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Evan

I am thinking it may be the equalizer network, but surprised you would see it on all 4 units. On mine a capacitor adjustment cured it, I was searching in all the wrong places. I had a pronounced dip on 40m.? Search on my call and you should find this cap in question. I think its in the driver to the PA and only affects cw output.??

Curt wb8yyy

Vic - I'll plug these values in and see what happens.? Not really that big of a deal but more curiosity as to if this was the "nature of the beast."

73,

Justin B.
KI5GKD

Evan,

I've changed the thread name to something I think more appropriate.
Should be sufficient to simply state that this is a continuation of the conversation here:
? ??/g/BITX20/topic/75464085#87372

As a summary of what we know thus far:

As shown in these posts from the old thread:
? ? ? ?/g/BITX20/message/87369
? ? ? ?/g/BITX20/message/87328
testpoint TP1 is quite dirty on 40m CW, but not on 30m CW or 80m CW.
This dirty 40m signal results in reduced output from the transmitter.
The reduced output only occurs during CW transmissions, not SSB transmissions.
CLK2 into the first mixer looks clean.
Issue has been seen in both v4 and v6 hardware, most units tested exhibit the problem.

Evan has found that reducing drive from RV1 does not clean up the TP1 signal,
so this is not an issue of coupling from the power amp.
Problem is most likely around the first mixer, the following steps will be taken to isolate the issue:

So, not as a solution but as a way of understanding the problem I might try the following steps:
1)? Remove L31, terminate that mixer port with 50 ohms to ground.? (There's no signal coming from the IF amp in CW mode.)
2)? Add a 6dB pad (150,33,150 ohms like R101,102,103) between T2 and L4 (perhaps? remove L4,C204 and tack it in there, avoids cutting traces)
3)? Replace the mixer with something else, perhaps an ADE-1.? Could be a resonance inside that mixer?

A question for Evan:
In post 87369 you sent this morning, what is being probed in the top two screen shots?

Jerry, KE7ER



On Thu, Apr 1, 2021 at 08:06 AM, Evan Hand wrote:

Dave,

Not sure how to answer your questions.? The final output of the ?BITX is a very nice sinewave.? I do have a good ground through the cold water line as I am in my basement.

The issue that Jerry is helping me with is the lower output on 40meters when using CW.? I had noticed that this is common on all 4 of the ?BITX that I have as well as others have reported.? Personally, I have seen this on 6 radios.? The scope traces are the first that I have seen that could possibly explain the issue.? We are now trying to work out the root cause and possible solution.? Everything is pointing to the mixer, with the next step of removing any connection to the BiDi amps to see if it is a termination issue with the mixer.

It is evident that I really need to start another thread.? The problem with that is there are a number of posts with pictures that are important to the issue that would also need to be copied.

73
Evan
AC9TU

Dave,

Not sure how to answer your questions.? The final output of the ?BITX is a very nice sinewave.? I do have a good ground through the cold water line as I am in my basement.

The issue that Jerry is helping me with is the lower output on 40meters when using CW.? I had noticed that this is common on all 4 of the ?BITX that I have as well as others have reported.? Personally, I have seen this on 6 radios.? The scope traces are the first that I have seen that could possibly explain the issue.? We are now trying to work out the root cause and possible solution.? Everything is pointing to the mixer, with the next step of removing any connection to the BiDi amps to see if it is a termination issue with the mixer.

It is evident that I really need to start another thread.? The problem with that is there are a number of posts with pictures that are important to the issue that would also need to be copied.

73
Evan
AC9TU

When I was using the S-meter provided in the main CEC sketch (not the outboard processor and -s version) I found the original values for calibration in the Memory Manager were pretty far off. They gave an indication, but not 6db/S unit and not really close to another rig that has very good S-meter calibration. I modified the numbers such that each reading was about 2x the voltage of the other (hence 4X power or 6db) and set the values such that they tracked a commercial radio pretty well. So, yes you can fiddle a bit and certainly get it above S9, but the scale isn't actually very linear and the display isn't that clear with only 8 divisions, so it isn't something to get too excited about. Here are the settings I used:


=Vic=

Hi Evan
Can you find any other clean sine wave in the HF? or VHF range in the uBITX of other device with your oscope? Is the uBITX grounded well. You have a beautiful scope, maybe your leads are picking up some other frequency and mixing with it. I see a bit of distortion on my 30 year old 20mhz oscope. Nothing like what you are seeing

Have you installed RF rated relays as the circuit does go through several sets of contacts. I noticed that on my v3 and v4 I could get .5-1 watt higher power output with no other modification.?
--
73
Dave

Jerry,

Woke up and could not go back to sleep, so tried the no output power test.? Following are the scope traces.

The signal is not as ugly, but lower in magnitude.
Full clockwise:


Full CounterClockwise:


I checked the other bands and the signal did clear up when at the fully clockwise setting of RV1.?

To see if the probe might be distorting the signal, I measured power with and without the probe.? It does affect the output power.? It increases when the probe is disconnected vs connected.?

Because the probe is close to the output when connected to TP2, I went back and measured the signal at TP1.? ?There there was not a change in the signal based on the RV1 setting.? Here is the trace for TP1 both 40meters and 30meters:



That I believe negates the theory that there is feedback with a phase shift involved causing a diminishment of the signal to the power amp stages.? There is something about the mixer that is diminishing the 40meter and 80meter signals on CW.? 40meters much more than 80meters.? 30meters is the least affected.? I need to do the board surgery to test your other points.

73
Evan
AC9TU

I have a few items that need to go.

2 x Bitx40's

? One looks pretty good the other needs some love. I have one VFO working and updated, the other didn't come with one. They have not been tested, one I know will need some TLC before smoke test (one of the LPF toroids will need reworking at the very least) the other should be pretty easy to get going.
?No further testing has been done.

? Picked them up from a fellow who never got them running and I just haven't had time to get them on the bench.

? 2 x 10-Turn pots included

Both for 85$ Shipped.

?

Foxx-3 40m with 7.030 crystal

?Tested and built by me

?Crystal is setup with a socket for changing within the 40m band

?50$ Shipped.

?

125$ shipped takes the lot.

I will ship over sea but ask a little extra for shipping and please be aware shipping over the pond has become unreliable and delayed.

Questions? Please ask. dherron at live dot com

Paypal preferred: no friends and family payments will be accepted.

?

Jerry,

I will try that and the other suggestions on Friday.? We watched our Grandkids today and are watching them tomorrow all day.? We are exhausted by the time we get home so no energy to play radio.

73,
Evan
AC9TU

Evan,

One issue with boosting power of the PA is that the 10 Watts at the IRF510's
is on the same PC board and power supply as the unshielded 0.0001 Watt signal
coming out of the first mixer.? Perhaps delays in the PA stages are such that
the coupling at 7mhz is out of phase, diminishing the weak signal?

I'm skeptical, but it's worth trying.
Turn RV1 fully clockwise for zero power to the rest of the PA,
see if TP1 still has such an ugly looking signal.

Jerry, KE7ER

My experience mirrors Curt's.? I couldn't make any contacts until I ran through the FX and BFO alignments.? I could barely hear any signals and when I did they were rather distorted IIRC.

Justin B.
KI5GKD

Evan,

I'm not really sure what all is going on with the power amp.
The simulation uses a rather simple model of the parts, certainly no more than can be gleaned from a datasheet.
Actual parts can work quite a bit differently, especially when you push them as hard as the uBitx does.

I've learned much from the forum as well.
Most of it when trying to answer somebody's question,
and realizing I don't understand the issue as well as I had thought.

Jerry, KE7ER

Jerry,

I ran your simulation of the ?BITX power amp circuit and there are decreasing results on power gained going much past 14volts.? How close to real-world do you believe the simulation to be?

Here is the message with the LTSpice model:
/g/BITX20/message/86233

I did play with increasing voltages in the pre-driver sections as well but still get a limited power increase.? That may be a model issue and not a real-world case.? That is the reason for the question.

Thank you for all of your very valuable information.? I have learned a lot from you on this board.
73
Evan
AC9TU

A good heatsink for the IRF510's is an aluminum enclosure.
Could just put a chunk of aluminum or copper between the IRF510's and the back panel of your enclose.
Or could mount the IRF510's to the bottom of the uBitx main board, bend them back 90 degrees,
and bolt the IRF510 tabs to the bottom of the chassis.??
Keep the IRF510 leads short, insulate them from the chassis and from each other.

Going for higher power?
Good luck, many ways this can fail if you push it.

Jerry, KE7ER

I will suggest that some rigs like mine arrive with the BFO not properly calibrated - so be careful with notion it is ready for airtime without adjustment.?

Early versions required more modifications and sometimes repair (from parts that later failed) than the newest version.?

While the board is pre-assembled, many opportunities to customize this rig to your liking.

Curt wb8yyy