HP 8648c Signal Generator PS replacement. - Page 1
Forum section: electronics / repair

author: Sam Cohen
author_rank: Newbie
timestamp: January 16, 2018, 01:39:33 pm
content: Hi fellow hobbyistThis is my first post, I just recently discovered this site with its very useful info.My name is Sam and I’m an electronic hobbyist who lives near Jerusalem Israel.I recently bought an HP 8648C RF generator from a local scrap yard, I paid 100$ for it and I’m now trying to fix it.It has a problem with its PS, I’ts not putting any DC on its outputs, other then the PS internally it looks clean and in decent condition.I took the PS Out and hooked up resistor load to its output and try to power it in attempt to identify the problem, this was unsuccessful.All the capacitors look OK (Using an ESR meter) and nothing seems burned.Strangely I couldn’t find electrical schematic for this PS which is what makes locating the failure very difficult.I did found some used original HP PS units on EBAY for 350$, for a 100$ purchased generator it seems a little ridiculous.So I thought of maybe using a PS made by another vendor that has more or less the same voltage rating.The original PS produces +5V, +15V, -15V and +38VDC, I have no idea what is the current drain on each of the voltages, I do know that most of the power is drawn by the 5VDC (the DC Cable uses multiple wires for the 5VDC).I have a Meanwell PS that has all the voltages except the +38V.It produces 11A on 5V, 2.5A on 15V and 0.5A on -15V.My plan is to use an LM2587 Based circuit to generate ~1A 38VDC form the 5VDC.Do you guys see a problem with going at this route?, do you have recommendation?.Any other ideas are welcome..BTW I plan of use this generator for general electronic repair.Thank youShmulik

author: Armadillo
author_rank: Super Contributor
timestamp: January 16, 2018, 04:23:22 pm
content: Hi,I think your idea is feasible. At least to power up the unit to identify if other parts of the generator are working properly, at the very least.Since you are in general electronic repair, then you should also repair the switching power supply.Unplug connector to mother board. Take out the board from the unit.Check for any missing components on the board, power it up and measure the output voltages.[remember to discharge capacitors before you touch the board again].

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 16, 2018, 06:57:09 pm
content: If you're getting no output at all (check with no load to and see if there are any phantom voltages), then check the primary side first. Check fuses, rectifiers, switching transistors exc.

author: Sam Cohen
author_rank: Newbie
timestamp: January 16, 2018, 08:28:02 pm
content: Hi Armadilo and CyberdragonThank you for your reply.I did took the PS Out, no voltage on any of the outputs with and without Loads.Fusses capacitors rectifiers looks OK above that its beyond my capabilities.Any way I did hookup that meanwell PS that I have with the 38V DC to DC converter and vwalla!! It works!!.I let it work for few hours and saw no signs of excessive heat nor did the voltage drop on any of the rails.So for now im putting the old PS aside and im working on mounting the replacement unit in.I did manage to check the generator functionality and everything seems to work fine.Attached is a picture I took.Thanks againSam

author: Cyberdragon
author_rank: Super Contributor
timestamp: January 17, 2018, 03:02:47 am
content: Make sure it's still calibrated since you changed the power unit.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: January 17, 2018, 03:16:21 am
content: Hi,I fixed one of the power supplies in an HP8648. I didn't take very good notes, but I can remember that the problem was a high value resistor, I think it was 470kThe resistor is part of the start up circuitry.If you post a good picture of the board, I might be able to help.Regards,Jay_Diddy_B

author: Sam Cohen
author_rank: Newbie
timestamp: January 17, 2018, 07:00:10 am
content: Hi JayJa_diddy_B: See attached picture of the Old PS unit.Cyberdragon: why do you think calibration is affected by the PS Change?, i dont see any data bus or something sophisticated running between the PS and the generator Board.Thx

author: jazzyjayzee
author_rank: Newbie
timestamp: July 21, 2020, 04:40:26 pm
content: Old thread, but +1, the 470k startup resistor is nested in behind the large power resistor by the main cap. Mine decided to simply not start, nothing burned, fuses intact. This resistor was open. Suggest larger wattage for long term reliability. Good luck!

author: yingziwin
author_rank: Newbie
timestamp: July 22, 2020, 01:24:53 pm
content: +1! I repair some 8648 power supply.The 470k startup resistor is open. Good luck!

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: July 22, 2020, 05:56:41 pm
content: Hi,Since this seems to be a common problem. Let me document it a little better. I have taken my HP8647A apart to photograph the repair.If you are working on these make sure that the main capacitor is discharged.OverviewThe resistor R1 is indicated by the arrow.The resistor is hidden near the power resistor.DetailsThis is the solder side of the board showing the pads for the resistor.RegardsJay_Diddy_B

author: Supsnoopy66
author_rank: Newbie
timestamp: December 26, 2020, 09:53:55 pm
content: Hi,I recently scored a 8647A in very nice condition out of ebay (as a repair project). However, it looks like Q2 is missing in the PSU (which has been fully recapped btw). I seriously doubt it is in working condition but before troubleshooting it, I'd like to make sure it's at least completeWould anyone happen to have a PSU in hand to snap a picture or maybe already know what are the references of Q2?Thanks a lot!Happy holidays!

author: kawal
author_rank: Frequent Contributor
timestamp: May 03, 2021, 06:42:13 pm
content: Great to have this post.My generator was fully working and all of a sudden it just will not turn. It turned out to the the resistor mentioned here which measured open out of circuit .Thanks for the post.