Re: How to troubleshoot a faulty 7B92A?

Dan G

On Sat, Mar 20, 2021 at 07:16 AM, Thierry Delaitre wrote:

U820 - Sweep control IC
- Pin 6 (auto delay) is 0.5V ; strange ?? other side of the R828 resistor
(180K) is 14.7V
- Pin 7 (trig D) is 0.5V
U820 contains comparator and discharge circuits on pins 6 and 8.

U820 allows C828 to start charging when it stops receiving trigger signals.
That is how it eventually reaches free-run in AUTO mode.

Your pin 7 being low suggests that your TRIG'D light is on. Is that correct?
It would explain why pin 6 is low, and why you never reach free-run mode.

If your 7B92A is still in the vertical slot, set for AUTO, AC, INT, then there
should not be any trigger signals, and the fact that U820 is in triggered mode
is suspect.

- Pin 8 (hold off timing) is 0V ; strange? according to the schematic there is
a pull-up resistor to the 15V line
Pin 6 and pin 8 cannot be reliably measured with a DMM, as they both
normally have positive-going ramps, and should never get much higher
than +5V. You will need to look at them with a 10X high-Z probe.
(Remember to use an IC probe tip adapter on your probe, or else
be extremely careful not to let the probe tip slip and short adjacent
IC pins. I find that it is usually safer to probe at some other location:
R828 for pin 6, R871 or R872 for pin 10, R834 for pin 8.)

U820 allows hold-off capacitors to start charging at the end of the
sweep, so if pin 8 is stuck at 0V DC, then U820 may think the sweep is
still in progress, or it may have never received a HOLDOFF START pulse
on pin 16.

- Pin 10 (hold off) shows a 4V DC voltage (no sweep)
- Pin 17 (trig disable) is 0V
- Pin 18 (lockout) is low
- Pin 19 (auto mode) is low as it should for auto triggered
Are pins 10 and 8 both constant DC, or is there a waveform visible with
a 10X probe?


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