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SB2 Q6 failure

Don, ND6T
 

The Slop Bucket II s/n 144 has been working flawlessly until this morning in mid-QSO I noticed that the modulation indicator wasn't flashing in tandem with the on-the-air lamp. I glanced at the antenna current and it was normal. I opened it to find that Q6, the RF sense detector diode had developed an open source-to-drain diode. Quickly replaced it with a 2N7002 that I had in the drawer and it is back on the air in less than an hour. One advantage of constructing your own equipment: You become familiar with the circuitry and with the maintenance techniques. A cheap and useful education!

Steven Weber
 

 

I hope that doesn’t become a problem. Using the 2N7002 as a diode isn’t exactly what it’s designed to do, but it saved from having to kit one diode just for that function. If they do start popping left and right, I guess tacking in a 1N4148 won’t be too hard to do.

 

Nice looking cabinet 😊

 

Steve KD1JV

 

 

 

 

The Slop Bucket II s/n 144 has been working flawlessly until this morning in mid-QSO I noticed that the modulation indicator wasn't flashing in tandem with the on-the-air lamp. I glanced at the antenna current and it was normal. I opened it to find that Q6, the RF sense detector diode had developed an open source-to-drain diode. Quickly replaced it with a 2N7002 that I had in the drawer and it is back on the air in less than an hour. One advantage of constructing your own equipment: You become familiar with the circuitry and with the maintenance techniques. A cheap and useful education!

 

Don, ND6T
 

I certainly hope not! Only time will tell. There is nothing in the specifications that suggest such a failure and it is clever way to avoid using the MELF (supposed to be "Metal Ended Leadless Faced" but really means "Most End up Landing on the Floor") packaged devices. Using an already included type device is another plus. I'm betting that this is just an anomaly. I seem to attract them. That is why I replaced it with another 2N7002 rather than a 1N4148. A good test and another learning opportunity.

Incidentally, that body diode was open and the gate junction appeared to have failed also. The device ended up acting like a bar of intrinsic silicon. Who knows the reason.

Thanks! The case used to house a scrapped data terminal device and the panels were cut from a junked case that once contained a Heathkit color-bar generator. There is no such thing as e-waste.

The SB2 is a real gem. I sincerely hope that you continue to offer them.

Don, ND6T
 

Yep! Problem, not a fluke. Q6 bit it again. This time it is properly burnt. Mind you, I am pushing the limits of the transceiver when I am powering it off of my 13.80 volt supply and running CW. It held for many hours and the failure did not affect operation. Just the RF indicator stopped. Comparing the 2N7002 with the 1N4148 I do not see enough of an advantage in changing. But looking at C23 and C29, both 100nF capacitors I get an uneasy feeling. Together they represent less than an ohm of reactance at 4 MHz. With an RF signal coming hot off of the drain of Q23 that represents a lot of power steered through Q6, far more than it can dissipate.

The easiest fix, in my estimation, would be to install 1 nF (.001 uF) capacitors in C23 and C29 positions. That would present an apparent 80 ohms in addition to the diode drop. In my rig that still lights the RF indicator LED at 7 watts (just under 11 volts supply) and does not affect receive that I can measure. I am still able to listen to extremely weak AM broadcast stations down around 800 KHz which seems to be a valid indicator that there is still some "wiggle room". My RF output now measures nearly a watt higher with the same current drain. Win-win!

In higher frequency rigs I would think that the problem might be more exacerbated since the reactance of those original capacitors will be considerably less than my 80m choice. There are more efficient detector topologies that would be more efficient and less stressful but changing those values seems to be an easy fix. The rest of the transceiver runs without issue. The PA heatsink is comfortably warm and harmonics are well within specification. No birdies, no images. Just admirable signal reports.

ajparent1/kb1gmx
 

I would just go with a 4148 diode and be done at that.  The SB-1 uses that and I've had no difficulties 
with it even at 16V DC (about 13-14W out!).

I think the failure mode is gate to source punch through not the body diode.

Allison

Don, ND6T
 

Hi Allison!
I would agree except the gate and source are tied together externally. Looking at the SB1 I notice that those two capacitors aren't 100 nF either, instead a 1 nF and a 10 nF are shown in the latest schematic.  At any rate, we shall see how it works out in practice. I held nicely for a time.

ajparent1/kb1gmx
 

On a technicality it would seem that way but gate to source or gate to drain is the same based on over voltage on the gate
relative to any pin (FYI that's 20V).  Its something that is variable in some circuits due to gate to source(or drain)
capacitance making a RF voltage divider.  It may be more likely due to two things your at 75M and likely the
IRF510 tends to put out a bit more power there.

Its a bit of nastiness I got to see doing some class D and E amplifier work.  Punch the gate oxide
and bad things happen.  Most of the time I got to see that when the drain exceeded the drivers
ability to keep the gate below 20V due to Miller capacitance.  Near instant poof.  It tended to
happen more at 6.77mhz than at 13.55 or 27mhz.   

Allison

Don, ND6T
 

Indeed. The voltage on that end is just about that level, too. I had to try, but yes, the next occupant of those pads will be a good ol' 1N4148.
The simplest of things often prove to have an incredible complexity.
Thanks Allison. Nothing beats experience.

ohwenzelph
 

So, not having built my SB2 yet and planning on 40 meters, should I simply replace Q6 with a 4148?

Steven Weber
 

 

Couldn’t hurt.

 

So, not having built my SB2 yet and planning on 40 meters, should I simply replace Q6 with a 4148?

 

ajparent1/kb1gmx
 

I went with the 4148. 

Based on older experience (and the SB1) I was more comfortable with that.


Allison

Gil
 

Hi, which way does the 4148 go? I haven't looked yet but starting on the build tomorrow... Thanks!
Gil.

Steven Weber
 

 

Cathode end towards the right, facing the resistor and cap.

 

 

Hi, which way does the 4148 go? I haven't looked yet but starting on the build tomorrow... Thanks!
Gil.

 

Jer Tres
 

i believe i put the band toward the right
jerry aa1of

On Fri, Apr 5, 2019 at 3:31 AM Gil <@gilgsn> wrote:

Hi, which way does the 4148 go? I haven't looked yet but starting on the build tomorrow... Thanks!
Gil.