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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Jan Sun 31, 2016 4:48 am 
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Location: MD-DC 20855
Carl 'burnt fingers' also weighed in with the snubber idea early on
(15 Mar 2013 11:35- see Page 3):

Quote:
...I would try oscillation snubbers at the grid, screen and plate pins,
A 10-22 Ohm carbon right at the grid, a 100 Ohm at the screen with a .01
disc bypass right close to either side.


Thanks, Carl! I am definitely going to put a 100 ohm resistor right at the
screen pin (even though the LF parasitic was already fixed). I think that
radio already has a plate VHF parasitic suppressor, so I'll leave well
enough alone there.

I really don't use it very often. Need to get that VFO project finished. Just
need to complete its packaging. So many projects here...

David

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Apr Mon 27, 2020 3:43 am 
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As a nice postscript to this thread, I'm pleased to report that my old T-60 ended
up seeing far more use than I might have imagined. I never did completely
finish that VFO project, though good progress had been made. I did come up
with a good enough booster amp circuit to drive the 6HF8 from a VFO.

Who would have guessed I would have ended up with so many 80 and
especially 40m crystals as I now have? The secret to my success- I built up
a little VXO fixture that plugs into the front of the unmodified transmitter.

That fixture will accommodate HC-49, HC-6, and FT-243. I have learned a
lot about VXO's and the details about 'pulling' crystals while maintaining a
clean waveform. It's a fairly 'fiddly' area. Sometimes there is advantage to
running 2 or 3 near-identical crystals in parallel, pretty easy now that the little
HC-49 types are so cheap and plentiful. I have surprisingly wide coverage
on 40 meters, with only a few small gaps, all the way from 7000 to 7130KHz.

80m is a bit more problematic. Crystals don't pull near as far down there.
I'm working on a heterodyne VXO project for 80m- difference mixing
with two crystals, the higher frequency being much more 'pullable' than
my 80m rocks.

David

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Apr Mon 27, 2020 3:31 pm 
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Nice postscript to a 4 year old string of postings. Glad it all worked out for you. Someday let us know when you get that VFO project running. :-)

Curtis Eickerman

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Apr Mon 27, 2020 5:16 pm 
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Hi, Curtis-

There were a couple of factors in play that eventually caused me to drop the VFO
project. One was the use of a TR switch for break in operation. The VFO runs at
7MHz and the back wave in my receiver was oppressive. Perhaps if I had designed
it to run at 3.5MHz and double? Ultimately that VFO can be re-purposed in some
heterodyne scheme, I imagine- moving it up or down a ways in frequency.

The other factor is that I got interested in the NRR activity (Novice Rig Roundup)
back in 2017. Extra points per QSO for Novice level power and crystal control-
thus my recently re-acquired interest in VXO's in general. Amusingly, I actually
placed first nationally in 2018 with my little T-60 after a serious effort on 80
and mainly 40 meters. That was startling... It's a fun, low pressure, week long
celebration of the old boat anchor rigs with an emphasis on recreating the
Novice experience- rag chewing encouraged... A number of those guys are
active homebrewers. A fair amount of tube type gear is still being built.
I personally took on a 6AG7 + 3D21B MOPA design and got it working and
on air. Cathode keyed oscillator and PA. Note sounds pretty good...

So a heterodyne VXO, difference mixing two crystal oscillators, is still arguably
'crystal control', though a far cry from what we all struggled with on 40 meters
amidst all the power house broadcasters back in the day. (grin) Thus far my
VXO work has been entirely pulling in-band 80 or 40 meter rocks with that
fixture I built. But taking inspiration from K8IQY and his "SAVXO" and SS-40
designs using VXO's, I figured out several mixing schemes that mainly use
standard, available microprocessor crystals and should be free of spurious
mixing products, according to the Collins Radio 'Mixer Spurious Analysis'
chart. You can pull one or several crystals in the ~10 to 20MHz range a
good ways. My focus is on 80, 40, and 30 meters mainly these days.
Sunspots? What the heck is a sunspot? (grin) I'll stick with solid state for my
VXO project, however. I'm not a purist. The driver and PA are vacuum tubes.
That's good enough for me.

David

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