Yet another 7L5 issue (7L5 / 7854) below 10 kHz

Cliff Carrie

The dot frequency displays with the decimal point in the wrong place below 10 kHz (Fine mode on); the displayed number must be multiplied by 10. The 7L5 tunes correctly to 9.750 kHz, for example, but displays 0.975 kHz
Tek documented this in Service TekNotes in November of 1982. Strangely, they never produced or published a fix as far as I can discover. Does anyone have a solution, or ideas for one?

Cliff Carrie.

Nenad Filipovic

Hi Cliff,

I wrote about a related problem in this message:
I also have the same issues with my 7L5/7854 as you reported (improper
reset and CF display below 10kHz), and my conclusion is that 7854 readout
processing does not conform to specification, so it's the 7854 firmware
issue. I summarized a part of it in the conversion document posted here:

So until someone dares to do brain surgery on 7854 ROMs, I doubt there's
much we can do. Tek did not touch it back in '82, so I figure it wasn't an
easy fix.
These recent microfiche scanning operations might reveal some project
documentation on 7854 firmware, though I'd be amazed if somebody fixed a 30
year old bug :) 7854 is still hard to beat even today.

Best Regards,
Nenad Filipovic

Nenad Filipovic

Hi Cliff,

Yes, in my opinion the "below 10 kHz" problem is purely on the 7854 side,
as my 7L5 displays CF correctly in both 7633 and 7104. I did not bother
much with it as I simply got used to it, and with GPIB data it is possible
to implement a fix for it in capturing software.

The reset anomaly is also present in mainframes other than 7854. In my 7854
it powers up at 3778kHz most of the time, but not always. In 7104 it does
1788kHz or something similar. I did not get into it deeper as it did not
bother me nearly enough as the very annoying mV issue in LIN mode on 7854.
The last is definitely a 7854 fault.

Is your 7L5 the older (SN below B069999) or the newer version? Mine is the
older. I think I remember I read somewhere that the older version is prone
to the reset problem, but can't say for sure.

Best Regards,

On Sun, Dec 8, 2019 at 5:34 PM Cliff Carrie <cliffcarrie@...> wrote:


I too have spent a lot of time on my 7L5, mostly in a complete overhaul of
the front panel hardware bits and correcting some assembly errors from the
last person who took it apart. This hobby is at least as much about
restoring these classic units as it is about using them.

Thanks for the links to your earlier posts. I had read them some time ago.
I can certainly agree that the "below 10 kHz" problem could be in either
the 7L5 or the 7854. I will try to test my 7L5 in a non-7854 7k mainframe.
I have an acquaintance locally who had at least one.

There is a big difference between the two problems, however. In the 10 kHz
problem, the 7l5 is correctly tuned. Only the frequency display is wrong.
In the power on problem, the 7L5 is actually tuned to 3778 kHz instead of
zero. I can't think of any signals inbound from the 7854 to the 7L5 that
can change the tuning. I still hope there may be a 7L5 solution to this
one. I will keep searching.