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 Post subject: SX100 product detector
PostPosted: Nov Sun 22, 2020 7:08 am 
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My sx100 has been modified by PO to include a product detector using a 3 transistor circuit. Anyone have info on this mod? Tnx


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 Post subject: Re: SX100 product detector
PostPosted: Nov Mon 23, 2020 1:26 am 
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AG8Z wrote:
My sx100 has been modified by PO to include a product detector using a 3 transistor circuit. Anyone have info on this mod? Tnx
I can't find any reference to a 3 transistor circuit, but I did find references to a 2 transistor product detector that was used in several receivers. https://www.qsl.net/w/wb4kdi//Heathkit/ ... ector.html

Now this requires a negative 6 to 12 V DC supply. So to use this circuit you might add a rectifier diode off of the 6 VAC filament supply, a couple of filter caps, a zener and a transistor as a voltage regulator. That would add up to 3 transistors.

Curtis Eickerman

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 Post subject: Re: SX100 product detector
PostPosted: Nov Mon 23, 2020 4:19 am 
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Actually, it shouldn't need to be regulated at all, since it has nothing to do with frequency stability and small gain shifts will be all but unnoticeable. But Curt is right: an included regulator might be the third transistor's purpose.

SteveH


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 Post subject: Re: SX100 product detector
PostPosted: Nov Mon 23, 2020 8:33 am 
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Hmmm. Might try one of these in my HQ-145A. It would be easy enough to add - and remove.

And if you used NPN transistors, you could tap a few ma. of B+ through a dropping resistor and a zener.

John


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 Post subject: Re: SX100 product detector
PostPosted: Nov Mon 23, 2020 5:11 pm 
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The third transistor could be a low gain amplifier to equalize output between the product and stock detectors.

Rodger WQ9E


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 Post subject: Re: SX100 product detector
PostPosted: Nov Mon 23, 2020 6:18 pm 
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Looks like a solid state
version of the Pullen mixer.

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 Post subject: Re: SX100 product detector
PostPosted: Nov Mon 23, 2020 6:43 pm 
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KX5JSC wrote:
And if you used NPN transistors, you could tap a few ma. of B+ through a dropping resistor and a zener.

John

I was just going to say that. A couple of 2N3904's should do it.

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 Post subject: Re: SX100 product detector
PostPosted: Nov Tue 24, 2020 11:22 am 
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Or 2N2222As. I have a few of both. They are like 1N4007s. You just never know when you will need a couple. :D

I wonder if it would be worthwhile to try to match a pair.

John


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 Post subject: Re: SX100 product detector
PostPosted: Nov Tue 24, 2020 5:47 pm 
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Or, an RCA CA 3028A should be easy to configure as a mixer as well... but I suspect they are scarce these days.

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 Post subject: Re: SX100 product detector
PostPosted: Nov Tue 24, 2020 6:45 pm 
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Lots of circuits like that got by without matched transistors. Or at least nobody fussed.

If you're going to use a CA3028, might as well just find an existing circuit for a product detector that used it, they were common at one time.

There was a time when you could get two transistors in one package.

Or RCA, at the least, had ICs that were just a few transistors. I think even one that had two differential amplifiers, well the transistors in that arrangement. CA3046 comes to mind but I may be wrong.

But most of that is long in the past.


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 Post subject: Re: SX100 product detector
PostPosted: Nov Tue 24, 2020 7:05 pm 
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If the PO's circuit is actually the 2-transistor PD or similar shown from the referenced web page, matching most likely will have no advantage at all, since the gain of one side is varied. And besides, the BFO and IF signals aren't even the same level, let alone the same source impedance. A pair of NPNs that can see lightning and hear thunder should suffice :lol:

SteveH


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 Post subject: Re: SX100 product detector
PostPosted: Nov Tue 24, 2020 7:19 pm 
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SteveH wrote:
A pair of NPNs that can see lightning and hear thunder should suffice :lol:

SteveH


That’s the funniest thing I’ve heard all morning, Steve. :D

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 Post subject: Re: SX100 product detector
PostPosted: Nov Thu 26, 2020 6:22 am 
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Circuit consists of 2 JFETs both N channel and P channel and a bipolar NPN Q. Two have inputs from IF but none from bfo. Strange. Output to s meter not audio. Will post circuit later


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 Post subject: Re: SX100 product detector
PostPosted: Nov Thu 26, 2020 3:46 pm 
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Since AGC is used in the SSB/CW mode, and since the BFO signal doesn't affect the AGC, I find that my SX-100 doesn't really need a product detector. I think this is because the radio has separate audio and AGC detectors, and the point at which the BFO is injected.

https://bama.edebris.com/download/halli ... -Schem.pdf

Dave


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 Post subject: Re: SX100 product detector
PostPosted: Nov Fri 27, 2020 2:53 pm 
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SteveH wrote:
If the PO's circuit is actually the 2-transistor PD or similar shown from the referenced web page, matching most likely will have no advantage at all, since the gain of one side is varied. And besides, the BFO and IF signals aren't even the same level, let alone the same source impedance. A pair of NPNs that can see lightning and hear thunder should suffice :lol:

SteveH

So, are you saying that transistor matching is not . . . critical?


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