[TenTec] Omni VI problems, continued

[Dr. Gerald N. Johnson]

On Tue, 2008-05-27 at 21:46 -1000, Ken Brown wrote:
> Hi all,
> 
> My Omni VI is not producing full power on all bands. I have not fixed it 
> yet,

 <SNIP>

> The low level driver board gets it's TX RF drive from the BPF/FRONT END 
> BOARD, and that gets TX RF from two sources, the 9 MHz IF and the XTAL 
> OSC LO MIXER BOARD. Now I am suspecting that the LO power is lacking on 
> some bands, though BPF switching problems might also be the cause of low 
> power on some bands. I've had some frequency instability on some bands 
> too, so I'm leaning towards a fault in the XTAL OSC. That board is not 
> easy to get to. I finally saw it for the first time tonight. It is a 
> really well built, good looking board, and the schematic shows only a 
> single electrolytic capacitor on the whole board. That electrolytic 
> capacitor is in the part of the circuit that "sets the bias point of the 
> oscillator stage to stabilize the oscillator output level", so it seems 
> plausible that a failure there could cause the problem I am having.
> 
Mixer output or gain is dependent on adequate LO drive. Low LO drive can
cause low output.

Remember that in the classic TenTec mixing scheme, 80 and 20 use the VFO
directly, while all other bands mix the VFO with a crystal before
driving the transmit mixer.

I'd expect oscillator tuning or crystal quality to have a bigger effect
on oscillator output than the electrolytic capacitor, and probably a bad
electrolytic might lead to random output irrespective of band on bands
where the crystals are used.

> I'll let you know what happens after I change C19 on the XTAL OSC LO 
> MIXER BOARD.
> 
> DE N6KB
> 
73, Jerry, K0CQ