Re: New member with a currently-dead 2465


On Fri, Jul 10, 2020 at 7:33 PM Vincent Mallet <vmallet@...> wrote:

Tested +5V and GND pins of U2556 and they are as expected.
I tested the 3-4 inverter by bringing pin 3 to +5V or GND (like you
mentioned, although using a 22R shunt briefly instead as this is what I had
laying around) and hmm I didn't see much reaction on its output 4.
That's not too surprising. The only inverter input where it is reasonable
to do is the 1-2 inverter. The input there is weakly biased, rather than
being positively driven from another logic output.

Surprising things happened next when I tested the 1-2 inverter. I started
by putting my scope probe on pin 1 to make sure my 22R shunt was having the
desired effect on the input. The input was sitting at about 1.05V. I
briefly applied +5V to pin 1 (via 22R) and bam! I started seeing a sine on
pin 1. The sine had a 840mV pk-pk amplitude sitting right above a 1V
offset, 10Mhz. I touched pin 1 again and a 2.24V pk-pk sine appeared (again
sitting right above 1V), and I saw something appear on the 2465 display!
The scope had booted up! Things vanished shortly after.

I redid the experiment after putting another probe on TP505 U2092-37 and I
saw different results. Once I got a nice 2.24V pk-pk sine on pin 1 which
gave me a 1.25Mhz (almost) square on TP505 which is matching the expected
value from the service manual. It lasted 5s, then the pin1 sine shrunk to
~840mV pk-pk again. Touched it again and I got a 400mV pk-pk (offset 1V)
but this one was ~30Mhz! The resulting TP505 more-or-less square wave was
at 3.75Mhz, three times the expected frequency. Touched it again and pin 1
sine became a not-so-sine wave, 10Mhz, 560mV pk-pk.

Sounds like there's something just marginal in the oscillator. If you catch
the oscillator napping, you can infer the values of the biasing resistors
(R2571/R2573/R2553) by the voltage drop across them. You also want to
validate that the output of the 1-2 inverter is making the right output
voltages. Low is below 0.8V and high is above 2.0V, if the output is in
between there, you're nowhere. Same for the inputs. You measured 1.05V,
which is in no-man's land, so now you need to figure out why. Either the
biasing resistors have drifted (this happens with carbon comp resistors in
particular) or your 1-2 inverter is marginal or bad, or there's leakage

I pulled the legs of C2565, C2566, C2572 to measure them with an LCR
C2565: 84.5pF (specs: 82pF, 5%)
C2566: 33.3pF (specs: 33pF 5%)
C2572: 100.1nF (specs: 100nF 20%)

I do not know how to measure leakage current yet.
If you catch the circuit in a non-oscillating state, you can measure the
voltage drops across the respective resistors. From Ohm's law and the
resistor nominal values, you should then be able to guestimate the current
going through them. If things don't add up, you either have resistors that
are out of spec, or you have leakage through the capacitors - measuring the
resistors will tell you which. You can probably get quite near the truth
by measuring the resistors in-circuit.

I should have access to a better desoldering tool soon and I'll pull the
resistors and U2556 so I can measure / test them out of circuit.

Good luck!

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