HP10525T Logic Probe: New Timing Capacitors

While putting together the PIR sensor, I had occasion to haul out the old HP10525T Logic Probe (a bookend for the Tek logic probe) to figure out why the shift register wasn’t updating; that was easier than hauling the breadboard to the oscilloscope. While it showed the problem (wire tucked into wrong hole, hidden behind a cluster of other wires), it didn’t seem to be blinking quite right. The HP10525T Logic Probe Operating and Service Manual says it should blink at about 10 Hz for any pulse train from about 10 Hz up through 50 MHz (yes, 50 megahertz), with a minimum pulse width of 10 ns (yes, 10 nanoseconds), but it didn’t do that for the PWM going to the RGB LED strip or the shift register clock.

Given a manual printed in February 1975, I’m sure you know where this will end up…

Unlike contemporary gear, the manual tells you how to dismantle the probe, using the needle tip as a tool. Doing so reveals a tidy circuit board with gold plated PCB traces:

HP10525T - original caps
HP10525T – original caps

The two tiny black rectangular capacitors just to the left of the 8 pin DIP IC are C1 and C2, rated 10 μF at 2 V (yes, 2 volts). As you might expect, they had ESRs in the 3 to 5 Ω range, rather than around 0.2 Ω. The catch is that the case doesn’t have room for anything much taller, but I did contort some solid tantalum through-hole caps into the space available:

HP10525T - replacement caps
HP10525T – replacement caps

Buttoned it up again and … it works fine. There really isn’t that much else to go wrong, is there?

This picture shows the incandescent lamp glowing half-bright to indicate that the lethally sharp probe tip (on the left here, with its stud on the right in the other pictures) sees a floating input:

HP10525T Logic Probe - glowing
HP10525T Logic Probe – glowing

I love happy endings, although I’m sure the accompanying HP10526T Logic Pulser needs recapping, too. When that project comes around, I’ll probably use SMD ceramic caps, because the pulser’s circuit board packs even more parts into the same volume.

Speaking of unhappy endings, HP used to be run by real techies: The Fine Manual’s body starts with Page 0 verso, after the title and two pages of front matter. ‘Nuff said.

9 thoughts on “HP10525T Logic Probe: New Timing Capacitors

  1. Fixed it with solid tantalums, eh? Hmm…

    I’ve had a few of those fail with the dreaded “ESR went to zero” problem. As in putting a short circuit across power supplies my 1979-vintage Tek 465B.

    Of course if you get a similar lifetime out of your replacements, that logic probe should still be going strong when you hand it down to the Larval Engineer someday:-)

    1. a few of those fail

      They’re probably about as old as the probe, so they’re getting along in years, too.

      They (allegedly) lack gooey electrolyte inside and should last roughly forever; of course, I could be totally wrong about that…

  2. I loved the HP gold-plated equipment from the ’70s. Had an original HP45 calculator that used gold plated traces as well as gold-leaded hermetic ICs. Things got, er, interesting once they started to use substitutes for gold in the products. Got a little funky with non-gold on keypads on calculators…

    Not familiar with Agilent’s test equipment, but that where the tech-heads ended up when HP decided to become a computer-only company. (A lot of the businesses were spun off Agilent, medical being first, with semiconductors following.) Note of irony: HP got into computers as a way to control the test equipment.

    1. when HP decided to become a computer-only company

      And should have left the HP name with the test equipment / tech biz, where it belonged. Ptui!

  3. I went to high school a long time ago, when there were televisions with mostly vacuum tubes. In my junior year, I went to work part time for a TV repair shop, thinking I would learn electronics from gurus. What I did learn instead was that they were basically salesmen, but, they had files full of Sam’s “Photofacts” which had all the information you would ever need to repair a set, even if you had no clue how it actually functioned. The photos would show where to put the scope probe, how to set the scope and pictures of the wave-forms. Most of the manuals that I saw had step by step directions on how to troubleshoot a set. On Saturdays they had a university student fix the sets that they couldn’t and I got to do the antenna work on the scary slate roofs. Today it is very hard to actually get the technical specs for most electronics gear much less repair it.

    1. much less repair it

      This may be the Golden Age of RF and analog hacking, but the fact is you can’t repair anything because all the functions reside inside a honkin’ big digital chip that you can’t probe or (un)solder or modify.

      If it ain’t bad caps or a broken connector, that thing is trash…

      1. If it ain’t bad caps or a broken connector, that thing is trash…

        From what I’m seeing, it looks like bad caps might be the biggest hardware failure mode for medium-old equipment. Well, alongside NiCd batteries that go toes-up. I’ve got a shaver that won’t power up anymore–glad I’m usually bearded. (Won’t get into firmware that doesn’t understand Y2K…) I’ve been cleaning the gubbage from the work room for a marathon fixit session–too many projects, too much bad weather.

        (BTW, got the Dust Deputy cyclone, just have to cut a decent 3″ hole through 1/2″ plywood and 16ga steel, preferably at the same time. My biggest bimetal hole saw is 2.5″, so I need to get creative. Might use the 8″ rotary table…)

        Gives me a little hope to keep some of my older

        1. bad caps might be the biggest hardware failure mode

          Either the bad electrolyte problem was a lot more prevalent than I thought or many manufacturers produced bad seals that let the caps dry out. Fortunately, I just fixed the power switch in a Tek scope that dates back to 1983 and all the caps were fine, so some folks got it right…

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