HP 721A - Page 1
Forum section: electronics / testgear

author: adnewhouse
author_rank: Contributor
timestamp: September 06, 2013, 04:59:34 pm
content: I picked up this power supply for a few bucks. All looked good for such an old piece of test gear. The meter was spot on and all switched/pots worked well. The output tested OK on my multimeter as well. I was beginning to think I had found a gem. I was wrong. Once I looked at the output with my scope I realized there was something wrong. Whats most likely the problem? I have to think it's the caps or maybe the rectifier diodes? Here's a few pics and a link to the manual:http://www.hparchive.com/Manuals/HP-721A-Manual.pdf

author: Unknown
author_rank: Unknown
timestamp: September 06, 2013, 05:11:41 pm
content: My guess would be that the large filter caps are shot.

author: mcleod
author_rank: Contributor
timestamp: September 06, 2013, 05:14:47 pm
content: I would start by testing C2.

author: willb
author_rank: Regular Contributor
timestamp: September 06, 2013, 05:26:44 pm
content: Caps. I own several vintage HP power supplies and most, if not all of them, have had some cap replaced at one point. Ive never looked inside a 721, but don't be surprised if you find a bad Mallory multicap and have to improvise to replace it, I've been there myself a few times.

author: adnewhouse
author_rank: Contributor
timestamp: September 06, 2013, 05:50:32 pm
content: I'll desolder one of the leads of C2 and test it. That was my guess as well.

author: adnewhouse
author_rank: Contributor
timestamp: September 06, 2013, 06:14:35 pm
content: C2 was indeed bad. Finding a replacement might not be too bad. 500uf at 75v. I found an original (http://www.ebay.com/itm/0180-0047-Agilent-HP-Capacitor-Fixed-500-uF-10-50-75V-20-85C-Sprague-D3-/251024139166?pt=LH_DefaultDomain_0&hash=item3a7234639e) but I'm not sure I want to get a used cap that could go at any time. Would somthing like this (http://www.digikey.com/product-detail/en/EGPA101ELL511MM30S/565-3427-ND/3528502) be a valid replacment?

author: Unknown
author_rank: Unknown
timestamp: September 06, 2013, 06:27:17 pm
content: I don't see any reason to spend $10 on an old cap, unless you're deeply concerned about keeping it original. People sometimes remove the guts of old caps and put the newer, much smaller cap inside to make it look original.That cap seems fine to me, but note that 500 uF isn't going to be particularly critical, so 470 uF is going to be just fine and possibly cheaper. Something likethisfor example.

author: mcleod
author_rank: Contributor
timestamp: September 06, 2013, 06:32:53 pm
content: I would not buy a new old stock or used capacitor, chances are they are not in good condition. Electrolytic caps have a finite shelf life, they can be reconditioned, but I prefer new. My opinion.Your digikey link should work well. I would test all the electrolytics in the supply, chances are most need to be replaced, this will help you meet the digikey minimum order too.

author: blackdog
author_rank: Frequent Contributor
timestamp: September 06, 2013, 07:02:11 pm
content: Hi,C2 can be 470uF and 63V.Also check C3, C4 and C5.Also check the output capacitors C8 and C9.After this replacement, it should be a fine powersupply :-)Kind regartsBram

author: PA0PBZ
author_rank: Super Contributor
timestamp: September 06, 2013, 07:12:44 pm
content: Quote from: adnewhouse on September 06, 2013, 04:59:34 pmOh man, I just looked at the last page of that manual and I'm feeling kind of strange now...

author: casinada
author_rank: Frequent Contributor
timestamp: September 06, 2013, 09:28:32 pm
content: Don't need to smoke anything to feel dizzy looking at that pageAmazing how things were so much simpler then..... a few discrete components and a nice and stable product.

author: PA4TIM
author_rank: Super Contributor
timestamp: September 06, 2013, 10:22:12 pm
content: This is my cutie, hope you get yours good. That ripple is indeed not normal,The good thing is,there is almost nothing in it.If you need some comparing measurements let me know.

author: adnewhouse
author_rank: Contributor
timestamp: September 07, 2013, 03:38:38 am
content: Thanks for all the help! The new caps are on the way. I ordered replacements for all the electrolytics just in case. Too bad the front panel doesn't look as good as it could. Good thing its more about how the instrument performs than how old and banged up it looks.

author: N2IXK
author_rank: Frequent Contributor
timestamp: September 07, 2013, 01:49:41 pm
content: Interesting that your unit has a 120/240V selector switch where mine (US 120V only version) has the power ON/OFF switch. Odd that they would put that on the front panel. Does yours actually have an on/off switch somewhere?Those little power supplies seem to go on forever, with the caps the only thing that ever seems to go bad.

author: PA4TIM
author_rank: Super Contributor
timestamp: September 07, 2013, 02:52:12 pm
content: That switch is the power switch. It is a 3 position switch. That blocking plate can be turned around and then it blocks the 220V position.I have seen more like mine. I do not know if that was an option (maybe for travelling techs) or an export version or just a newer or older variant.Maybe somebody knows. Mine has a european mains 3 sprong input connector at the side. I think this is not original.