Re: More Investigation of the Malfunctioning Time Base RESET Lamp in my HP-1421A.
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That's the action I see when using the LED circuit.? Yes, V105 illuminates as long as the RESET switch is pressed.? I don't have it in front of me, but I'm pretty sure mine is an 0803 prefix as well.
Thanks,
Barry - N4BUQ
My 1421A (serial 0803A03889), when I go to Single Sweep and press Reset switch S105, the Reset neon DS101 blinks and neon V105 illuminates as long as the switch is pressed.?
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??
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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