HP 54502A Oscilloscope fuse blows up - Page 1
Forum section: electronics / repair

author: habibrenjie
author_rank: Newbie
timestamp: December 13, 2016, 09:15:02 am
content: I have in my possession a HP 54502A oscilloscope, and one day it went bang and I got the familiar smell... It was the EMC which blew, and I had that replaced. Now every time I plug it in the mains the fuse blows... So I unplug the PSU from the scope, and it still blows. It doesn't blow when I plug in the EMC alone, so it must be a problem with the PSU. Has anyone had similar issues? If not any advice on how to start diagnosing the problem will be appreciated. Thanks all!

author: KhronX
author_rank: Frequent Contributor
timestamp: December 13, 2016, 09:46:13 am
content: And how exactly did you determine that the "EMC" blew?Assuming you mean the mains noise filter section (X2 caps, Y2 caps and common-mode chokes), those things rarely (if ever) blow, and if they do, there's something waaaaaaay more wrong downstream.Assuming you have a multimeter, you could start by "beeping out" the mains input to the power supply.After replacing the fuse at least, it might've been a good idea to connect the mains through a "bulb tester", ie. an incandescent / tungsten 60-100w lightbulb wired in series, for current limiting. If it lights up bright, you know there's considerable current draw, but the bulb should drop enough voltage to not have anything damaged in the DUT itself.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 13, 2016, 09:59:47 am
content: The service manual doesn't seem to contain any schematics, unfortunately.Any chance you could snap a few (well-lit and well-focused) photos of the inside of the power supply? There may be some helpful visual clues...

author: habibrenjie
author_rank: Newbie
timestamp: December 13, 2016, 12:55:01 pm
content: The EMC had a strong burnt smell, leading me to the conclusion that it blew. I followed your advice and "beeped out" the mains input pins. The only continuous pins were 2 and three, which go to a transformer. I tried the "bulb testing" with a 25w bulb with no light up. I'll try with 60w as soon as I can get a 60w bulb. My replacement part for the EMC is an FN393-6-05-11, which is the closest model I can find in my region, hopefully that isn't an issue. I'll upload the pictures once I have daylight, they don't look too good with current lighting.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 13, 2016, 01:36:38 pm
content: Well, now that i think about it, considering the age of the unit, the EMC filter may well have contained metallized paper X2 (and/or Y2) caps, which have been known to fail, with age.If you do try to power up the unit with the lightbulb in series, do remember to install a fuse as well, otherwise the circuit won't be closed

author: KhronX
author_rank: Frequent Contributor
timestamp: December 13, 2016, 07:23:16 pm
content: PS: It seems some photos have been posted here on the forum some time ago:https://www.eevblog.com/forum/reviews/hp-54502a-nearly-vintage-digitizing-scope/

author: orin
author_rank: Frequent Contributor
timestamp: December 14, 2016, 12:56:30 am
content: As far as I know, there is no current Schaffner offering that will work in this series of scopes.See:https://www.eevblog.com/forum/testgear/hp-54501a-restoration-and-calibration/If you are running off 220V, you could cut/remove the two wires that are shorted in 120V mode (I can't tell which terminals they go to from the datasheet). I fear something shorts out in the power supply with the -11 wiring in 220-240V mode.

author: habibrenjie
author_rank: Newbie
timestamp: December 14, 2016, 05:12:42 am
content: QuoteYes, that was the problem. I'm on 220v mains, and the thing didn't blow when I hooked to a 110v transformer. However, the scope isn't working properly, but that will be for another topic...Will I have to run it on 110v forever or is there a way to modify it to run on 220v? (I think the conclusion reached in the quoted thread was to use 110v)

author: orin
author_rank: Frequent Contributor
timestamp: December 14, 2016, 06:10:32 am
content: Quote from: habibrenjie on December 14, 2016, 05:12:42 amSure you can modify it. It's just a matter of cutting (removing) the two wires I mentioned. With the original Schaffner -12 part, they were open circuit for 240V, short circuit for 120V. With the -11 part, they are short circuit for 120V, but wired to live and neutral for 240V. Once you identify the two wires, you could wire them to your own 120/240V switch, but I don't feel that it's worthwhile.What's the scope doing run off 120V? Post a link to the new thread or I'll probably not see it.

author: habibrenjie
author_rank: Newbie
timestamp: December 14, 2016, 09:01:16 am
content: Thanks KhronX and orin for your help.Orin, pardon me if this seems like a dumb question, but I still don't fully understand which wires you're talking about. Pins 1 and 3 as well as 2 and 4 are connected in 110v, pins 2 and 3 are connected in 220v mode.Here is the link to problem:https://www.eevblog.com/forum/repair/hp-54502a-oscilloscope-screen-not-working/

author: orin
author_rank: Frequent Contributor
timestamp: December 15, 2016, 05:54:17 am
content: Quote from: habibrenjie on December 14, 2016, 09:01:16 amPlease define what you mean by which pins are connected - to each other or to the input side of the socket. Perhaps a picture? I have to take my 54510A apart to work out which wires are what and I haven't had time to do it.I looked at the other thread and if it's flashing on and off with a clicking sound then it's almost certainly the capacitors in the power supply that have leaked. I recently repaired the power supply from my 54510A (same power supply as the 54502A) by replacing all the capacitors on the secondary side and cleaning the leaked electrolyte off the PCB. I'll try and find the information on that for you.

author: orin
author_rank: Frequent Contributor
timestamp: December 15, 2016, 06:07:17 am
content: Quote from: orin on December 15, 2016, 05:54:17 amOK, the definitive post is in the hp_agilent_equipment group on groups.yahoo.com and is by Joel Setton. So go to groups.yahoo.com, join the hp_agilent_equipment group and look for posts by Joel Setton in the "54502a power supply" thread... then read the rest of the thread for additional information.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 15, 2016, 11:01:22 am
content: Considering the age of the unit, and the fact that it's a switching PSU, dead caps are near (if not at) the top of the list of suspects.That's how i revived a dead Tek 2225 i got for 35eu (~40$) last year...

author: paulr
author_rank: Newbie
timestamp: December 21, 2016, 12:12:36 pm
content: There is a reverse engineered schematic of the HP5403 power supply herehttp://www.radiocollection.be/images/restaurations_img/HP54503/supplyHP54503.pdfI understand that the Schaffner filters of that era are known to explode! There is a suitable replacement filter module available: Schaffner FN393-2.5-05-11 which is available from Mouser Electronics

author: KhronX
author_rank: Frequent Contributor
timestamp: December 21, 2016, 12:15:31 pm
content: You'll want to edit your post and put a space between the link and the fullstop at the endQuote from: paulr on December 21, 2016, 12:12:36 pm

author: paulr
author_rank: Newbie
timestamp: December 21, 2016, 12:40:10 pm
content: Noted, thank you.Paul

author: orin
author_rank: Frequent Contributor
timestamp: December 22, 2016, 01:16:09 am
content: Quote from: paulr on December 21, 2016, 12:12:36 pmThat won't work in 240V mode without modifying the scope's wiring.The -11 module is designed to connect two transformer windings in series for 120V and parallel for 240V, so short the two extra pins in 120V mode and wire them to live/neutral in 240V mode. The 545XX power supplies require the two pins to be shorted in 120V mode andleft open in 240V modewhich is what the discontinued -12 module does.The referenced schematic does not show this connection, but I believe it connects C4 in 120V mode and disconnects it in 240V mode, changing the input configuration from a bridge rectifier (240V) to a voltage doubler (120V). Depending on where exactly this connection is made, it wouldn't surprise me if the -11 module resulted in a dead short between live and neutral when set to 240V and used with one of these power supplies with the default wiring.