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.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
I redid the experiment after putting another probe on TP505 U2092-37 and ISounds 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 LCRIf 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 theGood luck!