Re: 7L5 dot frequency reset bug - any clues or a fix?

Dave Casey

I can test mine out tomorrow and let you know if it does the same thing.

Dave Casey

On Sat, Dec 7, 2019 at 4:46 PM Cliff Carrie <cliffcarrie@...> wrote:

The 7L5 manuals say the Dot Frequency should reset to 0.000 kHz at power
on. There have been previous posts about this not happening, but no mention
of a fix. I have personally seen two 7L5s, including the one I am currently
using in my 7854, that reset to a varying frequency near 3778 kHz. I have
forced a reset using a jumper to trigger the power-on reset timer. The Dot
Frequency goes to 0.000 kHz during the reset but goes back to kHz
when the reset timer ends. All other modes and indicators reset as
expected. I don't think this is unique to the 7854 power on since the
mainframe was already powered up before I did my test. I suspect a timing
race in U830 as the reset pulse turns off. I know this is a minor
annoyance, but the circuitry is there, and I'd like to make it work
properly. My 7L5 service manual is 070-2184-01. The reset timer is Q621 on
Drawing 5 lower left. U830 is on Drawing 8 (see pins 4 and 17).

1: Has anyone experienced this on the 7L5 in a mainframe other than the
2: Who else is experiencing this in a 7854?
3: Does anyone have a fix?

Cliff Carrie

Join to automatically receive all group messages.