Archive for December 20th, 2012

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.