Using a Pac-Rt® on an Astro-Spectra

Using a Pac-Rt® with an Astro-Spectra®
I've always liked the ability to cross-band repeat. My first use of a PAC-RT was at my
first paramedic job. Our radio system was a simplex VHF system with the tower located
in the far eastern end of the county. Portables were useless once you went out to the
"Westend" because it was as far as thirty-five miles away and over the Florida Trail
Ridge. With the pac, we could still communicate with dispatch once we got out of the
ambulance. I thought that was pretty cool.
I bought a used PAC-RT several years back and had it connected to a GE Delta. It
worked pretty well, although when used with the S-990 head the radio would continue to
scan while transmitting if you transmitted with the pac - not good. I always had to
remember to turn off the scan before using it. Then I got a Spectra®. It took a little
while, but I figured out how to get the pac to work with it.
Now I have an Astro-Spectra. This is a totally different radio. There are no usable
signals for a pac. I looked around for a used VRS-750, but I never saw one for sale. I
looked at a new one, but they were in the $1500 range, way more than I was willing to
spend. Then I looked at the Pyramid line, and the prices weren't much better. What I did
find on the Pyramid website, though, were instructions for connecting the Pyramid SVR200 to an Astro-Spectra. I knew I was on to something.
It's real simple & takes about an hour. You’ll need a low wattage pencil tip soldering
iron, a copy of the Pac-Rt pin out from Batlabs, and a copy of the parts layout diagram of
the Astro-Spectra Command Board.
On the Main Control Board (HLN5558 B-F):
1. Install a jumper between the Audio PA IC (TDA 7256) pin 10 (PA enable) and J503
pin 19 (VIP Out 1). This provides COR to accessory connector JU0006 pin 16. The pac
normally does not have to be supplied with a COR. It has its own squelch ckt. and
creates it's own COR. That won't work here (see step 4 for an explanation). You'll have
to bypass all that and use the COR provided here. Basically whenever the audio PA is
active, the pac will transmit. So if you are changing channels or adjusting the volume,
that tone will be transmitted by the pac. Alert tones from signaling get transmitted too.
The next steps involve moving some chip resistors on the command board. Carefully
remove the zero Ω resistors and then reuse them, unless you have some available.
Remember, this is surface mount stuff on a multi-layered PC board. It would be real easy
to screw up your command board, so be careful!
2. Remove JU521 and Install JU513 to enable external Mic Hi.
3. Remove JU518 and Install JU500 to enable external PTT.
4. Remove JU519 and Install JU528 to enable discriminator audio out. This isn't really
discriminator audio like you would expect from an analog radio. When the mic is hung
up, there is no noise at all, and what noise is there is at such a low level, the pac squelch
ckt. won't recognize it. What will happen is that the pac will just stay keyed until it times
out.
See the next page for the parts layout. I would recommend obtaining your own copy of
the Astro-Spectra detailed service manual as these scans are poor.
On the Interconnect Board (HLN6486D):
1. Install JU605 to connect PTT to JU0006 (Spare 1). It's located on second pad adjacent
to Pin 1 of J0006.
Once you have modified the radio, now wire up the cable from the pac to a 25 pin plug
like this:
PIN
1
9
12
16
18
18
22
COLOR
Black/Blue
Black/Yellow (shielded)
Brown (shielded)
Red
Yellow
Green
Orange
FUNCTION
mobile PTT
mobile discriminator
mic hi
COR
ground for PTT
digital ground
switched B+
Modifications to the Pac-Rt:
Take apart the pac plug. You will see the red wire coming out of the thick black cable.
You will also see an orange wire coming out of the smaller grey cable. This is a spare
line. Cut off the orange wire at the beginning of the grey jacket and solder it to the red
wire. This brings COR from the Astro-Spectra into the pac.
Now for the hard part. I don't have a pac manual, so it's kind of hard to describe this.
You'll have to look at the pictures.
On the main board in the pac, where the interface plug is mounted, there is a row of
solder terminals labeled "spares". The orange wire is connected to the one closest to the
outside of the case. Go ahead and connect a 6" piece of insulated wire to that terminal as
shown.
Now go to the point shown and cut that trace. This is the trace that takes the COR
generated by the on board squelch ckt. in the Pac to the controller ckt. It is +12 VDC
when active and 0 VDC when idle. Solder in your wire as shown. This supplies the
COR from the astro-spectra to the control ckt. of the pac.
Hook it all up and set your levels. It's that simple. The only oddity is that PL from your
handheld is very obvious on digital signals, but transmissions from a Pac through any
radio really don't sound very good. What is nice is that unlike a regular pac/radio
combination, this setup is not carrier squelch. I always found before that the pac would
retransmit everything the mobile heard, since it was working with raw discriminator
audio. This is not the case in this arrangement.
I take no responsibility for your success or failure in this project. You are on your
own and performing these mods at your own risk.