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Author Topic: Help with HBR-11 & BFO please?  (Read 465 times)
KC9KEP
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« on: March 10, 2009, 06:35:39 AM »

Hello Elmers,

I am building an HBR-11 receiver as originally featured in April 1963 QST magazine.

The BFO is a 100 kHz (I believe Hartley) oscillator and uses a tapped coil.

I had to wind my own BFO coil on a used slug-tuned form.

The oscillator is operating and can achieve 100 kHz but .. the bottom of the output sin wave is flattened.  It sort of looks as if the tube isn’t biased in the center of it’s load line.

Can anyone tell me how critical this is?  I would think that this could cause some non-linear distortion in the audio, but the radio is just receiving voice transmission, so perhaps it’s OK to leave it as is?

The BFO signal gets injected into a 6BY6 product detector, should that information be of any significance.

Thanks all!

--Tom Nickel AKA KC9KEP
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N2EY
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« Reply #1 on: March 10, 2009, 07:56:45 AM »

It's not critical at all. Don't worry about it. The oscillator isn't running Class A.

73 de Jim, N2EY
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KC9KEP
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« Reply #2 on: March 10, 2009, 08:40:12 AM »

OK, Jim,

Thank you for the info and prompt reply :-)

I could just imagine how much time & energy I
could have wasted by trying to analyze and
"fix" the circuit to change the tube bias unnecessarily.

73's

--Tom Nickel AKA KC9KEP
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W8JI
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« Reply #3 on: March 10, 2009, 09:16:28 PM »

Tom,

The ideal waveshape for a mixer is a square wave. :-)

The mixer has to be rapidly and heavily put into and out of conduction to be a good low distortion mixer.

You are only part way there, but it will be fine.

How did you make out on the critical coupling??

73 Tom

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KC9KEP
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« Reply #4 on: March 11, 2009, 05:55:30 AM »

Tom,

A "square wave" you say.  I did *not* know that!

But, that's one of the reasons that I enjoy building these projects so much .. all the things that one learns.

Are you referring to the "coupling" between the BFO and mixer?

I'm not that far along yet.

So far, I've fabricated a chassis & front panel, procured most of the parts, punched many holes and mounted components, have the filaments wired & most of the power supply wired.

The BFO was one of the first circuits that I had completed, because it resides in its own little enclosure and needed to be mounted onto the chassis.

I plan to wire the set "backwards", i.e., start with the audio amp & work back towards the front end of the receiver.

So far, I've learned that radios are mostly made of chassis "holes"! :-)

Thanks again,

--Tom Nickel KC9KEP

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W8JI
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« Reply #5 on: March 11, 2009, 08:04:10 AM »

When I used to build my homebrew receivers I would first lay out all the major parts and mount them. That would be the VFO capacitor, bandswitch, tube sockets, and transformers including IF transformers. I would start wiring at the power supply and then audio and work backwards.

I was asking you about your receiver with the double tuned input system. The one where you had two peaks on the input tuning.

73 Tom

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KC9KEP
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« Reply #6 on: March 11, 2009, 12:17:31 PM »

Oh, you were referring to the Simple-X receiver.

The last thing that I did with that project was to laser engrave a front panel, so at least it looks really nice now :-)

I was getting frustrated with the double peaks situation and wasn't making much headway.  Then, I read about the HBR receivers in QST magazine and got the bug to build one of those, so my Simple-X receiver is rear "burner-ed" for the moment.

When I get frustrated with my HBR-11, I'll come back to the Simple-X with vigor renewed and try again :-)

73

--Tom Nickel

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KC9KEP
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« Reply #7 on: March 11, 2009, 12:22:30 PM »

Tom,

Also, I did purchase a text book that was authored by the gentleman (who's name escapes me) who describes that tuned coupled circuit back in the ~20's/30's

He didn't say much more that what you had quoted in one of your earlier messages about analyzing the ciruit, but it's interesting reading.

Once again, a learning experience.

73

--Tom Nickel
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W8JI
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« Reply #8 on: March 11, 2009, 03:29:17 PM »

Thanks Tom. When you go on vacation to Florida stop by with it. I'm just a few miles from I-75.
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KC9KEP
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« Reply #9 on: March 11, 2009, 05:05:21 PM »

Tom,

Actually, we will be traveling thru Florida sometime in June.

We could stop by :-)

73

--Tom
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W8JI
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« Reply #10 on: March 11, 2009, 05:12:22 PM »

I'm near Macon GA. Look me up.
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