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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

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.

Argh!! I meant grounding Q109's collector (although grounding the base through a resistor has also shown the bulb will light as well). I think I may need to stop typing today... Barry - N4BUQ

BTW, when I placed the 1K across R137, that gave it enough of a path to ground through R139 as well as to -12.6V through R138 to make it light up so I presume grounding Q109's base will cause it to

Here's how I have done it for about 30 years. I have drill bits the size that work for my probes, I wind the wire around the bit to get a perfect fit every time. Sorry about the fuzzy picture. Mikek

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.

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 be?

(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 plug-ins? Jeremy N6WFO

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