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Author Topic: 34401a no display repair. How does one remove the power switch?  (Read 1221 times)

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Offline BobRyanTopic starter

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34401a no display repair. How does one remove the power switch?
« on: December 12, 2020, 09:45:17 pm »
I've just purchased a 34401a "turns on" which has no display. I've checked voltages to the front panel and +/-18v are present. Filament voltage is 5.4vAC with ribbon cable removed and 4vAC with the ribbon cable in place. Resistance across the filament is 10 ohms.

I'm suspecting a driver issue or perhaps a problem with the -13v regulation on the front panel but I can't get the damn thing off. I can't remove the display because the power switch is in the way and I'm afraid of breaking the front cover or the power switch prying at the edge.

I'm hopeful the unit is functional as the continuity tester seems to work. I've tried to blindly put the unit into RS232 remote mode but haven't been successful.

Can anyone give some advise on how to remove the long white pushrod? I have it disconnected from the switch at the rear of the unit but I don't see how to remove it.

Bob
« Last Edit: December 12, 2020, 09:53:45 pm by BobRyan »
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Offline BobRyanTopic starter

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Re: 34401a no display repair. How does one remove the power switch?
« Reply #1 on: December 13, 2020, 12:49:32 am »
After finding https://www.eevblog.com/forum/repair/how-to-hpagilent-34401a-display/ I was able to remove the power push rod. It required pushing the frame holding the transformer out a little bit but the front panel came off easily once the push rod was removed.

I'm still trying to figure out the display problem. It doesn't seem to be power or logic related which makes me wonder if the VFD itself has completely failed. There are a few ominous specks of ash(?) floating around in the display. I probed all of the pins and see what looks like +/-18v logic on most and power rails or the AC filament voltage on the others. Two pins (highlighted in red) display what looks like garbage but I haven't figured out the pin numbering yet
« Last Edit: December 13, 2020, 01:09:16 am by BobRyan »
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Offline Runco990

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Re: 34401a no display repair. How does one remove the power switch?
« Reply #2 on: December 13, 2020, 01:38:22 am »
Stop troubleshooting.... the VFD tube is open to air.  Replace it.  Meter is probably fine.

Look at the getter flash spots.  That is your problem.
« Last Edit: December 13, 2020, 01:40:12 am by Runco990 »
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Offline BobRyanTopic starter

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Re: 34401a no display repair. How does one remove the power switch?
« Reply #3 on: December 13, 2020, 01:43:38 am »
Thank you for the quick response. New VFD is on its way.
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Offline BobRyanTopic starter

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[SOLVED] 34401a no display repair. How does one remove the power switch?
« Reply #4 on: December 27, 2020, 02:05:01 am »
The replacement VFD I purchased off ebay fixed the issue. The unit I received looked brand new but the pins had obviously been soldered before.
Thanks again Runco990!
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Offline Runco990

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Re: 34401a no display repair. How does one remove the power switch?
« Reply #5 on: December 27, 2020, 04:35:09 am »
Sweet!  You're welcome!   :-+
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