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The ready lamp for SINGLE triggering mode in my HP 1421A isn't/wasn't working. ?I'm not sure what the problem is but I have removed the front panel to clean it and before I reinstall it, I'd like to determine whether the lamp is good as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not sure if that's when the lamp is supposed to be on or off so I'm not sure if that's enough to turn the lamp on.? I'm not finding much in the way of specs for an NE75 either.
Anyone know how I can test it?
Thanks,
Barry - N4BUQ
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Hi Barry,
NE75 information is in the link below:
Breakdown is 60-90V, maintaining is 55V, design current is 0.4mA
Voltages in the circuit condition are when the light is off as it is below holding voltage (resistive divider R135/R136 further divides the voltage to ~ 45V).
If you are using a bench supply to test the bulb you need to supply above 90V (worst case) and limit the current in holding state (when voltage across the bulb is 55V) to 0.4mA with a resistor. Do not apply voltage directly without a current limiting resistor.?
Ozan
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On Mon, Mar 25, 2024 at 09:33 PM, n4buq wrote:
The ready lamp for SINGLE triggering mode in my HP 1421A isn't/wasn't working. ?I'm not sure what the problem is but I have removed the front panel to clean it and before I reinstall it, I'd like to determine whether the lamp is good as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not sure if that's when the lamp is supposed to be on or off so I'm not sure if that's enough to turn the lamp on.? I'm not finding much in the way of specs for an NE75 either.
?
Anyone know how I can test it?
?
?
Thanks,
Barry - N4BUQ
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It should also light or flash when the 1421A is triggered, with the switch in normal.
The 1821A uses a similar circuit for the reset neon, but has it built into the reset switch, I had a similar problem with that, the neon was failing to light, I proved this by adding another neon in parallel, which worked fine. Fortunately I had a parts donor to repair the 1821A, as the switch is unobtainable.
To test neons I've used a current limited DC insulation tester (commonly known as a Megger in the UK), a voltmeter can be added in parallel to find the maintaining voltage, I use the 250V range on the Megger, if it doesn't light at that it's dead.
I could maybe use the Tek THS710 in parallel if I wanted to see the striking voltage too (might try this).
One of the Tek curve tracers could also be used, but I haven't got one.
As could a suitable? bench power supply, a series resistor is needed for this method.
David
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Ozan, David,
Thanks so much for the replies.? The two resistors (R135 and R136) are both mounted with the lamp so I can power it with R135 in series.? Since that lamp should be lit in both NORMAL and SINGLE and both modes appeared to be working, then I'm even more suspect of the lamp.? Testing it out of the circuit will confirm that one way or the other.
Thanks guys, Barry - N4BUQ
From: "Barry" <n4buq@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Monday, March 25, 2024 11:33:32 PM
Subject: How to Test an NE75 Neon Lamp
The ready lamp for SINGLE triggering mode in my HP 1421A isn't/wasn't working. ?I'm not sure what the problem is but I have removed the front panel to clean it and before I reinstall it, I'd like to determine whether the lamp is good as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not sure if that's when the lamp is supposed to be on or off so I'm not sure if that's enough to turn the lamp on.? I'm not finding much in the way of specs for an NE75 either.
Anyone know how I can test it? Thanks,
Barry - N4BUQ
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On 2024-03-26 05:33, n4buq wrote:
The ready lamp for SINGLE triggering mode in my HP 1421A
isn't/wasn't working. ?I'm not sure what the problem is but I
have removed the front panel to clean it and before I
reinstall it, I'd like to determine whether the lamp is good
as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not
sure if that's when the lamp is supposed to be on or off so
I'm not sure if that's enough to turn the lamp on.? I'm not
finding much in the way of specs for an NE75 either.
Anyone know how I can test it?
Thanks,
Barry - N4BUQ
50V on the lamp, if dark, is the idle state and will not turn the
lamp on.
If the +100V is present and you temporarily jumper point (4)? to GND
the lamp should turn on.
Arie
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Arie,
Okay - that makes a bit more sense to me.? Thanks so much for that!
Barry - N4BUQ
From: "Arie de Muijnck via groups.io" <mygroups@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Tuesday, March 26, 2024 8:30:09 AM
Subject: Re: [HP-Agilent-Keysight-equipment] How to Test an NE75 Neon Lamp
On 2024-03-26 05:33, n4buq wrote:
The ready lamp for SINGLE triggering mode in my HP 1421A
isn't/wasn't working. ?I'm not sure what the problem is but I
have removed the front panel to clean it and before I
reinstall it, I'd like to determine whether the lamp is good
as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not
sure if that's when the lamp is supposed to be on or off so
I'm not sure if that's enough to turn the lamp on.? I'm not
finding much in the way of specs for an NE75 either.
Anyone know how I can test it?
Thanks,
Barry - N4BUQ
50V on the lamp, if dark, is the idle state and will not turn the
lamp on.
If the +100V is present and you temporarily jumper point (4)? to GND
the lamp should turn on.
Arie
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BTW if it turns out the lamp is bad, it should be easy enough to find an equivalent, depending on how it's hooked up. If just wire leads, no problem. If it's on a base like a bayonet style, that should be common enough too, but if not, you can bust out the glass bulb and solder a regular leaded one into the base.
Another option is to put in a high efficiency LED, as long as the circuit has suitable current limiting and polarity. Simple mods can make it right if needed. It's quite amazing to see how much light you can get from high efficiency LEDs with currents below 1 mA, like you'd have in neon circuits. My favorites are the high power "white" LEDs commonly used en masse for lighting. They're made for up to hundreds of mA sometimes, but at tiny currents can run rings around a neon lamp when properly used in its place. The color won't necessarily match, but if it's in a fixture jewel or behind a filter, there are more options.
I have done this for neon replacement in optical choppers, and many times for various bulb and regular LED upgrading of indicator light fixtures.
Ed
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It's a typical leaded bulb characteristic of many neons of the era.? If it's bad and I can't find a suitable neon, I might go with an LED.? It would be easy enough to install.? The existing bulb is held in a soft plastic semicircle that helps lock the assembly to the front panel.? An LED would easily do the same thing.? A yellow LED would resemble the neon closely enough for me.
Thanks,
Barry - N4BUQ
BTW if it turns out the lamp is bad, it should be easy enough to find an equivalent, depending on how it's hooked up. If just wire leads, no problem. If it's on a base like a bayonet style, that should be common enough too, but if not, you can bust out the glass bulb and solder a regular leaded one into the base.
Another option is to put in a high efficiency LED, as long as the circuit has suitable current limiting and polarity. Simple mods can make it right if needed. It's quite amazing to see how much light you can get from high efficiency LEDs with currents below 1 mA, like you'd have in neon circuits. My favorites are the high power "white" LEDs commonly used en masse for lighting. They're made for up to hundreds of mA sometimes, but at tiny currents can run rings around a neon lamp when properly used in its place. The color won't necessarily match, but if it's in a fixture jewel or behind a filter, there are more options.
I have done this for neon replacement in optical choppers, and many times for various bulb and regular LED upgrading of indicator light fixtures.
Ed
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You would need to modify the circuit to use an LED, as it would be lit all the time with the resistors used, if you break the series resistor chain, the transistor might need changing too if it's not rated to switch 112V.
Closest currently available neon (without large minimum order quantity) would be the A9A. If you want one with reduced dark effect, A1D-T might do.?
David
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I got around to testing that lamp.? With R135 and R136 still in place, I connected to the end marked +100V and the other side of the lamp to ground.? The lamp fires very close to 90V so it's still good.? Looks like I get to start troubleshooting why it doesn't work (presuming it still doesn't) when I get the plugin reassembled.
Thanks, Barry - N4BUQ
From: "Barry" <n4buq@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Tuesday, March 26, 2024 8:15:24 AM
Subject: Re: How to Test an NE75 Neon Lamp
Ozan, David, Thanks so much for the replies.? The two resistors (R135 and R136) are both mounted with the lamp so I can power it with R135 in series.? Since that lamp should be lit in both NORMAL and SINGLE and both modes appeared to be working, then I'm even more suspect of the lamp.? Testing it out of the circuit will confirm that one way or the other. Thanks guys, Barry - N4BUQ
From: "Barry" <n4buq@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Monday, March 25, 2024 11:33:32 PM
Subject: How to Test an NE75 Neon Lamp
The ready lamp for SINGLE triggering mode in my HP 1421A isn't/wasn't working. ?I'm not sure what the problem is but I have removed the front panel to clean it and before I reinstall it, I'd like to determine whether the lamp is good as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not sure if that's when the lamp is supposed to be on or off so I'm not sure if that's enough to turn the lamp on.? I'm not finding much in the way of specs for an NE75 either.
Anyone know how I can test it? Thanks,
Barry - N4BUQ
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After reassembling the [now clean] time-base, DS101 is still not illuminated.? I've checked the +100V rail and that checks at 99.x volts (don't remember the exact fractional part).
I had not previously removed the top cover to observe the time-base.? With the cover removed, pressing the RESET switch causes a neon lamp (V105?) towards the back, right side of the main circuit board to illuminate so at least part of that circuitry is working.
I'm guessing the problem has something to do with Q109.? The -100V rail appears to be a crucial part of that circuit too.? I didn't check that rail but intend to do so tomorrow.
Thanks, Barry - N4BUQ
From: "Barry" <n4buq@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Tuesday, March 26, 2024 11:48:38 PM
Subject: Re: How to Test an NE75 Neon Lamp
I got around to testing that lamp.? With R135 and R136 still in place, I connected to the end marked +100V and the other side of the lamp to ground.? The lamp fires very close to 90V so it's still good.? Looks like I get to start troubleshooting why it doesn't work (presuming it still doesn't) when I get the plugin reassembled. Thanks, Barry - N4BUQ
From: "Barry" <n4buq@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Tuesday, March 26, 2024 8:15:24 AM
Subject: Re: How to Test an NE75 Neon Lamp
Ozan, David, Thanks so much for the replies.? The two resistors (R135 and R136) are both mounted with the lamp so I can power it with R135 in series.? Since that lamp should be lit in both NORMAL and SINGLE and both modes appeared to be working, then I'm even more suspect of the lamp.? Testing it out of the circuit will confirm that one way or the other. Thanks guys, Barry - N4BUQ
From: "Barry" <n4buq@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Monday, March 25, 2024 11:33:32 PM
Subject: How to Test an NE75 Neon Lamp
The ready lamp for SINGLE triggering mode in my HP 1421A isn't/wasn't working. ?I'm not sure what the problem is but I have removed the front panel to clean it and before I reinstall it, I'd like to determine whether the lamp is good as it's fairly messy to remove the front panel to get to it.
From what I can tell, there's about 50V across that but not sure if that's when the lamp is supposed to be on or off so I'm not sure if that's enough to turn the lamp on.? I'm not finding much in the way of specs for an NE75 either.
Anyone know how I can test it? Thanks,
Barry - N4BUQ
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Arie de Muijnck