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A very slow sweep speed (several seconds to sweep across the CRT) will give your DVM time to display the changing voltages at Q109.?

Jeremy?

On Sun, Apr 14, 2024 at 8:50?PM n4buq via <n4buq=[email protected]> wrote:

I need to double-check but I don't think that's exactly how mine is working (or, more properly, _not_ working).

In SINGLE mode and using an external trigger and the LED circuit, pressing RESET causes the LED to illuminate and it stays illuminated.? If I then trigger the sweep via the EXTERNAL input, the LED stays illuminated until the sweep finishes, at which point, it is extinguished and stays extinguished until RESET is pressed.? That process is slightly easier to follow at a slow sweep speed.? If I switch to NORMAL mode, the LED flashes in time with the sweep start and finish but it's a bit more difficult to tell exactly when it comes on and when it goes off.

My DMM isn't the best for checking those voltage changes due to it's relatively slow speed at which changed voltages appear.? I'll switch to watching that on another scope to get a clearer picture of those changes.

Thanks for checking that and the detailed voltage change points!

Barry - N4BUQ

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From: "Jeremy Nichols" <jn6wfo@...>
To: "HP-Agilent-Keysight-equipment" <[email protected]>
Sent: Sunday, April 14, 2024 10:24:00 PM
Subject: Re: [HP-Agilent-Keysight-equipment] More Investigation of the Malfunctioning Time Base RESET Lamp in my HP-1421A.

Barry, using either a very slow sweep speed (so there’s time to watch the voltages change) or Single Sweep mode.?

Q109

Collector +65V before sweep begins, changes to -9V during sweep, then returns to +65V

Emitter -9.39V changes to -9.09V and returns

Base -11.6V changes to -8.5V and return

DS101 illuminates during sweep (regardless of length of sweep) and only during sweep. V105 illuminates only while S105 is pressed.?

If S105 is pressed and held, the sweep DOES NOT repeat. During the sweep, DS101 illuminates. When the sweep ends, DS101 extinguishes, whether S105 is held closed or not. S105 must be cycled OFF-ON to sweep again.?

Jeremy?

On Sun, Apr 14, 2024 at 8:04?PM n4buq via <n4buq=[email protected]> wrote:

Jeremy,

Neither NORMAL or SINGLE will cause the lamp to light but the LED lights properly in either mode.

The voltage at Q109's collector is what's puzzling me.? When I first turn it on, the voltage is somewhere close to the value on the schematic (I don't recall the exact voltage); however, after a few seconds, the voltage drops significantly to somewhere close to what I listed and if I recall, it continues to slowly drift towards zero over a period of 20 or 30 seconds.? I'm not sure what that voltage will settle to if I turn it on in SINGLE mode and no triggering event occurs.

I know that neon lamps do not need the firing voltage maintained in order to keep the lamp on but 52V seems quite low and that doesn't take into consideration how much voltage R135 drops so that would be just a bit less than 52V across the lamp.

It would be interesting to know what you see for the voltage at Q109's collector in your 1421A.

Thanks again,

Barry - N4BUQ

Barry, what happens when you go to Single Sweep and press Reset switch S105??

My schematic (and yours) shows the Collector of Q109 at +48V but your measurement shows -0.33V??

Jeremy?

On Sun, Apr 14, 2024 at 7:20?PM n4buq via <n4buq=[email protected]> wrote:

IIRC, when I had the front panel off for cleaning, I inspected those and I _think_ they were very close.? Although I don't know why, they used 1% resistors there.? Of course, that doesn't prevent them from drifting but I do think they were quite close to spec.

Thanks,
Barry - N4BUQ

----- Original Message -----
> From: "Peter Gottlieb" <hpnpilot@...>
> To: "HP-Agilent-Keysight-equipment" <[email protected]>
> Sent: Sunday, April 14, 2024 9:07:32 PM
> Subject: Re: [HP-Agilent-Keysight-equipment] More Investigation of the Malfunctioning Time Base RESET Lamp in my
> HP-1421A.

> If you can lift Q109C then you could at least measure the sum of those two
> resistors.? But it would be nice to be able to measure the junction of them.
>
> On 4/14/2024 9:55 PM, n4buq via wrote:
>> I'll try grounding the collector of Q109.? Unfortunately, lifting one lead of
>> R136 isn't quick.? The front panel has to be removed to gain access to the
>> small assembly that holds DS101, R135, and R136.? It's too cramped to get to it
>> from the back side of the front of the chassis.
>>
>> Thanks,
>> Barry - N4BUQ
>>
>> ----- Original Message -----
>>> From: "Peter Gottlieb" <hpnpilot@...>
>>> To: "HP-Agilent-Keysight-equipment" <[email protected]>
>>> Sent: Sunday, April 14, 2024 8:10:33 PM
>>> Subject: Re: [HP-Agilent-Keysight-equipment] More Investigation of the
>>> Malfunctioning Time Base RESET Lamp in my
>>> HP-1421A.
>>> Let's go real simple.? You report the collector of Q109 is at essentially
>>> ground, right?
>>>
>>> So if you ground it, and ASSUMING R135 AND R136 HAVEN'T DRIFTED, the neon should
>>> see 86.9 volts, which should light it.
>>>
>>> Neons when new fire somewhere in the 65 volt range but when old can easily drift
>>> upwards, even higher than that voltage.? And resistors can and do drift out of
>>> tolerance.
>>>
>>> Check R135 and R136 values.? Then ground Q109 collector and measure the junction
>>> of R135 and R136 voltage.? If you see near 86.9 volts your neon is old and has
>>> too high a firing voltage.
>>>
>>> Quick and dirty, try lifting one lead of R136 and see if the neon lights then.
>>>
>>> Peter
>>>
>>>
>>> On 4/14/2024 8:59 PM, n4buq via wrote:
>>>> (New thread due to very incorrect subject line and opening paragraph in
>>>> previous thread.)
>>>>
>>>> I still have not determined why DS101 will not turn on in my HP-1421A
>>>> time-base.? I can ground Q109's base through a resistor and force the lamp to
>>>> turn on.? I can place a 1K resistor in parallel with R137 and also force the
>>>> lamp to turn on; however, nothing else works. I thought that the neon might
>>>> have aged to the point where there isn't sufficient striking voltage for it
>>>> but since it will turn on with these methods, that doesn't seem to be case(?).
>>>>
>>>> Setting the unit up as described in the manual for voltage tests, I get the
>>>> following:
>>>>
>>>> -100V F(B): -99.9V
>>>>
>>>> Q109
>>>> Emitter: -9.35V
>>>> Base: -8.78V
>>>> Collector: -0.33V
>>>>
>>>> Q110
>>>> Emitter: -8.01V
>>>> Base: -8.11V
>>>> Collector: -3.7V
>>>>
>>>> Given those values, it would appear (at least to me) that the lamp should turn
>>>> on but it doesn't.
>>>>
>>>> As a test, I disconnected R136/DS101 (the yellow (4) wire) from the board and
>>>> connected a battery, resistor, and LED between GND and the collector of Q109.
>>>> This works just as it should (i.e. the LED turns on when the sweep is
>>>> triggered, turns off at the end of the sweep, and repeats that).
>>>>
>>>> Is it possible that Q109 is not able to conduct enough current to turn DS101
>>>> on but enough to make the LED work?? That seems unlikely but, at this point, I
>>>> don't have any other explanation.
>>>>
>>>> Any help with this is appreciated.
>>>>
>>>> Thanks,
>>>> Barry - N4BUQ
>>>>
>>>>
>>>
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