5W Stentor Transmitter question...

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Albert H
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Re: 5W Stentor Transmitter question...

Post by Albert H » Mon Oct 05, 2020 1:54 am

I discovered one of the old "Stentor" transmitters in a box of miscellaneous junk the other day. I also found this thread:
RF-Head wrote:
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
Translation:

The Stentor came on the market around 1983 and that made it possible for anyone who wanted to broadcast on the FM Band, with all the consequences that entailed! In fact, so much has to be changed to this circuit for it to be good enough, that a different transmitter is preferable to this Stentor. It was also called "the disturbance" and "a lot of power for little money" for a reason. Out of nostalgia and for those who still have one lying around, I feel that I had to post this diagram, but again, it actually caused the "disturbance" complaints to increase and the FM Band became quite sick at the time!

Improvements:
* Connect a stable voltage source, cutting the oscillator supply to 9 volts with a 7809 regulator IC, and the remainder with 7812 to 12 volts.
* Always decouple the voltages on the "plus" properly and use RFCs.
* Include screens of PCB material or tinplate between the three stages and install the transmitter in A PCB material or tinplate box
* Connect a good filter and do not connect any further amplifier.
* Use a good antenna with a good radiation pattern, and good match, so don't just connect a dipole to the end of it!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

Albert H
proppa neck!
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Re: 5W Stentor Transmitter question...

Post by Albert H » Mon Oct 05, 2020 2:02 am

I fired up the one I found here. It's been modified with a 9V regulator for the oscillator and a 12V regulator for the rest of the board, and it has an extra output lowpass filter. It does just about 4 Watts at 98 MHz, and drifts around 40kHz from switch-on until it stabilises. When the trimmers are all peaked for the frequency of choice, the second harmonic is about 68dB down. It needs a PLL for proper stability, but as a quick, cheap rig, it's a hell of a lot better than the nasty Chinese efforts!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

radium98
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Re: 5W Stentor Transmitter question...

Post by radium98 » Wed Oct 07, 2020 2:42 pm

hhhh what a joke for chinese shits ,even they used japenese transistor ,they can not clone a 5w transistor lol ,what said for american ampelon etc..transistor even an old BLV25

XXL
no manz can test innit
no manz can test innit
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Re: 5W Stentor Transmitter question...

Post by XXL » Wed Oct 07, 2020 6:40 pm

Can’t you just add pll to this ? It would be easy and maybe useable then. I dunno how you 80s lot got on with vfo. It’s absolute garbage. Although I guess you didn’t need to be exactly on frequency unlike now days where all radios are digital tuning.

Albert H
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Re: 5W Stentor Transmitter question...

Post by Albert H » Thu Oct 08, 2020 2:32 am

XXL - some of us knew how to build oscillators! However, the serious amongst us used PLL circuits of one type or another.

One popular type of phase lock circuit would lock in 200 kHz steps up the band - the phase comparator would respond to multiples of 200 kHz. It was pretty good - the exciter frequency was adjusted to where you wanted, and it would settle on a 0,2MHz point up the band. The only problem came when someone over-modded the link, and the transmitter would jump up or down to the adjacent 200 kHz step!

A method I frequently used had a CB PLL IC, and a 10.240MHz crystal. The crystal was the reference for the PLL and was also multiplied by eight to 81.92 MHz. It was then mixed with another low frequency phase-locked oscillator to give the Band II frequency. There had to be a critically tuned filter in the way to reject the unwanted mixing products, but it would develop a good, clean and stable signal, using just one IC.

Another trick I discovered was to use a really cheap PLL-tuned receiver. I'd remove the front end to reduce its sensitivity, then tune the receiver to the exciter frequency, and use a voltage derived from the receiver's discriminator as an AFC for the transmit oscillator! This could also be used to provide a form of deviation limiting, too. It was a really cunning trick, and I used it in rigs from the early 80s well into the 90s. The receiver I used to use was a cheap "Korting" model or the same thing badged as "Philips". I got them from a friend in the Philippines (where they were made) for about £90 for a box of 12 of them!

I also discovered the 74AC series and 74F series of logic ICs. If you ran them at 6V, they'd easily divide down a Band II signal! This made PLLs really easy. I messed around with phase comparator circuits, and found a method using the four NAND gates in a 7400 and a dual bistable 7474. This drove an op-amp filter to provide the tuning voltage, and also gave a really reliable "locked" output. The logic PLL used a bigger board, but was really cheap to make.

I did do some PLL add-ons for the Stentor back in the 80s. Considering its simplicity, it was a remarkable little rig!
"Why is my rig humming?"
"Because it doesn't know the words!"
;)

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