It was fun, no question, and plenty of mental exercise. It missed the mark
for my intended use.
If my board was intact my list might be..
External balanced mod and filter as that cleans up the carrier leakage.
I built such a thing and put it haywired in a mint tin with a crystal
oscillator dialed in as needed and got excellent result. The circuit
was nearly in the end identical to bitx40 analog.
Mod both TX IF for no more than 11DB from the tx side of the bidi amp.
Pull the relays. Carefully remove all the RF tracks. and DC
running under them. Install those or replacements from the
underside and wire it up for 80 through 20 and forget the 10M filter.
Sounds nasty but fairly trivial bunch of wires. I put up the base circuit
for that three times. Go dig.
Replace the 33mhz low pass with a 20mhz low pass values. That will
take out most if not all of the lower spurs.
Edit the code to stop at 17M. Add the needed touch ups for the now
current relay scheme for the 80 through 17M use.
Rewire RX/TX to bypass the power amp input (rewire around K1). Use K1
for DC switching only and also to remove DC short the unused line to ground.
the whole switching scheme is nuts I'd try to use KT1 for that and dump K3.
Improve bypassing and add bead chokes to power lines all over the board.
The idea is less high power wandering around the board and leave the filters
in for RX as it helps with RX birdies. since that also means only 1 relay per
band is active it helps with power drain.
Mod the Power amp section:
Q90 a decent part mmbt2369 or other 600mhz FT part will do as we are stopping at 18mhz.
Play with C81 for more even power 80 to 17M.
Version A, what most might do:
Replace all the 3904s with 2n2222A and lower bias foor 20ma each idling.
Reinstall the IRF510s with shorted leads possible. Add insulator kits and real large heatsink.
Do yourself a favor hold the power to 10W or close. Pushing the amp to max power
just makes you a trash bucket operator as it will never be clean when doing that.
When you do that the RF gets back into other stages and creates more issues!
If you think you need more power get a used 100W radio!
Version B for those not terrified by now:
All the parts after Predriver removed. Rewind T8 on BN61-2402 6t and 2 turn secondary #30.
Replace pre-driver 2n3904s with one 2n2219A and bias it for 25ma. Rewind transformer
as 8 turns bifilar #28, connect as 4:1.
Get another Kitsandparts 5W CW amp, replace BD139s with IRF510 and bias network OR
get a QSX amp and be done.
Use coax (rg316) to connect from pre-driver output to amp module amp and from that amp
to the relay tree for filters. There will be power and control wires.
The above is an outline if you cannot think your way through it you will get over your head
fast. Buy an appliance. However if you build and like a challenge that is very nearly ground
up its very do able if you start with a working radio and work carefully. Its in the land of
building from scratch and nearly a kit. We left heathkit assembly back a while ago.
Result then is a decent 80 -17M radio. Forget anything over 20mhz the filters before
the amp is much more difficult and otehr issues conspire to make it a mess. We need
V5 to get that.
The plan for mine was a mil style man pack with 80/40/20/10 as primary bands (must have)
low power RX with AGC and 5-10W TX. As work on ubitx proceeded it was obvious getting
that in the package identified was not going to fit due to added blocks to make it work
and I found the power needs rather high for something I plan to run on battery.
I will have to come up with my own design to fill that need. I already have a breadboarded
version that needs to be size reduced to fit.