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Author Topic: Ten Tec Omni VI Frequency Hop  (Read 1133 times)

Posts: 13

« on: July 11, 2018, 08:50:44 PM »

Some of the this thread I have posted under Elmers.  More info now says put this in Mods and Repairs.  This is a Omni VI Option 3 Factory Installed Model 563.  Chips on logic board say Model 564.  Anyway the received signal on 30M will shift 500 Hz or so for a brief blip, maybe 50 milliseconds.  This happened irregularly, but something about 5 seconds apart.
What I know is other bands appear fine.  Says to me the 5.5 MHz tuned local oscillator is ok.  If I disconnect the coax #84 from the logic board, then the front end oscillator crystals run without discipline / varactor steering from the U13 PLL charge pump commands.  Without the steering command the receiver is solid, even on the problem 30M.  Corrections being made seem to be in the 100’s of Hz.
Monitoring the correction voltage, which is sent back to the front end local oscillator via the same coax used to send the sample signal, is that the level looks solid at some 5 volts.  Blips are too fast for the DVM to catch.
Is this a chip intermittent, serial programming on the ragged edge or what?

Posts: 13

« Reply #1 on: July 14, 2018, 12:19:23 AM »

I watched what was going on both more closely with a DVM and with a TEK 465B scope.  Findings:  The steering voltage back to the varactor diode on the L.O. Mixer board runs 3.something to 4.something for all selected bands except 30M.  There the voltage is against the 5V rail.  Watching the up / down commands (pins 14, 15) from the MC145170 PLL chip to the charge pump I see a balanced and stable pulse duty cycle except at 30M.  At 30M the duty cycle for up voltage walks progressively to longer on-shorter off until 100% is reached.  The process takes a few seconds.  Instead of staying hard over, there is an internal reset, and the process starts anew.  That is where the hop comes from. 

It seems that the 30M L O first mixer control crystal has drifted low in frequency beyond what the varactor loop can accommodate.  This would be a minor harm/no foul except for the system repeatedly doing a reset and attempted reconverging.

Also noted is the varactor diode is biased mid control range by a voltage divider on the LO Mixer board.  Which is to say the system will run with the PLL - charge pump coax disconnected.  Not elegant, but not really different from S-Line in service for many years.

Posts: 514

« Reply #2 on: July 26, 2018, 11:38:42 AM »

I had a similar problem on my VI ten tec.  There was an easy "fix" and also a more complicated proper fix.  E mail me at my QRZ e mail address if you still need the info, and I will dig through my old notes of over 20 years ago and try to find it.  My 30 ?? year old Omni still works fine without the frequency jumps I experienced first one one band (30 meters Huh) then later on another band or two. 
 Good luck    Rick    KL7CW

Posts: 13

« Reply #3 on: October 10, 2018, 07:38:40 PM »

Follow up to getting the Ten Tec going.  Hop was fixed.  Then the radio became slow to get audio up, leading to finally no audio.  Scope tracing found the audio was there ok from the detector circuitry.  But nothing got through the full-time DSP to the audio amplification.  Looked like an electrolytic was slow to wake up.  But not the case.  The DSP chip needs a -5v rail.  This is made with a 7660 charge pump IC.  Making a mockup of the circuit with new parts the problem indeed showed as a bad chip in radio.  A 75 cent part.  Tight quarters to work in.  I worked with a Swiss file to cut away pins on one side of chip body.  Then judicious bending back a forth got the 4 pins on the other side off.  Careful work with flux and braid got the pin remanants out and pads cleaned.  New installation went in with a dip socket.  Board was threaded back into radio, and the bazillion push on connectors redone.  Radio back on the air.
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