged in.Ed

author: bitseeker
author_rank: Super Contributor
timestamp: September 27, 2017, 12:47:05 am
content: Looking forward to seeing inside. Hopefully, just a line filter or something disconnected.