Will any of these mods be incorporated into uBitx v4 or even a new version soon?

--
72 and God bless
KD4EPG

Farhan,

For the 45mhz amp the only part suggested was BFR106 or maybe 2n2369/mmbt2369.
I'd not use 2n2222 there but its better than 3904.? Any suggestion of of 2n2222A would
have been in the power amp area predriver or driver.

Allison

I thought we needed better linearity here to reduce distortion, not improved response ?

开云体育

With better gain, you can afford more negative feedback to reduce distortion

开云体育

What is the expected level into the Tx bi-amp stage at 45MHz? I don’t have a feel for this and can’t check it as my uBITX is in pieces at present.

If it's a relatively high level then the 45Mhz amp stage maybe into limiting.

But, my testing on a proto build of these stages, shows that the bi-amp as it is, although fitted with a BFR106 in 1^st stage, is indeed, severely limiting at levels above about -13dBm.

?

Ie at -10dBm input level, 2^nd harmonic is ~21dB down, 3^rd is -22dB

And at -13dbm input, 2^nd harmonic is -33dB, 3^rd is -32dB.

Around a 10dB improvement in harmonics for a 3dB input level drop!

?

This is probably the reason why fitting the extra filter mod works reasonably well, as it's actually just reducing the gain of these stages, as Allison has surmised before.

If this is actually the case, then a simple ~3-4dB (or more?) pad at the input may be a solution although better would be to re-calc the Bi-amp stage for lower gain.? Currently the gain is about 18-19dB.

Glenn?? -vk3pe

Glen,

That was the first thing I did was try a pad instead of filter. But that did not help. It is the sane as reducing
the mic drive.

The filter helps to get rid of non IF signals (Harmonics) picked up from the PA. The ralay change does the
same but reduces the spurs and harmonics.

Last evening I was with Farhan, I was playing with a modded board and found spurs gone high.. baffled me!
Guess what ? I had soldered an unconnected wire to a ground spot on the bidi and when it was removed
the spurs came back to kosher levels. This explains it!

Raj

Raj, sorry, i didn't take in the detail of your previous post in this thread. I guess you tried shielding etc of the stages.
glenn

Just happened to have this little can which fits over the 45Mhz area.?? Or perhaps even more area needs to be shielded. If that actually works.?

Would adding the LPF as specified by Farhan provide approximately the same output power, something more than 10w at least on the lower bands?

Congrats to Farhan and Raj for the fine detective work in figuring out this issue.

--
Ryan Flowers - W7RLF

If one swaps the 3904s for BFR106s in the transmit side amp, is it a transistor swap, or will the job also require re-biasing?

I'm wondering if I should try this, or if it would be better to stay on the porch until the adults report back?? ;)?

Thanks!

Andy, KG5RKP

I had to re-bias after swapping the TX transistors (Q20, Q21 and Q22) in June.

I wondered why TX-power on SSB went down - and the reason was that Q20 was not on the correct operating point.
Allisson gave the right hint: At ideal bias the collector of Q20 (if a BFR106 is used) shall have about 6V.

And just by looking up that one, I am wondering now if a lot of solutions (or the right approaches) have not already been discussed in the past.

Sascha,

Have you checked the wiki and???

Info in both places.

The bias at Q20 should not be about 6V.? It should be more like 9V.
Reason for this is the ideal bias is about 10mA and at 6V collector its more like
36ma and that's very high.? ? Also the two transistors (Q21,22) cause a 1.5V offset
and we want the emitter of Q22 at more than 4.5v (+-1 V).

Allison

AndyH,

Yes rebias.

Allison

Thanks Ma'am.? I'm staying on the porch.? hi hi

Holy cow, Allison - I just worked through your RF power chain thread and you've covered all of the swaps and biasing there.
/g/BITX20/topic/rf_power_chain_mods_and/19399962?p=Created,,,20,1,240,0&jump=1

THANK YOU for teaching enough to get me off the porch!