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- HP8924-Family-Spectrum-Analyzers
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Repair of HP8924C
I have a HP8924C from Amtronix that has been in storage for awhile.? When I attempted to power on, it doesn't.? Could be the power supply, maybe not.? Unfortunately, my test bench and repair/diagnostic equipment are in storage and I don't have a troubleshooting space.? Is there anyone close to Huntsville, AL, Nashville, TN, or Bowling Green, KY that can do reasonably priced repairs on this unit? |
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Re: HP8935 Batteries
Yes, the 8935 has a coin type with tabs soldered to a memory board.
rick
On Apr 6, 2025 9:28 PM, Jeff Acree wrote:
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Re: HP8935 Batteries
Yes.
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On Sun, Apr 6, 2025 at 9:28?PM Jeff Acree?wrote:
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Re: hp8921a power supply
Since you have replaced the power supply the Misc Diags? Self test section should be run which provides a confirmation of the power supply voltages as well as the initial processor self test.
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The great thing about the 892X series was the built in diagnostics so you really need to take advantage of them whenever possible.
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None of the other vendors of this era provided anything close to that level of sophisticated diagnostics built into the system. |
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Re: hp8921a power supply
the second hand power supply installed testing radios on tx it seems to be working well however i wasnt able to get a signal on rx generation .i did some checking using a spectrum anylser and it shows it working but 8 megs off frequency ie -8mhz below the set frequency so i will check more ...... |
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Re: HP8920B Blowing Fuses
so the short is after the 300VDC rail ....the DC input merely powers a 12V to 300V DC? converter?
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I would still check for a short across the main switching? FET ....IRF840? ?...usually covered with heat shrink over the heat sink.
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I would also check the output filter caps ....I would pull a couple and look for leakage.
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The 8920B's were late enough that the caps have not started failing but all of the 8920 series is petty old by now.
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Pulling the power supply is a PITA ....the RF I/O has to come out first.
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Pulling the main PCB from the power supply chassis is another PITA ....have to release the plastic standoffs which means applying upward? pressure as the retainers are compressed.? ?That's of course after removing all of the other screws and clips.
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Never scrapped one yet and bought a bunch of parts units long ago that have all been used /sold after being repaired. |
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Re: HP8920B Blowing Fuses
Hi,
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No SMD components in the 8920x PSU. Q1 is a TO220 package MOSFET. It's highly recommended that replace all the low voltage electrolytics while you have the PSU apart. Last one I did the capacitors looked OK and tested OK in circuit, but a couple had started to leak and tracking was burning the PCB underneath. As there are several in parallel in the outputs, in-circuit testing is not conclusive.
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There is a circuit in the files section??
?/g/HP8924-Family-Spectrum-Analyzers/files/8920AB%20Power%20Supply%20Information%20Schematic Note there are some variations on this over the years. I do repair them but am in the UK so not really worth the additional postage over Amtronix if you decide not to do it yourself. It's not difficult, just involved. There is a lot of disassembly just to get at anything. If Q1 has gone short have a close look at C2 and associated resistors. A shorted gate on Q1 can take this out, and if you are unlucky, the tiny feedback transformer too. Robert. |
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Re: HP8920B Blowing Fuses
开云体育Amtronix know these inside out and do a great job. It all depends on your disposable time budget. They have a lot if the necessary spares too.From a satisfied customer.
Colin G4CWH? On 4 Apr 2025, at 03:07, Jeff Acree via groups.io <locustpointlight@...> wrote:
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Re: HP8920B Blowing Fuses
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.?
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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 it 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.?
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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.? ?I am in King George County VA.
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Thanks!
Jeff KC8VFN |
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Re: HP8920B Blowing Fuses
Hi Jeff,
Something has shorted in your unit. If you take it apart, be careful with the long push rod from the front switch to the power supply. Many people have broken that plastic rod and it's no longer available. That push rod needs to be pulled off the power supply in a straight motion with the power supply switch off. I'm guessing your power supply has shorted capacitors and / or a shorted Q1 power FET.? Leaking capacitors that short or burn is common. Generally, the unit emits smoke before it fails. The Q1 failure is rare but happens. There also is a bridge rectifier at the 120V input that can short. I would also check the rear AC/DC switch that goes open a lot. We have seen a lot of cases where the unit is working just fine, then after the rear AC/DC switch is cycled, the unit won't work. For that reason we recommend not using the rear AC/DC switch. If needed, Amtronix can repair your power supply and check your unit for $350. Rick |
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HP8920B Blowing Fuses
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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