The Electronics Universe
The CB Forum => CB Radio Repair Forum => Topic started by: KC4UMO on June 11, 2013, 08:45:12 PM
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Browning
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Part 2
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Sweet more stuff 8)
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it's gonna take me a little while to type this up, so bare with me,
:)
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no problem bro. take your time :)
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quote from you on the other site
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,
:)
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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?
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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,
:)
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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.
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BTW, I really need a digital cam and a video cam lol.
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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,
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Okay, I will scope those and see. Got my intersted now lol.
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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
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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
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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.
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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
well the one on the DSO looks clean enough to me, looks like i can see a little of the VFO fundumental riding on it, but it looks supressed enough, the other scope looks a little out of sync to me,
is that the output of the 1st mixer? looks like the freq says 25.something, can't make it out for sure,
and if it is, does the freq change with the VFO as it should?,
:)
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I never could get a good waveform on the 2235. The DSO did pretty good though. The frequency on the DSO was reading 29 to 30. Tomorrow I get some screenshot pics from the PC osciloscope funtion You will be able to read every thing and I will take a shot every few seconds so you see the changes. I like the built in FC in the DSO.
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I never could get a good waveform on the 2235. The DSO did pretty good though. The frequency on the DSO was reading 29 to 30. Tomorrow I get some screenshot pics from the PC osciloscope funtion You will be able to read every thing and I will take a shot every few seconds so you see the changes. I like the built in FC in the DSO.
ok at what point was this signal taken from? thats what i'm still not sure of,
oh and yeah man i love my DSO i would be lost without it, mine has a USB stick port so i can save a display pic to a flash drive, i really like that feature,
:)
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Sorry, yes that was the output of the 1st mixer. but cant remember which side of the cap I took it from lol. Think it was between the output and the cap going to the 2nd mixer.
I am a bit tired lol.
I dont think mine will alow me to save to my flash drive. When I stick the drive in the usb it does not come on.
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which DSO is it? i know you posted about it over at CBT but i don't remember,
with mine i had to format a USB stick to work with it, it had the wrong type of file system on it, i think it had to be formated with FAT32 to work with mine, tho i can't remember for sure, or maybe it was FAT 16, yup i believe that was it now that i think about it,
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which DSO is it? i know you posted about it over at CBT but i don't remember,
with mine i had to format a USB stick to work with it, it had the wrong type of file system on it, i think it had to be formated with FAT32 to work with mine, tho i can't remember for sure, or maybe it was FAT 16, yup i believe that was it now that i think about it,
Mine is the OWON PDS 6062. Just spent 20 minutes reading in the manual and nothing noted there about the usb port on the back. Bummer
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ok mine has the USB connector to hook it up to a computer like yours, but it also has a USB port to plug a flash drives into it as well, i just looked at your user manual and i don't think yours has the USB flash drive port on it,
its a real pita to get it to working anyway without linux to repartition the flash drive with a file system that it can use, i don't think froggy has gotten his to work with a flash drive yet, and his is just like mine, windows really limits you as time goes on, i doubt that the latest windows even has the ability to partition a flash drive, in linux its easy tho,
:)
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i'm gonna get off this thing for a while and see who's on the radio, catch you tomorrow bro,
l8r :)
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Okay bro, I fell asleep here in the chair. take care.
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lets see if this works.
Again this is 1st osc output.
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why is the scope reporting it to be 31.250Mhz instead of 31.4Mhz as it should be?
i know the frequency readout on my scope is the same as my best counter, so reckon the crystal is off frequency causing the problem,
notice the rise and fall in amplitude about every 6 to 7 cycles?? that is the VFO signal doing that,
now if you take the signal from the junction of C14 and C22 where the two signals come together, i bet the VFO signal will be easier to see there, it should cause the 31.4Mhz signal to rise and fall even more,
oh and by the way, nomad radio answered the question as to whether or not the crystal oscillator signal should be stronger than the VFO over at CBT, and i think he is correct,
:)
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I wanted to see if you noticed that about the frequency. :)
I am going to check more on itlater today. At the moment I have to run to town and had folks in here all day.
I think nomad hit it spot on also.
back in a bit
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Ok Laz.These 2 were taken at C14 and C22 juntion. First one is channel 23, second one is channel 1
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Another test mate.
I have seperated the 2 oscilators. Disconnected from input.
Channel 1 on scope is at top and you will see chan 1 data connected to the 31 meg osc.
Channel 2 is on bottom.
It is connected to the 4 to 5 meg osc.
You will see channel 2 oscilator dat in second pic
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Now tell me,
What do you see? :D
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it makes no sense to me, the red trace on the top looks like just a dirtier version of the green trace on the bottom, plus both scope pics look almost identical, i'm lost bro there should be alot more difference in the two signals, and all of em say 5.xxMhz, it makes no sense, looks like the VFO is all thats working in these pics,
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it makes no sense to me, the red trace on the top looks like just a dirtier version of the green trace on the bottom, plus both scope pics look almost identical, i'm lost bro there should be alot more difference in the two signals, and all of em say 5.xxMhz, it makes no sense, looks like the VFO is all thats working in these pics,
BINGO!
The scopes counter is reading 5 MHz from bother oscilators. There is very little difference in the two signals.
Now remember. I disconnected the 2 oscilators from the mixer and from each other. I am not sure if this is causing the oscilators to run away or what is going on. But, My guess is there is something wrong with the 31.4 MHz oscilator circuit.
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So checked the tubes and dont think the problem is with them. My tube tester says 40% but installed 2 new 12AT7 in the unit and does the same thing. No difference. I now have 3 units here with this problem.
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Got my new 12AT7's in. Going to try and get back on this today.
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Cleaned the sockets and no change. So decided to drag out the Sencore CS61 scope and take a few measurments. Seems that the Tek scope loaded the signal paths down. But the Sencore did the job. What I did was probe the output of bother oscilators, the 5 MHz and the 31 MHz where they combined. All I could see was the 5 MHz side as noted in below screen shots.
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After replacing R11 a 47K I now see 31 MHz