ordered a ubitx (V3) last May, wired it up and made one contact on 20
meter SSB. THEN, I started reading about the various issues with the
transmitter so stopped using it.
would really like to salvage the rig, so based on my reading of the
many comments and my limited understanding, here is what I have been
thinking about in the way of modifications in order to get to 'legal'.
addition to the basic version 3 ubitx with KD8CEC v1.095 firmware, I
have also experimented with using the VU2SPF/VE1BWV firmware with an
Atmega 2560 and Elegoo TFT display. I might need the Atmega for extra pins to implement mods below, but I would lose CW function because it is not in the software. I
also built and used K5BCQ’s external 20 watt amplifier and a home brew
lpf with with my old (circa 1995) White Mountain 20 meter SSB so I have a 'spare' external amp.
Like most everyone else, I would like to get a clean 10 watts output on every band 80 through 10 meters, preferably for both SSB and CW.
basic approach would (1) add the 45Mhz filter at R27, (2)
replace the 2N3904’s, rebias the replacement 2N2222’s and possibly redistribute gain by adding another stage of amplification in the pre-driver/driver signal stream (3) eliminate the current
onboard lpf and maybe (?) the existing IRF510’s (see 6b)...probably need to apply Allison’s
hacksaw methodology (4) bring RV1 to front panel control using shielded
cable in order to set drive level for each band to 500 mw at driver
stage (T10 transformer), (5) route signal from T10 through ND6T’s RF power monitor and
dummy load in order to set drive level via a tune/transmit switch/relay
before routing output to either (6a) external K5BCQ amplifier and
external lpf board or to (6b) existing IRF510’s and external lpf board. Would monitor the power at T10 since input to Kees's amp has to stay below 1 watt...probably not a problem here, but could be if I waited to monitor the power at the antenna connector as Don originally suggested.
think this configuration should accomplish (1) reducing spurs by using
45Mhz filter at R27, (2) improve gain at higher bands with the use of
2N2222’s, (3) reduce/limit harmonics with external lpf’s, and (4)
achieve level output across all bands by setting and monitoring individual band drive
with front panel RV1 pot, (5) reduce/limit IMD by adding another stage of amplification somewhere in the pre-driver/driver stages. Not sure I understand where to put it or if it is absolutely needed.
I am NOT an engineer, so I have a couple questions:
Will the re-biased 2N2222’s get me to a 'clean' 500 mw on the higher bands? The
block chain diagram from Henning (DK5LV) suggests I might need an extra
stage of amplification to get 500mw and lower IMD. Will spreading the gain across another stage of amplification solve the IMD issue because I would not be over driving any given stage? I'm not sure I understood that thread completely.
- I assume the earlier re-bias recommendation for the 2N2222's have to be changed to reduce the relevant stage gain if an additional amplification stage is added?
What is the best way to couple the output at T10 to an external amp
which I believe has a 50 ohm input impedance and a switch to run in
either class C or class A/B? Wish I had more details about Kees’s amp,
but the schematic is being held for ransom deep inside my filing
cabinet. Does T10 need to be re-designed for use with an external amp?
guess the bigger question is whether this basic concept and related
modifications would get my rig on the air legally? Am I missing
something? Finally, I have not even started to think about the software programming changes that might need to be made to accomplish the switching and monitoring I suggested above.
Thanks for any help you can provide as I am definitely in the category of "a little knowledge can be a dangerous thing".