5W Stentor Transmitter question...

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s2000

Re: 5W Stentor Transmitter question...

Post by s2000 » Sun Aug 14, 2016 7:25 pm

shorty wrote:Did your mrf237's come from a seller in Wales?
Blimey that is sounding familiar lol The ones I bought seem to have the emitter pin gounded to the case with the tab marking the collector. They're all good though, can easily get 8 watts out with a bit of over driving and a good heatsink. Wish I could get more! :tup

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Re: 5W Stentor Transmitter question...

Post by MiXiN » Sun Aug 14, 2016 10:23 pm

Thanks so much guys, particularly Sinus!

The tab that usually denotes the Emitter instead denotes the Collector as said ^. I never knew this - and I mustn't be the 1st to have tripped up on this.

I thought I'd tried the MRF237 both ways, but I was mistaken.

Anyway - it's now working and giving just under 6W at 13.8V, and the frequency is incredibly stable once the Stentor has been switched on for about 5 minutes; I set the frequency to 95.4 Mhz and it varied only between 95.4002 to 95.4007 Mhz at all times.

For the time being, I've fitted a tall TO39 heatsink (as used on NRG PLL rigs) as I've yet to drill the u shaped Aluminium heatsink provided for the final.
20160814_201704.jpg


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Re: 5W Stentor Transmitter question...

Post by sinus trouble » Sun Aug 14, 2016 10:53 pm

Glad to see its working Mr Mixin!
You should now be ok to bolt the U shape heatsink direct to the groundplane
Be cautious when drilling though! I believe the case of the MRF237 is 8.1mm??
Use a nice sharp 8mm drill bit and fine tune with maybe a few twists of a 10mm round coarse file?
If unsure? test on a bit of scrap ally first to achieve a snug fit :)
I am as stupid as I look! :|

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Re: 5W Stentor Transmitter question...

Post by MiXiN » Sun Aug 14, 2016 11:23 pm

sinus trouble wrote:Glad to see its working Mr Mixin!
You should now be ok to bolt the U shape heatsink direct to the groundplane
Be cautious when drilling though! I believe the case of the MRF237 is 8.1mm??
Use a nice sharp 8mm drill bit and fine tune with maybe a few twists of a 10mm round coarse file?
If unsure? test on a bit of scrap ally first to achieve a snug fit :)
Thanks, and an additional thanks for the MRF237 dimensions.

Much appreciated mate.

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Re: 5W Stentor Transmitter question...

Post by MiXiN » Sun Aug 14, 2016 11:31 pm

s2000 wrote:
shorty wrote:Did your mrf237's come from a seller in Wales?
Blimey that is sounding familiar lol The ones I bought seem to have the emitter pin gounded to the case with the tab marking the collector. They're all good though, can easily get 8 watts out with a bit of over driving and a good heatsink. Wish I could get more! :tup
If you check this PCB component placement of the Stentor, it's also showing the "normally" Emitter tab as the Collector on the MRF237/SD1127 so maybe this is the norm' for these 2 Transistors? Lol. http://www.senderbau.egyptportal.ch/yelstentor.htm

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Re: 5W Stentor Transmitter question...

Post by sinus trouble » Mon Aug 15, 2016 12:05 am

Mr Yellowbeard is a true ledge!!!
I discovered his site in the early years! Not only did he lead me to NRG, his non biased and experienced opinion taught me a few things about the designs i had used (pantek velleman etc)
At that time, there was not many willing to share their knowledge
I am as stupid as I look! :|

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Re: 5W Stentor Transmitter question...

Post by shorty » Mon Aug 15, 2016 8:14 am

I've started to think the sd1127 with tab as collector is probably been sold as mrf237 and 2sc1947, maybe the same transistors from china just marked up differently as i've had the same problem with 2sc1947's and i know on the 1947's emitter is tab but the ones i got didn't work and looked identical to the mrf237 and gave roughly the same reading on my meter, but theres no way i can now test as i binned them as i thought they were junk.

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Re: 5W Stentor Transmitter question...

Post by teckniqs » Mon Aug 15, 2016 9:54 am

You all beat me to it, as I mentioned before 237 was connected wrong way around, glad you got it working.

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Re: 5W Stentor Transmitter question...

Post by Albert H » Mon Aug 15, 2016 1:15 pm

shorty wrote:I binned them as I thought they were junk.
The 2SC1947 is a wonderful device. It's got a huge amount of gain compared to the previous generation devices (like the MRF237). The only real issue with them was the need for a big heatsink. The "Stentor" approach, which puts a big piece of earthed metal between the oscillator and the output, is probably the best one.

I looked at the "Stentor" and wondered if that approach could be modernised, getting rid of the trimmers and adding a PLL. I came up with a three transistor exciter, with a dual-gate FET as the oscillator, with its output amplitude adjusted by a diode-pump RF sensing circuit. The buffer stage was a BFR96, giving about 300mW. The final was a 2SC1947A, using the input filter trick that Stephen Moss developed so that the gain was fairly flattened across the band.

I used a Schottky Diode rectifier to derive a voltage proportional to the output power, and used this to control the gain of the dual-gate FET in the oscillator - this guaranteed exactly even power right across the band. It also gave the option of adding an extra transistor to turn off the power to the buffer stage if the SWR was very wrong.

The PLL was based on the "Pira" SAA1057 design, but with the code slightly modified to enable the "locked" output function. The modulator was scaled for 1V5 p-p for 75kHz deviation, and uses the control voltage to adjust the deviation sensitivity - it's not perfect, but the maximum for 75kHz deviation only varies by about 0V05 across the band.

The board develops 4 Watts exactly at 12V65 supply (a 7812 with a silicon diode in the ground lead gives exactly the right supply voltage!)

The whole thing fits on a 2½"×4½" PCB, and has a fairly big metal flange as the heatsink for the output transistor which also provides the screen between the output filter and the rest of the board. The cleanliness of the output is pretty good - no harmonics worse than -50dBc - and the in-band noise is very low. The only flaw is the PLL re-tuning noise. It's very low level, but it's irritating, and I don't really see a way of getting rid of it!
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Re: RE: Re: 5W Stentor Transmitter question...

Post by MiXiN » Mon Aug 15, 2016 3:52 pm

RF-Head wrote:A MRF237 is 100% emitor to ground.
if not you have fake MRF237s
I have found the original stentor board with papers in my father storage yesterday and will upload some picture of teh original 80s stentor board
RF-Head,

My friend bought a Stentor board off you a long time ago, but the PCB layout isn't like anything he's found on the internet - so he's given it to me as he can't build it.

The end stage (Amplifier) is easy enough to work out, but some of it is a mystery - so if you have a PCB component layout for this board it'd be great as I can also construct this as well. 8-)

Here's the board he bought from you...
20160815_151549.jpg
20160815_151607.jpg
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Re: 5W Stentor Transmitter question...

Post by Analyser » Mon Aug 15, 2016 4:31 pm

Albert H wrote: The modulator was scaled for 1V5 p-p for 75kHz deviation, and uses the control voltage to adjust the deviation sensitivity - it's not perfect, but the maximum for 75kHz deviation only varies by about 0V05 across the band.
Something I've always found challenging, because any variation in inductance (or any other tolerance) in the tank circuit causes a change in sensitivity. You almost need to trim every circuit individually to get it right, which is a pain.

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Re: 5W Stentor Transmitter question...

Post by shorty » Mon Aug 15, 2016 8:16 pm

@Albert H, i wasn't saying genuine 2sc1947's were junk they work well, i was referring to the fakes i got thinking they were genuine that didn't work so binned them.

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Re: 5W Stentor Transmitter question...

Post by Albert H » Mon Aug 15, 2016 10:08 pm

Analyser wrote:
Albert H wrote: The modulator was scaled for 1V5 p-p for 75kHz deviation, and uses the control voltage to adjust the deviation sensitivity - it's not perfect, but the maximum for 75kHz deviation only varies by about 0V05 across the band.
Something I've always found challenging, because any variation in inductance (or any other tolerance) in the tank circuit causes a change in sensitivity. You almost need to trim every circuit individually to get it right, which is a pain.
It's a real pain in the backside, but the trick is to only vary ONE parameter in the oscillator. That way you don't end up with ridiculously complicated equations with endless variable terms. It's not too difficult.

Start by designing an oscillator for 98MHz (or 100MHz to keep the numbers easy). Make sure that your varicap control Voltage goes up to close to 30V (use a voltage multiplier driven by a spare PLL output to give the elevated voltage) and goes way down to close to 0V. (My VCO tunes 97 - 108.5 MHz from 1.25V - 29.3V) . Add another varicap, biased from the 30V rail, for the modulation input.

Ordinarily, you'd have a single capacitor isolating the mod varicap from the rest of the oscillator. The trick is to make this capacitor an extra varicap, biased from the tuning voltage, but scaled down with a resistive network. You couple your mod varicap and its control varicap together with a (relatively) high value capacitor (1n or so). By scaling the tuning control voltage on to the extra modulation varicap, you can correct the sensitivity right across the band. With some work (and a couple of op-amps) you can actually make it perfect!
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Re: 5W Stentor Transmitter question...

Post by RF-Head » Tue Aug 16, 2016 8:50 am

@ Mixin,
Here a PDF that i made a few years back for the old website fmzenders.com
There is a picture of a stentor made on your PCB
This is the PCB from the original 80s production line :)
Also the schematic and some text about the stentor (only dutch)
Please let me know if you need more info

https://zonderva.home.xs4all.nl/data/stentor.pdf

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Re: 5W Stentor Transmitter question...

Post by MiXiN » Thu Aug 18, 2016 3:22 pm

RF-Head wrote:@ Mixin,
Here a PDF that i made a few years back for the old website fmzenders.com
There is a picture of a stentor made on your PCB
This is the PCB from the original 80s production line :)
Also the schematic and some text about the stentor (only dutch)
Please let me know if you need more info

https://zonderva.home.xs4all.nl/data/stentor.pdf
Thank you very much mate.

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Re: 5W Stentor Transmitter question...

Post by MiXiN » Tue Nov 22, 2016 10:00 am

My Stentor has a frequency range of around 91 > 98 Mhz but I want to cover about 100 > 108 Mhz.

To get it higher in frequency, would I just move the tapping on the inductor near the Oscillator FET? If so, which way - or would I reduce the number of turns on the inductor?

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Re: 5W Stentor Transmitter question...

Post by Albert H » Tue Nov 22, 2016 6:21 pm

Stretch the inductor, so the turns are further apart. You'll find that it goes up the band. If you can't get it to go far enough up, take a turn off the oscillator coil. You may also have to stretch the coils towards the output to get the full power.
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Re: 5W Stentor Transmitter question...

Post by MiXiN » Tue Nov 22, 2016 11:26 pm

Nice one Albert.

I'm getting 4W down at 91Mhz & just under 6W at 98Mhz as it is, but I'll do as you said, have a fiddle around and report back in the next few days.

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Re: 5W Stentor Transmitter question...

Post by Maximus » Wed Nov 23, 2016 1:19 am

Albert H wrote:Stretch the inductor, so the turns are further apart. You'll find that it goes up the band.
This^
I'm not sure if it's the same for LPF filters. I've been told that sometimes you've got to spread the coils out.

Reminds me of when I mistakenly read a 2.2uf cap for a 22uf cap.

Really silly mistake. I could see the full swing voltage of the pll ic going up and down on the scope.

Took me ages to realise the mistake. I think the cap was between pin 2 or 3 to ground.



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Re: 5W Stentor Transmitter question...

Post by RF-Head » Wed Nov 23, 2016 8:52 am

@mixin
For what i see on your stentor pcb is that you used complete turns on your coils but you need to use half turn like 5 1/2 turn
see pictures of other stentors on google

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