The ESSB Audio Forum
Where all your Hi-Fi Audio
Questions are Answered
TX Monitor Circuit Description:
After much experimenting here at the Voodoo-LABS,
We think this is going to be the best fix for the Monitor phasing phenomenon that happens when changing bands.
In other words, the monitor volume is very low on some bands compared to the others,
and it also changes in volume with each subsequent transmission.
This mod also protects the Receiver circuits from any stray RF during transmissions.
The TX Monitor signal is derived on the SDX by sampling a portion of the
Transmitted RF directly after the first TX Band Pass Filters, before the driver amplifier on the RF Unit.
This RF sample is injected into the 1st mixer of the Sub-Receiver for demodulation.
The problem appears to be that Final Transmitted RF is also making its way back into the
Sub-Receiver´s 1st mixer circuit on the RF Unit by various other pathways including the designed pathway.
This is causing a phase mixing effect on certain Bands,
namely on 17 and 21 meters, with the effect rolling thru 10 meters.
The main effect is very low monitor volume, which could
change in pitch, and volume with each successive key down.
It was noticed that when we disconnected the RCA RANT (Receive Antenna IN) Cable from
the PA Unit, the phenomenon stopped completely.
That´s when we deduced that Transmitted RF was leaking back into the RF Unit circuits.
We tried considerable stages of ground strapping, ground trace cutting, RF bypassing,
and shielding to try to isolate this phenomenon with little effect.
After much experimentation, the best solution we came to find is as follows.
While trying to isolate this problem, it was noticed that there was a
much higher level of Transmitted RF on the RCA RANT Cable center conductor then on the Chassis Ground.
We deduced this higher RF level is being induced from the T/R Relay.
No amount of RF bypassing could reduce this level, but we could ground this center conductor and
make it the same potential as the Chassis Ground to alleviate the Phasing problem.
So we designed a circuit that will ground the
Receiver Antenna Input directly to chassis ground potential using a relay.
This relay is engaged when in Transmit, and if in SSB mode only.
It will not engage in any other mode as it is not necessary.
This is accomplished by using the SSBB (SSB TX +15VDC) signal.
Here is the schematic of the Relay Circuit we designed:
Gotta have PARTS!!!
Monitor Phasing Mod Parts
DigiKey - Part Numbers - Manufacturer
|Q1 and Q2 - 2ea Needed
NPN General Purpose Transistor
Digikey - BC547BFS ---- BC547B
|D1 and D2 - 2ea Needed
Digikey - 1N4148FS-ND ---- 1N4148
|R1 and R2 - 10K ohm - 2ea Needed
1/6 watt axial Resistor
Digikey - 10KEBK-ND ---- CFR-12JB-10K
|R3 - 82 ohm - You will need 1ea of these
1/6 watt axial Resistor
Digikey - 82EBK-ND ---- CFR-12JB-82R
|C1 - .01uF - 1ea Needed
Radial Lead Capacitor
Digikey - BC1146CT-ND ---- K103Z15Y5VE5TL2
|RL1 - 1ea Needed
G6L-1P SPST Relay
Digikey - Z1229-ND ---- G6L-1P DC12
|CN1 Plug - 1ea Needed
Digikey - 455-1000-ND ---- EHR-2
|PINS - 3ea Needed
Crimp Pins for Connector Housing
Digikey - 455-1042-1-ND ---- SEH-001T-P0.6
|CN1 Jack - 1ea Needed
Digikey -455-1611-ND ---- B2B-EH-A(LF)(SN)
We built this circuit on a piece of Perf-Board, and then
mounted it to the RF Unit using a plated-thru ground hole.
By utilizing a 2 Pin connector, the complete RF Unit is still removable.
The SSBB signal wire comes from the Signal Unit after a jumper is added to that circuit Board.
An unused pin is used on CN10 to accommodate this new signal wire.
Here are some pictures of the completed Mod.
Here are some views of the Circuit Board.
You will notice on some of the pics that the .01uF Cap is not installed yet.
Next is to modify the Signal Unit to bring the SSBB Signal to the RF Ground Relay Board.
You will place a jumper from Pin 9 of CN1 to Pin 10 of CN10 of the Signal Unit.
This jumper wire is placed on the bottom side of the Signal Unit so it will have to be removed.
We used Pin 10 of CN10 because its an empty, unused hole.
We got another wire with the contact pins from another Kenwood wiring harness the right length,
and inserted that into the wiring harness to transfer the SSBB signal from the Signal Board to the
new RF Grounding Mod CB on the RF Unit.
You could also just manufacture your own wire and crimp the required Pins (see Parts List) onto the wire.
This is the bottom of the Signal Unit.
The White Wire is the Jumper for the SSBB Signal we need.
It goes from Pin 9 of CN1 to Pin 10 of CN10.
Ignore the Orange wire as that is from a previous Mod that was removed.
Thats all of it.
You should be able to accomplish this with no problems.
You should here the relay click when when you key the rig in SSB Mode only.
In all other mods it should be silent.