Author Topic: R27 receiver  (Read 14647 times)

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Offline KC4UMO

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R27 receiver
« on: June 11, 2013, 08:45:12 PM »
Browning

Offline KC4UMO

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Re: R27 receiver
« Reply #1 on: June 27, 2013, 05:57:41 PM »
Part 2

Offline The Radio Shop

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Re: R27 receiver
« Reply #2 on: June 27, 2013, 06:08:30 PM »
Sweet more stuff  8)
Buddy kc4umo

Offline Lazarus

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Re: R27 receiver
« Reply #3 on: June 27, 2013, 06:39:24 PM »
it's gonna take me a little while to type this up, so bare with me,

:)

Offline The Radio Shop

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Re: R27 receiver
« Reply #4 on: June 27, 2013, 07:11:36 PM »
no problem bro. take your time :)
Buddy kc4umo

Offline Lazarus

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Re: R27 receiver
« Reply #5 on: June 27, 2013, 07:33:04 PM »
quote from you on the other site
Quote
31.4 + .455 - 4.890 = 26.965
31.4 being the crystal, .455 the IF and 4.890 being the vfo.
So since I still do not get this, what should I see between C14 and C22. that is part that really gets me atm.
ok using the above example for channel one,

lets start at the 31.4Mhz crystal oscillator, and the VFO, and how they are mixed,

the signal from the 31.4Mhz crystal oscillator and the signal from the VFO are being mixed passively at the junction of C14 and C22, at that point there is a total of 4 signals present all at once, the two fundamentals which are:

1: the 31.4Mhz crystal oscillator signal,

2: the 4.89Mhz VFO signal, 

and two heterodyne frequencies which are:

3: the sum heterodyne at 31.4 + 4.89 = 36.29Mhz

4: the difference heterodyne at 31.4 - 4.89 = 26.51Mhz

now these heterodyne signals at this point are kind of like phantoms, and they don't really exist yet as distinguishable signals that can be seen on a scope or frequency counter, (i bet they would show up on a spectrum analyser though), but that will soon change after the signals passed through the 1st mixer, which is really not acting as a mixer in the conventional sense at all (because both fundamental signals are going into only one input), but is simply acting as an amplifier with a low Q or wide band tuned circuit on its output which is tuned to allow only the deference heterodyne frequency range to pass, therefore both the fundamental frequencies and the sum heterodyne are prevented from passing, and because of the flywheel or ringing effect of the tuned circuit at the output, what comes out is a real signal at the deference heterodyne frequency of the two fundamentals, which is 26.51Mhz,

now lets assume that there is a strong 26.965Mhz signal being injected into the antenna jack,

this 26.51Mhz signal from the 1st mixer is then mixed at the grid of the 2nd mixer with the incoming 26.965Mhz signal, and again two heterodyne frequencies are produced from those two fundamentals same as before, so we have two fundamental frequencies and two heterodyne frequencies at the input of the second mixer, and again at the output of the second mixer is another tuned circuit, this time a very hi Q or narrow band one, which is tuned to allow only the deference frequency of 455Khz to pass,

so here is the math:

31.4 - 4.89 = 26.51Mhz <--output from the 1st mixer

then

26.965 - 26.51 = .455Mhz or 455Khz  <--output from the 2nd mixer

i hope that helped,

:)

Offline The Radio Shop

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Re: R27 receiver
« Reply #6 on: June 27, 2013, 07:52:35 PM »
Now that is what I am talking about!
That helps alot on explaining how this thing downmixes the signal. Good post bro.
So there are mixed signals from the 31 meg osc and VFO osc. Since these signals are mixed passively at the junction of C14 and C22 only the 31.4 MHz signal can be detected there.  Although there are actually 4 signals?
Buddy kc4umo

Offline Lazarus

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Re: R27 receiver
« Reply #7 on: June 27, 2013, 08:04:16 PM »
Now that is what I am talking about!
That helps alot on explaining how this thing downmixes the signal. Good post bro.
So there are mixed signals from the 31 meg osc and VFO osc. Since these signals are mixed passively at the junction of C14 and C22 only the 31.4 MHz signal can be detected there.  Although there are actually 4 signals?
at the junction of C14 and C22, i don't really think you should be able to distinguish any one particular signal, assuming that both fundamentals are equally strong as they should be, it should look like a really big mess on a scope, so in my opinion if the 31.4Mhz signal is very dominant at that point then i think perhaps the VFO signal strength is suffering for some reason,

but also bare in mind that i've never worked on one of these radios, so i really don't know for certain what the various signal levels should be, i'm only going from experiences with other radios, so you would probably be the better judge of whether or not the 31.4Mhz signal strength should be dominant at that point,

:)

Offline The Radio Shop

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Re: R27 receiver
« Reply #8 on: June 27, 2013, 08:20:16 PM »
Now that makes sence to me.
The VFO signal is coupled via a 1pf cap while the 31 meg signal is couple via a 5pf cap.
I think I should try and repalce those caps. They could be causing the issue I am having. I will throw the scope on it tonight or in the morning. These caps are very old style (looks like a resistor) and since they are value my cap meter reads 56pf on them.
Buddy kc4umo

Offline The Radio Shop

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Re: R27 receiver
« Reply #9 on: June 27, 2013, 08:21:24 PM »
BTW, I really need a digital cam and a video cam lol.
Buddy kc4umo

Offline Lazarus

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Re: R27 receiver
« Reply #10 on: June 27, 2013, 08:28:43 PM »
Now that makes sence to me.
The VFO signal is coupled via a 1pf cap while the 31 meg signal is couple via a 5pf cap.
I think I should try and repalce those caps. They could be causing the issue I am having. I will throw the scope on it tonight or in the morning. These caps are very old style (looks like a resistor) and since they are value my cap meter reads 56pf on them.
also i think it would be interesting to see what the 26.51Mhz signal from the 1st mixer looks like going into the second mixer with no signal going into the antenna jack, it think it should be a pretty clean 26.51Mhz sine wave, but i'm curious to see just how well the output tank on the 1st mixer suppresses the fundamentals,

Offline The Radio Shop

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Re: R27 receiver
« Reply #11 on: June 27, 2013, 08:31:45 PM »
Okay, I will scope those and see. Got my intersted now lol.
Buddy kc4umo

Offline Lazarus

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Re: R27 receiver
« Reply #12 on: June 27, 2013, 08:33:45 PM »
BTW, I really need a digital cam and a video cam lol.
i used to have a really good one, a sony handy cam, took real good stills as well as vids, but the time came that i was going to have to eat it a few years ago, so i sold it to my brother for less than half of what i gave for it, it probably wouldn't have tasted all that great anyway, lol

Offline KC4UMO

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Re: R27 receiver
« Reply #13 on: June 27, 2013, 08:58:47 PM »
I know you hated that.  But like you d\said, got to eat.

ok, that signal is nasty looking. I post what I see on the Tek 2235 and the DSO
The first is the DSO. Sorry for the super imposed image of me taking the pic. I dont have the lap top out here do could not send file to it. And main pc too far away.

Second is the Tek.Set at 50 Mu per Div and .1 Ms per Div

Offline The Radio Shop

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Re: R27 receiver
« Reply #14 on: June 27, 2013, 09:16:22 PM »
I will take the lap top out there tomorrow and connect it to the DSO and capture the wave forms.  Really need a 10 fot UBS cable to connect it to the main pc. That feature in the DSO is sweet. I should use it more in radio repair but I dont.
Buddy kc4umo