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

Cliff Carrie
 

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

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



Cliff Carrie
 

At 7L5 SN B060000 Tek changed U830 from PN 155-0117-00 to 155-0198-00. This may be a reset bug fix, but I have not found any Tek info on it. Also, there is a typo in Diagram 8. U830 shows as PN 155-0177-00. The parts list (and my 7L5) show 155-0117-00.

Dave Casey
 

My late 7L5 (B09...) powers up in my 7854 at various frequencies. Also
sometimes it powers up in 10dB LOG mode and other times in LIN mode. I
powered it off and on a few times (with varying off times) and observed the
following:
1266.75KHz, LOG
2000.00KHz, LIN
3777.25KHz, LIN
3767.25KHz, LOG
3795.00KHz, LOG
Sometimes I tuned it down to zero before power cycling, sometimes I didn't.

This 7L5/7854 combo has the known readout bug, and this 7L5 has not been
modified for the 7854 CHOP mode issue.
I have another 7L5 I can dig out to test, but I would expect to see similar
results.

Dave Casey

On Sun, Dec 8, 2019 at 1:09 PM Cliff Carrie <cliffcarrie@...> wrote:

At 7L5 SN B060000 Tek changed U830 from PN 155-0117-00 to 155-0198-00.
This may be a reset bug fix, but I have not found any Tek info on it. Also,
there is a typo in Diagram 8. U830 shows as PN 155-0177-00. The parts list
(and my 7L5) show 155-0117-00.



Cliff Carrie
 

UPDATE:
The change of U830 (dot frequency counter) from PN 155-0117-00 to 155-0198-00 at 7L5 SN B060000 changes this part from a leadless ceramic carrier to a leaded (DIP) ceramic carrier. I don't know yet if the reset bug was fixed in the newer chip, Having recently acquired one of the later 155-0198-00 leaded chips, I wonder if it could be installed, temporarily for a test or permanently in the leadless carrier socket without damaging it. Pinout seems unchanged. I visualize a plastic backup block under the chip between the two rows of pins to hold them against the leadless socket contacts. I can easily create a pressure pad between the chip and the adjacent circuit board to ensure it doesn't pop out of the socket.

Has anyone ever tried this (installing a DIP leaded chip) in any leadless socket (not necessarily in a 7L5)?

I certainly don't want to replace the 40 pin socket for a test that may show no change and/or some new incompatibility.

One more note: the serial number on a 7L5 does not tell the whole story about the circuit level. Tek released at least 3 versions of a major update kit 040-0872-xx. If your 7L5 has an added front panel "B - save A" lighted pushbutton it is B070000 or higher or has this kit installed. Mine with serial B03xxxx and the 040-0872-01 kit is equivalent to at least SN B081400. Two boards (Storage and Digital Averaging) were also replaced with a single board. Note that there are other serial numbers where boards changed part numbers, but auditing my 7L5 against the manual, there don't seem to be any circuit changes or improvements not covered in the update kit.

Cliff Carrie

Dave Casey
 

Cliff -

Did you see my response from December 8? I have a late 7L5 (> B090000) that
doesn't sound like it works any better than yours with respect to the reset
behavior.

Dave Casey

On Mon, Jan 20, 2020 at 12:19 AM Cliff Carrie <cliffcarrie@...>
wrote:

UPDATE:
The change of U830 (dot frequency counter) from PN 155-0117-00 to
155-0198-00 at 7L5 SN B060000 changes this part from a leadless ceramic
carrier to a leaded (DIP) ceramic carrier. I don't know yet if the reset
bug was fixed in the newer chip, Having recently acquired one of the later
155-0198-00 leaded chips, I wonder if it could be installed, temporarily
for a test or permanently in the leadless carrier socket without damaging
it. Pinout seems unchanged. I visualize a plastic backup block under the
chip between the two rows of pins to hold them against the leadless socket
contacts. I can easily create a pressure pad between the chip and the
adjacent circuit board to ensure it doesn't pop out of the socket.

Has anyone ever tried this (installing a DIP leaded chip) in any leadless
socket (not necessarily in a 7L5)?

I certainly don't want to replace the 40 pin socket for a test that may
show no change and/or some new incompatibility.

One more note: the serial number on a 7L5 does not tell the whole story
about the circuit level. Tek released at least 3 versions of a major update
kit 040-0872-xx. If your 7L5 has an added front panel "B - save A" lighted
pushbutton it is B070000 or higher or has this kit installed. Mine with
serial B03xxxx and the 040-0872-01 kit is equivalent to at least SN
B081400. Two boards (Storage and Digital Averaging) were also replaced with
a single board. Note that there are other serial numbers where boards
changed part numbers, but auditing my 7L5 against the manual, there don't
seem to be any circuit changes or improvements not covered in the update
kit.

Cliff Carrie



Jean-Paul
 

Cliff and Dave:

On my 7L5, SN B09..., same: Random init dot, never 0, sometimes 3777.
Be fine to engineer a fix!

How many out there have the tracking gen, OPT 25, a very useful option?

Jon

Cliff Carrie
 

Thanks for the data, Dave.

I guess the only remaining question in my mind is whether the reset bug ever shows up with a 7L5 in NON-7854 mainframes.

Now I have to start chasing the two apparent design errors more in-depth. My 7L5 is now in bug-free condition except for the "reset" and "below 10KHz display" errors. I have two extender cables. Time to do some serious debugging.

Cliff Carrie

Dave Casey
 

I can try one in a 7603 at some point in the not too distant future.

Dave Casey

On Fri, Jan 24, 2020 at 10:55 PM Cliff Carrie <cliffcarrie@...>
wrote:

Thanks for the data, Dave.

I guess the only remaining question in my mind is whether the reset bug
ever shows up with a 7L5 in NON-7854 mainframes.

Now I have to start chasing the two apparent design errors more in-depth.
My 7L5 is now in bug-free condition except for the "reset" and "below 10KHz
display" errors. I have two extender cables. Time to do some serious
debugging.

Cliff Carrie