[R-390] CU-872A/U RX multicoupler

[Roger Ruszkowski]

Chris,
 
Does any one have a CU872 they would part with?
I would like to own one for my R390's.
 
Once upon a time 68 -75 I use to take care of whole bunches of these items.
 
Today we have op amps and digital displays. 
We care not about sensitive meter movements.
The CU872 has a 50 micro amp like 1 x 10 -6 meter for self test.

It has 20 6922 tubes dual triodes 10 deep cascode string.
It is 1 RF in 8 RF out. Zero gain. But you spread out to 8 so the gain is 8.
But port to port is zero gain.

Range is 500 KHz to 30 plus mega hertz. 
They were used world wide with lots of big antenna farms and many hundred R390 receivers.

We would run then 3 deep to fan one antenna out to 512 receivers.
You could do it. We just never got that many receivers and operators all interested
in the same geographic area a big rhombic antenna covered.
We did run three deep to get out to eight different rooms and then more than 8 receivers in a room.
Maybe 14 receivers on 7 positions in one room all looking at the same region of the world.

The point is we though the reception out of the units for all receivers was as good as a receiver would
get if it was just coupled direct to the antenna. And these test were run often to show the problem 
was not the antenna, receivers cable or the CU872 couplers. 

The noise floor was excellent.

Inside the unit has a filter that cuts out every thing below 2 MHz. 
This keep the AM broad cast band and cross modulation products out of everyone's receiver.

You can re cable the BNC connectors and by pass this filter if you want to listen to the AM band.

The unit is two identical amplifier pairs. The one input is split and drives two set of 10 tubes.
to four outputs. So 1/2 input yields 1 output for a real gain of 2.

You can save half of your tubes and just run a 4 output device if you want.

You can re cable the amps into a series for a gain of 4 to four receivers.

You have to think about the impedance matches as you are only driving one amp from the 
antenna and not two amps in parallel. And you are taking one output back to the second amp
where you again have the same mismatch ratio. If you are only work a mega hertz of interest,
the transformers should be doable.  

The 6922 are just low noise 12 au7 or 12ax7 with a gain of just 33.
The tube is 12 volts only it doe snot have a center tap filament to run on 6.3 volts.
 12 AT 7 have a gain of 100.

You can repopulate your CU872 with "hot" 12AU7 12AX7 12AT7 type dual triodes.
I do not know what will happen to the noise floor or the gain.

We run 6922 24 x 7 for about 2 years and then just changes a whole batch of 20.

We would grab 4 units on a semi annual twice a year basis and just give one of them 20 news tubes and put it back in the rack.

We would then run 80 tubes through the tube tester and select the best 60 to re populate
the other 3 units. We sort of chose the top 20 next best 20 and third best 20.

The top 40 would all look good. The last 20 would be the best of 40 and you could tell
they were not as conductive as the best tubes.

It was a method to keep every thing running and it worked.

Hundreds of operators had CU872's inserted between their antennas and their receivers 
and could not hear the difference as you swapped them from one to two or three deep 
or from port to port or direct to the antenna. 

Cu872's were in the world from before my time in 68 and were still going strong 
through the 70's into the 80's.

For every 4 or 5 receiver in the Julian Creek Massacre there should have been one CU872.

No one knows what they were or why anyone would want one.
Those of who do know have no reason to speak of such things.

If you have one I would like to discuss acquiring one from  someone.
One would be a life time supply.
 
Antenna to one receiver gain is nil.
Antenna to one receiver loss is nil.
Antenna to one receiver noise floor change is nil.
Antenna to one receiver sensitivity  change is nil.
Antenna to 8 receivers is like 8 receivers on 8 antennas.

Roger AI4NI 


Do any of you have one of these that is in active duty?    I'm curious about
its performance, etc.  
Since Goggle didn't turn up anything in the way of conversations about this
unit, it may be that the going price of 6922's caused these to be
sacrificed.
Many of you frequent the same forums and lists so please excuse the
multi-posts.  I am casting a wide net hoping to catch a bonafide user to
talk with.
Thanks es 73,
Chris
W7JPG

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