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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

I'm not sure exactly what you're asking about. The signal on Q109's base is tied to the base of 1/2 of a Schmitt Trigger through R140. Aside from the fact that the schematic has the wrong value (it

OK, let me pull my 1421A manual. Jeremy [email protected]> wrote:

Well, one effective way to get a decent ground connection is to wrap bus wire around the probe ground right at the tip and then solder it to the groundplane of the board.? I got that idea from the

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

If you can turn it on, it's not bad.? If you can provide a signal to the transistor that turns it on, it's likely not bad. Where does the signal come from, is it there when it is supposed to

(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

You are correct! It's the lamp in my 1421A, not my 140B. Apologies. I'll start a new thread. Thanks so much, Barry - N4BUQ

I hate to be stupid about this, but my HP-140B (serial 1119A00489) doesn’t have a DS101, only a DS401 (the Power ON lamp). Might you be referring to one of the

The lesson learned is that probing high-impedance, high-frequency circuits with an ordinary scope probe will not result in accurate readings, and may very well disturb the circuit under test. To make

I am still unable to get DS101 to turn on in my HP-140B. 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

Very interesting, jmr, and I haven't really given it much thought until now. Most of my work these days, at my day job, anyway, is frequency domain, 6 to 18 GHz, so microwave. Not using a scope too

Hey All, I just picked up a 3245a (waiting for it to be delivered today) and I'm wondering if anyone ever acquired a copy of the original calibration software? I know that Illya over on xDevs has a

Ok thanks. If it helps any further see the link below: Tektronix Oscilloscope Probe Circuits Concepts (w140.com) ( https://w140.com/tekwiki/images/6/62/062-1146-00.pdf ) This is an ancient document

Ha, I can relate, Jinxie!? Between 12 hour workdays and about 2.5 hours a day on the freeway every day, I have zero time for extracurricular activities during the week.? Weekends I'm watching our

JMR, thank you very much for the very thorough and well set out reply (and all your contributions on the topic as well of course). I'd be interested to see if I can duplicate your results with my own

I went right through the manual again. The "Wform Save" button saves the waveform to one of 4 novolatile memories. I cannot do a measurement on them, in fact the manual says you can't. So this is not

I also found an old VNA measurement of my 10073A scope probe. You can see from the blue trace that Cp falls up at VHF and UHF. It falls to about 5pF. You can also see the knee in the green Xp trace

In case anyone claims that a 50R VNA can't reliably measure small capacitances at V/UHF, I've dug out an old VNA measurement I made of an exotic 3.3pF SMD cap up to 2GHz.? I measured it on a VNA and

This is getting ridiculous now... The Cp and Rp of a scope probe is very easy to measure up at VHF using a regular 50R lab VNA. Here's another graph from Tektronix showing how Xp changes up at VHF.