Topics

Farhan's new PA for ubitx

Tim Gorman
 

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

ajparent1/KB1GMX
 

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

Jerry Gaffke
 

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.




On Tue, May 29, 2018 at 06:32 pm, Tim Gorman wrote:
Is that 0 capacitor for C261 and C262 *not* a 0ohm jumper?

Vince Vielhaber
 

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

Vince.

On 05/29/2018 09:52 PM, Jerry Gaffke via Groups.Io wrote:
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.




On Tue, May 29, 2018 at 06:32 pm, Tim Gorman wrote:

Is that 0 capacitor for C261 and C262 *not* a 0ohm jumper?

atouk
 

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


On 5/29/2018 9:45 PM, ajparent1/KB1GMX wrote:
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

Jerry Gaffke
 

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.



On Tue, May 29, 2018 at 06:52 pm, Jerry Gaffke wrote:
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.

Jerry Gaffke
 

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.


On Tue, May 29, 2018 at 07:17 pm, atouk wrote:

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

Jerry Gaffke
 

Separate universes.


On Tue, May 29, 2018 at 07:03 pm, Vince Vielhaber wrote:
Yes, it's recommended you put the two pads in separate rooms.

Jerry Gaffke
 

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.
. . .

 


On Tue, May 29, 2018 at 07:03 pm, Vince Vielhaber wrote:
Yes, it's recommended you put the two pads in separate rooms

Ashhar Farhan
 

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=yahoo.com@groups.io> 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.
. . .

 


On Tue, May 29, 2018 at 07:03 pm, Vince Vielhaber wrote:
Yes, it's recommended you put the two pads in separate rooms

Skip Davis
 

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


Skip Davis, NC9O 

On May 29, 2018, at 22:25, Jerry Gaffke via Groups.Io <jgaffke@...> wrote:

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.


On Tue, May 29, 2018 at 07:17 pm, atouk wrote:

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

Tim Gorman
 

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=
yahoo.com@groups.io> 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


VE7CWS WRSeiler <waltrseiler@...>
 

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

ajparent1/KB1GMX
 

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




 

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

At 30/05/2018, you wrote:

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?

Terry Morris
 

I have the v3 PCB dated 02/18 silk screen but under the mask the date is 2017. I don't have plans for assembly until end of August. Too many other projects. Ashhar says version 4 PCB is available. Would it be advisable that if there are more board revisions that I purchase the board before beginning the remainder of assembly?

Thanks,
Terry KB8AMZ

Jerry Gaffke
 

You have a complete kit for the v3 uBitx radio, may as well just build it.
When v4 comes out, it will also be a complete kit, not just a pc board.
Looks like v4 will have virtually no circuit changes except around the audio amp, not a big deal. 

That 02/18 date is likely when the board was fabricated, something added by the board fab house.
The 2017 date in copper is when it was designed.

Jerry


On Tue, May 29, 2018 at 10:36 pm, Terry Morris wrote:
I have the v3 PCB dated 02/18 silk screen but under the mask the date is 2017. I don't have plans for assembly until end of August. Too many other projects. Ashhar says version 4 PCB is available. Would it be advisable that if there are more board revisions that I purchase the board before beginning the remainder of assembly?