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
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----- 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 groups.io 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
