79 Repeater Blog
May 31, 2008 - I drug out the one 7950 I didn't
sell at Huntsville last year. This was one of three that I bought for almost
nothing from a dealer of used amateur gear last year -- all worked fine but were
rough cosmetically. The last one that I kept has repainted covers and a cable
with an RJ-45 plug protruding from the back. This has turned out to be somewhat
of a blessing in disguise:
That RJ-45 was apparently put in for packet service. One of the lines on it was
for discriminator audio. I traced it out on the schematic and saw that the line
was going to a point just before the audio pre-amp and low-pass filter. After
hooking it into the repeater wiring, it sounded great. In fact, I can probably
lose a couple of the resistors I'd had to put in line with the Mitrek.
That left COR. The best thing available for COR was to tap the line going to
the display that lights the "C. TUNE" LED -- the TR-7950 version of a "BUSY"
light. It is driven from the squelch switch which is in turn driven from a
noise amplifier. I figured that COR usually came from some sort of fancy
carrier detect circuit, but I'm beginning to think that it's just audio-derived
in many radios...
It seems to work fine, but I'm afraid it's going to be really stingy on a signal
that flutters the squelch; that is, it's really going to have to hold the
squelch open to keep the repeater keyed up. I could be wrong on this, but
that's how it seemed to act at the squelch threshold when the LED flickered.
We'll see how it works.
I think the receiver will perform fairly well. I don't know about sensitivity,
but selectivity should be great. It has a helical resonator in the front end,
which is rare even in modern "repeater receivers." It was designed to receive
only between 140-150 MHz, which in repeater service is vastly preferable to the
wide-open front ends of modern DC-daylight receivers.
It's temporary in any event, but as we know from Murphy's Law, "A temporary
installation tends to become permanent, and a permanent one never is."
I'll try to have it all put together and back on the air sometime tomorrow,
though I may not make it tomorrow.
May 31, 2008 - Mitrek had a crispy-critter
tantalum cap in the SWR protection circuitry, which may actually be a symptom,
not a cause. I'm not even going to try to figure out why it's locked in
transmit.
Bad news. The Mitrek is not modular -- both TX and RX are on the same PCB.
This probably helps explain the crappy duplex performance. The 50 watt power
level was also a problem which might have been solved by duplexer retuning--we
did move 10 kHz, after all...
I'm scrapping the Mitrek entirely. I may press a Kenwood TR-7950 into service
as a receiver temporarily until I can get another FT-2500 or something similar.
May 31, 2008 - I brought the 79 home again this
morning. Problems started some time between 7 a.m. and 5 p.m. yesterday
(5/30/2008). The MITREK, which I was using only as a receiver, locked in
transmit. I bet it's done this before & that's why its finals are blown. I
could find no reason why it did this. All I can figure is that it has something
to do with how the duplex mod was done and/or something came loose inside...
The repeater was working, but it was clearly being desensed as I could bring it
up locally, but not from my house or around Russellville.
A temporary fix may be to gut the Mitrek of all the transmit hardware, but the
whole thing's getting replaced eventually, probably by another Yaesu
FT-2500M. I'm so pleased at how well it's working as a transmitter, I bet one
will make a good receiver too.
May 20, 2008 - Repeater is on the air and open for general amateur use! The repeater is operating below coordinated limits as follows: the antenna is currently very low, and I am running 25 watts instead of the coordinated 50. My priority with this machine is now getting the antenna higher!
May 17, 2008 - The 146.79 repeater is currently on my workbench. The audio problems started last year when I wired in a Yaesu FT-2500M as a transmitter due to problems that arose with the Mitrek's transmitter (and now, somehow, the Mitrek's finals have been blown). I now have the repeater sounding right by using the direct modulator connection on the FT-2500's control unit PCB instead of the ultra-sensitive microphone input. No more courtesy tone overdeviation!
Believe it or not, I am seriously considering purchasing another FT-2500M to use as a receiver. The radio is generally regarded as commercial-grade anyway (there is an identical commercial version, as a matter of fact) and has an excellent reputation for its strong, well-filtered, and intermod-proof receiver, as well as a super-clean transmitter. In fact, here's a high-profile repeater that uses an FT-2500M as a transmitter: http://www.ussc.com/~uarc/rptr/76rpt.html
The FT-2500M has two other features that should endear it to repeater & remote base owners: an internal jumper puts COR voltage on pin 1 of the microphone connector, and there is also a PCB pad for discriminator output. Though these features were intended for use by 9600-baud packeteers, they are perfect for the repeater builder too!
I will probably put the repeater back on air as-is soon, and wire in the new receiver later. The antenna is still at just 10', but the coverage is surprising. After I put the new receiver in service, a better and higher antenna will be the next project, and a new controller might soon ensue as well.
October 2007 - The 146.79 repeater is ON THE AIR on a temporary antenna. The audio levels still aren't quite right. In fact, the beep level is so high that the courtesy tone and CWID overdeviates. However, the repeater should be somewhat usable for the time being. More updates to come...