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 3778.xxx 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
7854?
2: Who else is experiencing this in a 7854?
3: Does anyone have a fix?

Cliff Carrie



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