Hi,?

I have had success with the WSPR beacon mode using Dr. Lee's uBITX firmware. On each WSPR band, I matched my antenna for low SWR as best as I could. On some bands, the finals get VERY HOT so I would kill power to the uBITX hopefully before I destroyed the finals. I do realize that digital transmission modes are high duty cycle loads.?

In an attempt to reduce output the uBITX RF power, I turned RV1 clockwise 1/8 turn to reduce output power on the finals - that helped prevent overheating.?

Is adjusting RV1 the best way to reduce output power with a stock uBITX build?

Regards,

Vick KG7UNJ

Being a symbol of a capacitor, it would mean ZERO pF or open circuit.

If you put 0 ohms then you surely will blow the finals or fuse!

Raj

Actually the IRF510s have higher gain that the RD16s.? At least at lower frequencies.
though the difference is generally within 3db. Read the spec sheet and not run with?
myth and lore.? The RD16HHF is a fine device but limited to 12V systems and?
has a slightly better IMD, it is not a panacea.? Most of the issues with IRF510s
is people trying to get a lot of power but putting little in or from unstable sources
then wondering why its flakey and oscillates.? ??

Keeping the feedback lowers the gain and keeps them stable at lower
frequencies with reactive loads.? I'd rather see the driver make a bit more power.?
As the low power out has not been an IRF510 issue its a drive to them that is
being worked. Example I was getting 1.6W but the drive was barely 70 milliwatts.
The IRF510 was doing very well at 10M knowing that.? the problem was why
only 70mW of drive and worse from 4 devices pulling 100ma idleing?

Generally to make a stable amplifier chain you don't run devices wide open
if possible. Lower gain and more devices to make it generally is safer as
your not forcing power by forcing gain especially in high power stages.

Right now speculating on intermediate work I've done is not in ones best
interests as you might have to take it all out.? The mods around q90 help some
and its the only for certain mod I've done is to put a BFR106 there for more
even gain plus the peaking mods suggested by Farhan.? It also insure that
at 28 mhz it can produce the design gain.? The 2n3904 could not do that
though it was doing better than the book and theory claims.


Allison

If the process is flawed then surely the outcome will most certainly be flawed!

Thank you!

So R261 and R262 aren't used either, right? They could also be removed?

tim ab0wr

On Wed, 30 May 2018 08:15:30 +0530
"Ashhar Farhan" <farhanbox@...> wrote:

0 pf is 'no pf', that is, it is open. Take out the capacitor. It has
been placed on the circuit to be used if needed with PA transistors
with higher gain like the RD16HHF1s, etc.

- f

On Wed, 30 May 2018, 08:06 Jerry Gaffke via Groups.Io, <jgaffke=
[email protected]> wrote:

It's really tough to make a 0pf cap (or 0 ohm resistor) that is
accurate to within 1%
Or 100%.



On Tue, May 29, 2018 at 07:26 pm, Jerry Gaffke wrote:

Separate universes.
. . . <#m_4917506501796616938_quoted-43406361>



On Tue, May 29, 2018 at 07:03 pm, Vince Vielhaber wrote:

Yes, it's recommended you put the two pads in separate rooms

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Tim, Farhan’s suggestion was to remove C261 and C262 not short them out which will remove the feedback on the IRF510’s.

Nice job Allison. My PA print is about the same size, 2.5 by 5 inches and that includes one corner 1.5 by 1.5 " cut out to leave access to the BFO/AF mixer below and also 2 reasonably big heatsinks of about 1.5 by 1.2" each placed horizontally on but isolated from the board. Next to the heat sinks I placed a vertical PCB strip for shielding and behind that was just enough room for the LPF band filter. So you actually have very little space left to place all the components. It does required some planning ahead but you can fit it all on there easily, I even managed to position each coil in another direction. But I doubt they will influence each other's field much, it's just to be on the safe side.

That is why I am hesitant on replacing the T50-6 toroid coils and make new ones using T50-2. I glued them in and it is a tight squeeze there, I don't want to damage anything surrounding it when taking them out. The T50-2 would result in a better Q for these coils but do I really need that? My IF is 10 MHz and I left the BFO a little below that, so the upper side band passes through the crystal filter. The DDS works from 17.0 to 17.3 MHz so I get 27 and 7 MHz mixed before the BPF. The BPF only lets the 7 MHz part through and turns USB into LSB and vise versa. Worst case scenario there may be some unwanted 27 (17+10) and 14 (7x2) MHz signals. They are much higher in frequency than the desired 7 MHz so I wonder: do I really need better coils with a higher Q and thus steeper slope? To me it seems impossible it will ever interfere in the 20 meter or CB band.?

Maybe, when everything else is finished and I want to use some more power, I could consider adding a new bandfilter and simply bypassing the old one.?

It's really tough to make a 0pf cap (or 0 ohm resistor) that is accurate to within 1%
Or 100%.

Separate universes.

Final could be weeks, still waiting for Allison's intermediate comments on changes.
I suggest you leave the rig stock unless you are keen to do your own development.

First thing I would experiment with is Farhan's suggestion of adjusting the value
of C81 and R83 near Q90 to 470pf and 2.2 ohms as per his new v4 schematics.
Should even out the power across the various bands somewhat.
Then re-adjust RV1 for no distortion on any band after doing any mods.

What should be changed around the IRF510's is still open for debate.

A bit ambiguous, but I believe Allison is suggesting you might want to leave
those 0.1uF caps at C261 and C262 in place, as they were originally.
And while you're at it, she suggests leaving those resistors at R97,R97 as they were with 47ohms.

I suggest you sit on your hands for a few days or weeks till this shakes out a bit further,
and we have some consensus as to what is the best way(s) to upgrade the uBitx final.

Unless you have a fair understanding of what's going on here and the instruments to see it.?
Or at least a nice stock of spare IRF510's on hand.

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Is there a best and final solution yet or are we still waiting for Allison's final comments on changes?

Yes, it's recommended you put the two pads in separate rooms.

Vince.

--
Michigan VHF Corp.

That's a cap, so the zero means 0pf.??
Be sure to place the two PCB pads infinitely far apart.? ;-)

Farhan is disabling the negative feedback around the IRF510's.

Last time I built a bitx20? PA I use a strip of copper clad (2sided) and it was about 2" by 5"?
input at one end and the PA transformer at the other.? everything evenly between.? I still
have it in a different radio runs about 7W and the final is biased at 100ma.? It was built
stock form the circuit.? Layout at that amount of gain is everything and packing it close
makes for issues.


Allison

those are .1 uf caps, pass rf block dc.? Says so in small print on the schematic.

That is the negativefeed back to keep the mosfets tame at low hf.? Without the cap its full
on forward bias likely will not hurt them but it will suck down many amps as under that
condition the mosfets look like s short to ground

Chaning R97/98 to 220 ohm is going the wrong way and not advised.?

Allison

My favorite AM radio is my Hallicrafters SX100 or the highly modded S120
of for just being odd ball my really old (1953) RCA am potable AC/DC/battery
using tubes.? ;)

I still wish I had the old RBO II shipboard AM RX.? ?It was heavy enough to
hold down a truck.


Allison

I made the changes listed for the PA of changing R97 and R98 to 220ohm.

I thought the change of C261 and C262 meant going to a 0ohm jumper but
when I made that change the PA now immediately starts pulling over
5amps at turn on and blows my 3 amp fuse.

Is that 0 capacitor for C261 and C262 *not* a 0ohm jumper? From drain to
ground I get about 220ohms which is what I would expect from the two
220ohm resistors R98 and R261 in series paralleled with R97 and R262 in
series.

Any comments, suggestions, ideas?

It's not hard to replace C261 and C262 but I don't want to try any other
fixes till I have an idea of what is going on.

tim ab0wr