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  1. HP8924-Family-Spectrum-Analyzers
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HP 8924C power supply issue

 

Hello group, thanks for letting me join,? My name Is Bill and call is N7OQ.?


I got a free HP-8924C that the guy said needed a new power supply. Well I put Some power to it and nothing, then I removed the PS and opened it up looking for any obvious problems. I cleaned it up everything looked good, no burned components or over heating areas so I put it back together and plugged it in and the green light came on. Now the unit powers up, had the clean display adjustment post to keep a display Going and now that works good too.?


Well this morning I hit the switch and nothing, it was in the garage and was cold, so I put a little heat into the PS with a hot air gun and it came right back to life. Has anyone seen this problem? As long as I keep the unit plugged in even with power off it stays warming enough and will fire right up. But who how long that will last. Thanks
 

It is a very common issue with these power supplies. All the electro caps go faulty. Especially the little ones on the small boards. You need to change them all for 105 degree rated caps. The old 85 degree caps dry out. Also do the caps in the regulator PCB - its separate to the power supply. You may also see the odd tantalum cap smoke, bang or go faulty in the rest of it. Takes allot of time to replace them all but well worth it. The very best general purpose analogue test gear ever made.
 

there is a write up in the files section ...or at least there used to be from the old hosting service.

usually you are missing the green led on the rear of the unit which is the 15 volt standby supply.

That should be there anytime the unit is plugged in ....which is why it dies ...used to power the internal reference as well as the on/off voltage IIRC

This comes from a small daughter board on the back corner and the cap on that board fails.

not usually all of the caps ...just the small one on the daughter card.
 

As others said, it is most likely the capacitors.? About 90% or more switching supply problems are the capacitors.

I had an 8924c that acted similar.? I took a hot air gun from a SMD rework station that had some small tips and could blow hot air on one capacitor at a time.? Found the one that was causing the problem,but replaced several other ones while I had the 8924c open.? After going through all the trouble of removing all the screws I felt it worth while to replace many of the electrolytic capacitors .

Ralph ku4pt



On Saturday, April 17, 2021, 03:34:03 AM EDT, William Verstelle <bverstelle@...> wrote:


Hello group, thanks for letting me join,? My name Is Bill and call is N7OQ.?


I got a free HP-8924C that the guy said needed a new power supply. Well I put Some power to it and nothing, then I removed the PS and opened it up looking for any obvious problems. I cleaned it up everything looked good, no burned components or over heating areas so I put it back together and plugged it in and the green light came on. Now the unit powers up, had the clean display adjustment post to keep a display Going and now that works good too.?


Well this morning I hit the switch and nothing, it was in the garage and was cold, so I put a little heat into the PS with a hot air gun and it came right back to life. Has anyone seen this problem? As long as I keep the unit plugged in even with power off it stays warming enough and will fire right up. But who how long that will last. Thanks

._,_
 

Thank you everyone, I¡¯ll order 105 degree caps and do a recap. So far everything else seems to be working. This is a great forum.
Bill N7OQ

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