BK Precision 1801 Frequency Counter...what is missing on the back?

[W3KW]

The back panel of the BK Precision looks like there are two options that are not installed. There appears to be a slider switch punch out and possibly a BNC knock out. They are pre-pressed but not open. Does anyone have a version that has something in those knock-outs? I may add a rear BNC and use the counter with ham radio. I don't really care if the addition pulls the frequency of the counter off a bit. I'm in the thing now and have replaced the electrolytics, so while it's torn apart, why not? These are cheap and I own two anyway.

Anyone know what the knock-outs are for? Anyone add a rear BNC?

Wes
W3KW

Manual with Schematic is attached.

[W3KW]

Photos attached.

[W3KW]

Front.

I don’t know how to post multiple photos using my iPad.

[Gertjan]

The answer to your question  is in the manual you posted.
In the introduction, page 4: "Provision has been made for the user to convert the instrument to operate from an external time base when extreme accuracy is desired."

See also page 11, under the header "E Modifications".

The details are in the schematic, drawn under the power supply.
The internal / external switch is drawn under the power transformer, with A,B and C. Ext Ref output is F, and Ext Ref input is D.

Regards, Gertjan.

[W3KW]

Wow. Humbled. I studied the schematic fairly thoroughly and did see A-D and 1-4 but didn't put 2 and 2 together. A and B are linked by a trace on the board. I assumed were a switch an option there, a jumper would have been used. I assumed poorly. I merely scanned the photos for I/O on the front and rear panels in the manual, thinking the answer would be there. READ the manual man! Sorry. Thanks for pointing out all my laziness! Ha.

I will reach out to BK and ask if the Modification Instructions are still available and post them here if I can get my hands on them.

Thanks for the help.

Wes