[Collins] 312B-4 Directional Coupler Failure

Tue, 29 Jul 2003 21:24:29 -0400

Hi Hank:

The 312B-4 uses the classic Breune bridge circuit.

Some inexperienced ops occasionally use the bridge in a very high SWR
circuit (highly reactive) instead of 50 ohms, and that can cause very high
reactive voltages to be applied to the bridge's voltage sensing caps C1 and
C2. These are concentric glass-dielectric variables, and they can short. So,
check those with your ohmmeter.

Also look at the associated diodes CR1 and CR2. Check these for proper
forward/reverse readings on your ohmmeter.

The circuit is very simple, and I've repaired a number of these bridges, as
well as homebrewed my own versions. You shouldn't have any trouble finding
the bad part/s. Once you replace them it's an easy matter of adjusting the
caps using a resistive dummy load.

If you need it, I can send you a copy of the schematic.

Vy 73, AI2Q, Alex in Kennebunk, Maine
http://users.adelphia.net/~alexmm/ai2q.htm

 .-.-.

-----Original Message-----
From: [email protected]
[mailto:[email protected]]On Behalf Of Maciog Henry J Col
AFC2ISRC/CCG
Sent: Tuesday, July 29, 2003 4:52 PM
To: '[email protected]'
Subject: [Collins] 312B-4 Directional Coupler Failure

Dear fellow Collins enthusiasts,

My 312B-4 directional coupler does not read forward or reflected power.
Transmitted power passes through it, but the values do not register on the
unit's meter.  I'd like to repair the directional coupler but need some
hints as to likely causes (as well as where to get parts).  Also, any
pitfalls associated with making the repair.  Thanks for the help!

Sincerely,

                                                                        Hank