EMRFD Message Archive 5773

Message Date From Subject
5773 2011-01-23 15:39:36 alan7114p Modular Direct Conversion Receiver--Help Neede
I am trying to build the Modular DC Receiver on P. 8.14 in EMRFD (latest printing). My trouble is with the VXO. It oscillates just fine (I can hear it in my TS-850) and there is an output of about 100+ mV p-p at the output of the first transistor. However, at the output of the second transistor it falls to under 20 mV p-p. There is essentially no output where it is supposed to connect to the TUF-3. I have carefully built this module twice. Outputs between the two are approximately the same.

I suspect my problem may be with the transformer that is attached to the second transistor. I did not have the bead described in the article but wound 12 turns on an FT 50-43 and also tried a bino core type 43. I used trifilar turns (twisted a few turns to the inch before winding). But I am really not sure how to connect the various ends. I tried to duplicate the direction of the wire in the schematic, but I'm not sure I did it correctly. I'll describe what I did if that will clarify:

I'll call my wires brown, green and red (as that's what I used) and say they have a number 1 end and a number 2 end. Brown 1 is input, Brown 2 to Green 1, Green 2 to Red 1, Red 2 to 12 volts. The Green 2/Red 1 end is my output. Is this right?

I can't figure out anything else I could have done. In one version I used a poly 10 nF (.01uF, right?) at the output from the transformer. In the other I just used a ceramic cap. Didn't seem to make any difference. All other parts seem OK. Have checked the wiring many times.

One other thing: voltage between top of 4.7V Zener and ground is actually 5.22V.

Any help will be greatly appreciated.

73 de Alan K6ZY
5774 2011-01-23 19:57:22 Rex Klopfenstein Re: Modular Direct Conversion Receiver--Help Neede
I am building the Modular DC Receiver and I just applied power to the VFO this
afternoon. I too got a signal from the 1st transistor and not much after that.
At that time I quit for the day! I plan to trace the circuitry tomorrow. I will
keep my eye on the list!
Good Luck

Rex Klopfenstein

5775 2011-01-24 05:08:20 kb1gmx Re: Modular Direct Conversion Receiver--Help Neede
Ok, having built that osc circuit many times and found it to be
a good one that just works I'd go straight for the transformer.

the winding should be series connected as in:

End of first wire goes to start of second
end of second goes to start of third.

collector is start of first.

Output tap is the point where end of second goes to start of third.

End of third is DC power.

Amy core should work (FT or BN) and I've used both.

However.. the edges of binocular cores can be sharp and
cause wires to short against themselves. A single turn
short will really lower the output.

The voltage variati
5776 2011-01-24 10:07:00 Wes Re: Modular Direct Conversion Receiver--Help Neede
Hi Alan,

We assume that the measurements use a 10X probe with a suitable bandwidth oscilloscope. You are probably OK there.

I don't recall any figures showing a pictorial of a trifilar winding, but a bifilar winding is shown in two figures on page 3.34.

All the transformer really does in the output of the VXO of Fig 8.22 is to create a 3:1 turns ratio. These things are amazingly uncritical and they do not have to be duplicated exactly. Grab a ferrite toroid and put 12 turns on it to feed the collector. Then put a four turn winding
5777 2011-01-24 19:42:23 Wes Re: Modular Direct Conversion Receiver--Help Neede
Hi Alan and group,

Wow, this is weird. You (Alan) posted a question and I thought I'd offer a comment. I hit the reply button and got the usual screen where I then typed the response. I clicked on Send and I thought that the deed was done. Well, my response never made it to the group. The only thing that did happen was that my name and address appeared to the right, suggesting that it was my post and not yours. Your question was never addressed. So, let me try again.....

Anyway, I assume that you were using a 10X probe with your oscilloscope and that the scope had enough bandwidth. That's probably not an issue, but that's the first thought that comes to mind.

I don't recall a pictorial that shows a trifilar winding, but there are a couple showing bifilar windings on emrfd page 3.34. In this particular application, all that the trifilar winding does is to make an auto-transformer where the output is extracted from a tap 1/3 of the way up from signal ground. So put "dots" on the windings with the dots all on the bottom. Alternatively, you can use another ferrite toroid core. Place a 12 turn winding between the decoupled Vcc and the 51 Ohm resistor. Then put a 4 turn winding
5778 2011-01-24 22:44:52 alan7114p Re: Modular Direct Conversion Receiver--Help Neede
OK. Turns out that the problem is my beginner's interpretation of the schematic. Some wires that I thought were connected aren't (they just cross in the schematic). Thanks for all of your help.

I particularly want to thank Steve AA7U who was the one who actually figured out what happened.

And, Rex, send me an email and I'll tell you what I learned (if you haven't already figured out the problem yourself).

73 de Alan K6ZY

5779 2011-01-25 07:58:52 Rex Klopfenstein Re: Modular Direct Conversion Receiver--Help Neede
This question was in the back of my mind about the crossing wires on the
schematic! In the "good old days", a crossing wire had a hump in it and an
intersection was joined with a dot. I have also seen a small break in crossing

Is the intersection in question where the 4.7K resistor, 51 resistor, 10uF cap,
and transistor collector meet? I assume that the transistor collector-51
resistor is one circuit and the 4.7K resistor-10uF cap the other?

These is another area on Fig 8.22 with similar interpretation issue. The circuit
in question is in the Low Noise Audio Amplifier in the area of collector of
2N3904. Is one circuit is the10K resistor - 3.3K resistor and the other is the
wire between the collector of one transistor and the base of other transistor?

Rex Klopfenstein

5780 2011-01-25 08:20:54 Tim Re: Modular Direct Conversion Receiver--Help Neede
I too was confused by this schematic convention which appears occasionally in EMRFD and in the Kanga documentation packages.

I believe the "secret decoder ring", is that in this dot-less hump-less convention, a four way intersection represents no connection between perpendicular wires. But a three way intersecti
5781 2011-01-25 09:05:17 davidpnewkirk Re: Modular Direct Conversion Receiver--Help Neede
5782 2011-01-25 13:31:13 Tim Re: Modular Direct Conversion Receiver--Help Neede
I fully agree with your sentiments as to how schematics might be drawn to avoid confusion by having no 4-way intersections.

But every issue of QST I picked up in the past few decades, I can identify several four-way connections in just a few seconds. e.g. Nov 2010, p. 56. Jan 2011, p. 43 Fig. 2. Feb 1984, p. 36 Fig 2.

Even the ARRL handbook, which does manage to often avoid 4-way connections, still occasionally does it. e.g. page 18.30 in the 2006 handbook.

To their credit, all the above examples have great big humongous black dots at both 3-way and 4-way intersections.

I prefer the way QST does things (even including 4-way connections) over the dot-less convention, if for no other reason just decades on decades of habit.

The gotcha with the dot-less convention shows up with e.g. the Kanga R2Pro audio output amp schematic, e.g. http://www.kangaus.com/pdf%20files/R2Pro%20Documentation%20Package.pdf Figure 6. No dots anywhere in the transistor biases, yet the 4-way intersections are truly connections. (In violation of the "secret decoder ring" that I myself promulgated just an hour before.) That circuit took me for a loop until I figured it out.

So my preference is to show the dots and not be dogmatic if someone shows a 4-way connecti
5783 2011-01-25 13:37:23 Jim Tonne Re: Modular Direct Conversion Receiver--Help Neede

I am a mere "lurker" on this group but I just had to
bust in and tell of an interesting thing I learned
decades ago.

I was working on a job for RCA Broadcast in
Meadowlands, Pennsylvania, USA. They in fact
had a schematic drafting standard that addressed
this issue. When four wires meet and are joined,
one of them is displaced or "jogged" a bit so that
only three wires actually met so as to avoid this

- Jim W4ENE

> how schematics might be drawn to avoid confusion by having no 4-way
> intersections.
5785 2011-01-25 22:19:43 davidpnewkirk Re: Modular Direct Conversion Receiver--Help Neede