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?I am still befuddled by this. My BitX40 has Q13 replaced with a 2N2222A, and has R135 turned almost fully CCW in order to get the desired drive level with this replacement. I added the back-to-back diodes at K1 pin 12 to ground to avoid a repeat issue with Q13. I am currently running 13.6v to the rig, with the main board and Raduino powered off a 12v regulator, delivering 11.5v at this input power. The PA is supplied the full 13.6v, or 24v through a separate voltage booster. I am reading the desired voltage on the tab (source) of the PA transistor, either 13.4 or 24 as selected, and 3.9v on the gate and 0v on the drain at either voltage during transmit. Bias is set to about 100ma and PA current draw goes a bit over 1a with modulation. When I transmit at 13.4v I am getting 7-8 watts out, as I expected, but when I switch to 24v I am only seeing 6w out. The 24v does not sag. I have traced the wiring, measured voltages, made sure all grounds are common, and frankly just don’t see why this is. Any ideas?

?

I do notice, at least on the DVM I am using, a spike in PA source voltage with modulation that might peak at close to 70v, while measured at the booster or power supply it stays pretty steady. Is that normal? I am guessing it is RF getting into the DVM. Otherwise, everything seems to be working fine. The transmitted audio sounds fine and I have made a number of contacts.

?

=Vic=

Vic:

Is there any difference in the distance achieved with QSO's
using 12v. vs 24v? Or is there any difference in the reported
received S level when using 24v vs 12v?

There are several possible explanations.

1) There is oscillation at 24v and this is contributing to a false
sense of power output. It is most likely being removed by the LPF.

2) There is insufficient drive at 24v -- This is usually corrected by
increasing the mic gain. For a good solution, see K.P.S. Kang's
solution at .

3) The LPF or output transformer is killing the signal. This is unlikely
since it works at 12v very well and no one has reported anything similar.

4) There is really no 24v on the drain of the IRf510. This is unlikely
since you are reporting a good voltage there. (Incidentally you mixed up
source and drain).

6) Your measuring equipment is not telling you the truth. Really, you need
an oscilloscope to measure the true RF output at each stage. A good RF
probe will do the same thing, however, and it is easy to homebrew.
Why are you using a DVM to measure RF?

Since you report good audio with QSO's, I suspect the last, though I would
not discount oscillation as the culprit. You need to know the RF level at each stage
through the PA system from drive to output at the antenna. It is also possible that
your antenna is not matched well enough. (That would mean mysterious increased
heating of the heat-sink at 24v; you probably should increase the size of the
heat-sink anyway if you are going to use 24v. Easy enough to do.) Measure the
SWR at 12v and 24v, BEFORE the antenna tuner (if it is used) and AT the antenna.

Really, I think you need to rethink your measuring equipment first.

john
AD5YE

I should have made note that all my tests in the opening message were in to a dummy load and reading on a dual-needle wattmeter with a 30w full scale reading. No reverse power was seen.

I tried reducing the drive, and succeeded in only reducing the power out similarly on both high and low power settings...say 3w on 13.5v and 2w on 24v, or something similar. I had to increase that drive from mid-scale right after replacing a failed Q13 with a 2N2222A that I had available.

I sort of ran out of steam today...brought back a bad cold or flu from Frostfest, the first hamfest I attend each year, but I may try to develop a table of voltages to compare to K7AGE's list and see if anything stands out. Interestingly, the current draw by the PA drops from just over 1a at full modulation at 13.5v to about .95a with the 24v supply. I feel something must have changed as I was reading an appropriate gain that matched the square of the voltage ratio before.

=Vic=

More follow-up. Voltages are all approximately what is shown in the K7AGE table for the Q13-15 devices, slightly less as I have 11.5v out of a regulator to the main board. I bypassed the regulator and ran the main board off the incoming supply, about 13.5v, and no real change other than some voltages were slightly higher. I checked in to a 40m net with a station about 450 miles away, and there is no observable difference between "high" power and standard power, confirming that "high" is only a couple of watts lower. He commented that the audio sounded good (I have made the VK3YE type of mod, ending up with 30pf).? I also have a 100pf NPO cap across L7, but I don't see how that should change things.

Truly a puzzle.

=Vic=

Alas, in my curiosity and inability to leave well enough alone, I came up with a brilliant plan to determine why I cannot get more power out at a higher PA voltage. Again, working on the suspicion that RF getting into the voltage booster is causing it to misread the output voltage and throttle it back to something less than 13.8v…hence the lower output with a theoretical higher supply…I bypassed the little booster and ran a 12v battery in series with the 13.8v supplying the rig, getting a bit more than I? wanted (close to 26v), but close enough, I thought, to see if that was the issue. Eureka! I go from maybe 7w out on the supply voltage into a dummy load to almost 20 in a quick test! So THAT is the problem, I need to figure a way to stabilize that booster or isolate it or something.

?

The alas part? As I was unplugging various clip leads, one brushed a speaker terminal on the open cover…and I heard a slight pop. Now I have no audio out from the rig <sigh>. The clip lead must have still been hot or something and perhaps popped the little AF amp…I sure hope not, though, as that will be a major booger to try and replace. All this after admiring how well it worked and what a nice report I got from ECARS this morning when I checked in…

=Vic=

HI Vic,
Interesting observations.... I am having a similar issue with my uBITX and a booster, Ill try the 24v battery solution....

73. Nik

Well, gentlemen, we have a true ham here.

If RF is getting back into your booster and changing the output voltage,
there are 2 solutions; either block the RF with a filter (and shielding),
or change to a linear power supply.

I like linear supplies because they are almost stone silent and
handle RF very well. But they are heavy and consume more power.

It should be a rather simple matter to block the RF back into the power
supply with an adequate filter. The problem is finding one that works
over the entire transmit range. But it can be done. Another problem
might be that your booster cannot handle the output wattage at higher
levels. You may need to replace it with something better.

Keep it up; you're doing fine.

john
AD5YE