N3CRK Linked Repeaters - 224.5 and 443.675

As of 12 April 2018, the two repeaters are linked.


It took many many hours of work and experiment. I went through at least four iterations of the system as I thought of ways to make it simpler and better at the same time.

I decided to run what is confusingly called "flat" audio. That actually means there is no de-emphasis in my receivers and no pre-emphasis in my transmiters.
A user's transmitter provides the only pre-emphasis which is carried from the repeater receiving antenna(s) unchanged to the transmitting antenna(s).
Another user's receiver then provides the only de-emphasis.

For more information on "flat" audio see this excellent article on repeaterbuilder.com

Obviously any audio added in the repeater(s) (like voice announcments) would have to be pre-emphasised to match. This determined my choice of controller.
I seriously considered the Arcom RC210 Repeater Controller which is designed to handle up to three repeaters/Link Radios.
Ken Arck was very prompt in answering emailed questions. However, he told me that, although the RC210 has synthesized speech and digital voice recording, there is no provision to pre-emphasise it.
He could only suggest using pre-emphasis in my transmiters. But that would defeat all the advantages of "flat" audio.

That took me back to my favorite controller, the Ham Gadgets ID-O-Matic IV. It is only $39.95 for the kit or $59.95 built.
I would only recommend the kit to an experienced kit builder who has a small, needle-point soldering iron.
His shipping is $5.97 but that can include other small items as well. I bought a Voice Recorder Board ($10.00 assembled only) and two LM386 Amplifier Modules ($7.95 each, assembled only).
The voice recorder board is an add-on to the ID-O-Matic IV. That means the audio wire is accessible to add a pre-emphasis network.
The CW ID, being only a single frequency, does not need pre-emphasis. It only needs setting to a suitable level via a pot on the board.


The Hardware

The ID-O-Matic IV is only intended to control one repeater. It took a bit of outboard interfacing to link the two repeaters.

I like reed relays. They are simple, reliable and static proof. I also have a junk box full of them.
Since I am using detector outputs which are unsquelched, I needed to add outboard squelch to each receiver.
I also needed to boost the relativelely weak detector outputs to a higher level and lower impedance.
With a 10:1 voltage divider on the output, the Ham Gadgets LM386 Amplifier Modules do the job perfectly.
A reed relay in the audio path takes care of the squelch. Both run on the 12 Volts I have available in each box.

(Project boxes aren't as easily available as they used to be and have gotten rather expensive.
I use electrical boxes from my local builder's supply. They are not pretty, but they do the job.)

The color code I chose for the 6-conductor, shielded cables from my junk box, that plug in to the controller is as follows:

DB-9 M pin 1 Red - 12 VDC from ID-O-Matic IV power supply
DB-9 M pin 2 N.C.
DB-9 M pin 3 Blue - Audio to ID-O-Matic IV from receiver (amplified and squelched)
DB-9 M pin 4 Yellow - Audio from ID-O-Matic IV to deviation potentiometer
DB-9 M pin 5 Brown - Push-to-Talk to ground to transmitter
DB-9 M pin 6 Violet - Carier Operated Relay to ground from receiver
DB-9 M pin 7 N.C.
DB-9 M pin 8 Black - Ground
DB-9 M pin 9 Shield - Ground


Interfacing the two Icom F221s radios to the ID-O-Matic IV.

The left two cables connect to the two Icom Radios.
The right 6-conductor, shielded cable connects to the ID-O-Matic IV.

At left center is the LM386 Amplifier Module mounted with thick, double sided tape.
The detector output of the receiver is connected directly to it. The output goes through a 470 Ohm resistor inside a piece of spaghetti.
(I save all the jackets I strip from wires to use as spaghetti.)
The resistor connects with a blue wire to one of the relay contacts.
The other contact connects to a blue wire in the cable to the ID-O-Matic IV where it is shunted by a 47 Ohm resistor to form a 10:1 voltage divider.
The potentiometer (which works backwards) on the LM386 Amplifier Module controls the receive audio level into the ID-O-Matic IV.

The two reed relays are in parallel and connected from the receiver COS output to +12 Volts.
The other relay provides a contact closure to ground to the ID-O-Matic IV COR input via a violet wire.
The pair of relay coils is shunted with a diode to prevent high voltage flyback from damaging the receiver COS output.

A yellow wire (hot) and black wire (ground) from the ID-O-Matic IV bring audio to a 15K potentiometer which is used to set deviation on the transmitter.
In the case of the Icom, it is all the way up.

All external wires are shielded and the shields are grounded to the box.


Interfacing the BCR Repeater to the ID-O-Matic IV.

The left 6-conductor, shielded cable connects to the BCR Repeater.
The right 6-conductor, shielded cable connects to the ID-O-Matic IV.

This box is basically the same as the other one.
The main exception is that the detector output of the BCR has DC riding on it which went straight into the amplifier module.
A series 33uf capacitor (seen on the terminal strip) solved the problem.
The output of the amplifier also goes through a 470 Ohm resistor and relay. It connects to the same 47 Ohm resistor in the controller for a 10:1 voltage divider.
The 15 K deviation potentiometer for the BCR ended up turned pretty far down.

All external wires are shielded and the shields are grounded to the box.


This is what the various boxes look like mounted on the wall behind the repeater rack.

The bottom left two are the radio inteface boxes just described.
The bottom right relay is used to switch between the CW ID and the voice information messsage.

The top left box contains a 15 Amp lighting relay that can be used to shut down both repeaters completely from the house.
The 12 Volt wall wart powers the ID-O-Matic IV.
The top right box contains convenience outlets for the two repeaters and a relay controlled fan outlet.
The wall wart runs a 12 Volt fan behind the Icom transmitter.


The inside of the controller box.

At the top right is the ID-O-Matic IV.
It has been modified by connecting a 4.7K resistor to the mixing bus at the junction of R15, R18 and R19. This is where audio from the receivers will be inserted.
4.7K won't load down the bus and there is plenty of level to make up the extra loss.
Also Q5 has been removed so the original receiver input will never be squelched on the board.

Squelched audio from both receivers enters through the blue wires which are tied together across a 47 Ohm resistor to ground.
This lowers the impedance of the blue wires to 47 Ohms, forms half of the two 10:1 voltage dividers and keeps the wires from losing high frequency response or picking up noise.
The received audio then goes through a 3.3ufd capacitor and into the 4.7K mixing resistor.

The two violet wires from the COR relays in the interface boxes are tied together and to the COR input on Pin 7 of the green connector.
This works because they are simple relay contact closures to ground.

The audio out on pin 11 connects to both yellow wires which go to the two deviation potentiometers.
I know from experience that it can easily handle the two 15K pots in parallel.

The orange wire from pin 3 (PTT) connects through parallel coils of the two reed relays in in the upper left to +12 Volts. They are shunted by a diode.
A brown wire from each PTT relay goes to one of the transmitter PTT inputs. That keeps the transmitter PTT inputs isolated from each other.

The orange wire connected to pin 10, ALT MSG input connects to the tip of a 1/4 inch jack.
Grounding this will switch from the CW ID to my voice information announcement.

The relay in the lower right connects to the Beacon Indicator output on pin 5. This is programmed for fan control.
The contacts ground one side of the external fan relay which connects to 12 volts on the other end.
Both the coil and the contacts are shunted with diodes.

Finally on the lower left is the voice recorder board with it's pre-emphasis network.
The power, ground and control leads are all connected in the normal way to the ID-O-Matic IV.
The (green) audio lead goes through a pre-emphasis network. This can be seen on the terminal block under one of the mounting bolts.
From there it goes to the original receiver audio input on pin 8. I need the extra gain of this input to make up for the loss in the pre-emphasis network.
This is also why I had to remove Q5 - so it would not cut off the voice annoucement.
There is not quite enough gain, but I prefer the voice information annoucement to be at a slightly lower volume anyway.

All external wires are shielded and the shields are grounded to the box.


The back of the controller box.

The two 6-conductor, shielded cables, with the exception of the brown PTT wires, are paralleled inside.
The right two terminals bring in 12 VDC power. The next two go to the 12 Volt fan relay.
The 1/4 inch jack is wired to the ALT MSG input on the ID-O-Matic IV. Grounding it will switch from the CW ID to my voice information announcement.
My CW ID is programmed into the CWID message. My voice information announcement is programmed into the ALT message.
Dan at Ham Gadgets says this will not work, but it works just fine when the ALTMSG polarity is programmed to zero.

This box has undergone so many modifications that I need to start over with a new box one of these days.


I set levels with a VU meter connected to the speaker jack of an old Radio Shack Scanner with both repeater talk-out frequencies programmed.
I already knew that to get a usable voice announcment I need it's pot all the way up and the 443.675 deviation pot all the way up.
Next I programmed HT's with touchtone pads for each repeater's talk-in and talk-out frequencies.
I transmitted on 443.675 holding the "5" button down while I set the scanner volume to a reference point on the meter.
Then I switched the HT to the repeater input and did the same thing while I adjusted the 448.675 amplifier module for the same meter reading.

I switched the scanner to 224.5 and set the meter to a reference point with a touchtone from an HT on 224.5.
I put the scanner back on 443.675, put the HT on 222.9 and adjusted the amplifier module on the 220 receiver for for the same reference reading.
Finally I put the scanner back on 224.5 and, using the 224.5 transmitter deviation pot, set it to match the refence reading on the scanner.

As a final check I ran through all the above steps, but talking on the HT's.
I am a recording engineer from way back and can read a bouncing meter quite well. It all looked good.
The voice information announcement was a little lower in volume but sounded good on both repeaters.


Does it work?

When I announced I was done with my testing, I immediately got a call from WA8TGG.
He said he could not tell any difference between the two repeaters.
Since I had spent several hours getting the pre-emphasis on the Voice Recorder Board to sound good, I asked him how it compared with my talking.
He said he thought I was just talking. He didn't know it was a digital recording.
His reports made me so happy after all the many, many hours of work I had put into linking the repeaters, I nearly dislocated my shoulder trying to pat myself on the back.



N3CRK - 21 April 2018