Omnibus F4 Pro Softserial

by admin

Hojo and the case of the deaf Tentec Omni V

  • OMNIBUS F4 Pro (V3) Upgrade version of OMNIBUS F4, the OMNIBUS F4 Pro (Some shop call it as OMNIBUS F4 PRO V2) added SD card supports, has 5v3A BEC, LC filter for Camera and VTX, build in Current sensor for high Integration Frame.
  • The Omnibus F4 offers a SoftSerial port under software control that can be used for SmartPort telemetry. In order to use the soft serial port you must connect the SmartPort signal from the receiver to the CH5 pin in the figure below, and then connect a 1kΩ resistor between the CH5 and CH6 pins.

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A friend from the local ham club handed me his Tentec Omni V. He hadn't used it in a while, and was preparing to sell it to another ham when he noticed it appeared to be very weak on receive. He asked me to take a look.


Power supply


Right off the bat, I checked the power supply. It was reading low. The trimmer pot was right next to the activity LED, so I couldn't resist dialing it up to a proper voltage.

Softserial

Initial problems

I powered up the rig and found the LED display blank. I opened the covers and wiggled a few things directly behind the LED display. It flashed back on. The board was not seated very well, apparently.
At the same time, one of two incandescent bulbs behind the S-meter was intermittent. I snugged up the fixture with a pair of pliers, and the bulb became more reliable.
Omnibus
Having taken care of those little physical problems, I could finally try injecting a signal into the radio. Sure enough, it was quite hard of hearing. I needed a signal of about -20 dbm to be discernible. I confirmed the problem existed across multiple bands.

Diagnostics

I opened up the radio and started tracing the signal. I injected a tone on 14 Mhz (1) and followed the path. It got through the antenna selection circuits and initial attenuator just fine. It reached the front-end mixer board (2), and exited that board mixed to 9 Mhz (3). I also verified that the signal disappeared when the injected signal was stopped. So far so good. The signal passed through the 9 MHZ IF board (3) -> (4) unmolested and was amplified slightly. From there, it went into the Pass Band Tuning board (4), and finally into the AF/IF board (5). There's where the trouble began.
I measured a good signal coming into the board (5 above). It passes through a few amplifier stages first thing. I checked at the end of that stage and found no signal. Tracing back, I found the signal disappeared after passing through a J310 Fet.
I next confirmed that there was power on the '+REG' rail. It was reading 8.78 volts. Unfortunately, I had no idea what the voltage was supposed to be. After about 40 minutes of tracing the +REG rail back to a power board, I found a knockoff sentence in the service manual that +REG should be 8.5 volts. So, it was a touch high, but shouldn't have been a problem for this situation.
Having narrowed it down to this Fet, I checked my parts bin, and voila, I had a few in stock!

The repair


The board had approximately a jillion connectors on it. I carefully photographed everything before removing it.

Now that I had a better view of the FET in question, I confirmed the model, and that my parts-bin FET matched.

Pro



Testing

Omnibus F4 Pro

After putting things back together, the radio was hearing loud and clear. My Service Monitor can generate signals as low as -130 DBM. The signal was clearly discernible all the way down.
Talking with the owner, he was satisfied that the repair was sufficient, so I stopped there, rather than attempting to find alignment instructions.

Omnibus F4 Pro Soft Serial Download


Omnibus F4 Pro V2 Manual

Success!