[time-nuts] Extron 3 80 video distribution amplifier

Didier Juges didier at cox.net
Wed Sep 6 23:18:47 EDT 2006

Hello fellows Time Nuts,

Following a question of mine here a few weeks ago about using a video 
distribution amplifier for frequency reference distribution, and lots of 
good advice I got in response, I just got one of those distribution 
amplifiers from eBay and there are good news. The box is very sturdy, if 
not very RF proof, and all the electronics of the unit itself fits on a 
PWB that is mounted along the rear panel, the rest of the box (about 85% 
of it) is EMPTY, with many mounting studs on the top and bottom panels, 
a hacker's dream.

The box has plenty of room for a GPS receiver, an HP 10811 OCXO, a 
microprocessor board and a display, if you see where I am going.

Of course, the original wall-wart will not power the new guts, but a 
cheap laptop type switcher from www.allelectronics.com will do the job 
nicely for few $. I need an 18 or 20V unit, then I will put a small 
switching step down regulator for the GPS receiver, it would be wasteful 
to use a linear regulator to step from 18-20V down to 5V at 300-400 mA 
(I have used the LT1375HV for that in many projects).

Since I hate making enclosures from scratch for my projects (the least 
fun part in my opinion), this one will be well taken care of.

If you are interested in using this amplifier, here is some more info 
than is available on the Extron web site.

The Extron 3 80 has 4 inputs (R, G, B and SYNC) and each drives 3 normal 
outputs and a LOOP output. The R-G-B channels are analog and the SYNC 
input drives TTL gates, so depending on your application, it may or may 
not be suitable for time or frequency distribution. Each input has a 
corresponding LOOP output, which is simply in parallel with the input, 
for looping through another device. You could use the LOOP outputs to 
loop a single 10 MHz signal through all 3 inputs and have 9 outputs 
right off with no modification (just put the switch to LOOP and put a 
termination on the last LOOP output to terminate the line) and low skew.

I need at least 5 outputs at the moment, so 9 will suffice for a while 
for my purpose. Also, I only need it for frequency reference, not for 
timing, so delay is not a big concern to me at the moment (when I 
graduate to full-fledged [time-nut], this may change :-) It would not be 
too hard to modify the unit internally for one input and 16 outputs all 
in sync (give or take a nS or two), and 3 spare connectors.

The delay between each input and any of the corresponding outputs is 
about 10-11 nS (measured with a sine wave input from my HP 8657B on a 
TDS210 60 MHz scope, I will use the HP 5334B for more accurate reading 
later.) The delay between each input and the corresponding LOOP output 
is 1-2 nS.

There is a switch labeled 75 ohm/LOOP which simply makes the R-G-B 
inputs high-Z in the LOOP mode, 75 ohms otherwise.

There is another switch that adjusts the gain of the R-G-B channels to 
1:1 or 1:0.7. In addition, each analog channel has an adjustment 
potentiometer inside that allows to adjust the gain from 0-120% of rated 
(assuming 50 ohm loads on the outputs). Each R-G-B channel also has an 
internal jumper, I have not determined what it does yet. A schematic 
would be handy.

The wall wart is a 9VDC unit, the amplifier has an internal negative 
voltage generator and two regulators to drive the video amplifiers which 
operate from +/- 5V.

I do not have the schematic, and it will be hard to pull because of all 
the connectors that clutter the PWB. If anyone has the schematic, that 
would be appreciated. Note that the PWB is common with another unit that 
has 6 outputs (and no LOOP output) for each input, the PWB is 
differently populated. I guess either schematic would do.

The video amps are labeled 687J and do not have identifiable 
manufacturer label. I could not find a link to a spec sheet. There also, 
any help would be appreciated.

Didier KO4BB

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