Stentor Design Questions

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Stentor Design Questions

Post by mountainman » Wed Sep 27, 2017 1:55 am

I put together a Stentor based on Yellowbeard's design:

http://homepage.eircom.net/~yellowbeard/5watters.htm

I didn't get to do much testing before work but the initial add power and feed input resulted in nothing coming from the antenna it was attached to.

Before we get too far into this, I am still learning a lot about radio design and function.

I had to make a few changes to his drawing to suit what I had available and am wondering which change(s) that I made that may have caused the non-functioning unit:

-The 3 leg 65p trimmer caps were replaced with 2 leg versions. I figured that because 2 of the legs always attached to the same trace/ground, it should still adjust the same amount of capacitance, right?
-I can't get any good info on what to use for a through-hole substitute for a BF245. I put in a 2N3904 and hoped for the best.
-The 12k resistor right next to the pair of 68k resistors was replaced with a 10k
-L1 and L2 look like they are supposed to be 5 and 6 turns respectively, with a center tap going to the middle hole. I think I got pretty close but the taps may not be in the exact center.
-I'm not sure I am applying the 12v (13.8VDC) in the right spot.

Here's some pics of what I came up with using the thermal transfer method for making the PCB.

IMG_20170926_110242.jpg
IMG_20170926_110221.jpg
IMG_20170926_110137.jpg
IMG_20170926_110120.jpg
Please critique and give me some ideas on what I did right/wrong.
You do not have the required permissions to view the files attached to this post.

User avatar
Analyser
tower block dreamin
tower block dreamin
Posts: 319
Joined: Fri Sep 26, 2014 9:11 pm

Re: Stentor Design Questions

Post by Analyser » Wed Sep 27, 2017 7:03 am

The 2N3904 is a general purpose NPN transistor but the BF245 is a JFET, so totally different devices.
You may be able to get away with putting a 10K resistor from base to +12v on the 2N3904 to get it to run but I'm not sure from just looking.
The best option is to buy a JFET, similar ones are the 2N3819 or J310, but study the datasheet carefully before fitting because the pinouts are different.

shorty
no manz can test innit
no manz can test innit
Posts: 123
Joined: Thu Sep 04, 2014 11:20 am

Re: Stentor Design Questions

Post by shorty » Wed Sep 27, 2017 8:49 am

Check the pinout of the mrf237, there are versions of this transistor from china with the collector/emitter the opposite way round not as per layout in the mrf237 datasheet.

shorty
no manz can test innit
no manz can test innit
Posts: 123
Joined: Thu Sep 04, 2014 11:20 am

Re: Stentor Design Questions

Post by shorty » Wed Sep 27, 2017 10:33 am

Ignore my post above, i didn't read you had used a 2n3904 instead of a fet, after you replace the oscillator transistor with a fet, it's still worth checking the pinout of the mrf237.

In future i'll read the whole post before jumping in with a comment, i can be a numpty some times.

mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Wed Sep 27, 2017 7:06 pm

2N3819 ordered! It'll be a week.

Any comments on replacing the 3 leg caps with the two leggers?

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Stentor Design Questions

Post by Albert H » Thu Sep 28, 2017 2:22 am

Analyser wrote: Wed Sep 27, 2017 7:03 am The 2N3904 is a general purpose NPN transistor but the BF245 is a JFET, so totally different devices.
You may be able to get away with putting a 10K resistor from base to +12v on the 2N3904 to get it to run but I'm not sure from just looking.
The best option is to buy a JFET, similar ones are the 2N3819 or J310, but study the datasheet carefully before fitting because the pinouts are different.
This circuit MUST use an FET for the oscillator to work. E301, J310, BF245 will all work. A bipolar silicon transistor (like your 2N3904) just won't do anything in this circuit.

The types of components are usually specified for good reasons! You can't just go changing one transistor type for another (unless you know what you're doing!).

The soldering on your board looks like you didnt clean the copper before soldering - rub the tracks down with a bit of "Ajax" scouring powder, then wash off the residue with water. If you want to get your fingerprints off the board too (usually a good idea since the greases from your fingers won't help the solder to flow!) use some Isopropyl Alcohol (available at your friendly local Pharmacy) and give it a final wash before soldering.

Also make sure that your iron is hot enough, and DON'T use that "lead-free" crap that Maplin will try to sell you. Get proper 60 / 40 Tin / Lead solder with a flux core (the best is Ersin Multicore if you can find it).

The three-legged trimmers work just the same as the two-legged variety - two of the legs are connected together. They're three-legged for mechanical stability.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Stentor Design Questions

Post by Albert H » Thu Sep 28, 2017 2:25 am

mountainman wrote: Wed Sep 27, 2017 7:06 pm 2N3819 ordered! It'll be a week.

Any comments on replacing the 3 leg caps with the two leggers?
The 2N3819 might not work too well in this circuit, unfortunately. Give it a try, but if it fails to oscillate, you'll have to get a real RF FET like the BF245!

See my comments in the earlier post about the trimmers.

Please heed what I said about the soldering.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Thu Sep 28, 2017 6:06 pm

The BF245s I'm finding on the interwebz are units being shipped straight from China, I'm worried about fakes. It looks like the only available ones are:
BF-
256B
256C
246B
247B
244B
410B


I guess I could order 10 of each along with some J310s and do a lot of plugging in to try to find what works. Surprised no one else has run into this issue on the Stentors recently, as the BF245 has been out of production for going on 10 years ago.

After ethching, I used acetone and the scour pad of the sponge to remove the etch resist. Then I used 220 grit to rough up the copper and then did a final wipe with Iso alcohol after the sanding. The smudges and remaining oil in the pics is probably from handling while soldering. I guess I need to throw on a pair of nitrile gloves while I'm soldering. I'm using 60/40 .8mm with 2% flux, got a big roll at a storage sale.

I have a Veronica 5w board going in the acid this morning. I'll use your procedure when the etching is complete.

I'm wondering if I have an old CB laying around that has a FET in it. I did a lot of AM/CB/SSB stuff before playing with FM so I have a few parts rigs laying around......

User avatar
yellowbeard
tower block dreamin
tower block dreamin
Posts: 305
Joined: Wed Jul 08, 2015 5:40 am

Re: Stentor Design Questions

Post by yellowbeard » Thu Sep 28, 2017 6:47 pm

I have an 80's one in my hand here that I dug out, your coils look a little fat but that may be the size of the trimmers. They should be 6mm inside diameter, and L1 is 5.5 turns tapped at 2.5 turns from the earthy end and L2 is the same but the tap is 3.5 turns from the 2N2219. I have never used an equivalent BF245, if you use another type be sure of the pin-out. The MRF237 can get sweaty - the original heatsink is a length of U shaped aluminium, that one you got on it may not be enough. One of our Dutch friends has started selling this kit again - see here:

Code: Select all

http://www.amateurradioshop.nl/products.php?385&cPath=55&sid=1mfbd8f7s07v2qin347roi9gb7
Have fun and let us know how you get on! :tup

mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Thu Sep 28, 2017 7:28 pm

yellowbeard wrote: Thu Sep 28, 2017 6:47 pm I have an 80's one in my hand here that I dug out, your coils look a little fat but that may be the size of the trimmers. They should be 6mm inside diameter, and L1 is 5.5 turns tapped at 2.5 turns from the earthy end and L2 is the same but the tap is 3.5 turns from the 2N2219. I have never used an equivalent BF245, if you use another type be sure of the pin-out. The MRF237 can get sweaty - the original heatsink is a length of U shaped aluminium, that one you got on it may not be enough. One of our Dutch friends has started selling this kit again - see here:

Code: Select all

http://www.amateurradioshop.nl/products.php?385&cPath=55&sid=1mfbd8f7s07v2qin347roi9gb7
Have fun and let us know how you get on! :tup
Coils are definitely too fat then. Measuring close to 10mm. I tried to base the size off of the picture in relation to the MRF, guess I was a little off. I'll wind some new ones. I also have a few slug tuned 4 and 7 turn TOKO types if you think I could get them to work for the non-tapped coils. The way I did the tap was to wind the coil around the former first, then scrape the paint off the coil near the middle on the side closest to the pcb. Then I soldered another piece of EnCo wire to the exposed copper pointing down so it would drop in the holes drilled on the PCB. I'm actually having to build this thing backwards (or maybe inverted?) because I did not flip the image I got from the yellowbeard site before ironing on and doing the etch. I suppose I could do it again if it would help me out with simplicity. I like the yellowbeard layout and once I found the PCB dimensions and got my scaling right, the holes are pretty much perfect for component layout.


Are you THE yellowbeard?

I had seen that dutch kit and was trying to use their site for any advice on how to put this homebrew together. There isn't much documentation on that site for the product, just one pic of the unassembled kit. I think I will try to document as much as I can during this build and post up a PDF somewhere for people like me who got in way over their heads.

Another thing I forgot to mention before was that I had to use a BB409 varacator instead of a BB109. That was probably a stupid thing but I'm trying to learn.....

mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Thu Sep 28, 2017 8:31 pm

yellowbeard wrote: Thu Sep 28, 2017 6:47 pm I have an 80's one in my hand here that I dug out, your coils look a little fat but that may be the size of the trimmers. They should be 6mm inside diameter, and L1 is 5.5 turns tapped at 2.5 turns from the earthy end and L2 is the same but the tap is 3.5 turns from the 2N2219. I have never used an equivalent BF245, if you use another type be sure of the pin-out. The MRF237 can get sweaty - the original heatsink is a length of U shaped aluminium, that one you got on it may not be enough. One of our Dutch friends has started selling this kit again - see here:

Code: Select all

http://www.amateurradioshop.nl/products.php?385&cPath=55&sid=1mfbd8f7s07v2qin347roi9gb7
Have fun and let us know how you get on! :tup

What should the coil spacing look like? I made some new ones on 5mm bolts which makes a nice 6mm ID. Based on the thread pitch, the L3,4,5, are only 6mm long from the one leg to the other. Should these be stretched out to the original mounting holes based on the Yellowbeard PCB layout or should I drill new holes?

I'm going to tackle the tapped coils after work, I'll check back on this board while I'm there.

Got a 4 day weekend coming up, would like to finish this project off if I can find a FET in one of these old CBs!

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Stentor Design Questions

Post by Albert H » Fri Sep 29, 2017 2:34 am

Stretch the coils so that the spacing between turns is just slightly more than the wire diameter. You'll find that any inaccuracy in the coils can be tweaked out with the trimmers, so don't worry too much about getting them dead right.

When you've got it going, DO NOT connect it directly to an aerial!! You're going to have to wind three more coils for a lowpass filter. Let us know when you get the thing working into your dummy load and I'll give you the dimensions, part details and layout for a good filter.

I remember connecting a frequency counter to one of these, and watching the numbers count like a rocket launch countdown as it drifted. When it achieved equilibrium, it was pretty stable (for a free-running oscillator). I messed with the components in the oscillator stage in an effort to make it more stable, and reduced the drift somewhat. I replaced the trimmer in the oscillator with fixed NPO ceramic capacitors, and roughly tuned the oscillator by stretching and compressing the oscillator coil. The stability was much better. I also used fatter wire for the oscillator coil to enhance its rigidity.

The final stability "fix" was to use a frequency counter module and monitor the last digit, deriving a control voltage that was fed to the varicap control line. This was a (sort of) crude PLL, and resulted in a Stentor that would tune up the lower end of Band II in 200kHz jumps as you turned the fine tune pot!

It went into an Eddystone box, with the MRF237 flange in contact with the case lid for heatsinking, and the frequency counter LED display (a calculator type) peeking through a slot so you could see what frequency it was on. The output trimmers were accessible through holes in the box, so it could be tweaked for maximum urge into whatever aerial it was feeding. It was used by a station in Belgium every weekend for a few years, running from a car battery and loaded into a Slim Jim up a tree. They used to get pretty good reception for about 10 miles around their site.

The only thing missing from the basic circuit is pre-emphasis. An audio buffer / pre-emphasis stage is a useful addition.
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

MiXiN
proppa neck!
proppa neck!
Posts: 612
Joined: Sun Sep 07, 2014 4:20 pm

Re: Stentor Design Questions

Post by MiXiN » Fri Sep 29, 2017 2:45 am

I made a thread on the 5W Stentor on here a while ago. A search will find it.

I bought mine from the Dutch seller above, and although the PCB was really decent, the MRF237 was one of the many fakes with an incorrect datecode.

I used a legit' Motorola MRF237 and got something like 4.5 Watts out of it at the lower end of the band, and 6 Watts out at the upper end and it was incredibly stable considering the design.

The only downfall was that it could only cover about 5 Mhz of the band if I remember correctly.

For coil formers, I used drill bits and they worked a treat.

As said above - use a more substantial heatsink on that final Transistor, because that one you have is insufficient.

By the way - there are loads of BF245 FET's on eBay in various gains (BF245A, B, C) made by Philips and Fairchild.

mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Fri Sep 29, 2017 5:12 am

MiXiN wrote: Fri Sep 29, 2017 2:45 am I made a thread on the 5W Stentor on here a while ago. A search will find it.

I bought mine from the Dutch seller above, and although the PCB was really decent, the MRF237 was one of the many fakes with an incorrect datecode.

I used a legit' Motorola MRF237 and got something like 4.5 Watts out of it at the lower end of the band, and 6 Watts out at the upper end and it was incredibly stable considering the design.

The only downfall was that it could only cover about 5 Mhz of the band if I remember correctly.

For coil formers, I used drill bits and they worked a treat.

As said above - use a more substantial heatsink on that final Transistor, because that one you have is insufficient.

By the way - there are loads of BF245 FET's on eBay in various gains (BF245A, B, C) made by Philips and Fairchild.
Your thread is what got me started on the build, aided by Yellowbeard's PCB drawings.

Which BF245 did you use? I have about a dozen parts CBs with several fets in them, gonna do a cross reference on specs when I get home and see if anything is close to the BF245. If not, I'm heading to dump to dig through piles of old stereos and cassette players and try to find one there. I got another week til payday so I'm doing free or nothing at this point while looking for the FET. :smoke

MiXiN
proppa neck!
proppa neck!
Posts: 612
Joined: Sun Sep 07, 2014 4:20 pm

Re: Stentor Design Questions

Post by MiXiN » Fri Sep 29, 2017 8:58 am

I'll dig out the Stentor later and have a look, but I'm pretty sure it was the BF245A that was supplied in the component kit.

radium98
proppa neck!
proppa neck!
Posts: 907
Joined: Fri Aug 26, 2016 7:01 pm

Re: Stentor Design Questions

Post by radium98 » Fri Sep 29, 2017 4:44 pm


mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Sat Sep 30, 2017 4:23 am

I like that Stentor layout radium. Im on the edge of printing a new board from that silk screen and transferring everything over to the new layout. I'll probably try this one first and then build a second (I bought a dozen each of 2219 and MRF237 from electronic surplus-They are really cheap on that stuff if you don't mind stuff with short leads being cut out of something) stentor with what I learned from the first.

Looking at putting a JF1033B JFET from a Cobra 139XLR in place of the BF245. The 3819 is still at least a week out and I'm flipping through what I have available now. The 1033 looks farily similar to the BF245 by looking at datasheets, and has noise figures calculated @ 100mhz, leading me to believe it would work in the first stage. Similar maximum dissipation ratings (350mA vs 315mA) and it is already being sourced from a transmitter, even if it is a 27Mhz AM xmitter.

Thoughts?

Albert H
proppa neck!
proppa neck!
Posts: 2737
Joined: Tue Apr 05, 2016 1:23 am

Re: Stentor Design Questions

Post by Albert H » Sat Sep 30, 2017 10:04 am

That FET should work - just be sure to get the pin-out correct!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

radium98
proppa neck!
proppa neck!
Posts: 907
Joined: Fri Aug 26, 2016 7:01 pm

Re: Stentor Design Questions

Post by radium98 » Sat Sep 30, 2017 5:31 pm

you can fir a BF494 or BF199 instead but it is npn so you must redesign a bit yes the one i posted is pretty good for a VFO and are you going to couple a PLL eg the one posted on PIRA ?

mountainman
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 46
Joined: Tue Sep 26, 2017 2:34 am

Re: Stentor Design Questions

Post by mountainman » Sat Sep 30, 2017 7:45 pm

Albert H wrote: Sat Sep 30, 2017 10:04 am That FET should work - just be sure to get the pin-out correct!
Just pulled two of them out of the old crippled Cobra. It had a bad PLL chip in it so I don't feel too bad stripping down a 70's classic CB. We'll give them a try here in a bit once I decide which PCB to use. With the incorrect coils and shoddy solder pads on the first go, I'm liable to go ahead and print one of the ones radium posted and build off of that. If I'm reading the sch right, it looks like there is only one tapped coil to make on that version.

radium98 wrote: Sat Sep 30, 2017 5:31 pm you can fir a BF494 or BF199 instead but it is npn so you must redesign a bit yes the one i posted is pretty good for a VFO and are you going to couple a PLL eg the one posted on PIRA ?
I don't think I quite ready to go designing circuits with PLLs on my own just yet. I did a lot of logic pin mods on the CB PLLs to give me different freqs based on the channel selector but those were extremely well known modifications and I was just following directions. I think I am going to start over based on the PCB layout you posted and try to get this thing going sometime tonight.

Quick question:

I need to build a small dummy load for testing this guy. I know that if you put resistors in parallel, you basically find the mean average of the resistors that gives you your total resistance. eg (2) 10 ohm + (3) 50 OHM= 170 OHM/5 Resistors = 34 ohm.

I have a few .5 ohm 4 watt resistors from another scrap project but these obviously are not going to give me a 50 ohm dummy load. So my question is, if I do (4) .5 OHM (4W) + (1) 230 OHM (.25W) = 232 OHM/5 Resistors = 46.4 OHM. Will I still end up with a 16.25W capacity or will all the juice go to cooking the single quarter watt resistor?

I have a bunch of 1 and 2W resistors but I'd prefer not to have to put (20) 1W 1k ohm resistors in parallel to get a 20W 50 ohm DL. If I can use the 4W resistors, it would save a lot of space and time.

EDIT: Not sure my math was quite on point on that last calculation.....

Post Reply