Re: New member with a currently-dead 2465

Vincent Mallet

It lives!!

I'm definitely in here for the journey but it's nonetheless interesting to
see how someone with experience would zero in on the potential culprit and
how they'd move forward with the investigation.

Here I put things back together, removed all the probes except TP505 (far
enough from the circuit not to influence it) and tried a few power on/off
cycles to try to catch the oscillator napping... It was mostly hyper (i.e.
30MHz at pin-1, 3.75MHz at TP505). Caught it napping once, went to measure
voltage drop over R2559 with a DMM and that was enough to wake it up!

Gave up on that plan; I lifted a leg of each of the five resistors in there
to measure them out of circuit; they look impeccable:
R2549: 685R (specs: 680R 5%)
R2564: 330R (specs: 330R 5%)
R2571: 986R (specs: 1K 5%)
R2573: 997R (specs: 1K 5%)
R2553: 10K01 (specs: 10K 5%)

I removed U2556 and replaced it with a new one from TI that I had around
and ta-da! It lives! Super exciting!

I managed to remove U2556 without turning it into a sacrifice so I will try
to exercise it offline to see how it behaves.

I had a couple of questions on the oscillating circuitry:

1/ R2553 is a 10K tied to -5V. What does that do? Why do we need to bring
-5V into the picture?

2/ Figure 3831-50 “Test waveform setup information” for A5 shows 3
waveforms, including “1” taken at U2556-4 and “2” taken at U2468-5. Both
waveforms seem to be centered around 0V with a +/- 2V swing around 0. How
do I interpret this? Why negative? If I probe U2556-4 (now that it lives) I
get a waveform that looks like what’s in the manual but it swings from 0V
to ~3.6V. Is it because I am supposed to be AC-coupled and ignore the

Now I need to find some probes for this scope so I can try to quantify its
performance. The only probes I have are the ones bundled with my Siglent
DSO ("PP215 200MHz") and I don't know how good or bad they are.



On Sat, Jul 11, 2020 at 8:21 AM Siggi <siggi@...> wrote:

On Fri, Jul 10, 2020 at 8:02 PM Tom Miller <@tmiller>

Just replace U2556. It is a cheap part. Cut the leads close to the chip
package, remove the package, then unsolder and remove each pin one at a
time. Clear the holes and put in a new 74LS04. I wager a cheeseburger
that is the problem.
In an over-the-counter repair shop, this would be sage advice, and I'm not
sure I'd take you up on that bet.
I believe Vincent is, however, more interested in the journey than the
destination, and by measuring and thinking through all the things, you
learn more :).

Join to automatically receive all group messages.