iC 756 Pro3 input overload

IC 756 broken due to another transmitter in the shack transmit in the same band.

Test results :

After some checking and repairs, it was found that there are 3 broken functions:

1. TR switch and RX mute circuit in broken.

2. Diode switches in BPF broken, causing bad BPF response.

3. Resistor failure causing Diode switches in BPF circuit not working.

Repairs done :

1. TR switch and RX mute

This is a common problem when 756 hit by large signal input at the antenna.
The transistor (Q25) failed and left open, and/or diodes (D22, D23) are also left open.

Just need to replace those components. Then verify that the signal coming from antenna input is attenuated when in SEND mode (transmitting).

2. Diode switches in BPF broken, causing bad BPF response.

Symptom :

Signal received is lower than other radio (normal radio).

Diagnostic :

Here is BFP response for 80m. Note the attenuation is reduced on higher frequency :

Here is BPF response for 20m band :

Another response for 160 :

Lucky, there is another 756 Pro III that can be used for comparison.

BPF response for 20m :

BPF response for 15m :

Cause :

Broken diodes used for band switching and causing disturbance on frequency response.

10m Diode Switch was broken shortened

6m Diode Switch also shortened

Solution :

Replacing the diodes, BPF responses are now back to normal.

3. Resistor failure causing Diode switches in BPF circuit not working.

This is a difficult bug to locate. After fixing BPF swiches and RX Muting, the radio still exhibit a high power output even the power control was set to minimum.

Symptom :

On some band(s), transmitter will produce maximum power output even the power control set to minimum.

One might think as runaway feedback.

Diagnostic :

During transmitting, the buffer input were filtered using the same filter as during receiving.

D104 is supposed to shut signal coming from Antenna (leak from PA output), coming back to the buffer input.

But in this case, the R108 was broken and can not provide enough current to reverse bias the D104 (and make the diode switch open). This is verified by transmitting with PA disconnected, and measure signal level at the Cathode of D104. The signal at Anode is similar in level as signal on Cathode.

So, signal coming from PA will arrive at buffer input and amplified again thus making a loop and causing RF power to go uncontrolled.

Solution :

Replace the broken R108.

Summary :

The problem caused by RF input overloading to the receiver is quite severe. Not only components near the antenna switch, but can also go through to more deeper in the receiver.

So, one must be very careful not to transmit while another radio receiving next to it.