Hojo and the case of the deaf Tentec Omni V
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.
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.
Initial problemsI 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.
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.
DiagnosticsI 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!
I pulled the FET and replaced it.
TestingAfter 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.