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HP8920B Blowing Fuse
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Friends,
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My beloved HP8920B RF Communications Test Set started blowing the 5A AC fuse today.? It blows as soon as you push the power switch.? I am wondering if there is a common problem I could look for.? Tomorrow I may try to see if it will run on 12VDC.? It has a connector and switch for DC operation.? Any advice appreciated.
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Thanks!
Jeff?
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I would check at the input of the power supply for a shorted filter cap. or a shorted rectifier diode.
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On 2025-04-02 9:01 pm, Jeff Acree via groups.io wrote:
Friends, --
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You have a failed power supply/ Almost certainly the main switch Q1 a MOSFET. This can also take out the feedback transformer T2 which is a PITA. You can ty DC input, see /g/HP-Agilent-Keysight-equipment/message/151023 for the connector detail, its the same on SA and 8920x. Better to use 24V if you have it the current is high at 12V. If it doesn't run on DC it is probably Q1. The DC input runs a DC-DC converter with high voltage DC output. If it runs on DC it is likely the mains bridge rectifier. A failed Rifa is possible but unlikely as they self heal and you can't miss the smell....
Join the 8920 group too /g/HP8924-Family-Spectrum-Analyzers/topics ?
Robert. |
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This morning, I tried running it using a DC power supply and that led to it blowing the 15 Amp external fuse for DC power.? As I mentioned yesterday, if you try to use on AC, it blows the external 5 Amp AC fuse.? I have never had the cover off, but that is the decision I need to make now.? Send it off or have a go at myself.? I have a well-equipped lab but am not sure about a schematic for the power supply.? I will need to look for one.? I won't work on much of anything without one.? From what I've seen in response to my inquiries, it seems like Q1 is a possibility.? Will it be a SMD?? I have some SMD rework equipment, but not a ton of experience with it.? I normally work on Ham Radio Equipment, mostly repeaters, not HP test equipment.? Recommendations welcome.?
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Thanks!
Jeff |
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Hi Jeff. I feel your pain. And FYI, the power supply is a pain to work on. The PCB is held by a number of plastic stand-off type things that you have to squeeze closed and work the board out. But if you go this route, Q1 is a likely suspect.
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I have attached the schematic of the power supply, at least this is what I have seen in the 8920's I've worked on.
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Good luck!
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Mark
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