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29th Jan 2006, 12:33 am | #41 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Some more progress today / tonight. I could not resist doing a test assembly of the major parts to get a sneak preview of what the final result will be:
http://gargnas.net:3000/bilder/dassrull/P1290003.JPG http://gargnas.net:3000/bilder/dassrull/P1290004.JPG Ed, can you see your meters?! (Thanks, and thanks again, they are awesome!) Build exitement level 8(10)! =) /Oskar |
29th Jan 2006, 5:45 pm | #42 |
Dekatron
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Location: Newcastle upon Tyne, Tyne & Wear, UK.
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Oskar, the meters cleaned up nicely. Looking at the tuning cap I remembered that there used to be articles about making your own in the 20's magazines. These could easily have been adapted to suit the higher voltages and spacings required for transmitting use.
Ed |
29th Jan 2006, 8:01 pm | #43 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Ed,
Yes, I took them apart and cleaned the glass carefully with cotton "tops" dipped into window cleaner (not sure what you call them, they are supposed to be used to clean your ears : ) ). I had to use a razorblade to remove the most persistant tape remains. The tuning cap is indeed homebrew. At least I think so. It might be something sold in a kit form to hams. brg Oskar |
30th Jan 2006, 11:29 pm | #44 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Just some work done to the VFO this evening.
http://gargnas.net:3000/bilder/dassr...o/P1300004.JPG The mechanics of it is basically sorted. I am not sure if I really should be using pressed paper for the oscillator coil-form. The alternative is some grey PVC tubing I found in the garage. Good night ... zzz /Oskar |
1st Feb 2006, 9:28 pm | #45 |
Hexode
Join Date: Apr 2003
Location: Cotswolds, UK.
Posts: 465
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Use something stable with low rf absorbtion for the coil. Polythene or similar. The cardboard won't be physically stable enough to keep the coil form stable and hence the frequency will not be stable.
Robin |
2nd Feb 2006, 10:23 am | #46 | |
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Quote:
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2nd Feb 2006, 12:09 pm | #47 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
My usual technique is to wind the coil and test it, and then cover it witch laquer. I think it should work. If the coil is not covered in laquer it is also easy to accidentally mess up the turns and move them apart for example.
/Oskar |
2nd Feb 2006, 1:35 pm | #48 |
Dekatron
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Gents, on the topic of coil formers, Philips had some very good ribbed glass ones. The design aim is to get a stable (mechanically) former that has an Er (permitivitty) near 1 (air). This is why a ribbed former is often used.
Cardboard, well lacquered is a good substitute, with 6 or 8 ribs of varnished hardwood glued on. These were some of the tricks used for home construction in the early days; they appeared to work very well. HTH Ed |
2nd Feb 2006, 1:47 pm | #49 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
I am having a language difficulty here. =) Could you explain "ribbed" Ed, how such a coil form looks? And how would the wooden ribbs be applied?
/Oskar |
2nd Feb 2006, 4:05 pm | #50 |
Hexode
Join Date: Apr 2003
Location: Cotswolds, UK.
Posts: 465
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Imagine a number of fins all glued together in the middle to form a star. The outside of the ribs are slotted to take the turns and locate them.
The wire is then wound round this. The wire in these cases is often guite large gauge. Robin |
2nd Feb 2006, 4:11 pm | #51 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Aaah, ok now I get it! Thanks!
Unobtainable though. I will make a test coil on the cardboard form to be able to test the VFO. Maybe later I will find more suitable coil form material. Glass or ceramic would be nice. (If I could only get my Snowmobile together so I can go on building! Track suspension was busted in all sort of ways ) /Oskar |
6th Feb 2006, 7:38 pm | #52 |
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Oskar,
That former looks massive! I would doubt you need anything like that size for a VFO working on 80m - I might have some nice ribbed ceramic formers here, but nowhere near that big.... Cheers Sean
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10th Feb 2006, 8:41 pm | #53 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi all,
Not much done as far as building is concerned I'm afraid. I have made a coil and tested the VFO though. It is working! But I have to arrange a capacitor combination to limit the band coverage. Right now it tunes all over the place! Oh, that former was intended for 160m. I'm quite curious on that band, so I'll make coils for 80 & 160. When the time comes, 160m seems like a better place to test the transmitter, not so crowded That way I am less likely to make someone angry The transmitter will not be instantly band-switchable. But could rather be rebuilt in a couple of minutes for different bands. B.t.W .. OT. I got my Snowmobile together at last, runs like a dream .. ah. =) Have to trow clothes into washer .. they're soaked with two-stroke-fumes .. :P /Oskar |
20th Mar 2006, 12:06 am | #54 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi all!
Just a quick progress update before I go to bed Did a test-assembly of the PA on a scrap chassis. No disasters fortunatly .. phew! (the magic smoke is still there!) Looks like I have some tweaking to do though. I could not get the plate current to get any higher than ~35mA. I have to do some measurements, to see if it's my grid resistor that is to large, or if the grid drive is not enough. Also the grid resistor feels a bit warm (it is a 3W part!), I will have to check this. Could the low plate current be down that the 807 I am using is weak? I took it from a old variable DC supply*. That unit used two 807's, and they were both marked 70mA. Anywhay, it looks promising! The 807 gets a really pretty "blue-glow" when the P.T.T is closed! (I do not think the valve is gassy, the glow is just present on the inside of the glass, not inside the valve structure, and this is quite normal (correct me if I'm wrong!).) *Don't worry, I did not scrap a nice piece of equipment, I found it at the local dump, and it had been run over by a tractor! Good night! /Oskar |
20th Mar 2006, 7:07 am | #55 | |||
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Oskar,
Quote:
Quote:
In the PSU you got the tube from, it was probably running in pure DC mode. Any lack of excessive emission would go completely unnoticed. So it can be very weak for your application indeed, without having caused any problems in its old job. Among my collection of eight used 807s, only one of them has emission strong enough to be able to run class C at full ratings. Quote:
Best regards Frank N. |
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20th Mar 2006, 8:04 am | #56 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Wait a minute!
Should I not have an RFC between the grid and the grid resistor? Maybe the grid resistor is dampening all of the grid drive ... hmm (could be the RF that is making it warm?). I'll have to look at the voltage at the grid with the scope to see how it looks. Measuring the grid current could reveal a few things also for sure. I did not want to connect my very nice meters into this prototype until I had everything measured up. I should also try to get a couple of NOS 807s'. /Oskar |
20th Mar 2006, 9:43 am | #57 | |||
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Quote:
Quote:
Any chance of a few snapshots of your component layout, both topside and below the chassis? Quote:
Did I mention your next step must be measuring the grid current? Frank N. |
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20th Mar 2006, 12:45 pm | #58 |
Pentode
Join Date: Oct 2005
Location: Gargnäs, north of Sweden
Posts: 105
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Yes, I will definitely measure the grid current tonight!
Here's some photos: http://www.gargnas.net:3000/bilder/d...l/P3200024.JPG http://www.gargnas.net:3000/bilder/d...l/P3200023.JPG http://www.gargnas.net:3000/bilder/d...l/P3200022.JPG http://www.gargnas.net:3000/bilder/d...l/P3200016.JPG http://www.gargnas.net:3000/bilder/d...l/P3200017.JPG A bit of a lash-up really, I will probably need to tidy it all upp to get some decent results out of it. The 807 plate RFC is about 2mH, and I'm testing now 160m. The tuning cap for the ouput PI filter is a bit (quite alot actually) oversized, but hey, it makes future expansion possible . I also have a strange problem. Sometimes when I fiddle about with the VFO frequency, and 807 grid tuning, something starts squeling (sounds like it is coming from the 807 grid tuning cap actually!) and the waveform on the scope goes crazy. I then quickly change frequency, and the problem is gone. Parasitics? I was thinking of adding parasitics supressors to the VFO also. /Oskar |
20th Mar 2006, 1:13 pm | #59 |
Retired Dormant Member
Join Date: May 2005
Location: Aarhus, Denmark
Posts: 281
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Oskar, you have a PM.
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20th Mar 2006, 2:30 pm | #60 |
Dekatron
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Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
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Re: Homebrew transmitter project (Frank vs. Oskar :) )
Hi Oskar,
It looks very nice!, However, that may be the cause of the problem, well, one or two of them anyway...... RF construction is very different to AF construction! single earth points as close to the valve as possible are the norm, and short interconnects are a must! I could probably detect your VFO/PA radiation in the UK Still, it is heading in sort of the right direction Cheers Sean
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