[TenTec] No SSB from Omni D

[Jay Eimer]

Okay, I borrowed a scope and checked some things.  One thing I confirmed is
that my mic is getting to the SSB Gen board.  Turns out, out of 4 mics that
I tried only one was working with one other marginal.  Once I tried with the
good mic I got output enough that a friend was able to hear me on 28.330.
Unfortunately, he was on his HTX-10 (radio shack) mobile sitting in my
driveway.

He heard me up to about 2 miles away, then my signal faded.  I could hear
him about twice that distance.  Note that he was on a base loaded vertical,
and I was using an 132 ft. dipole and a tuner so the polarization was
mismatched.

I tried making some contacts last night with the good mic, no go.  It seems
that I'm getting out, but very, very weak (5W or less).

So I did some other checking with the 'scope.  Here's some of the anomalies
I found.

I have modulation at the MIC test pin on the SSB Gen board.

I have modulation at pin 7 of the dual op amp IC1.  This is the output of
the first stage amp IC1b.

I have an unmodulated 4V dc at pin 3 of IC1.  This doesn't make sense,
especially since the manual lists a test voltage (SB-N receive with no
drive) of 0V dc.

I have no output (unmodulated ~4V) out of pin 1 (final output of the second
stage op amp, IC1a) that feeds into IC2.

I'd suspect the IC, except for a further anomaly.  I appears that there is a
voltage divider/step down circuit.  Out of pin 7 on IC1, it goes through R25
and feeds pin 6 (feedback) and divides to R26.  This feeds one side, while
Reg+ feeds the other side of voltage divider through R19 and R23.  R23 feeds
R24 and then pin 3 (+ Wiper?) and R19 feeds R18 and then Pin 1 to Pin 1 of
IC2.  Where this is wierd is that every resistor shows 4V (no drop) on both
sides.  This doesn't seem right, but I'm no expert.

So, is this the same IC that you had a problem with?  Or do I have a bad
component somewhere else on the board?  I suspected a bad cap was blocking
modulation, but I can't see which one that would be, since the problem
appears to be between output of pin 7 to input of pin 2 and or 3 of IC1.

What I think I need most is a test plan to isolate which of these might be
the problem.

Thanks,

Jay
AD5PE (just got my new call last Thursday!!!)

-----Original Message-----
From: tentec-admin@mailman.qth.net
[mailto:tentec-admin@mailman.qth.net]On Behalf Of Kenneth Stringham
Sent: Tuesday, September 30, 2003 11:02 AM
To: tentec@mailman.qth.net
Subject: Re: [TenTec] No SSB TX from Omni D


Hi Jay,

My first question is: do you have the Operator's
Manual. This includes a detailed description of the
circuitry and the schematic. If you do not, I have one
here that you can purchase, I sold my 546C recently
because of my need for cash.

You problem will be located by looking at the SSB
generator board. This radio generates the CW signal by
moving the Carrier Oscillator 750Hz so it is inside
the filter pass band and then it unbalances the IC
balanced modulator. My guess is that you have problem
with the circuitry that moves the oscillator frequency
for LSB/USB. The frequency is offset to far down the
filter pass band. One of mine had a defective balanced
modulator IC which I replaced and all was well.

rest of original snipped....