On Thu, 2008-05-29 at 19:10 -1000, Ken Brown wrote:
> Hi all,
>
> Changing C19 on the XTAL OSC LO MIXER BOARD did not make any big change
> in the power output, and I can still hear a periodic "relocking" sound
> when on 30 meters (maybe on other bands too). By scoping the output of
> the LO board, I can see that on bands with low TX power output, the
> output of the LO is also low (relative to other bands, I don't know what
> it should be), so I am suspecting the that the last TX mixer, U1 on the
> BPF/FRONT END BOARD is starved for LO power. I also see on some bands
> there the LO signal appears on the scope as multiple tones. I am doing
> this all in CW mode, so I don't expect to see anything but constant
> amplitude sine waves anywhere in the transmit signal path. I am not too
> concerned if they are not pure sine waves, but if they look like
> multiple tones, or modulated it seems like something is wrong. Maybe the
> carrier balance in the LO MIXER is off, letting some of the 5 MHz VFO
> signal through.
In the LO Mixer path there is never anything but sinewaves except at the
output of mixers before the output filter. There is no SSB or keyed
signal. You may find fewer signals in receive mode so your scope can't
pick up PA output by stray coupling.
>
> The next things I'll try is to get all of the XTAL OSCillators adjusted
> for "minimum stress", which is to have the PLL correction voltage as
> close as possible to nominal, whatever that is, and I'll check the LO
> mixer carrier balance.
As noted by others, finding the limits of the control voltage range and
setting to the middle of the range is effective.
LO mixer balance could be critical to a clean signal.
> Based on the values of R25 and R26 (which are 1k
> and 470k) I think I can connect a 10 Meg scope probe at connector 84 to
> monitor the DC voltage there, without severely disrupting the circuit.
> Does anyone know what the DC voltage at 84 is supposed to be? I guess I
> can determine the full range of DC voltage there, and then set the XTAL
> osc tuning coils for the center of that range. I'd sure rather have a
> specification to aim for.
A WAG would say 2.13764587 volts, give or take .139 volts. ;<)
>
> By the way, if anyone is wondering why I don't just sent the rig to
> Ten-Tec for service, there are a number of reasons. Shipping cost both
> ways from Hawaii would be many times the cost of any parts that need to
> be replaced. If I had Ten-Tec fix it I would not learn much.
I think I've heard these complaints before and the cure was to tune the
crystal oscillators.
>
> DE N6KB
>
73, Jerry, K0CQ
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