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February 12th 04, 04:00 AM

In article , Leo
writes:

On 10 Feb 2004 10:14:34 -0800, (N2EY) wrote:

snip

Heath SB-400. The Pride of 1964

Oh yeah - and my basement!

Amazing how well a 40 year old box full of glowbottles is still
useful, huh?
15 Hz stability from a one-tube Hartley is pretty decent.

With a narrowband signal like this, it doesn't take much drift! I'm
seeing a frequency decrease of up to 15 Hz, beginning a few seconds
after keying. B+ to the VFO appears to be well regulated - maybe not
tight enough though. Might have to replace the 0A2 with a few zeners
- haven't tried that yet....

First try replacing the 0A2 regulator and 6AU6A oscillator tubes -
different
ones may have slightly different characteristics.

That was one of the things I tried when I rbuilt the TX back in the
summer - ended up with an 0A2WA regulator and a 6136 in the LMO. That
brought it much closer...that and some conductive silver grease in the
bearings of the LMO tuning cap (I didn't even know they made that
stuff till I dug into the Web and discovered that TRW used it when
they built the LMOs for Heath, and it dries out over time.....)

Lotta subtleties in old gear. Not as "primitive" as some folks try to tell
us...

But the real problem may be that the heater voltage is changing when
transmitting, rather than the B+. Another possibility is that heat
from the
6146s and power supply may be causing a bit of drift, because PSK31 is
a very
high duty cycle mode and a lot of heat gets dissipated on transmit.

That is very possible - it's also possible that the drift is in one of
the other oscillator sections other than the LMO. Haven't had time to
tear it apart and see yet....one of these days!

Two xtal and one tunable oscillator in that heterodyne scheme.

Upwards drift in the het osc will make the operating freq go higher
Upwards drift in the LMO will make the operating freq go lower
Upwards drift in the BFO will make the operating freq go lower

So there's a bit of compensation *if* the oscillators all drift the same way.

Thanks for the info!

You're welcome.

73 de Jim, N2EY